Note: Descriptions are shown in the official language in which they were submitted.
20-TR-1309
~1--
CHOPPER TYPE PROPULSION SYSTEM WI'rH
_
LOW SPEED ELECTRICAL BRAKING CAPABILITY
FOR TRACTION VEHICLES
_
Back~round of the Inventlon
The present invention relates generally to
electrical propulslon systems for traction vehicles,
and it relates more particularly to means for providing
improved electrica~ braking of a system using electric
power choppers to control the magnitude of current -in
sel~ excited d-c traction motors.
Large electrically driven traction vehicles
such as locomotives or transit cars are propelled by a
plurality o~ traction motors mechanically coupled to
the respective wheel sets of the vehicle. Such motors
are usually of the direct current (d-c) type. A d-c
traction motor comprises a stator, a rotor, armature
windings on the rotor, and rield windings (either con-
nected in series with the armature or separately ex-
cited) on the stator. In order to control its tractive
effort, there is associated with the motor suitable
~eans for regulating the magnitude Or direct current in
the motor arm~ture. Electric power apparatus commonly
~G known as a chopper is an energy conservlng means for
regulating arn!ature current.
A chopper ls essentiall~ a controlled switch
connected in circuit with the motor armature to meter
current from a ~ource Or relatively con~tant ~-c elec-
,
20-TR 1309
-2-
tric power to the motor. The swltch is cyclically op~
erated between open and closed states, and by appropr1-
ately controlling the timlng of the successive tran-
sitions between these alternate states the magnitude of
armature current can be varied or maintalned substan~
tially constant as desired. Assuming the chopper ls ln
series with the motor and the propulsion system is op-
erating ln its motoring mode, during closed periods of
the chopper the motor armature windings will be con-
nected to the d-c power source through a path of negli-
gible resistance, whereby virtually the full magni~ude
of the source voltage is applied to the motor armature
and the current tends to increase. During the open
periods of the chopper, the motor is disconnected from
the power source and armature current, clrculating
through a free wheeling pathg decays ~rom the magni-
tude previously attained. In this manner, pulses o~
voltage are periodically applied to the motor, and an
average magnitude of motor current (and hence torque)
is establîshed. The rate of change of current is
limited by the circuit inductance.
The ratio o~ the closed time (toN) of the
chopper to the sum of the closed and open times
(toN + toFF) during each cycle of operation is the
2~ duty ~actor of the chopper. ~or a 0.5 duty ~actor,
the repetitive closed and open periods of the chopper
are equal to each other, and the width of each voltage~
puise has the same duration as the space between suc-
cessive pulses. In practice, so long as the chopper fre-
quency is relatively high (such as, ~or example, 300Hz) the circuit inductance (lr.cluding the inductance
provided by the armature windi.ngs of the traction motor
itself) wlll smooth the undulating current in the motor
armature sufficiently to preven~ untoward torque pul-
20-TX-1309
~3-
sations, whereby the vehlcle is propelled without any
uncomfortable amount of ~erking or lurchlng. ~y
varying the duty ~actor of the chopper~ the average
chopper output voltage ~as a percentage of the d-c
source voltage) and consequently the average magnitude
o~ current can be increased or decreased as desired.
This is popularly known as tlme ratio control or pulse
control.
A propulsion system using choppers can be
adapted for electrical braking by reconnecting the
power circuits so that each chopper is connected to the
d-c power source in parallel rather than ln series with
its associated motor. In the braking mode of opera-
tion, a traction motor behaves as a generator, and the
magnitude of its generated voltage (electromotive
force) i3 ~roportional to speed and field excltation.
The excitation of a series field machine is a function
of the magnitude o~ armature current. With the
chopper reconnected in parallel w~th the motor, during
its closed periods the chopper provides a low resis-
tance path for armature current which therefore tends
to increase, whereas during its open periods the arma-
ture current path includes the power source and the
free wheeling path, whereby current tends to decrease.
The electric power output of the motor is either fed
back to the source (regenerated), or dissipated in a
dynamic braking resistor grid that can be connected
in parallel with the chopper, or a comblnatlon of both.
In either case, the average magnitude of armature cur
rent (and hence braking efrort) can be controlled as
desired by varying the duty factor of the chopper.
In the present state of the art, choppers for
traction vehicle applications use high-power, solid-
state controllable switchlng devices known as thy-
35 ristors or sllicon controlled rectifiers (SCRs), A
z~
20-TR 1309
- thyristor i~ typically a three-electrode de~ice haYing
an anode, a cathode, and a control or gate terminal.
When lts anode and cathode are externally connected ln
series with an electrlc power load and a source of ~or-
ward anode voltage (i.e., anode potential i5 positive
wlth respect to cathode), a thyristor will ordlnarlly
block appreciable load current until a ~iring signal
is applied to the control terminal, whereupon lt
switches from its blocking or "off" state to a con~
ducting or "on" state in which the ohmic value of the
anode-to-cathode resistance is very low. Once trig-
gered in this manner and latched in by conducting load
current of at least a predetermined minimum magnitude
prior to removal of the firing signal, the thyrlstor
can be turned off only by reducing the current through
the device~to zero and then applying a reverse voltage
across the anode and cathode for a time perlod suffi-
cient to allow the thyristor to regain its forward
blocking ability. Such a device forms the main load-
current-carrying switching element of the chopper, and
suitable means is provided ~or perlodically turning it
on and off.
In practical applications the main thyristor
of the chopper is periodically turned off by means of
a "commutation" circuit connected in parallel there-
with. A typical commutation circuit is a "ringing"
circuit, i.e., the circuit contains inductive and capa-
citive components that develop an oscillatlng or
ringlng current. A chopper commutation circuit may in-
clude, for example, a precharged capacitorg an in-
ductor, a diode, and the inverse parallel combination
of another diode and an auxiliary thyristor. In a
voltage turn-off type of chopper, these components o~
the commutatlon circuit are so interconnected and
,
~L4Z~Z~
20-TR-1309
--5--
arranged as to divert load current from the maln thy-
ristor in response to turning on the auxiliary thy-
ri3tor, and the main thyristor current is soon reduced
to zero. The ringing action of the commutation clrcuit
temporarily reverse biases the main thyristor which ls
consequently turned off, and during the reverse bias
interYal the current in the auxiliary thyristor 05-
cillates to zero so that the latter component will also
be turned off. For an ensuing brief lnterval, load
current will contlnue to flow through the capacltor
and a series diode in the commutation circuit of the
chopper, thereby recharging the capacltor from the d-c
source to complete the commu~ation process. Now the
chopper is in an open or non conducting state, and it
cannot return to lts closed or conducting state until
the main thyristor is subsequer,tly turned on by ap-
plying another firlng slgnal.
The duty factor or percentage on time of the
chopper is determined by the time delay between firlng
the auxlliary thyristor and subsequently firing the
main thyri3tor during any full cycle of operation.
The shorter this delay, the higher the duty factor~
whereas the longer this delay, the lower the duty
factor. Practical limits are imposed by the nature
of the switching devices used in the chopper. For
example, the maximum duty factor is approxlmately .91
~or a chopper using a main thyristor rated 1100 amps
(average) and 2000 volts (peak forward voltage) and
operating at a constant frequency of approximately 300
Hz. A higher duty factor cannot be safely obtained at
that chopping ~requency because the aforementioned time
delay must be at least 300 microseconds to make sure
that the ~.ain thyristor ls not re~lred prematurely,
l.e., beYore the auxiliary thyristor has time to be
completely turned off during the commutation process.
,
20-TR-1309
~6--
For the same assumed parameters, the minimum duty
~actor would be approximately ~09. Thls is because
the mlnimum pulse width per cycle is determ~ned by the
recharging tlme of the capacitor ln the o~cillatory
commutation circult. Con~equently, so long as lt is
belng operated in a constant rrequency ~ariable pulse
width mode, the chopper is effective to control motor
current only in a limited range between its predeter-
mlned minimum and maximum duty factors.
It is generally desirable to be able to vary
the chopper duty factor over substantially the ~ull
range between 0 and 1Ø In U.S. patent No. 3 1 944 9 856,
a constant frequency oscillator ordinarily determines
the ~ree running frequency of the chopper, but at high
motor speeds a pair of frequency dividers are comblned
with appropriate logic components to effect a two-step
reduced frequency, maximum pulse width mode of opera-
tlon, thereby extending the range of duty factor vari-
ations above the maximum attainable when the chopper
is operated in lts constant high ~requency pulse width
mode. At the lower chopping frequencies the minimum
delay time required a~ter turning on the auxlliary
thyrlstor before refiring the main thyristor iB a
smaller fraction of the whole period of each cycle.
By thus increasing the duty factor, the percentage of
the available d-c source voltage that the chopper can
apply to the motor armature is desirably increased. In
the referenced patent the chopper frequency is reduced
in two discrete steps that are ~ust equal, respec-
3 tively, to one-third and one-half o~ the constant high
frequency, and this technique ls not optimum for con-
trolling armature current during low speed electrlcal
braklng o~ a chopper type propulsion system on a large
traction vehicle.
~14Z~i~g
20-TR-1309
--7--
Smooth continuous varla~ions o~ ~he duty
factor up to 1.0 are desirable during the braking mode
o~ operation to obtain high, constant braking effort
when the vehicle is traveling at low speeds. The
higher the duty factor, the lower the minimum speed
at which the maximum magnitude of armature current can
be sustained during braking. Once the vehicle decel
erates below this minimum speed~ braklng ef~ort wlll
decrease or fade out, The lowest posslble minimum
brake fade out speed ls generally desirable.
Before changing from motoring to braklng
modes of operation9 it is good practice to reduce the
chopper duty factor to zero so that there is no current
in the armature of the motor at the time the propulslon
syste~.n is reconnected for braking operation. If the
vehicle were moving slowly when the motoring-to-braking
transition is desired, it ~rould be di~ficult to turn
on the chopper after the transition. This is because
at low speeds the voltage generated by the motor is
low, especially ln a series field motor with zero cur-
rent. The low voltage may be insufficient to ~orward
bias the main thyristor ln the chopper. Even if the
main thyristor were successfully trlggered, a rela-
tively long time is required for current to build up
to an appreciable level in the armature current path,
and there is a possibllity that latching current will
not be attained during the period of the firing signal
that is normally applied. Raising the voltage o~ the
motor by using the prior art technique of boosting its
field at the beginning of a braking mode of operation
is helpful but does not completely solve the problem
Lengthening the period of the normal ~iring signals
is not a desirable solution because o~ the attendant
energy loss and lsolation problems. Shunting the ~ree
wheeling path with an inversely poled auxiliary thy-
ristor that temporarily conducts current rrom the d-c
~ ~4~i~D
20-TR-1309
--8--
source for au~menting current flowing through the main
thyristor when initially trlggered, a~ suggested in
prlor art U.S, patent No 3,748,560, is not a practical
solution.
Summary Or the_ Invention
Accordingly, lt is a general ob~ective of
the present inventlon to provide a chopper type of d-c
traction motor propulsion system wherein the duty
~actor of the chopper is smoothly variable over a
.10 wide range extending to a maximum of 1Ø
Another ob~ectlve of this invention is the
provision of a traction vehicle propulsion system that
is characterized, when operating in its braking mode,
by an unusua~ly low minimum brake ~ade out speed.
A further ob~ect of the invention is to pro~
vide a propulsion system characterized, when operating
in its braking mode, by smsoth, constarlt, relatively
high braking effort as the traction motors decelerate
to a low brake fade out speed.
Yet another object.ive is the provis~on of lm-
proved means for effecting electrical braking of a
chopper type of d-c traction motor propulsion system
wherein initial turn on of the chopper at the be- .
ginning of the braking mode of operation can be ob-
tained at relatively low speeds.
In carrying out our invention in one form3
a propulsion system having motoring and braking modes
of operation includes a chopper and means e~fective
when the system is opera.ting in its mo~oring mode for
connecting the chopper, in series with the armature
and field windings of a d-c traction motor, to a d-c
electric power source that includes a filter capacitor.
In normal operation, the chopper is alternately turned
on and off in response to periodic gating signals o~
short duration that are produced by cyclically operative
means. During the off lntervals of the chopper, current
.~
20-TR-1309
--9-
in the armature o~ the motor circulates through free
~heellng rectifier means that i~ connected in circult
t~erewith~ Upon commanding a motoring-to-~raking
transition, ~rake set up means i3 operat1ve for re-
connecting the propulsion system to establish an arma-
ture current path comprising the field winding in series
~ith first and ~econd parallel branches, the first
~ranch including the chopper and the second branch
including the fil~er capacitor ~n series with the free
lO wheellng rectl~ier means. At the same time the po-
larity of the connection of the series field winding
relative to the armature i5 reversed.
In one aspect of k~le invention, we provide
burst firing means effective in response to the re-
15 connecting operation of the brake set up means for~upplying an extended chopper turn on signal having
a duration substantially longer than the aforesaid
short duration of the normal gating signals, thereby
ensuring that the chopper turns on and conducts arma-
20 ture current to begin the braking mode of operation ofthe propulsion system. In another aspect of the inven-
tion, the gating signal produclng means is controlled
by a variable control signal whose value determines the
duty factor of the chopper, and it is arranged so that
25 for control signal variations within a predetermined
range, the gating signals are produced at a predeter-
mined constant frequency while the off time of the
chopper is gradually decreased toward a predetermined
minimum as the value of the control signal approaches
3 a high end of that range, whereas for control signal
variations beyond the high end of the afore~aid range
the gating signals are produced at an average ~requency
that decreases from the predetermlned constant frequency
to zero as the value of the control signal increases
' '
20-TX~1309
-1 0
~hlle maintalning the predetermlned minlmum of~ time,
A~ a result, smooth variation of the chopper duty factor
up to l,0 is obtained.
T~e invention will be better understood and
its various ob~ects and ad~antages will ke more fully
appreciated ~rom the following description taken ln
con~unction with the accompanying drawings.
Brie~ ~es-cri~ion of the ~ra~ings
Fig, l ls a ~unctional block diagram of a
traction vehicle propulsion sys~em having a plurality
of chopper/motor units connected in parallel to a
d-c bus;
Fig. 2 is a schematic clrcuit diagram of
the filter and dynamic brake shown aæ single blocks
in Fig. l;
~ ig, 3 is a schematic circuit diagram of
a chopper/motor unit shown symbolically in Fig. l;
Fig, 4 is a graph showing the armature current
vs. speed characteristic of the Fig. 1 propulsion
system;
Fig. 5 is a functional block diagram o~ the
master controls and the No. l chopper control shown as
single blocks in Fig. l;
Fig, 6 shows the interrelationship of Figs, 6A
and 6B;
Figs, 6A and 6B are schematic diagrams of
the logic and contactor actuating mechanisms in the
brake control block of Fig, 5;
Fig, 7 is a chart showing the sequence in
which the various components of the brake control
are operated during a motor-to-braking transition;
Flg, 8 is a schematic diagram of the burst
firing block of Fig, 5;
.; , .
.
. i , . . . . . .
~l~4Z~i~
20-~R-1303
Fig, ~ ig a ~chematic dlagram of the chopper
reference block of Fig, 5;
Fig~ 10 ~s a schematic dlagram Or the chopper
pulses block of Fig, 5,
Fig, 11 Is a graph showing the relationshlp of
t~e output frequency to the ~nput voltage of the V~F
converter ~lock of Fig. 10;
Fig. 12 is a simplified schematic circuit
diagram of the armature current path of the illustrated
system after being reconnected ~or electrical braking
operation;
Fig, 13 is a diagram similar to Fig, 12 but
showing an alternative propulsion system;
Flgs, 14A, 14B, and 14C are time charts of
three different duty factors; and
-Fig. 15 is a graph showing the manner in
which armature current and electromotive force of the
traction motor vary with speed as the vehicle is elec
trically braked from 45 to 2.7 miles per hour.
Description of the Preferred Embodime_t
Fig. 1
Fig. 1 depicts a propulsion system comprising
at least two d-c traction motors 11 and 21 suitable
for propelling or retardlng a large traction vehicle
such as a locomotive or transit car. The motor~ 11
and 21 are shown symbolically in Fig, 1 and are res-
pectively labeled "Ml" and "M2", It wlll be understood
that each motor has conventional armature and series
field wlndings (see Fig. 3), The motor rotors a~e
mechanically coupled by speed reducing gears to sepa-
rate wneel sets or the vehicle (not shown), and the
armature windings of the motors Ml and M2 are elec-
trically connected via duplicate electric power
choppers 12 and 229 respectively, to a common d-¢
2~-rrR-l30s
-12-
power hu~ 31~ ~Person~ ~killed ln the ~rt ~111 h~
awar~e that additional chopper~motor units can he
readlly connected to the bus 31 in parallel w1th the
two units that are illustrated ln Fig, 1~ The d-c
~us 31 ls coupled to a suita~le source of d-c electric
power. Conventional filtering means 32, including a
s~unt capacitor ~see Fig, 2), is connected between bu~
and source for isolation purposes and to provide a by-
pass of the source for high-frequency, chopper gen-
erated currents.
Preferably the d-c power source for the pro- -
pulsion system includes a controllable electric power
converter 33, means lncluding a contactor 34 for con-
necting the input o~ the converter 33 to a source 35
of relatively constant voltage, and regulating means
36 effective when the propulsion system is operating
in a motoring mode for controlling the converter 33
so as to limit the average magnitude of voltage across
the shunt capacitor in the filter 32 to a predetermined
level (e,g,, 1750 volts) during light load conditions
when the capacitor voltage would otherwise tend to
rise higher. In the illustrated embodiment of the
invention, the voltage source 35 is stationary and
feeds alternating voltage of relatively high magni-
tude and commercial power frequency to an alternatingcurrent (a-c) line 37 comprising a catenary or third
rail located along the wayside of the traction vehicle,
The magnitude of the a-c line voltage may be, for
example, 25,000 volts rms, and the frequency may be
3 60, 50 or 25 Hz. Onboard the vehicle there is a power
transrormer 38 to step down this voltage~ The primary
winding of the power transformer 38 is connected by
way of a high voltage circuit breaker 39 to a current
collector 40 (e,g,, a pantograph) that makes sllding
26z~3
20TR 1309
- 13 -
contact with the wayside line 37. The secondary
winding of the transformer 38 is connected by way of
separable contacts of the contactor 34 to a set of
a-c input terminals of the converter 33.
Preferably the converter 33 is a phase-
controlled rectifier circuit utilizing controllable
solid state electric ~alves such as thyristors or
silicon controlled recitifers in selected leys of a
full-wave bridge rectifier configuration, and the
associated regulating means 36 is constructed and
arranged in accordance with ~he teachins of
United States Patent Number 4,152,758 issued
May 1, 1979 to R. B. Bailey, T. D. Stitt~ and
D. F. Wil~iamson and assigned to the General Electric
Company. As is indicated in Fig. 1, a capacitor
voltage feedback signal is supplied from the d-c
bus 31 to the regulating means 36 on a line 41, and
a~n alternating voltage feedback signal is supplied
to the regulating means 36 on a line 42 which is
coupled through a potential transformer 43 to the
input terminals of the controlled rectiier circuit
33.
In order to meter the current in the arma-
tures of the motors Ml and M2 that are connected in
parallel array to the d-c bus 31, each of the respec-
tive choppers 12 and 22 is alternately turned on
(closed) and turned off ~opened). For the first chopper
12 this pulsing type of operation is controlled by an
associated No. 1 control means 13 which normally
supplies chopper No. 1 with alternate turn on and turn
off signals on lines 14 and 15, respectively, and the
second chopper 22 is controlled by a similar No. 2
control means 23 which normally supplies it with alter-
.
20-TR-1309
-14-
nate turn on and turn off signals on ll,~es 24 and 25,
respectl~ely, The chopper turn on and turn off signals
are s~nchronized with a train of' discrete clock pul~es
that are generated at a constant high frequenc~ Ce,g "
300 H~) 'oy a master clock 44. The clock 44 is con-
nec~ed to the control means 13 and 23 by lines 45 and
46J respectively. The clock pulses supplied on line
46 to the No. 2 control means 23 are phase shifted or
staggered with respect to the clock pulses that are
supplied on line 45 to the l~o. 1 control means 133
here~y the two or more choppers used ~n the illus-
trated propulsion system have their respective turned-
off periods sequentially initiated at substantially
equally spaced intervals during each cycle of opera-
tion. By operating the choppers in sequence rather
than in unison, the amplitude of' ripple current in the
filtering means 32 and the rms current in the filter
capacitor are desirably reduced, thereby minimizing the
size of the flltering components that are required to
provide a desired degree of electrical isolation be-
tween the choppers and the wayside power line 37.
In each of' the motors Ml and M2 the average
magnitude of armature current (and hence motor torque~
will depend on the duty factor of the associated
chopper. As will soon be explained in more detail,
each of the control means 13 and 23 is arranged to
vary the duty factor as necessary to minimize any dif-
ference between a current feedback signal and a current
reference signai. To provide current feedback signals,
conventional current transducers 17 and 27 in the arma-
ture current paths of the respective motors Ml and M2
are connected vla lines 16 and 26 to the control means
13 and 23, respectively. The current reference s~gnal
in each control means is derlved from a current call
Z~
20-TR-1309
-15-
~ignal recei~ed on llne 47 from a master controls block
5Q, In accordance with one aspect of the present in-
vention, the chopper control means 13 ~and 23) has the
capability o~ smoothly varyi~g the duty factor of the
chopper 12 (and 22) over a continuum that extends all
the way between zero at one extreme (chopper turned
o~ continuously~ and 1.0 at the opposite extreme
(chopper turned on continuously). This will be more
apparent hereinafter when Fig, 10 is descri~ed.
rrhe master controls 5O, shown as only a
single ~lock in Fig. 1, perform several funckions that
will now be briefly summari~ed~ The construction and
operation of these controls will hereinafter be ex-
plained in more detail in connection with the descrip-
tion of Figs. 5 and 6. One function of khe master con-
trols is to provide the aforesaid current call signal
on output line 47. The value of this signal is varied
as a function of the setting of either a manually op-
erated throttle 51 or a manually operated brake con-
troller 52~ which is mechanically interlocked with thethrottle, and it is also a function of the speed of
the vehlcle. The vehicle speed is indlcated by speed
sensing means 18 and 28 which are respectively coupled
to the wheel sets of the vehicle or to the armatures
of the motors Ml and M2. These speed senslng means
typically are tachometer generators, and they feed
back to the master controls 50 on lines 19 and 29 sig-
nals representative of the angular velocities of the
armatures of the respective motors.
26 Another function of the master controls 50
is to provide a voltage reference signal for the regu-
lating means 36 that controls the phase-conkrolled
rectifier circuit 33 in the d~c electi~ic power ~ource
of the illustrated propulsion system~ rrhis signal i~
27 supplied over a line 53 from the rnaster controls ko the
regulator 36. Its value, which is set in the masker
.
.,
20-TR-1309
-16-
controls~ determines the limit level of voltage across
the ~hunt capacitor in the fllter 32.
A thlrd function of the master controls 50
is to carry ou~ an orderly transition of the propulsion
system between its motoring and braking modes on com-
mand. This entails actuating the contactor 34 that
connects the input term~nals of the controlled recti~
fier circuit 33 to the secondary windings of the power
transformer 38, and accordingly the master controls are
shown connected to the contactor 34 by a line 54, It
also entails actuating certain additional contactors
and a reverser in the armature and field circults of
the motors 11 and 21. These additional contactors and
the reverser for the first motor 11 are shown in Fig, 3
which will soon be described,
ln accordance with a second'aspect of the
presen~, invention, at the start of a braking mode of
operation the master controls 50 will momentarily boost
the motor fields and will supply a burst firing signal
20' on line 55 to the chopper control means 13 and 23.
The burst firing signal causes each of the control
means 13 and 23 to supply an extended turn on signal
to its associated chopper~ thereby ensuring that the
chGpper in fact turns on while the field ls being
boosted.
When the illustrated propulsion system is
operating in its braking mode, electric energy from
the motors Ml and M2 (now behaving as generators) is
dissipated in a resistor grid that needs to be con-
nected to the d-c power bus 31 for this purpose. The
braking resistor grid is represented in Fig, l by a -'
block 56 labeled "Dynamic Brake," and the master con-
trols 50 are connected to this block by a line 57 in
order to act-late a contactor that will connect certain
resistors in the grid in paral,lel circuit relatlonship
.
.
62~
20-TR 1309
-17-
wlth the shunt capacitor in the ~ilter 32 in response
to a transition ~rom motoring to ~raking modes or
operation, It should ~e noted that a single dynamic
brake 56 is shared by all of the chopper/motor unit~
that are connected in parallel to the d-c bus 31,
~here is also provided ln the master controls mean3
effective during braking for actuating additional
"staging" contactors in the dynamic brake ~lock 56
for changing, in three discrete steps, the amount of
resistance connected to the d~c bus as necessary to
prevent the generated energy from.charging the filter
capacitor to an unacceptably high level of voltage~
Fig, 2
The dynamic brake block 56 and the ~iltering
means 32 of the propulsion system have been shown in
more detail in Fig, 2 which will now be descri~ed.
The d-c power bus 31, shown as one line in Fi~, 1, is
actually a pair of conductors 31p and 31n which are
respectively connected via the filtering means 32 to a
pair of d-c output terminals 33p and 33n of the phase-
controlled recti~ier circuit 33, The terminal 33n and
the conductor 31n are both at ground potential, and a
potential that is positive with respect to ground ls
developed on the terminal 33p and on the conductor 31p,
The positive conductor of the d-c bus is connected to
ungrounded power terminals 12a and 22a of the res-
pective choppers 12 and 22.
' As is shown in Fig. 2~ the filtering means 32
comprises a voltage smoothing capacitor 60 connected
between the conductors 31p and 31n and a current
limiting inductor 61 connected between the positive
conductor 31p and the corresponding source terminal
33p, Although the filter capacitor 60 is illustrated
and referred to in the singular, ln practlce this com-
ponent will usually comprise a bank of parallel capa-
citor elements. ~he capacitor 60 provides a current
~Z~2~
20~TR-1309
-18-
path for any instantaneous dlfference between source
current and total load current during the motoring
operation of the propulsion system, thereby attenu-
atlng both line-frequency ripple and chopper-generated
harmonics. For thlS purpose a fllter capacitor having
a capacitance value of 6000 mlcrofarads iB contem-
plated in one practical application of the invention.
The inductor 61 can have an inductance value of the
order of 6 millihenrys. The capacitor voltage feed-
back signal line 41 is connected to the positiveconductor 31p of the d c bus 31. As can be seen in Fig. 2, the dynamic brake
resistor grid 56 preferably comprises two reslstors 62
and 63. These resistors have nearly equal ohmic values,
with resistor 62 being divided into two serial elements
62a and 62~. One pole of a normally open two-pole con-
tactor BB is connected between the upper end of the
resistance means 62 and positive conductor 31p of the
d-c bus, a conducting path 64 is connected between the
lower end of the resistance means 62 and the upper end
of the resistor 63, and the second pole of the con-
tactor BB is connected between the lower end of the
resistor 63 and the grounded conductor 31n, whereby
all of the resistor elements 62a, 62b and 63 are con-
nected in series with one another across the d-c bus
31 when the contactor BB is aetuated to lts closed
position A d-c motor 65 that drives a blower ~not
shown) for forcing cooling air across the resistor
grid is connected in parallel with the element 62
of the resistance means 62 for energization b~y the
voltage drop across this element when conducting
current. If and when an approxlmately 50 per cent
reduction in the amount of resistance across the d-c
bus is desired, a flrst sta~ing contactor Bl can be
2~ ~
20-TR-1309
-19-
closed to connect the lower end of the reslstance
means 62 to the grounded conductor 31n of the d-c bus 9
thereby short circuitin~ resistor 63~ Thereafter, 1
and when ano~her similar decrement in resistance is
desired, a second staging contactor B2 can be closed to
connect the upper end of resistor 63 to the positive
conductor 31p. A diode 66 in the path 64 is reverse
biased when both of the sta~ing contactors are closed
so that no current can flow in the path 64, and now
the resistors 62 and 63 are effectively connected in
parallel with each other across the d-c bus. The con~
tactor BB, Bl, and B2 are coupled by broken lines 57a,
57b and 57c, respectively, to brake control means shown
in Fig. 5. Their opened and closed positions are res-
pectively determined by the brake control means, andthe mechanisms for activating these contactors are
shown in more detail in Fig. 6B. The operation of
the staging contactors Bl and B2 to prevent the braking
energy from overcharging the filter capacitor 60 will
be explained in connection with the description of
Fig. 6.
~ig. 3
Turning next to Fig. 3, a preferred embodi-
ment of the first chopper 12 will now be described.
The illustrated chopper is of the type disclosed in
U.S. patent No. 4,017,777 issued on April 12, 1977,
to R. B. Bailey and assigned to the General Electric
Company. In brief, it comprises a main thyristor 70,
an oscillatory commutation circuit 71 connected across
the main thyristor, and an auxiliary or commutating
thyristor 72 in the commutation circuit. The main
thyristor 70 is connected between the power terminals
12a and 12b of the chopper, with a commutating inductor
73 belng dis~osed between its anode and the terminal
20-TR-1309
20-
12a, AB ~as previously mentloned, the anode terminal
12a o~ the chopper 12 is connected directly to the
positive conductor 31p of the d-c power bus 31~
The commutation clrcuit 71 of the chopper in-
cludes, in addition to the thyristor 7~, a commutatingcapacitor 74, an inductor 75, and a diode 76. The
positive plate of the capacitor 74 is connected di-
rectly to the terminal 12a) and the negative plate
of this capacitor is connected to the terminal 12b
through a diode 77 that is pole~ to block capacitor
dIscharge current when the main thyristor 70 iæ turned
on. The auxillary thyristor 72 is connected across the
commutating capacitor 74, with the inductor 75 being
connected between its anode and the positive plate of
the capacitor. The commutating thyristor 72 is shunted
by the inversely poled diode 76, and its cathode is
connected through a resistor 78 to ground~ The gate
or control electrode and the cathode of each of the
thyristors 70 and 72 are connected to gate and cathode
terminals G and C, respectively. While each of the
thyristors 70 and 72 and each of the diodes 76 and
77 has been shown in Fig. 3 as a single element, it
will be understood that in practice, if required in
choppers having high voltage and/or current ratings,
additional elements of l~ke kind could be connected in
series and/or parallel with the illustrated elements
and operated in unison therewith.
Normally the chopper 12 îs turned on by
firing the main thyristor 70. This is done by applying
3 a discrete signal of appropriate magnitude and duration
across its gate and cathode terminals, With the main
thyristor 70 turned on and the commutating capacitor 74
charged, the diode 77 is reverse bias~d and there is
no current in the commutation ci,rcuit 71. Subse-
quently the commutating thyristor 72 is fired by
20-TR-1309
-21-
applylng acro~s its gate and cathode terminals a di~-
crete chopper turn off signal of appropriate magnitude
and duration, Now the commutating capacitor 74 wlll
discharge through the inductor 75, The resulting
ringing action of the commutation clrcuit 71 soon
forward biases the diode 77, whereupon current ln
the main thyristor 70 is reduced to ~ero and the main
thyristor is temporarily reverse blased. This turns
off the main thyristor 70. During the reverse bias
interval the current in the commutation circuit 71
oscillates to zero and reverses direction, While
current is flo~ing through the diode 76, the commu-
tating thyristor 72 is reverse biased and consequently
turned o~f. For an ensuing brief interval, current
continues to flow through the commutating capacitor
74 and the diode 77, thereby recharKing the capacitor
from the d-c source to complete the commutation pro-
cess. Now the chopper is turned off, and it wlll re~
main in this state until the main thyristor 70 is re-
fired by the next turn on signal,
So long as the propulsion system is operatingin its motoring mode, the chopper 12 is periodically
turned on and off to regulate the average magnitude
of current flowing from the d-c power source to the
armature and series field windings of the associated
motor Ml. In Fig. 3 the armature of this motor is
shown at 80, and the series field winding is shown
at 81. The chopper 12, the armature 80 9 and the field
81 are connected in series with one another between
the terminal 12a and ground, and thiæ series combi-
nation of components is therefore connected across the
filter capacitor 60 (Fig, 2). As is shown in ~i~. 3,
the means for serially lnterconnecting these components
includes a current smoothing reactor 82 and the current
20-TR-1309
-22-
transducer 17, both of which are connected between the
cathode terminal 12b of the chopper and the armature 80,
and a contactor M which connects the series ~ield 81 to
ground, The contactor M is closed Cas shown) during the
motoring mode of operation and ls open during the
braking mode of operation. The lnterconnecting means
also lncludes a reverser RR that determines the po-
larity of the connection o~ the series field winding
81 relative to the armature 80.
The reverser RR is illustrated as a double-
pole double-throw contactor. When this reverser is in
a first position, the movable contact comprising one
of its poles engages a stationary contact Fl and the
movable contact comprising its other pole engages a
stationary contact F2, whereas when the reverser is in
a second, alternative position, the first-mentioned
movable contact engages a stationary contact Rl which
is connected to contact F2, and the other movable con~
tact engages a stationary contact R2 which is connected
to contact Fl, Either the armature 80 or the series
field winding 81 can be connected between the contacts
Fl and F2. In the illustrated embodiment of the in-
vention, it is the field winding 81 that is so con-
nected.
During intervals when the chopper 12 is
turned off, armature current IA in the motor Ml is
conducted by free wheeling rectif'ier means FWR which
is connected in circuit with the armature 80 and ~ield
81. In Fig, 3 the f'ree wheeling rectifier means is
shown as a simple diode having its anode connected to
ground and its cathode connected to the cathode termi-
nal 12b of the chopper 12. Whenever thls element is
conducti.ng current, terminal 12b is at nearly ground
potential, If' desired, the free ~Iheeli.ng rectifier
means FWR can comprise a thyristor instead of the 11-
- lustrated diode. If a thyrlstor were used, its f'iring
.
`
~ ~Z~i2~
20 TR 1309
- 23 -
can be controlled by the improved gate means described
and claimed in United S-tates Paten~ No. 4,284,938,
R.B. Bailey, dated August 18, 1981, and assigned to the
General Electric Company.
To change from motoring to braking modes of
operation, the contactor M is opened and a companion
contactor B is closed. As is shown in Fiy. 3, the con-
tactor B, when closed, connects the last-mentioned
movable contact of the reverser RR to the anode terminal
12a of the chopper 12 (and hence to the positive
conductor 31p of the d-c bus 31). Consequently, when
the contactor ~1 is actuated to its open position and
the contactor B is actuated to its closed position,
the propulsion system is reconnected to establish an
armature current path comprising the field winding 81
and the contactor B in series with at least two parallel
branches. A first one of these parallel branches
is provided by the chopper 12, and the second parallel
branch is provided by the filter capacitor 60 (Fig. 2)
in series with the free wheeling rectifier means FWR.
The conducting direction of the free wheeling rectifier
means in the second parallel branch enables armature
current to charge the filter capacitor 60 when the
chopper 12 is turned off but blocks discharge of this
capacitor through the chopper when turned on. A third
branch paralleling the first and second branches of
the armature current path i5 provided by the resistor
grid 62, 63 (Fig. 2) whenever the dynamic brake contactor
BB is closed.
During the transition from motoring to
braking modes of operation, the reverser RR is actuated
so as to reverse the polarity of the connection o~ the
series field winding 81 relative to the armature 80 of
the motor Ml. With the field 81 connected to the
reverser RR as shown in Fig. 3, actuation of the reverser
20-TR-1309
-24-
wlll reverse the direction of current ln the fleld 81
and thereby reverse the polarity of the electromotlve
~orce generated in the armature windings 80 during
the braking mode Or the operatlon (when the motor Ml
is behaving as a generator). As a result~ the electro-
motive force will be applied across t~e chopper 12 with
the proper polarity to forward bias the main t~yristor
70, Alternatively, if the reverser RR were connected
across the armature 80 instead of the field 81, the
polarity of the generated electromotive force would be
the same during braking as during motoring but the
pOSitiVe motor brush would be reconnected through the
reverser and the contactor B to the anode terminal 12a
of the chopper.
The opened and closed positions Or the res-
pective contactors M and B and reverser RR in the arma-
ture and field circuits of the motor Ml are determined
by brake control means (shown in Fig 53 to ~hich they
are coupled by broken lines 12~ and 153, and the mecha-
nisms for actuating these components are shown in more
detail in Fig. 6B. The same mechanisms can also be
coupled, respectively, to similar contactors and to
a similar reverser that are connected in the armature
and field circuits of the second chopper/motor unit
22/M2, whereby the second unit of the propulsion
system is reconnected for braking operation and the
polarity of its field is reversed with respect to lts
armature connection simultaneously with the occurrence
of these events in the Fig. 3 chopper motor unit.
Fig 3 also illustrates means for boosting
the fields of the traction motors Ml and M2 This
means comprlses a transformer having a primary windlng
83 and multiple secondary windings 84 and 85, a
suitable source 86 of alternating voltage, and a
normally open double-pole field boost switch FS con-
nected between the source 86 and the primary winding 83.
s3
20~TR-1309
-25~
The voltage across the transformer secondary windlng 84
when energized is rectifiecl by a pair of diodes 87 and
applied to relatively positive and negative output
terminals 88p and 88n. As ls shown in Fig, 3J opposlte
ends of the secondary winding 84 are connected through
the respective diodes 87 to the relatively positlve
terminal 88p, and a center tap of the secondary winding
is connected directly to the relatively negative output
terminal 88n, A similar rectifier circuit connects the
other secondary winding 85 to a second pair of output
terminals 89p and 89n,
The first pa~r of output terminals 88p and 88n
of the field boost means are respectively connected to
the upper and lower movable contacts of the reverser RR
and hence to the associated field winding 81 of the
motor Ml. -A current limiting inductor is serially in-
serted in this connection~ and a resistor 91 is con-
nected in parallel with the field to minimize the ef'fect
of chopper-induced ripple on motor commutation. When-
ever the switch FS is closed, the transformer primarywinding 83 is energized by the source 86 and the field
boost means then supplies current of desired magnitude
(e.g,, 60 amperes) from its output terminals 88p and
8~n to the field winding 81. This will increase the
field excitation of motor M,
The movable contacts of the field ~oost
switch FS are actuated by a suitable mechanism (shown
in Fig, 6B) to which they are coupled via broken line
185. As will be more fully explained hereinafter in
connection ~rith the description of Fig, 6, this
actuating mechanism is automatically operative to close
the switch FS temporarily, for a predetermined period
of time (e,g,, approximately one se~oncl), in response
to a motoring-to-kraking transition of the propulsion
system, whereby a momentary lncreas~ Or field current
20-TR 1309
_26-
is obtained at the beginning of the braking mode of
operation, At the same time, the ~ield boost means
will also momentarily increase current in the field
wlnding ~not shown) Or the second motor M2 to which
the second pair of output terminals 89p, 89n are
connected, The additional excitation during the period
of field boost results in more electromotive force
being generated by the traction motors, whereby the
voltage across the armature of each motor is desirably
increased, This armature voltage increase is signifi-
cant if the braking mode of operation is initiated with
the vehicle moving at a relatively low speed, At low
speeds without field boost the electromotive force
might be insufficient to force current to build up ln
the armature and series field of the motor.
During the above mentioned period of time
that the field boost means is operative to increase
field excitation at the beginning of the braking mode
of operation, a burst firing signal from the master
controls causes the No. 1 chopper control means 13 tQ
apply to the gate terminal of the main thyristor 70 in
the chopper 12 an extended firing signal having a dura-
tion substantially longer than that of the discrete
turn on signals that are periodically supplied by the
control means in normal operation, This solves a prob-
lem of effecting initial turn on of chopper 12 when
electrical braking is initiated at low speeds. Even
with the previously described operation of the field
boost means, the increased voltage across the armature
80 of the motor Ml is not high enough at low speed
~e,g,, 3 miles per hour) to force current in the arma-
ture current path to increase abruptly as soon as the
leadlng edge of the first firing signal is applied to
the gate terminal of the main thyristor 70, The in-
ductance o~ this path and the appreciable voltage drop
, .
z~
20-TR-1309
-~7~
across the main thyristGr when conducting less than
lt~ 'tlatching" current can result ln delaylng current
building up to the minlmum latching level for an inter-
val much longer than the duratlon of one of the normal
periodlc flr~ng signals~ The extended firing signal
that is applied during the field boost period in
accordance with the present invention has a duration
longer than the maximum anticipated time that will be
required for armature current to attain the latc~llng
level of the thyristor 70, and therefore successful
turn on of the chopper is ensured when braking is
initiated at low speed.
Flg. 4
In Fig. 4 the solld-line trace 94 depicts the
relationship between the magnitude of armature current
IA and the-vehicle speed (MPH) during the braking mode
of operakion of an electrlc locomotive embodying the
present invention, with the brake control means being
set at its maximum ~1.0) rate, At speeds above the
corner point 95 (approximately 21 MPH), the prcpulsion
system is operating at constant horsepower~ whereas
at speeds between the corner point 95 and a predeter-
mined minimum point 96 the propulsion system is
operating at a constant current and hence constant
brakin~ effort. A m~nimum speed as low as 3 MPH can
be obtained in practice. Electrical braking can be
successfully initiated at any speed above this low
mlnimum, and the maximum braking rate can be sustained
without fadeout as the vehicle decelerates to the
minimum point 96. The improved electrical braking per-
formance of the present invention can be appreciated
by comparing it with the armature current vs. speed
characteristic of a typical prior art electric locomotive
during the bralcing mode of operation. The latter
3y characteristic is shown in Fig, 4 by the broken-line
20-TR-1309
-28-
trace 97. Below the corner point speed 95, armature
current in the prior art propulsi,on system tends to fall
o~f linearly with decreasing speed, but the ~raking
range is extended to a minlmum point 98 (e.g., 8 MPH)
by using staging contactors to reduce the ohmic ~alue
of the braking grid resistors in four discrete 3teps.
Below the mi~imum speed point g8 the braking effort of
the prior art propulsion system fades out.
Fig, 5 (Master Controls)
The master cont,rols ~0 of the propulsion
system as well as the No. 1 control means 13 that
periodically turns on and of~ the first chopper 12 have
been illustrated in Fig. 5 in functional block diagram
form. As i6 shown in ~lig~ 5, the master controls com-
prise a reference generator 100 that receives inputs
from both~the throttle 51 and the brake controller 52
and that also receives a th~rd input, representative Of
vehicle speed, on a line 101 from an averaging circuit
102 which is coupled by lines 19 and 29 to the speed
sensors (tachometer generators 18 and 28) associated
with the repective motors Ml and M2. The re~erence
generator 100 is operative to produce a current call
signal I~ which is fed via the output line 47 to both
the No. 1 and No. 2 control means. In practice, the
current call signal is passed through a conventional
power limit circuit ~not shown) which proportionately
reduces its value in response to either lo~ voltage on
~he fiiter capacitor or overcurrent or overtemperature
in the po~er transformer. The master controls further
comprise a brake control block 103 coupled by llnes
104 and 105 to block 106 labeled "Reference Level &
Ramp Up" and coupled by the line 104 and another
line 107 to a block 108 labeled "Chop Enable," The
brake control block 103 receives lnputs on line 110
from the throttle 51 and on lines 111 and 112 from
the bralce controller 52.
2~ciZ9
20-T~ 1309
29-
In the illustrated embodlment of the present
invention, the throttle 51 and the brake controller 52
are mechanically interlocked wlth each other and with
a manually operated, two-position forward/reverse
controller 113. Together with the reference generator
100, these components form conventional command means
~or the propulsion system of an electric locomotive.
The command means has alternative motoring and braking
states. In its motori.ng state, the brake controller
52 is locked in an "off" posltion and a manually
operated hand].e (not shown) of the throttle 51 can
be selectively moved in eight steps or "notches" be-
tween a low power position (notch 1) and a maximum
power position (notch ~). The reference generator 100
ls suitably constructed and arranged to vary the
current ca;.l signal I* as a function of both the
selected power notch of throttle 51 and the average
speed of the traction motors of the propulsion system.
The graph shown in Fig 5 in the upper half of the
reference generator blocX 100 indicates the manner in
which the value of I* varies with speed for various
different power notches of the throttle 51.
In the braking stat.e of the command means,
the throttle 51 is locked in an "idle" position and a
manually operated handle (not shown) of the brake con-
troller 52 can be freely moved through a "brake on"
sector between a predetermined low limit or 0 brake
(minimum braking rate) and a predetermined maximum
limit 1.0 (maximum braking rate). The reference
3 generator 100 will now vary the current call signal
I* as a function of both the setting of the brake con-
troller 52 (in its brake on sector) and the average
-speed of the +raction motors. Brake position 1.0 re-
! sults in I* being maintai.ne~d at a c~nstant maximum
~; 35 value (whicll corresponds, e g., to armature current o~
L4'~ 9
20-TR~1309
~30-
approximate1~ 880 amps) throughout a low speed range
and being varled inversely ~rith speed above the cor~er
polnt, This maximum characterist~c is indlcated by
the graph shown in Fig, 5 in the lower half of the
reference generator block 100. 'At other positions of
the brake handle, I* is reduced proportionately to the
setting of the brake controller 52 (e.g., at 0.5 brake,
the highest I~ is approximately 440 amps).
The interlockin~ of the throttle 51 and the
brake control 52 make it necessary for the operator to
follow a prearranged sequence of operations in order to
change from motoring to braking states. The throttle
handle must first be move,d to its notch 1 position from
any higher notch (which therefore reduces the current
call signal I~ to its lowest value for the given speed
at which the vehicle happens to be movin~), and it can
then be moved from notch 1 to "idle." In the idle
position of the throttle 51 the brake handle is no
longer locked in its "off" position. Now the brake
controller 52 can be moved to a brake "set up" positlon
which locks the throttle in its idle position. In
this state, referred to hereinafter as the transition
state, the illustrated command means is in between its
motoring and braking states. Subsequently the operator
can advance the brake handle from its set up position
to the brake on sector, starting with the aforesaid
low limit. When a return to the motoring state is
desired', this sequence is reversed, The interlocks,
also prevent the forward/reverse controller 113 from
chan~ing position except when the throttle ls in idle
and the brake is off.
The throttle 51 and the brake 52 are coupled
-to the brake control block 103 by means Or lines 110,
111, and 112 in order to supply the latter component
with indications of their respective positions.
.
,
.
2~ ~
20-TR-1309
-31
Expressed in digital terms, the throttle 51 provides
on line llO a logic signal that is low or tlOII whenever
the throttle is in its idle position and high or "1"
when the throttle is in any one of its power notches,
Similarly, the brake controller 52 provides on line
lll a logic signal that is low or "0" whenever the
brake is off and hlgh or "l" when the brake ls in
either its set up position or its brake on sector.
The brake controller also provides on the line 112
a logic signal that is low or "0" when the ~rake is
in either its off or set up position and that is high
or "1" whenever the brake is in the brake on sector
(0 brake to 1,0 brake). The ~orward/reverse controller
113 is also coupled to the brake control block 103 by a
line 114 on which is provided a logic signal that is
low or "O".whenever thls controller is in a reverse
position and high or "l" whenever a forward position
is selected.
The brake control block 103 responds to a
state change of the command means by appropriately
actuating the various contactors shown in Figs, l, 2
and 3, More details of a practical embodiment of the
brake control block are shown in Figo 6 which will soon
be described. From the subsequent description of
Fig, 6 it will be apparent that the contactor 34
~Fig, l) connecting the propulsion system to the power
transformer 38, which contactor is coupled via the
line 54 to the brake control block 103 in Fig, 5, ls
temporarily opened and then reclosed during a state
3 change of the command means. The opening of the
contactor 34 is indicated by a reset signal on the line
104 from the block 103, In dlgital terms, whenever
the contactor 34 is open the reset signal on line 104
is low or "0", ~ut when the contactor 34 is later re-
closed and certain other inputs are normal, the signalon the line 104 changes to a high or "1" level,
20-TR-1309
32-
The reset signal line 104 1B connected to
the reference level and ramp up block I06. This com-
ponent of the master controls ls designed to produce
and to set the value of a voltage reference slgnal
VREF that is supplied over the line 53 to the pre-
viously described regulating means 36 ~Fig. 11~ The
block 106 is reset in response to a 0 signal on the
line 104, whereby VREF is reduced to a low value Ce.g.,
zero~ whenever the contactor 34 opens to disconnect
the controllable converter 33 from its voltage source.
Upon reclosing the contactor 34, the signal on the
reset line 104 changes from 0 to 1. A 1 signal on this
line enables the block 106 to increase or ramp up VR~F
to a predetermined value which depends on whether the
command means has been changed to its motoring state or
to its braking state. The reference level and ramp up
block 106 is suitably constructed and arranged so that
VREF increases at a predetermined rate (e.g., 2500
volts per second) until it attains either a first level
corresponding to approximately 850 volts if there is
a low or "0" signal on the line 105 (indicating a
braking mode of operation) or a second higher level
corresponding to approximately 1750 volts if the signal
on line 105 is high or "1" (indicating a motoring mode
of operation~. After VREF reaches its present level3
the regulating means 36 is effective to control the
converter 33 so as to prevent the voltage across the
filter capacitor 60 from falling below the aforesaid
first level when the propulsion system is operating in
its braking mode or from rising above the aforesaid
second level when the propulslon system ls operating
in its motoring mode.
As ls shown in Fig, 5, the reset signal line
104 from the brake control block 103 iæ also connected
aZ~ji,~
20~TR-1309
~33-
to one lnput of the chopper enable block 108~ Another
input of the block 108 is connected to the line 41 for
recelving the capacitor voltage feed~ack signal ~rom
the filter 32, The latter signal ls representati~e o~
the voltage across the filker capacitor 60 ~Fig. 2),
which voltage is identl~ied by the reference character
VF in Fig. 5 and hereinafter. The output of the block
108 is supplied on the line 107 to the brake control
block 103 and also to the No. l (and No, 2) chopper
control means 13 ~and 23).
The chopper enable block 108 comprises
bistable means that is suitably constructed and
arranged to detect the magnitude of the input on line
41 and to change states as this magnitude traverses
predetermined "pickup" and "dropout" levels. It is
in a dropped ollt state when VF is relatively low
(e.g " below 500 volts) and ~n a picked up stake when
VF is relatively h~gh Ce.g., above 725 volts). So
long as the bistable means is in its dropped out state3
a low or "0" signal is provided on the output line
107 of the chopper enable block 108. But when the
bistable means is in its picked up state and in res-
ponse to the presence of a 1 signal on the reset line
104, the chopper enable block is effective to provide
a high oOr l signal (hereina~ter referred to as the chop
enable signal) on the output line 107. ~us a chop
enable signal indicates that the contactor 34 is
closed, the filter capacitor 60 is charged to an ap-
preciable voltage level, and certain other inputs are
normal.
In the brake control block 103, operation of
the means that temporarily closes the field boost swi~ch
FS (Fig. 3) is prevented ~nd no burst firing signal can
be supplied on line 55 in the absence Or the chop
enable signal on line 107. Upon receipt Or this signal,
.
, , i-, .. .
20-TR-1309
-34-
the field boost period commences, pro~iding that the
command means ls then In it~ braking state and the
propulsion s~stem has been reconnected by opening the
contactor M and closing t~e contactor B, and provlding
further that the speed feedback signal on the llne 101
indicates that the average motor speed is not below
a predetermined low magnitude (e.g., an angular
velocity corresponding to a vehicle speed of appror.i-
mately 3 MPH). In delayed response to the commencement
of field boos~, the brake control block 103 supplies
the burst firing signal on line 55 to the No, 1 (and
No, 2) chopper control means 13 ~and 23). The brake
control block 103 is also effective during the braking
mode of operation to supply to these control means a
signal designated B' on a line 115.
- Figs. 6A and 6B
Details of a preferred embodiment of the
brake control block 103 are shown in Fig,x 6 which
will now be described. This figure is a combination
of two contiguous figures 6A and 6B, Fig. 6B is lo-
cated on the same sheet as Fig, 6 and shows the actu-
ating mechanisms of the various contactors and the re-
verser. Fig. 6A is on a separate sheet~ and it shows
a schematic diagram o~ logic circuits and other com-
25 ponents that are used to control these actuatingmechanisms. In Fig. 6A each of the encircled dots
symbolizes a conventlonal and logic function (output
is high or 1 only when all of its inputs are 1, other-
wise output is low or 0), and each of the encircled plus
30 signs symbolizes a conventional OR logic function
~output is 1 if any one or more of lts inputs is 1, and
output is O only when all inputs are 0~. Known elec-
tromechanical hardware or solid state electronic equi-
valents can be used to lmplement these logic functlons.
35 Typically, cam actuated interlock contacts on the
~l~Z6~5~
20-TR-1309
-35-
throttle and brake controller and conventional inter-
locks of the contactors and reverser are appropriately
interconnected for this purpose.
The ~rake controls shown in Figs, 6A and 6B
comprise three main parts: means for opening and re-
closing the contactor 34, brake setup means, and field
boost means. The first-mentioned means includes an
AND logic circuit 120 connected by a line 121 to an
OR logic circuit 122 whose output is coupled via a
line 123, a time delay component 124, and a line 125
to a mechanism 126 (Fig. 6B) that actuates the movable
main contacts o~ the contactor 34 ~Fig. 1). The cir~
cuit 120 has two inputs: one is received on the line 110
and indicates the position of the throttle 51, and the
other is received on a line 127 from the output of an
AND logic circuit 130. The l~tter circult in turn res-
ponds to two inputs. The first input of circuit 130 is
coupled via a line 131 to an interlock contact (not
shown) of a mechanism 132 (Fig. 6B) that actuates the
contactor M (Fig. 3) to which it is coupled via broken
line 129, and a 1 signal on the line 131 ln~icates
that the contactor M is closed. (Note in Fig. 6B that
the same mechanism 132 that actuates the contactor M
is also coupled by line 129 to the contactor B This
mechanism has a flip flop type of control which is
set in a first stable state in response to a 1 signal
on a first input line 132a and which is reset to an
alternative stable state in response to a 1 signal on
a second input line 132b. In the first state the
mechanism 132 will maintain M closed and B open, where-
as ln the alternative state the mechanism maintains B
closed and M open. The first input line 132a Or this
mechanism is connected directly to the 1 ne 110 so that
M is closed and B is open when the throttle 51 is
-
~lfl2~2~
20-TR-1309
--36--
positioned in one o~ its power notche In some appll-
cations of the invention it may be de~irable to o~tain
the same results by using separate mechanlsms to actu-
ate the respective contactors M and B )
~he second input to the AND logic circult 130
of the contactor opening and reclosing means is re~
ceived on a line 133 from the output of an OR logic
circuit 134. As will soon be explained, the output
~rom the circuit 134 is 1 whenever the reverser RR
~Fig, 3~ i5 in the correct one of its two positions.
Consequently a 1 signal on the output line 127 of the
logic circuit 130 indicates that the propulsion system
is properly connected for motoring operation. So long
as there are 1 signals on both lines 110 and 127,
there is a 1 signal on the output line 121 of the AND
logic circuit 120, and this signal is passed through
the OR logic circuit 122, the time delay component 124,
and the line 125 to the contactor mechanism 126. In
response to a 1 signal on line 125, the mechanism 126
maintains the contactor 311 closed. The 1 signal on
the output line 121 of the logic circuit 120 is also
passed through an OR logic circuit 135 to line 105
which is coupled to the reference level and ramp up
block 106 (Fig. 5) so as to set VREF at the value
desired during the motoring mode Or operation.
To start a motoring-to-braking state change
of the command means, the handle of the throttle 51 i8
moved from power notch 1 to idle This event is indi-
cated by a signal change frolr. 1 to O on the line 110,
and as a result the signal on the output line 121 of
the logic circuit 120 changes from 1 to 0. This
causes a corresponding signal change on the output
line 123 of` t~le logic circuit 12?, providing that
there is then a O signal on a second input line 136 of
this circuit:. (As will soon be explained, the state
.
~4Z¢i~
20-T~-1309
~37-
o~ the signal on line 136 is determined by the brake
set up means. This signal is 0 during the motorin~
mode of operation but changes to 1 in response to the
brake controller 52 being moved to i~s set up or on
position and t~e propulsion system being properly re-
connected for braking operation,~ A short time after
the signal on llne 123 changes from 1 to 0, the signal
on the output line 125 of the delay component 124 goes
to 0, and the actuating mechanism 126 opens the con-
tactor 34 in response to a 0 signal on line 125. ~he
de].ay introduced by component 124 allows time for the
choppers 12 and 22 to reduce armature currents in the
respective traction moto~s at a controlled rate to a
low magnitude or zero before the contactor 34 discon-
nects the propulsion system from the power source.
Such armature current reduction occurs at the end of
the motoring mode of operation in response to the
current call signal I~ (Fig, 5) being reduced by move-
ment of the throttle to its power notch 1 and to its
idle position.
As soon as the contactor 34 opens, there i8
a signal change from 1 tc 0 on a line 137 that is
coupled to an interlock contact (not shown) of the
contactor actuatlng mechanism 126. This line is
connected as an input to an AND logic circuit 138
whose output supplies the aforesaid reset signal to
the line 104. Thus the signal on line 104 changes
from 1 to 0 in response to the opening of the con-
tactor 34, and this will reset the reference leYel
and ramp up block 106 as previously described. When
the contactor 34 is su~sequently reclosed, the signal
on line 137 changes from 0 to 1. Hence the reset
signal on line 104 returns to a 1 state~ providing
that at the same time a 1 signal is being supplied
from a terminal 140 to the second input Or the AND
20 TR-l3og
-38-
lo~ic circuit 138. The terminal 140 i8 adapted to
~e connected to means ~not shown~ for supplying an
input signal that is 1 so long as certain condi~ions
are normal. Such conditions can compromise, e.g.,
the powèr transformer 38 being energized, proper con~
trol power being present, and a master shutdown swltch
being in a run or on state.
Upon operation of the brake set up means
~described below~, the si~nal on the llne 136 changes
from 0 to 1, and this 1 signal is passed through the
OR logic circuit 122, the delay component 124, and
the line 125 to the actuating mechanism 126 which auto
matically recloses the contactor 34 in response thereto.
The delay component 124 is suitably designed so that
it does not delay this 0 to 1 signal change. The 1
signal on ~he line 136 is also supplied to inverting
means 141 whose output is connected to the OR logic
circuit 1359 and there~ore the reference level signal
on the line 105 changes from 1 to 0 at the same time
reclosure of the contactor 34 is initiated by the
brake set up means~ A zero 0 signal on line 105 sets
VREF at a value that is desired during the braking
mode of operation.
In the brake set up means of the brake con-
trol block, the signal on the line 111 is fed through
a time delay component 142 to a line 143 having three
branches, One branch of the line 143 extends to
Fig, 6B where it is connected not only tp the second `'
input line 132b of the mechanism 132 that actuates
3 the contactors M and B (Fig. 3) but also to the input
of an actuating mechanism 144 that is coupled by
broken line 57a to the two-pole dynamic brake con-
tactor BB (Fig, 2), When the signal on the line 143
changes from 0 to 1, the mechanism 132 actuates the
contactor M to its open position and actuates th~
-
20-TR 1309
-39-
contactor B to its closed position, there~y recon-
necting the propulsion system for braking operation~
and at the same time the mechanism 144 closes the con-
tactor BB to connect the dynamic brake resistor grid
across the filter capacitor. This signal change on
the line 143 takes place in delayed response to move-
ment of the brake controller 52 from off to set up
positions, The delay introduced by the component 142
ensures that armature current in the contactor M has
time to decay to a low magnltud~ or zero befor~ the
contactor M opens,
Another branch of the line 143 provides
inputs to two AND logic circuits 145 and 146 that
are respectively labeled R and F in Fig. 6A, The
second input to the first circuit 145 is received on
the line 1-14 and indicates the position of the
forward/reverse controller 113 (Fig. 5), whereas
the second input to the second circuit 146 is received
on a line 147 from the output of inverting means 148
whose input is connected to the line 114, Conse-
quently, if the controller 113 is in a forward position
when a motoring-to-braklng state change of the command
means ta~es place, the output signal of the first cir-
cuit 145 will reflect the O to 1 change on the line
~5 143, but if the controller 113 were ln a reverse
posltion at this time, the output signal of the
second circuit 146 would reflect such change, The
output of the circuiks 145 and 146 are fed, respec-
tiveIy, through an OR logic circuit 150 to a line
152b and through an OR logic circuit 151 to a line
152a~ The second input to the loglc circuit 150 is
supplied by an AND logic circuit 155 whose flrst
-input is received on the line 110 and ~!hose second
input is received on the line 147, and a second input
to the logic circuit 151 i8 supplied ~y another AND
20-TR-1309
-IJO-
loglc circuit 156 whose ~irst input i5 also received
on the line 110 and whose second input is received on
the line 114.
The llne 152b from the output of the OR logic
circuit 150 and the line 152a from the output of the
OR logic circuit 151 are both connected to an actuating
mechanism 152 (Fig. 6B) that ls coupled by broken line
153 to the movable contacts of the reverser RR (Fig. 3)
associated with motor Ml and to the contacts of a cor-
10 responding reverser (not shown) associated with motorM2. This mechanism has a flip flop type control; it
maintains the mova~le contacts of the reverser in their
first position (engaglng stationary contacts Fl and F2
in response to a 1 signal on the line 152a, and it
15 maintains the movable contacts of the reverser in their
second position (engaging stationary contacts Rl and
R2) in response to a 1 signal on the line 152b, The
reverser RR is shown in its ~irst position in Fig, 3.
Hereinafter this will be called position F, and the
20 second position of the reverser will be called position
R.
In operation, assuming that the vehicle is
being propelled in the forward direction, the mechanism
152 will actuate the reverser RR from position F to
25 position R in response to a motoring-to-braking trans-
ition of the command means, and it wlll actuate the
reverser from position R to position F in response to
a braking-to-motorin~ transition. Assuming lnstead
that the vehicle is being propelled in the reverse
30 dlrection, the mechanism 152 will actuate the re-
verser RR fro~ position R to position F in response
to a motoring-to-braking transition or the command
means, and it Yill actuate the reverser from posltion
F to posltion R in response to a braking-to-motoring
~, 35 transltion. In each case, this operation of the brake
~.
2Ei29
20-TR-1309
~41-
set up means reverses the polarity of the connection
Or the series field winding 81 relatlve to the motor
armature 80~ Whlle not shown in Figs. 6A and 6B,
the mechanism for opening and closing the Rl,R2 con-
tacts of the reverser could be separate from themechanism for opening and closing the Fl,F2 conkacts~
and in some applications of the invention it will ~e
desira~le to provide additional. lnterlocking to ensure
that the reverser changes positlons only when both o~
the contactors M and B are open.
To indicate the postiion o~ the reverser
RR, interlock contacts (not shown) associated with the
actuating mechanism 152 are coupled to lines 157 and
158. A 1 signal on the line 157 indicates that khe
reverser is in position F, and this signal provides an
input to an ANn logi.c circuit 160 whose other input is
ta~en from the input line 152a of the mechanism 152.
As is shown in Fig.-6A, the output of the circuit 160
serves as one input to the OR logic circuit 134 whose
output is connected to the line 133, and it is 1
whenever a 1 signal on line 152a is directing the
mechanism 152 to actuate the reverser RR to its posi-
tion F and there is a 1 signal on line 157 to indicate
that the reverser in fact is in this position. On
the other hand, a 1 signal on the line 158 l~dicates
that the reverser is in position R, and this signal
provides an input to another AND logic circuit 161
whose second input is taken from the input line 152~
of the mechanism 152. The output of the circuit 161
serves as another input to the OR logic circuit 134,
and it is 1 whenever a 1 signal on line 152b is di-
recting the mechanism 152 to actuate the reverser to
its position R and there is a 1 signal on line 158 to
indlcate that the reverser in f'act ls i.n this posikion,
Thus, the slgnal on the output llne 133 of the clrcuit
2~ ~
20~TR~1309
_42-
134 is in a l state whenever there is proper corres-
pondence between the directed position and the actual
position of the reverser RR. This signal i~ red to an
input o~ the previously described AND logic circuit
130, and it also is fed to an input of another AND
logic circuit 162. The latter circuit responds to two
additional inputs received on lines 163 and 164, res-
pectively. Line 163 is coupl~d to an interlock contact
(not shown) of ~he M/B actuating mechanism 132, and a
l signal on this line-indicates that the contactor B
ls closed. The other line 164 is coupled to an inter-
lock contact (not shown) o~ the mechanism 144 that
actuates the dynamic brake contactor BB, and a 1 slgnal
on this line indicates that the contactor BB ls closed.
The signal on line 164 is also employed to
activate staging means 165 for controlling a pair o~
actuating mechanisms 166 and 167 that are respectively
coupled by lines 57b and 57c to the staging contactors
Bl'and B2 in the dynamic brake circuit 56 (Fig, 2).
As is indicated in Fig. 6B, there are three lines
41, 168, and 169 connected to the staging means 165
for respectively supplying it wikh the capacitor
voltage feedback signal, a f'irst reference signal
represent~r.g a maximum level of voltage (e.g.g 1650
volts~ that is permissible on the filter capac~tor
during the braking mode of operation, and a second
reference signal representing a desired minimum capaci-
tor voltage (e.g., 1200 volts), The staging means 165
is contentionally constructed to close and open the
contactors Bl and B2 as necessary to minimize excur-
sions of VF above the maximum level or below the minl-
mum level.
Witil a l signal on each of` the lines 133,
163, and i6~J, all three inputs of' the AN~ logic circuit
20-TR-1309
~-43-
162 are 1 and consequently there ls a 1 signal on the
output line 170 of this circuit. A 1 signal on the
output llne 170 lndicates that the propulsion system
has been properly reconnected for ~raking operation.
5 In other words, the system is ready to begin a braking
mode of operation. The line 170 is connected to one
input of an AND logic circuit 171, and the third branch
of the line 143 i5 connected to the other input of this
circuit, The output of the circuit ]71 is connected to
10 the line 136, and consequently the signal on the output
line 136 is in a 1 state whenever the system ls set up
for braking and the brake controller is eitner in its
set up position or in its brake on sector. As was pre-
viously mentioned, a 1 signal on the line 136 is
15 passed throug'n the OR logic circuit 122, the delay
component i24, and the line 125 to the mechanism 126
which responds thereto by actuating the contactor 34 to
its closed position.
The field boost means of the brake control
20 block includes an AND logic circuit 172 having three
inputs. As can be seen in Fig. 6A, the first input of
the logic clrcuit 172 is received on the line 112,
and it changes from O to 1 at the start of the braking
state o f the command means when the brake control]er
25 52 is moved from lts set up position to its brake on
sector. The second input af the circuit 172 is re-
ceived on the line 170, and it changes from O to 1 in
response to operation of the brake set up means to
reconnect the propulsion system for braking~ The
30 third input of the circuit 172 is received on the
line 107, and it changes from O to 1 when the chop
enable signal is provlded by the chopper enable block
108 (Fig. 5) in response to a 1 slgnal on the reset
line 104 and a high filter capacitor volta~e VF-. The
35 output of the circuit 172 is cr:nnected to .,he line 115,
;~ 20-TR-1309
~411--
and it will be in a 1 state whenever there are 1 s1g-
nals on all three inputs of thls circult, as is true
in the braking mode of operation. The 1 ~ignal on the
line 115 is the aforesaid B' signal w~lich is supplied
5 to the chopper control means 13 and 23, This Rignal
is also supplied as one input to an AND logic circuit
174 whose other input is received on a line 175 from
level detecting means 176, The level detecting means
176 is connected via the line 101 to the speed aver-
10 aging circuit 102 (Fig, 5), and ~its input is a speedfeedbacI; signal ~ representative of the average
angular velocity of the armatures of' the respective
traction mlotors. So long as this reedback signal
exceeds a particular value corresponding to a pre-
15 determined low vehicle speed (e.g,g approximately3 MPH), the means 176 wlll supply a 1 signal on the
line 175 to the logic circuit 174, but the signal on
line 175 is in a 0 state when the signal ~ is below
this value. Thus a 1 output from the logic circuit
20 174 is prevented whenever speed is below the pre-
determined low level.
The output of the AND logic circuit 174 læ
connected via a line 177 to the input of a one shot
block 178, The component 178 can be a conventional
25 monostable multivibrator which produces a 1 output
signal having a predetermined f'ixed duration (e.g"
0,5 second) once triggered by the s~gnal on line 177
changing from 0 to 1. The output of the component 178
is connected on a line 179 to a "B" input of a second
30 one shot block 180 similar to the component 178 except
that it is triggered by the signal on line 179 changing
from 1 to 0, whereby the 1 signal on the output line
181 o~ the componer,t 180 begins at the end of the 1
output signal on line 179. The output signals on the
35 lines 179 and 181 are respectively noted ~y the
reference letters X and Y in ~ig. 6A, Both are con-
.
20-T~-1309
-45-
nected through an OR logic circuit 182 to a line 183
w~ich is connected to a mechanism 184 (Fi~o 6B~ for
actuatlng the field boost switch FS (Flg, 3), 1~e
mechanism 1849 which is coupled to the movable contacts
of` the switch FS ~y a ~roken llne 185, ls operative
to close the switch FS only when there is a 1 signal
on the line 183, as is true for a limlted period of'
time (e.g., approximately one second~ after the one shot
component 178 is triggered.
The one shot block 178 is initially triggered
at the start of the braking modé of operation (as soon
as the brake controller is moved to its brake on sector
and the brake set up means produces a l signal on the
line 170 and a chop enable signal is supplied on line
107). So long as the system remains in its braking
mode, the component 178 will be automatically retrlg-
gered anytime the vehicle slows down to a speed lower
than the aforesaid predetermined low speed (e.g., 3 MPH)
and subsequently accelerates to a speed above this
threshold, whereupon the signals on the lines 175 and
177 change states from 0 to 1 and the field boost meanæ
responds by temporarlly reclosing the switch FS whicb
momentarily increases field excitation of the traction
motors (now behaving as generators). Such triggering
is desirable because the electromotive force of these
machines may have been too low to sustain armature
current (and hence braking effort) when the vehicle was
traveling slcwer than 3 MPH.
The output of the second one shot component
180 is connected on the line 181 to the input of yet
another one shot block ]86 similar to the f'lrst com
ponent 178 eY.cept that the fixed duratlon of lts 1
- output signal ~F is much shorter ~e.g~, 2 milli-
seconds). Thls output signal is the af'oresaid burst
Z~
20-TR~130
~4~-
flring signal which i~ supplied on the line 55 to the
chopper contr~l means 13 and 23. The third component
186 produces it~ output signal when triggered ln re3-
ponse to a 0 to l change of signal Y from the second
one shot block 180, wh~ch event is delayed with res-
pect to the initial triggering of the first one shot
block 178 by an interval equal to the duration of the
signal X on line 179. ~herefore the signal BF com-
mences approximately midway through the period of timeO that the field boost switch FS is clo~ed.
Fig. 7
The operation of the brake control means
during a motoring-to-braking state change of the command
means will now be summarized with the aid o~ Fig. 7,
The motoring-to-braking sequence is begun by reducing
the throttle setting to its lowest power notch (which
reduces the current call signal I* to a low magnitude)
and then moving the throttle handle to idle. This
step of the transition process causes the signal on
the line 110 to change from l to 0, and in response
thereto the contactor 34 is opened, the signal on the
voltage reference reset line 104 is changed from l
to 0, and the chop enable signal on line ln7 is termi-
nated (l.e., changed from l to 0). The first step also
unlocks the brake controller 52 which can now be moved
from its off position to the brake set up position.
The command means is now in its transition state,
In Fig. 7 the beginning of the transition
state is noted as time to~ At this time the signal
on the line lll changes from 0 to 1, and in delayed
response thereto a 1 signal on the line 143 causes
four e~ents to take place. Two of' these events
are implemented by the M/B actuating mechanism 132
which is directed by the l signal or. line 143
(and hence on line 132b) to open the contactor
20-TR-1309
1~7_
M and to close the corttactor B, The third
e~ent ls implemented ~y the RR actuating mech-
anism 152 which is directed by the concurrence Or
the 1 signal on line 143 and a 1 sigrtal on elther line
114 or line 147 to reverse the position o~ the reverser
RR. Assuming that there is a 1 signal on the llne 114,
thls event will open the Fl,F2 contacts and close the
Rl~R2 contacts of the reverser RR. The fourth event
ls implemented by the BB actuatlng mechanism 144 which
is directed by the 1 signal on line 143 to close the
dynamic brake contactor BB. These four events do
not necessarily happen simultaneously~ and whichever
one that takes place last will cause the brake ready
signal on line 170 to chan~e ~rom 0 to 1, as iB indl-
cated at time tl in Fig, 7. The resulting 1 signal online 136 (and hence on line 125) directs the contactor
actuating mechanism 126 to reclose the contactor 34,
whereupon the signal on the reset line 104 changes from
0 to 1 (a 1 signal on terminal 140 is assumed). Now
the voltage reference signal VREF ramps up to the de-
sired level ~as set by a 0 signal on the reference
level llne 105), and as soon as the capacltor filter
voltage VF attains a sufficiently high magnitude (at
time t3 in Fig. 7), a chop enable signal is supplied
to line 107.
Anytime after to the operator can move the
brake controller 52 to its brake on sector, thereby
terminating the transition state and starting the
braking state as indicated ~y the signal on the line
112 changing from 0 to 1, conld take place earlier or
later than t3. In Fig, 7 it i5 shown at a time t2 that ---
is earlier than t3. As soon as the signal on line 112
is 1 and both the ~ralce read~ signal on line 170 and
the chop enable signal on llne 107 are 1, the B' signal
on line 115 changes from 0 to 1. This mark~ the start
2~
20-TR-130g
-48-
o~ the braking mode of operation of the propulsion
3ystem3 and it also triggers the first one shot block
178 of the field ~oost means (assuming that speed is
not below 3 MPH), Therefore the signal X on line 179
changes from 0 to l concurrently with the signal B'.
When the signal X changes from 0 to 1, the
resulting l signal on line 183 directs the FS mechanlsm
184 to close the field boost switch FS, and at time t4
the movable contacts of this switch reach their closed
circuit poSitiOn to start a brief period of increased
current ln the series field windings of' the kraction
motors, Subsequently, at time t5, the signal X auto-
matically reverts to its 0 state, thereby triggering
the second one shot block 180. The signal Y on the
output line 181 of block 180 now changes from 0 to 1.
This maintains a 1 si~nal on line 183 until the signal
Y3 at time t6, automatically reverts to its 0 state,
whereupon the FS mechanism is directed to open the
field boost switch FS. The switch FS returns to lts
open position at time t7, thereby terminating opera-
tion of the field boost means. Signal Y changing
from 0 to 1 at time t5 also triggers the third one shot
block 186 which is then effective to produce on line
55 the burst firing signal BF that is shown by the
bottom trace of Fig. 7.
In the illustrated embodiment of the present
inventior., the period of time that the field boost
means is operative to close the switch FS (from tLI
to t7 in Fig. 7) is approximately one secondg and the
the time (t5) at which the one shot block 186 becomes
effective to produce the signal BF is delayed until
approximately 0.5 second after t4. Thls delay allows
time for the increased field currerlt to overcome re-
sidual excitation in the f'ield poles of each motor and
to develop therein an appreciable reverse magnetic
Z~i~
20-TR-1309
_1~9_
~ield be~ore the associated chopper is turned on ln
the armature current path to begin the braking mode
o~ operation. Alternatively, lf the reverser RR
were connected across the armature rat~er than the
5 ~ield windings of the motor, less time ~ould be needed
because the direction o~ fle]d ~oost agrees with the
direction of residual exci~ation, and therefore the t4
to t5 interval could be made much shorter by corres-
pondingly shortening the fixed duration of the signal
X. In some applications o~ the invention it mlght
even be desirable to start operation of the field boost
means before the armature reversing aspect of the oper-
ation of the brake set up means is completed.
Fig. 5 ~Chopper Controls)
.4s is shown in Fig. 5, lines 55 and 115 from
the brake control means shown in Flg. 6A are connected
to the No. 1 (and No. 2) chopper control means 13 (and
23). The No. 1 control means 13 comprises a block 191
labeled 'IBurst Firing," a block 192 labeled "Chop.
Ref," a block 193 labeled 'IChop. Pulses," and a pair
of blocks 194 and 195 each labeled "Gate Drive.~' The
line 55 conveys the burst firing signal BF to an input
of the burst firing block lgl. Another input o~ this
block receives on the line 16 the current.feedback
signal representative of armature current IA in motor
Ml. The burst firlng block 191 has two output lines
196 and 197 connected to the chop pulses block 193;
and it is suitably constructed and arranged (see Fig~
8) to supply on the line 196 a d~c gate signal that ls
3 contemporaneous with the burst firing signal on llne
55 and to supply on line 197 a commutation suppressing
signai that is initiated by the burst firin~ signal
and terminated when ~he magnitude o~ IA increases to at
least a predetermined threshold.
The line 115 conveys the B' signal to an
lnput of the chopper reference block 192 in the No. 1
:.,i:,, i ~
:
.
i2~
20-TR-1309
-50-
control means 13, Other inputs of this block receive,
respectively, the chop enable signal on the llne 107
~rom t~e chopper enable means 108, the current call
signal I* on line 47 from the reference generator in
the master controls, the current feedback signal on line
16 ~rom the current transduçer 17 in the armature cur-
rent pakh of the motor M1, and certaln additional
slgnals from a terminal 198. ~he chopper reference
block 192 is suitably constructed and arranged Csee
Fig, 9) to process these inputs and produce therefrom
1~ a ~ariable control signa] Vc representative of the de-
sired duty factor of the associated chopper 12~ This
control signal is supplied on a l~ne 199 to the chspper
pulses block 193.
The chopper pulses block 193 has five inputs
lS that are respectively connected to lines 45, 1999 107,
1973 and 1~6, and it has two output llnes 201 and 202.
~etails of 2 preferred embodiment of this component are
shown in Fig. 10 which will soor. be descri~ed. Nor-
mally the chopper pulses block 193 is cyclically opera-
ti~-e to produce on its output line 201 a train of first
periodic gating signals of relatively short predeter-
mined duration (e.g., 10 microseconds) and to produce
on its second output line 202 a train of periodic
second gating slgnals of the same short duration. The
first gating signals are supplied on line 201 to the
input of the ~ate driver l9lJ whose outpuk ls coupled
via the lines 14 to the gate and cathode terminalæ G
and C of the main thyristor 70 in the No. 1 chopper 12,
and the component 19l~ is suitably constructed and
arranged to supply a firing signal to this maln thy-
ristor in respGnse to each of the flrst gating signal~
received on line 201. The periodic s~?cond gating
signals from the chopper pul~es ~lock 193 are supplied
on the llne 202 to the input of the companion gate
20-TR-1309
~51-
driver 195 who~e output i~ coupled vla lines 15 to the
gate and cathode ~erminals G and C of the auxillary or
commutatin~ thyristor 72 in the No, 1 chopper, and th~
component 195 is sultably cons~ructed and arranged to
supply a firing signal to thls commutating thyristor in
response to each of the second gating signals received
on line 202, As will be apparent hereinafter from the
description of Fig. 10, the first gating signal~ on
line 201 are produced alternately with the second
gating signals on llne 202, whereby the gate drivers
are effective to alternately turn on and turn off the
chopper. The chopper pulses block 193 includes means
for synchronizing the ~eco~ld gatlng signals with the
clock pulses on line 45 and means responsive to the
value of the variable control signal Vc on line 199
for influencing the timing of the first and second
gating signals so as to determine the duty factor of
chopper No. 1.
ht the beginning of a braking mode of opera-
20 tion, the d-c gate signal on line 196 is passed through
the pulses block 193 to the output line 201 in the form
of an extended chopper turn-on slgnal that effects
firing of the main thyristor 70 throughout the period
of the burst firing signal on line 55, which period
25 is substantially longer than the duration of a first
gating signal that the block 193 periodically produces
ln normal operation. At the same time the commutation
suppressing signal received on line 197 is effective
in the block 193 to prevent the production of any
30 second gating signal on the line 202 until the magni-
tude of armature current increases to at least the afore-
said predetermined threshold.
Fig, 8
With reference now to Fig, 8, a preferred
35 embodiment of the burst; firing ~lock 191 of the No~ 1
Z0-TR-1309
-52-
chopper control means 13 will be de~cr:Lbed~ This com-
ponent comprises an AN~ logic circuit 2~4 havlng two
inputs: one is the burst firing signal BF received on
the line 55, and the other is received on an output
line 205 of level detecting means 206. The level
detector 206 ls supplied via the line 16 with the
current feedback signal that indicates the actual
magnitude of armature current IA in the motor Ml~ and
~t is suitably constructed and arranged to produce on
its output line a signal that is 1 so long as IA is lesæ
than a predetermined threshold magnitude (e.g., 100
amperes) and that is 0 when the magnitude of lA in-
creases to at least this threshold. As was previously
explained, lA will be low or zero at the start of the
field boost period, and thérefore both inputs of the
logic circuit 204 wi]1 be 1 when the burst firing signal
appears on llre 55. At this time the output signal o~
the circuit 204 changes from 0 to 1.
The output of the AND logic circuit 204 is
connected by a line 207 to an input of another AND logic
circuit 208 whose second input is received on a line
209 from a high frequency clock 210 that generates a
train of discrete "1" pulses. By way of example, the
frequency of the pulses on line 209 is 21,6 KHz, and
each pulse can have a duration of 10 to 20 microseconds.
The output signal of the logic circuit 208 there~ore
comprises a burst of high-frequency pulse~- that lasts
for an interval equal to the duration of the 1 output
signal from the logic circuit 204. The duration of
the latter signal normally corresponds to the period of
the burst firing signal on line 55, namely, approxl-
mately 2 milllseconds. The output signal Or the circult
208 ls the d-c gate signal that is ~upplied over the
line 196 to the cyclically operatlve chopper pulses
block 193 in the No. I chopper control means 13 (~ig.
2~
20-TR~1309
~53-
5~, and during lts presence on line 196 the block 193
wlll be effective to supply an extended chopper turn on
signal to the gate driver 194 of the main thyristor 70
in the chopper 12. The duration of the latter signal
is substantially longer than the predet0rmined dura-
tion of the per~odic first gating signals that are
normally produced by the pulses bolck 193, Preferably
the duration of the extended chopper turn on signal ls
at least 100 times longer than that of the first
gating signals, and in the exam~le glven herein lt is
approximately 200 ti.mes longer. This ensures that if
a motoring-to-braking transition were commanded while
the vehicle is mo.ving at a relatively low speed, the
initial firing signal for turning on the chcpper will
not expire prematurely, ~efore armature current has
time to attain the latching level of the main thy-
rlstor, and it consequently ensures that the chopper
is in fact turned on and conducts armature current
to begin the braking mode of operation of the propul-
sion system durlng the period of time that the seriesfield of the motor Ml is being boosted to increase the
electromotive force that is being generated ln the
armature of this machine. Once turned on, the chopper
will freely conduct current in the armature current
path of the motor Ml, and the rise af current in this
path, including the series field, will augment the
field boost so that the electromotive force rapidly
increases, This further enhances the buildup ~f arma-
ture current which soon attains its 100~amp threshold,
ordinarily within less than one-half second of the
time that the burst firing signal BF is produced,
As can be seen in Fig, 8, the burst fi.ring
signal BF on the line 55 in the burst ~iring means
].91 is also supplied to a cloc~ input of a conventlonal
flip flop device 211 whose "D" input is connected to
z~
20-TR-1309
-54-
a d-c control power terminal 112 which is positive
Ce~g~, ~10 volts~ with respect to a predetermlned
re~erence potential. The Q Outpllt of this device i3
COnnected to the line 197, and it changes from 0 to 1
when the cloc~ input signal changes from 0 to 1 on
receipt Or the signal ~ Su~sequently this output
ls chan~ed back to 0 by applying a 1 signal to the
reset lnput of the device 211. The reset input i~
connected via a line 213 anrl inverting means 214 to
the line 205, whereby its 0 to 1 change coincides with
the output of t~e level det~cting means 206 changing
from 1 to 0 as a result of armature current attaining
the 100-amp t,hreshold, The Q output signal o~ the
device 211 is the commutation suppressing signal that
is supplied over the line lg7 to the pulses block 193
(Fig, 5), and in its 1 state this signal is effective
to disable the puises block 193 and thereby prevent it
from producing an~ gating signals that would otherwi~e
turn off the chopper 12. The commutation suppressing
signal on line 197 is in its 1 state for an interval
that begins at the same time as the burst firing signal
BF and that ends as soon as armature current attains
its 100-amp threshold. During this interval the
chopper is turned on in response to the d-c gate signal
on line 196 and then remaln on continuously, but once
the interval explres the pulses block lg3 can resume
normally producing gating signals to alternately turn
off and turn on the chopper.
~ig, 9
Turnin~ next to Fig, 9, a preferred embodi-
ment of the chopper reference means 192 will now ~e
described. Thls means, whlch was shown as a ~lngle
block in ~ig. 5, includes a summing point 216 havlng a
first input connected to a current reference signal llne
217, a second input connected to the current feedbacl{
.
20-TR~1309
~55-
slgnal line 16, and an output connected to an error
signal line 218~ The current reference slgnal on line
217 1s representative of the desired magnitude IR~F f
armature current in the motor M1, the current ~eedback
Signal on line 16 is representative o~ the actual
magnitude IA of this current, and the error signal on
line 218 is therefore representative of the difference
- between IREF and IA. Preferably the quiescent value o~
the error signal ~i.e., its value whenever bot~. the
desired an~ actual magnitudes OL armature current are
zero) is negative wit~ respect to the predetermined
re~erence potential of the control power.
The error signal on line 218 is processed
by a suitable gain network 219 having a proportional
plus integral transfer characteristic, whereby a zero
steady-sta~e error can be obtained. The galn of the -~
network 219 is varied as a function of speed ~indicated
by the speed feedback signal on line 19) 3 current (in~
dicated by the current feedback signal on line 16) J
and whe~,her the system is in a motorlng or braking mode
of operation (indicated by the B' signal on line 115).
In the braking mode, the transfer function of this
component has a faster time constant and a higher gain
than in the motoring mode. The output of the gain net-
work 219 provides the variable control signal Vc whichis fed on line 199 to the cyclically operative chopper
pulses block 193 in the No. 1 chopper control means 13
(Fig. 5), The value o~ Vc varies as a function of any
to assume whatever value results in reducing this
difference or error between IREF and IA and will tend
difference to ~,ero. The value of Vc can vary between
predetermined first and second extremesy and it is
varied in a sense approaching the second or high extreme
(e.g., ~10 volts on an analog ~asis) ~rom its rirst or
low extreme (e.g., -1.5 volts) so long as IA is less
20~TR-1309
~56-
than IREF. The timing of the alternate firsk and
second gating slgnals that are perlodically produced
~y t~e cyclically operative chopper pulses block
193, and consequently the duty factor Or the chopper
12, are determlned by the value of Vc on line 199.
~en the value of Vc is at lts low extreme, the duty
factor is zero (chopper turned off contlnuously~, and
~hen Vc is at its high extreme the duty factor i8 1. O
Cchopper turned on continuously).
In order to provide the aforesaid current
reference signal, the line 217 of the chopper reference
means 192 is connected through three blocks 220, 221,
and 222 in tandem to the line 47 on which the current
ca]l si~nal I~ is received from the master controls.
The block 22Q is designed to be effective only ln a
braking mode of operation, as indicated by a Bl signal
on line 115~ to prevent the current rererence signal
on line 217 from falling below a certain minimum value
that corresponds to a predetermined magnitude of arma-
ture current (e.g., 100 amperes). This is deælrableto maintain self excitation of the traction motor Ml
and to ensure a minimum braking effort in the event
the operator were to move the handle of the brake con-
troller 52 to the lowest or zero position in its brake
on sector The block 221 is labeled l'Jerk Limit," and
it performs the conventional function of preventing
the value of the current reference signal on the line
217 from being changed too fast. By way of example,
the maximum rate of increase of the reference signal
can be limited to a rate corresponding to 200 amps
per second, and the maximum rate of decrease can be
limited to a rate corresponding to 1000 amps per
second.
The block 222 of the ch~pper reference ~eans
192 is suitably constructed and arranged to perform
6~
20 TR-1309
-57
dual runctions. I~s first functlon is to proportion-
ately reduce the current call signal either in the
event Or a whee] slide involving the wheels that are
coupled to the motor Ml or ln response to low voltage
on the commutating capacitor of the chopper 12 compared
to the magnitude of armature current that has to be
commutated, In Flg 9, the two lines under the refer-
ence number 198 respectively represent the wheel slide
and the commutating capacitor voltage lnputs to the
block 222. The second function of this block is to
reset the current reference signal to a value corres-
ponding to zero current whene~er there is no chop
enable signal on the line 107. For this purpose the
block 222 includes means for clamping its output to a
low or zero value in response to the signal on line
107 changilig from 1 to 0, as happen.s at the beginning
of a motorir,g-to~braking transition of the command
- means when the contactor 34 is opened in response to
the throttle handle being moved to its idle position.
If the current reference signal on line 217 were not
already reduced in response to the handle of the
throttle 51 being moved to power notch 1 and then to
ldle, it would no~r be driven at its maximum rate (as
limited by the block 221) to a reset level that ls
slightly negative with respect to ground, thereby
altering the value of the control signal Vc as neces-
sar~ to ensure that this signal attains the aforesaid
lo~r extreme. As a result, the chopper duty factor
and hence I~ are rapidly reduced to zero. Subse-
3o quently, when the chop enable signal returns to line107 ~at time t3 in Fig, 7), the output of the block
222 is unclamped and the current reference signal on
line 217 can increase to whatever value is being called
for by the signal I~ on line 47.
.:
.
,
20-TR-1309
-5~-
Figs, lO and ll
Flg, 10 illustrates the preferred em~odiment
o~ the chopper pulses block 193. In this component the
~arlable control signal ~C on line l9~ ls supplied as
one input to a summing polnt 224 where it is compared
with a saw-tooth reference signal produced ~y a ramp
generator 225, The ramp generator 225 i8 connected
to the master clock 44 ~y a line 45a, and lt ls period-
ically reset ~y a phase l clock pulse on this line,
The clock 44 generates a train o~f phase l pulses on
the line 45a, each pulse being in a l state for a pre-
determined duration (e.g., 3ao microseconds~ and
successive pulses recurring at a constant frequency
(eOg., 300 Hz).
The ramp generator 225 comprises integrating
means for changing the value of the reference signal at
a predetermined constant rate and means operative in
synchronism with the phase l clock pulses for period-
ically resetting the reference signal to a predeter-
mined base value which is substantially equal to the
aforesaid high extreme of the control signal Vc Ce.g "
~lO volts), After being reset, the reference signal
changes in a sense approaching the aforesaid low ex~
treme value of Vc, and the rate of change is selected
so that the reference signal excursion is approxi-
mately lO volts during one period of the clock pulses.
This reference signal is subtracted from Vc in the
summing point 224, and the difference is supplied on
a line 226 to a zero crossing detector 227 whos~ output
is fed on a line 228 to an AND logic circuit 230. In
digital terms, the signal on the output line 228 is
low or "0" so long as the value of the reference signal
produced by the ramp generator i~ greater Ci.e,, more
posltive) than the value of the control signal Vc, and
20 TR 1309
- 59 -
it is high or "1" whenever the latter signal is greater
than the former. When Vc is at its high extreme, the
signal on line 228 is 1 continuously. When Vc has a
negative value the signal on line 228 is 0 continuously.
When Vc is in a range between zero and its high extreme,
the signal on line 228 will change states twice each
cycle of the master clock; from 1 to 0 when reset by a
phase 1 clock pulse, and from 0 to 1 concurrently with
the value of the reference signal equalling the value of
10 Vc~
The variable control signal Vc on line 199
and the phase 1 clock pulses on line 45a are also
supplied as inputs to a voltage-to-fr~quency converter
231. This component is suitably constructed and
arranged to periodically produce at its output F a train
of discrete 1 signals having an average frequency
that is related to the value of Vc in accordance with
the graph shown in Fig. 11. For variations of Vc between
its low extreme (-1.5V) and a predetermined first
intermediate value (e.g., -0.5 V), the frequency of
the output signals F varies between zero and the clock
frequency -(300 Hz) as a direct linear function of the
value of Vc. For variations of Vc between its high
extreme (+10 V) and a predetermined second intermediate
value (e.g., +9.1 V), the frequency of the output
signals F varies between zero and the clock frequency
as an inverse linear function of the value of Vc. For
variations of Vc in a predetermined range that
is defined by the aforesaid first a~d second
intermediate values, the frequency of F is constant
and equal to the frequency of the master clock. A
V/F converter well suited for this purpose is disclosed
and claimed in United States Patent No. ~,256,983
issued March 17, 1981, R.J. Griffith and
R.D. Stitt and assigned to the General
.
'
2 ~ ~ 20-TR-1309
-60-
Electric Company. Such a converter is so arranged
that a 0 to 1 change of its output signal F always co-
incldes with the leading edge of a phase 1 clock pulse
on the line 45a. This converter receives addltional
inputs via lines 45~ and 45c from the mas~er clock 44.
The clock is designed to generate on line 45~ a train
of phase 2 pulses that are characterized ~y the same
~requency and duration as the phase 1 pulses on llne
45a but are displaced in time therefrom by a predeter-
mined fraction of the period of the clock (e.g., by1/3 period, or 1/900 second), and to generate on line
45c a train of phase 3 pulses that are sim~lar to but
further displaced in time from the phase 1 pulse ~e.g~,
by 2/3 peri.od or 2/900 second). A 1 to 0 change of
each output signal F produced by this converter coin~
cides with the leading edge of the phase 2 clock pulse
that is next received after the output signal was ini-
tiated. In addition, this converter is arranged to
produce at a second output E a signal that is 0 only
when the value of Vc is between its low extreme and
the aforesaid first intermediate value and that other-
wise is 1,
The output signal F of the converter 231 ls
connected by a line 232 to a first input of an AND
logic circuit 233. Another input of the latter cir-
cuit is connected through a line 234 and inverting
means 235 to the line.197 which receives the commutation
suppressi.ng signal from the burst firing block 191 (see
Flgs, 5 and 8), Thus there is a 1 signal on llne 234
except during intervals when the burst firing means
is effective to supply a 1 signal on line 197. As is
shown in Fig. 10, the third input of the logic circult
233 is connected via a line 236 to the Q bar output of
a conventiorlal D type flip flop 237. The set input of
the latter component is connect~d through inverting
. ; ,
.
.. , .,,. , .,, > .,.,., . ,.. , . ~ .. ~
20-TR-1309
--61-
means 238 to the line lQ7 ~hich receives the chop ena~le
slgnal from the chopper ena~le means 108 (Fig, 51, and
the clock input ls connected tTlrough lnverting mean~
239 to t~e output line 232 of the V/F converter 231.
As will soon be explalned, the flip flop 237 serves a
pulse steering purpose ~rhen the signal on line 107
changes from 1 to 0, Du-rlr.g normal operation the chop
enable signal is 1 and the Q bar output of 237 is in a
high or 1 state,
1~. With 1 signals on bothiof its input lines 234
and 236, the loglc circuit 233 will pass a 1 signal to
its output line 240 concurrently with each of the
perlodic 1 signal from the output F of the converter
231, The line 240 is connected through an OR logic
circuit 241 to the input of a one shGt block 242 which
produces a-l. OUtpllt signal having a relatively short
predeterri~lned duraticn ~e,g " 10 microseconds) whenever
it is trigggered by the signal on the line 240 changing
from O to 1. I~he bl.ock 242 is connected by a line 243
to suitable amplifying and isolating means 244 which
is effective ~Jhile the output signal on this line ls l
to forward bias the base-to-emitter Junction of an NPN
trans:Lstor 245. The collect.or and emitter of the
transistor 245 are coupled via terminals 202a and 202b
25 to the input of the gate drive block lg5 (Fig. 5), and
when this transistor is forward biased its collector
current is the aforesaid second gating signal that
periodically causes the gate driver 195 to fire the
commutating thyristor 72 in the No, 1 chopper 12,
30 This happens each tlme the output signal F of the con
verter 231 changes from 3 to 1, providing that 1 slg-
nals are then pl~eSent on both lines 23lJ and 236, Thus
the frequency Or the second gating sig.n.als is t,he ~ro-
quenc~ of th~e output si~nal F,
,
6;Z~3
20-TR-1309
-62-
The output llne 243 of the one shot hlock 242
is also connected to a reset input of another D type
flip ~lop 247, As can ~e seen in Fig, 10, the clock
input of the latter component is connected to the out-
put line 248 and the D input is connected directly tothe positlve control power terminal 212. The Q output
of this flip flop is coupled over a line 250 and an OR
logic circuit 251 to the input of another one shot
block 252 which produces a 1 output signal having a
10-microsecond duration each time it is triggered ~y
the signal on the llne 250 changing from O to 1. ~he
block 252 is connected by a line 253 to suitable ampli-
rying and isolating means 254 which ls effective while
the signal on this line is 1 to forward blas a tran-
sistor 255 whose collector and emitter are coupled via
terminals 201a and 201b to the lnput of the gate drive
block 194 (Fig. 5~. When the transistor 255 is forward
biased, its collector current is the aforesaid first
gating signal that periodically activates the gate
driver 194 which then fires the main thyristor 70 in
the No. 1 chopper. This happens each time the signal
on line 250 from the Q output o~ the flip flop 247
changes from O to 1.
The output o~ the flip ~lop 247 is reset to
zero by the signal on line 243 each time a secondgating signal is produced, and it thereafter is re-
turned to a 1 state upon receipt of a 1 signal on the
line 248 connected to the clock input. Once returned
to 1, the Q output remains in this state until reset
3 by the next 1 signal on line 243. As a result, in
normal operation the signal on line 250 periodlcally
changes from O to 1 at a frequency that is the same as
the frequency Or the second gating signals, and the
I'irst gating signals will alternate with the s~cond
æating signals~
- , ~
.
lZ6~
20-TR~1309
-63-
The clock input of the ~lip flop 247 la
connected by the line 248 to the output of the AND
loglc circuit 230. This circuit has four inputs: one
ls received on the llne 228 ~rom the output of the pre-
~iously described zero crosslng detector 227; anotherinput is received on the line 236 from the Q bar output
of the flip flop 237; the third ls received on a line
256 from the E output of the V/F converter 231; and
the fo~lrth is received on a llne 257 which is connected
through inverting means 258 to the line 45a. The sig-
nal on line 257 serves a lockout function; it prevents
a 1 signal on line 248 while each of the phase 1 pulses
on line 45a is 1~ which is the case for an interval of
approximately 300 microseconds following the lnitlation
f each oP the second gating signals. This interval,
referred t~ hereinafter as the lockout interval, is
required to make sure that the first gating signal is
not produced prematurely, i.e., before the commutating
thyristor has time to be completely turned off during
the commutation process of the chopper.
So long as there is no phase 1 ~ulse on the
line 45a, the signal on line 257 is 1, and assuming 1
signals on both of the lines 236 and 256, the signal
on the output line 248 of circuit 230 will now reflect
the state of the signal on line 228. As was previously
explained, the signal on line 228 changes from 0 to 1
whenever the saw-tooth reference signal produced by the
ramp generator 225 decreases to the value of the con-
trol signal Vc on line lg9, Consequently, so long as
3 ~C has a value in a range between 0 and ~9.lV, the
gating signals are produced at the constan~ frequency
of the master clock (300 Hz) and the time interval from
the production of one of the second gatlng signals for
firing the commutation thyristor to the production o~
the succeeding rirst gating æignal for flring the main
z~
20~TR 1309
-6
thyristor varies lnversely with the value f ~C~ Thl~
interva] ls referred to as the off tlme (toF~ o~ the
chopper 12. It decrease~ toward a predetermined mlni-
mum as the value of Vc approaches 9.1 V. The minimum
turn off time i.s the same as the aforesaid lockout in-
terval (e.g., 300 microseconds~.
The duty factor of the first and second
gating signals is equal to l -f x toFF, where f i~
the frequency of the output signal F of the V/F con-
verter 231. So long as this converter is operating inits constant 300 Hz mode, the minimum of~ time of 300
microseconds restricts the maximum duty factor to
approximately .91. As Vc i.ncreases from 9.1 V to its
high extreme of +lO V, the duty factor is increased
from .91 to nearly l.0 by reducing the average frequency
of the periodlc output signals F of the converter 231
whl.le maintaining the off time substantially equal to
the aforesaid minimum.
The minimum duty factor of the chopper is
also restricted in the constant ~requency operating
mode of the converter 231, even when Vc ls reduced
to zero or to a negative value. This is because each
time the commutating thyristor is fired it will conduct
a pulse of load current having a minimum duration or
width which is determined by the recharging time of the
commutating~capacitor 74 in the oscillatory commutation
circuit 71. This minimum 'ton" time therefore depends
on the parameters of the commutation circuit~ and in a
practical embodiment of the invention it resul.ts in a
minimum duty factor of approximakely ,09 at a chopping
frequency of 300 Hz, For variations of Vc from -0,5
to its lo~ extreme of -1.5 V, the duty ~actor is de-
creased to nearl~ ~ero by reduci.ng the average frequency
of the periodic output slgnal.s F Or the converter 231,
During this va.riable frequency, mi.n.l.mum pu].se width
20-TR-1309
-65-
mode of operation, the first gating signals for flring
the main thyrlstor are lnhibited by the 0 signal on the
line 256 which disables the ~ND logic circuit 230 and
prevents it from supplying a 1 signal on line 248 to
the clock input of the flip flop 247. Consequently no
gating signals are supplied by the chopper pulses block
193 to the main gate driver 194, but the chopper is
alternately turned on by firing its commutating thy-
ristor and turned off by self commutation. The
commutating thyrlstor 7 S periodically fired in
response to the second gating signals which the
block 193 is now supplying to the gate driver 195
at a reduced frequency th~t varies wlth the value of
VC and that is zero when Vc is at its low extreme, and
each time the commutating thyrlstor is fired it con-
ducts armature current for a mi.nimum on time (toN)
un~il automatica].ly extinguished by the rlnglng action
of its oscillatory commutation circuit. The duty fac
tor, which can be expressed as r x toN, ls proportional
to the frequency of the output signals F of the V/F
converter 231. It will now be apparent that the chop-
per pulses block 193 has the capabiltiy of smoothly
varying the duty factor of the chopper over a con-
tinuum that exter,ds from 1.0 when V~ is at its hlgh5 extreme (~10 V~ to zero when Vc is at its low extreme
i V).
As was previously explained, normally the
signal on the chop enable line 107 is 1, but during a
motoring-to-braking transition lt is temporarily 0.
3 Whenever this signal changes from 1 to 0, a 1 signal
is applled to the set input of the flip flop 237,
thereby changing the Q output o~ this component from
0 to 1 and the Q bar output from 1 to 0. ~he Q output
is connected on a line 2~0 to a first input of an AND
20-TR-1309
~66-
logic circuit 261 whose other input i8 connected to the
line 250 and whose output is connected vla a llne 26
and the OR logic circuit 241 to the input of the one
shot block 242. Conse~uently, if the chopper were in
a turned on state (as lndicated by a 1 signal on li.ne
250) at the time the flip flop 237 ls set, the n to 1
change of the Q output on line 260 would trigger the
one shot 242 and steer one last gating ~ignal to the
gate ~river 195 of the commutating thyristor, thereby
turning ofl the chopper 12. At the same tlme, the 1 to
O chan~e of the Q bar output on line 236 disables the
AND logic circuits 230 and 233, and no further gating
signals can be produced by the chopper pulses block
193 so long as ~here is no chop enable signal on line
107. Later, after the chop enable signal is restored
to i.ts 1 state, the flip flop 237 will return it~ Q
output to the 1 state and its Q bar output to the O
state upon receipt of a 1 signal at its clock input
~indicating that the F output of the V/F converter 231
is 0), and no~ the chopper pulses block 193 can re-
sume normally producing gating signals to alternately
turn on and turn off the chopper with a duty ~actor
determined by the value of Vc
To ensure initlal turn on of the chopper 12
during the period of time that the field of motor Ml
is being boosted at the beginning of a braking mode of
operation, t.he d-c gate signal on line 196 is connected
through t.he OR logic circuit 251 to the one shot block
252. Preferably, as was pointed out above in connec-
tion wlth the description of Fig. 8, this d-c gate
signal is actually a short (e.g., approximately 2 milli-
seconds) burst of hlgh-freauency (e.g.~ 21 6 K~z) dis-
crete 1 pulses. Such pulses wlll repetitively trigger
the block 252, and conse~uently a corresponding burst
Or gatlng signals is prod.uced at terminal.s 201a and
~ z~
20 TR 1309
- 67 -
201b of the chopper pulses block 193. This burst of
gating signals has the same frequency as the pulses
comprising the d-c gate signal, and it is re~erred to
herein as the extended chopper turn on signal. When-
ever the burst firing means is effective to supply the
gate driver 194 with this extended chopper turn on
signal, the gate driver responds by supplying a
correspondingly extended initial firing signal to the main
thyristor of the chopper 12. A gate driver well suited
for this purpose is described and claimed in Canadian
patent application S.N. 362,551 filed October 16, 1980,
R.B. Bailey and assigned to the General Electric Company.
concurrently with the extended chopper turn on signal,
and for whatever additional time is necessary in order
for IA to attain the aforesaid 100-amp threshold, the
commutation suppressing signal on line 197 is in a
1 state (and the signal on line 234 is 0), thereby
disabling the AND logic circuit 233 and preventing
the chopper pulses block 193 from producing any second
gating signals that would otherwise cause the gate
driver lg5 to fire the commutating thyristor and turn
off the chopper.
With one exception~ the chopper pulses block
for the No. 2 chopper control means 23 is the same
as the block 193 shown in Fig. 10. The one exception
involves the connections to the master clock 44.
Where Fig. 10 shows a line 45a supplied with phase 1
pulses from the master clock, the corresponding line
of the No. 2 chopper pulses block should be supplied
with phase 2 pulses, whereby the resetting of its ramp
generator and the production of an output signal
F by its V/F converter will be delayed one-third of
the period of the master clock with respect to the
occurance of these events in the No. 1 chopper pulses
,~
Z6~
20-TR-1309
-68_
bl~ck Similarly, the pulses block in the contrGls
for a third chopper (not sho~n) should be synchronized
with the phase 3 pulses of the master clock. In
propulsion systems using six chopper/motor units,
the master clock could be provided with a 6-phase out~
put. In this manner the respective choppers are turn~d
off in sequence rather than in unison. By thus stag-
gering the of r times of the respective choppers, the
amplitude o~ ripple current in the filter 32 i3 de~ir-
ably minimized.
Figs. 12 - 15
Having described the various power and con-
trol components of the illustrated propulsion system,
the operation of the system during electrical braking
will no~r be summariæed with the aid of Figs. 12-15.
T'lig. 12 i6- a simpli~ied diagram of the filter capacltor
60, the d-c bus 31, and the flrst chopper/motor unit
12/M1 after reconnection for the braking mode of opera-
tion. In this figure the dynamic brake resistor grid is
shown at 264, the lumped reslstances of the motor arma-
ture and field and of the cab]es in the armature cur-
rent path are represented by a single resistor 265,
and the lumped inductances of these components are
represented by a single inductor 266. While not
shown in Flg 12, other chopper/motor units of the
propulsion system are of course connected across the
d-c bus conductors 31p and 31n in parallel ~ith the
unit 12~Ml. The practice of the present invention
is not limited to the particular propulsion system
shown in Fig. 12, and it is useful, for example, in
propulsion systems Or the type disclosed in U.S. patent
No 4,051,J-121 issued on September 27, 1977, to
T R. Brinner and T. D. Stitt and assigned to the
General Electric Compan~. ~ig. 13 illustrate6 such
an alternative system in its, electrical braking con-
20-TR-1309
-69~
figuration, Switching means 267 connects a dynamic
brake resistor 268 across the parallel branch Or the
armature current path that includes the chopper 12, and
additional resistance 269 shunted by second switching
means 270 can be inserted in the armature current path
by opening the latter switching means when desired.
In this embodiment the armature current path during
braking includes a flyback diode 271 instead of the
contactor B, A separate set of resistors 268, 269
and switches 267, 270 is required for each of the
indlvidual chopper/motor units of the propulsion
system,
In electrlcal braking, the motor Ml behaves
as a generator. Its armature is driven by mechanical
inertia of the vehicle and exerts on the wheels to
which it is coupled a negative (braking) torque that
is a function of the generated current, Thus IA is a
measure Or bra~ing ef~ort. The armature current path
for the motor Ml in Fig, 12 and the corresponding paths
for the other ~raction motors ~not shown) o~ the same
propulsion system include the dynamic braking resis-
tance 264. The electrical energy generated by each
motor is disslpated in the form of heat by the IR
losses in the armature curr.ent path. While part of
the power loss takes place ln the armature and ~ield
windings of the individual motors, in the associated
chopper (when turned on), and in the resistance 265,
most of the braking energy o~ all of the motors is ln-
tended to be dissipated in the resistor grid 264,
3 (Although the illustrated embodiment o~ the invention
employs dynamic braking, the invention can be practiced
equally well in regenerating systems where braktng
energy is fed back to a receptive source,)
The voltage across the fllter capacitor is
. ~
-
20-TR-1309
7o-
~F. Wlth contactor BB closed3 the ~ame voltage 18 lm-
pressed across the resistor grld, and its average magni-
tude is equal to the square root of the product of
the resistance (in ohms) o~ the grid times the power
~in watts) being dissipated thereln. When the chopper
12 is turned off and constant current is conducted by
the free wheeling rectifler FWR, the voltage V~b across
the lumped inductance 266 and the reactor ~2 in Fig, 12
ls equal to VF - VQ, where VA equals the electromotive
for~e generated by the machlne Ml less the sum of the
voltage drop across the lumped resistor 265 and the
forward voltage drop across ~WR. ~he magnitude o~ the
electromotive force varies directly with motor speed
and also as a function of IA which excites the series
field of the motor Ml. (Note that if the chopper 12
were never-turned on, IA and hence braking effort would
be zero so long as VF exceeds VA, as would be true at
low motor speeds.)
In order to control IA as speed is reduced
and therefore achieve relatively high and constant
braking effort at low speed, the chopper 12 is period-
ically turned on during the braking mode of operation,
and its duty factor is varied as a function of the
value of the control signal Vc. When the chopper is
turned on and conducting constant armature current, Vab
is positive and equal to VA (assuming the forward volt-
age drop across the chopper is not materially different
than the forward drop across FWR). If V~ is greater
than VA~ during the off time of the chopper Vab is
3 negative. In order to regulate armature current IA
(and hence braking effort) to a preset constant average
magnitude determined by the current call signal I*, the
average ma~nitude of Vab must be zero. Otherwi~e, IA
would be either increaslng (if the average were posi-
tive) or decreasing (if the average were negative). A
20-TR-1309
-71~
zero average requires that VF, be greater than VA, and
it requires a chopper duty ~actor equal to 1 VA~VF.
Fig. 14 shows the required duty ~actor durlng
two cycles of steady state operating of the chopper 12
for each of three di~ferent speeds in the braking mode
of a propulsion system having six chopper/motor unlts.
High speed operation (e.g., 42 MPH) is illustrated by
the trace 273 in Flg. 14A~ At this speed with typical
traction motors and other practical parameters, VA will
be approximately 820 volts and IA is approximately 490
amps. Assuming that the resistance of the resistor
g`rid is approximateiy 1.1 ohms, VF will be approxi-
mately 1640 volts and the duty factor is seen to be
0.5. In Fig. 14B the trace 274 illustrates Va~ during
operation at the corner point speed (e.g., 21 MPH) 3
where VA is approximate].y 460 volts and IA is 880 amps.
The power (6 x VA x IA) di~ipated in the resistor grid
at this speed is substantially the same as at t~e
higher speed. Therefore VF remains nearly 1640 volts
and the duty factor is approximately .72. In Fig. 14C
the trace 275 illustrates Vab during operation at a
much lower speed (e.g., 4 MPH) with IA maintained at
880 amps. VA is now appro~imately 34 volts, while VF
has fallen to the minimum le~el of 850 volts that i5
maintained by the controllable converter 33 during
braking. Therefore the duty factor is now .~6 which
ls higher than the maximum obtainable when the chopper
is in its constant frequency mode. The desired duty
factor ls or~tained by reducing the chopping frequency
3 (to approximately 267 Hz) while maintaining the minimum
off time.
Fig. 15 traces the arma~ure current IA and
the electromotlve force Vemf generated by the motor Ml
during an ~l~ctrica~ bra~clng process t~lat be~lns at
20-T~-1309
-72~
45 MPH and that continues at the maximum braki~g rate
until the vehicle has decelerated to a low speed of
2.7 MPH. (A level track ls assumed.) Point 1 marks
the initial turn on of the chopper 12 by the bur~t
- 5 firing means of this invention, whlch takes place ap-
proximately one-half second after the chop enable slg-
nal on line 107 enables the current reference signal
IREF in the chopper reference means 192 (Flgs, 5 and 9)
to begin ramping up, at a rate of 200 amps per secondt
from its negative reset level t~ a value correæponding
to t~le desired magnitude of armature current, which
value will ultimately be determined by the current call
signal I* from the reference generator 100 in the
master controls (Fig, 5), Prlor to this initial turn
on of the chopper, the fleld boost means is operative to
increase Vemf, but there is no current in the armature
current path because the filter capacitor voltage VF
is greater than Vem~ and the free wheeling rectlfier
FWR is reverse biased. ~hererore IA starts rising from
0 at point 1, and it quickly attains the 100 amp-
threshold at point 2 in Fig, 15, whereupon the commu-
tation suppressing signal on line 197 terminates and
the chopper pulses block 193 (Figs. 5 and 10) ~s able
to produce a second gating signal for firing the com-
mutating thyristor and hence turning o~f the chopper 12.
Thereafter the chopper pulses block 193 alternately pro-
duces its first and second gating signa]s at a duty
factor d'etermined by Vc, whereby IA proceeds to track
IP~EF
As IA increases, so does field excitation,
and this causes the volts per RPM to increase along
the field saturation curve of the motor M1, As the
generated voltage increases, so does VA. The power
to b~ dissipated ln the dynamic braking resistor grid
20-TR-1309
~73-
ri3es with the product of IA and VA~ and this pumps
up the filter capacitor voltage VF At high speeds,
VA is su~ficiently high when IA traverses point 3 ln
Fig. 15 to raise VF a~ove 1200 volts (assuming slx
chopper/motor units and braking resistance o~ at least
four ohms), and between points 3 and 10 the staging
means 165 in the brake controls (Fig. 6B~ will switch
the staging contactors Bl and B2 (Fig. 2) as necessary
to maintain VF in a range between 1200 and 1650 volts.
For example, at point 4 V~ reaches 1650 volts and the
staging means responds by c]osing contactor Bl to re-
duce the dynamlc brake resistance. Again at polnt 5,
VF reaches 1650 volts and the staging means responds
by closing contactor B2 to further reduce the resist-
ance to its minimum value (e.g., 1.1 ohms).
At point 6 in Fig. 15, armature current in-
tersects the constant power segment of the current
ca]l slgnal curve that is set by the reference generator
100, and from this point I~EF tracks I*. Maximum power
is now being dissipated in the resistor grid, and VF i3
nearly 16~0 V. At point 6 the duty factor of the
chopper is approximately 0.5 (see Fig. ll!A). The
vehicle is being retarded by the braking effort that
increases with IA, and as it decelerates (above corner
point speed) I* is increased exponentially by the
reference generator 100. Fig. 15 reveals that speed
decreases, Vemf decreases, and power remains substan-
tially constant as IA increases from point 6 to the
corner point 7.
From point 7 to point 12 of Fig. 15~ the
vehicle is braking at maximum, constant current. The
chopper duty factor at the corner point is illustrated
ln Fig. 14B , As speed decreases from the corner
point 7, the generated voltage decreases and the duty
factor will be increased in order to regulate IA to the
20-TR-130g
74-
constant magnitude called for. As vol~age decreases,
sc does the power to be dissipated in the reslstor
grid, and consequently VF decreases. At polnt 8 VF
reaches 1200 volts, and the staging means responds by
opening the ccntactor B2 to increase the dynamic brake
resistance. Although IA tends to decrease when B2 is
opened because of the resulting step increase in VF,
the chopper reference means 192 quickly responds by
increasing Vc, and hence the duty factor of the
chopper, as necessary to mainta~n the called for magni~
tude of current, and there is no noticeable torque
bump in the braking process. At point 3 VF again falls
to 1200 volts, and the staging means opens Bl to insert
the maximum resistance of the grid. As speed decreaseæ
from point 10 to point 11, VF drops below 1200 volts
and decreases to a minimum of 850 volts, whereupon the
controll2ble converter 33 begins charging the filter
capacitor 6n from the power source so as to maintain
this minimum level. The minimum level of VF is se-
lected to ensure sufficlent voltage for successful op-
eration of the commutating circuit in the chopper 12,
whereby proper operation of the chopper can continue
during low speed electrical braking when the generated
voltage is very low.
When speed has decreased to a minimum of
approximately 2.7 MPH, at point 12 in Fig. 15, Vemf is
Just equal to the sum of the voltage drops across the
chopper 12 and the lumped resistance of the armature
current path, and therefore VA ls 0. The duty ~actor
is now 1.0, and the chopper can no longer control IAwhich rapidly decays from its called for magnitude.
Decreasing current in the motor field results in less
electromotive force, and the system col~apses~ Thi.s
is the lo~r speed brake fadeout point of the lllustrated
propu]sion system. Additional braking at such low
20-T~-1309
~75-
speeds can easlly be erfected by conventional frictlon
or air brakes. Note that the minimum fadeout speed
of electrical braking ls e~en lower than 2.7 MPH i~ the
brake controller ~s calling for less than maximum IA,
Note also that the fade out speed of electrical braking
would be appreciably higher than 2.7 MPH lf the duty
factor were limited to a maximum of .9l.
While a preferred embodiment of the invention
has been shown and described by way of example, many
mGdifications wlll undoubtedly occur to persons skilled
i.n the art. The concluding claims are therefore in-
tended to cover all such modifications as fall within
the true spirit and scope of the lnvention.
