Note: Descriptions are shown in the official language in which they were submitted.
. 20~C~368
8~
This invention relates to a new and .improved wheel
. slip control for electric drive systems.
More particularly, the invent.ion relates to an im-
proved wheel slip control for electric drive systems of the type
employed on electric driven vehicles such as diesel-electric
locomotives, and which employ an alternating ~urrent supply
and series-type direct current traction motorsO The improved
wheel slip control provides for the selective redu~tion in
tractive ef~ort of any one of a plurality of series-type
direct current traction motors to arrest whael slippage without
requiring that the tractive effort of non-slipping motors
be reduced, unless the connection is applied simultaneously to
;~ two or more motors, and accomplishes this in a reliable and
- relatively low cost manner.
An improvsd whePl slip control system for electric
locomotives and the like employing series-typ~ direct current
trac ion motors~ is disclosed in U~ S. Patent ~o. 3,737,745 -
issued June 5~ 1973 to Russell Mo Smith and Rene J. Chevaugeon
for WHEEL SLIP CO~TROL SYSTEM, Assigned to the Genexal Electric
Co. The wheel slip control system described in Patent No.
3,737,745 is intended for use primarily with traction motor
drive systems having relatively stable (stiff) supply voltaga
~, sources of either direct or alternating current. Because of
their nature, diasel-electric locomotives and like e~uipment
: employing series~type direct currant traction motor drives, do
not have available a stable supply voltage source that can be
raadily used with wheel slip control systems of the type dis-
closed and claimed in Patent ~o~ 3,737,745. To overcome this
problem, the present invention was developed.
It is, therefore, a primary object of tha present
. .
~: invention to provide a new and improved wheel slip control system
~ for controlling slippage of individual ones of a plurality of
"', ~e~ ~
. .,
. .
,
:. .
.. . ..
20-LC-368
series-type direct current motors comprising a traction motor
drive system, without requiring that the tractive e~fort of
non-slipping motors be reduced.
A further object of the invention is to provide such
a wheel slip control ~ystem which is capable of use with rela-
tively unstable, variable voltage power supply sources of the
kind available with diesel-electric locomotives and like equip-
- ment~
A ~till further feature of the invention is to provide
a wheel slip control system having all of the above-set-forth
characteristics and which is reliable in operation, easily
- maintained and relatively inexpensive to manufacture and install.
In practicing the invention, a wheel slip control
systam for electric traction motors employing series-type
direct current traction motors having field and armature windings
connected in series electrical circuit relationship, power
rectifier means and means for supplying alternating current
to-`-the power rectifier means, is provided. The series-type
traction motors are connectad across the output from the powar
rectifier means for ths supply of normal excitation direct
current to the motors~ The improvement comprises current trans-
former m~ans having primary and secondary windings wound on a
common core. The primary winding is serially connected in the
supply connection between the maans for suppLying alternating
:
current and ths power rectifier means. Secon~ary rectifier
meanq are provided having the output thereo~ (upon excitation)
coupled to supply auxiliary fi~ld current to the field winding
of each series-type direct current traction motor in addition
to the normal direct current excitation from the power recti~ier
... ~
means~ The polarity of the auxiliary field current is such
~;- that upon addition to the normal excitation direct current,
the total current flowing in the field winding is maintained
.
~ -2-
. ~
.'
--~ 20-LC-368
48~27
.`...... at a substantially constant value or slightly less from that
which had bean Elowing prior to the occurrence of a wheel
slippage conditionO Means are provided which are responsive
~ to the occurrence of a wheel sli.ppage condition for effectively
supplying output current from the secondary winding of the
current tran-~former means to the~ sscondary rectifier means
to excite the same whereby the aluxiliary field current is
`:~ supplied to the series field winding o the direct current trac-
. tion motor that is slipping. The current transformer means is
- 10 designed such that it has a current transformation ratio
: corresponding in number to the number oi~ serie~ connected direct
... curren~ traction motor excitation circuit paths connected in
parallel circuit relationship across the output of the main
. power rectifier means.
.: These and other objects, features and many of the
-. attendant advantages of this invention will be appreciated
.. more readily as the same becoi~es better underætood by reference
~` to the following detailsd description, when considered in con-
:, nection with the accompanying drawings, wherein like parts in
....
~ 20 each of the several flgures are identified by the sams refer-!
ence character, and wherein:
Figure 1 is a functional block diagram of a new
.~ and improved wheel slip control system constructed according
to the invention;
- Figure 2 is a functional block diagram of a modified
.. ~ form of the wheel slip control system shown in Figure l;
Figure 3 is a functional block diagram of still a
, different form of wheel slip control system constructed
.:~
~;~. according to the invention, and which requires lower cost and
fewer power rated components than the embodiments of the in-
~ ..,i
vention shown in Figures 1 and 2; and
Figure 4 is a detailed schematic circuit diagram of
.,'~
.~ -3-
:
;~!
~' ' ' '' , ' "
. '. ,, ' ' ; . ', ,
; 20-LC-368
a preferred form of the invention which obviates the need for
a separate wheel slippage detector and associated control
circuitry.
Figure 1 is a functional block diagram of a wheel slip
correction system constructed according to the preæent inven-
tion. In this system, a conventional, large, power rated ~hree-
phase alternator is shown at (11) which may be of the type
gensrally found in diesel-electric locomotive and like e~uip-
: ment. The alternator (11) ~upplies variable voltage, three-
.-~ 10 phase altarnating current electric power across the conductor~
(12, 13,14) to a three-phase, full wave rectiier bridge ~15)
comprised by the diode rectifiers (Dl-D6). The diode rectifier
.~ bridge (15) is connected across a palr of direct currant power
~upply terminal~ (16,17) ~or supplying ~ull wave rectified
direot current voltage to traction motors (Ml-M4).
The traction motors (Ml-~4) are of tha ~eries type
wherein the armature winding indicated at (Ml, M2, etc,) are
connected in series circuit electrical ralationship with the
corrasponding field windings (Fl, F2, etc)~ In the traction
.
drive arrangement shown in Figure 1, the seri~s traction motor
~: (M1) and its associated serieæ connacted fie~d winding (Fl) is
-~ connected in series circuit relationship with the series field
.~.,.
: winding (F3) and armature winding of series traction motor (M3)
.: and the series circuit thus comprised, is connected across the
;~ direct curr~nt power supply terminals (16, 17). For convenience ~:
and simplicity of illustration, the usual ~peed ragulating,
series-parall~l connected resistor speed controlling network,
: :.
and other ~ontrol f2atureæ normally associated with traction
motor drive systems hav~ not baen illustrated since they do
.:
not ~omprise a part of the present invention~
The improvement made available by the present in~ention
comprises a wheel slip correction system formed by a pair vf
.'`~
~ 4-
' .'
:',
:~,, . . :
-- 20-LC-368
IL6~4~ 7
cur~ent transformers (CTl and CT2) whose primary wi.ndings are
connected in series circuit relationship in the conductors
~12, 14), respectively, betwesn alternator (11) and the diode
rectifier bridge (15). Each of the current transEormers is
comprised by a common core member (18A, 18B), a primary winding
(19A, l9B) and a secondary winding (21A, 21B~. The primary
and secondary windings of each currant transformer are wound
on separate legs of their respective core members (1~3A, 18B).
In the embodiment of the invention shown in Figure 1, the core
members (18A, 18B) have been illustrated as oval or square
- shaped with csntral openings therein7 however, the current
transformers may have any desired physical configuration and
may comprise conventional, commercially available current trans-
-~ formers having an appropriate current transfcrmation ratio as
described hereinafter~
Each of the secondary windings (21A, 21B) of current
transformers (CTl, CT2) are connected across one set of dia-
gonally opposite terminals of an auxiliary diode xeckifier
bridge ~22 or 23), respectively. The remaining set of diagon-
ally opposed terminals of the respective auxiliary rectifiar
bridges (22,23) are connected across the series connected field
. windings (F2, F4) of series traction motors (M2, M4) and the
; series field windings (Fl, F3) of series traction motors
' (Ml, M3), respectivaly~ Ths polarity of the connection of ths
auxiliary diode rectifier bridges (22, 23) is such that when
the secondary rectifier bridges conduct, they supply an auxiliary
direct current which is circulated through the respective sets
of field windings (F2, F4) and (Fl, F3) in an aiding direction
, That is to say~ normally, the same armature and field current
circulates through the respective series traction motors such
as (Ml) and (Fl) and thereafter through (F3) and (M3) in series
between the direct current positive p~wer supply conductor (16)
--5--
, , .
--- 20 LC-368
3 l.~ 7
-':
and nega~ive supply conductor (17). The auxiliary current
supplied rom the auxiliary rec1:.ifier ~23) augments or adds
to whatever field current is flowing in the field windings
(Fl, F3) under the conditions to be described hereinafter. A
similar connection is provided i-^or the auxiliary field current
~ed to the ield winding~ (F4, F2) in series fxom tha auxiliary
iode rectifier bridge (2~).
. :-
The secondary windings (21A, 2lB) of current trans-
,: '
formers (CTl, CT2) have their outputs connected across the dia-
gonally opposite terminals Qf their respective secondary rect-
.. ifier bridges throu~h conductors (24, 25) and conductors (26,
27), respectively~ Each set of auxiliary supply conductors f
(24, 25) and (26, 27) are short circuited by a pair of reverse
polarity, parallel connected silicon control rectifiers
(SCRl, SCR2) connected across the respective supply conductors.
' The short circuiting silicon control rectifiers (SCRl, SCR2)
.~ for supply conductors (24, 253 have their control qates con-
nected to the output of a wheel slip detector (29) for the
traction motors (M2, M4), and the silicon control rectifiers
`. 20 (SC~l, SCR2) connected across upply conductors (26, 27) have
their control gates connectad to the output from a wheel slip
detector (28) for the traction motors (Ml, M3). The wheel slip
. detector~ (28, 29) may comprise any conventional means for
detecting a wheel slippage condition of any one of the traction
~, .
.; motors (M1- M4). For example, th~se devices may comprise
i tachometer generators, voltage measuring bridges, etc.~ of the
,- type described in greater de~ail in the above-reference U, S.
:: Patent ~o~ 3,737,745. The connection of the wheel qlip detectors
: to the respective short circuiting silicon control rectifiers
(SCR1, SCR23 is such that the SCRs normally are conducting in
~- ~he absence of a wheel slippage condition~ Upon the occurrence
of a wheel slip condition in either of the series connected
-6-
, . .
20-LC-368
dl ~ A 1~
motors of a set, such as (M~, M3), the associated short cir-
cuiting SCRs will have the gate signal removed therefrom so that
they aCsume a current blocking (nonconducting) condition. That
is to say, the SCRs become open circuited and any voltage
appearing across the auxiliary supply terminals (26, 27) will
be applied across the diagonally opposite terminals of aux-
iliary rectifier (23) and will result in the supply of an
auxiliary field currsnt through the series connected field
windings (Fl, F3).
; 10 In operation, the wheel slip correction system shown
in Figure 1 (as well as in the other figures~ attempts to keep
the field current of a series typa traction motor detected to
be slipping approximately constant after wheel slip occurs.
Consequently~ the motor is made to exhibit shunt motor charac-
taristics resulting in reduction of the tractive effort quite
rapidly as the wheel slippage causes its speed to sxceed that
of the locomotive or other unit being driven by the traction
motor drive system~ In the wheel slip control systems describ
:~.
ed in Patent ~o~ 3,737,745, it is necessary that the supply
voltage to the traction motors be maintained approximately
constant~ This condition is difficult to satisfy in a diesel
electric locomotive or other similar traction motor drive sys-
tem so that some other method of varying field excitation to
t provide a decreasing torque characteristic as wheel slip occurs,
becomes necessary~ In the wheel slip correction system herein
described, after wheel slip is detected, the field current
. .
~ of the slipping traction motor is either increased so that the
, .
torque of the motor falls off even more rapidly than that of
~- a comparable shunt motor, or as wheel slip occurs, the field
current of the slipping motor is allowed to decay but at a rate
which is less than that of the armature current. In other
words, the auxiliary field current introduced from the auxiliary
.
- 20-LC-368
power supply keeps the field current well above -the armature
current but still allows it to dlecrease. This results in slowly
decreasing the tractive effort of the slipping motor so that it
`~ i5 self-correcting without raquiring the ramo~al of large
amount~ of power from the entire traction motor drive sy~tem,
or from other, non-slipping traction motors.
In the arrangament sho~wn in Figur~ 1, the current
transformers (CTl, CT2) sense the current being supplied to the
traction motor drive system through the alternating supply
conductors (12, 13, 14)~ In normal operating conditions with-
out wheel slippage, it is expected that the total aurrent
supplied to the traction motor drive system will split about
~` evenly between ~he two parallel circuit paths comprised by ths
series connected motors (Ml, M3) and the series connected motors
(M~, M2), respectively, and the current tran~formers (CTl, CT2)
must be designed to accomodate currents of this magnitude.
In this arrangement, a current transformation ratio of 2-1
which pro~ides a current flowing in the secondary winding that
is one half of the current flowing in the primary winding,
provides auxiliary field current tha~ is more than ad~quate
to meet anticipated needs.
As stated earlier, the short circuiting silicon control
rectifiers (SCRl, SCR2) are normally conductin~ so as to short
circuit the secondary windings of the current transformers and
provide lit$1e or no loading on the overall system during nor-
mal operationO Only enough voltage is developed across
sacondary windings (21A, 21B~ to make up or losse~ in the
~ystem, ~owever, upon the occurrence of a slipping condition
in ona o~ the motors, for example, Imotor ~1)' the armature
current through the motor will decrease rapidly due to the
increased back EMF ge~erated by reason of the slipping condition.
The slipping condition will be detectad instantaneously by the
-8-
'`
: . . . .
''- .
20 LC-368
wheel slip detector (28~ which thereafter turns off short
circuitiny SCRl and SCR2. Just prior ~o the establishment
of the above-described conditions, the normal current flowing
in the primary winding (19B) of current transformer (CT2)
will have astablished a magnetic flux condition in the core
which will require that the secondary current then flowing in
: .
secondary winding (21B) be maintained in accordance with
Lenæ's Law. Consequently, with the short circuiting (SCRl, SCR2)
turned off, a rather large voltage is develop~d across the
~; 10 secondary field winding (21B) which supplies current through
auxiliary rectifier (23) to the series connected field windings
(Fl~ F3) to maintain the current through the field windings
at a substantially constant value or perhaps somewhat less than
, ......
the value previously maintained under normal, non-slipping
operating conditions. As a result of the supply of this aux-
iliary field current in aiding relationship to the suddenly
reduced, normally supplied field excitation current, the series
type traction motor will be caused to exhibit shunt motor
characteristics, and the tractive effort of the slipping motor
:;,
will be gradually reduced until the slipping condition i9
corrected~ The particular phenomenon whereby this reduction
in tractive effort is achieved, is described in greater detail
in the abova-referenced~Patent No. 3,737,745 and particularly
.,.
with respect to Figure 3 thereof. While Figure 1 illustrates
. . .
- what is essentially a single phase arrang~ment o~ the current
tran former and auxiliary rectifierst a complete three-phase
arrangament readily could be provided by one skilled in the
art in the light of the above teachings~
The circuit shown in Figure 1 has some faults in that
the short circuit silicon control rectifiers (SCRl, SCR~) must
rate the full KVA of the power to be supplied to the traction
~ motor field windings ~Fl, F3), and further they must have a high
.,, .j
,... .
,. .~
'~. 9_
. .
,.~..
:
, ~ . 20-Lc-36a
8~27
current and low voltage rating. The circuit shown in Figure
2 of the drawings provides a better impedeance match for the
shoxt circuiting SCRs; however, they still must rate the
- full KVA o~ the power to ba supplied to the traction motor
field winding The arranyement shown in Figure 2 is sub~tan-
tially indentical to that of Figure 1 with the exception that
two ~econdary windings t21A, 21A') and (21B, 21B'), are provid-
ed for each of the current trans~ormars (CTl~ CT2). The
secondary windings (21A, 21B3 are connected directly across
the diagonally opposite input terminals of the auxiliary rect
ifier bridges (22, 23), respeatively, without any short cir-
cuiting SCR devicesO The additional secondary windings
J- (21A' J 21B') are connected directly across the respe~tive sets
of short circuiting silicon control rectifiexs (SCRl, SCR2) with
the gates of the short circuiting SCRs being connected to the
output from the wheel slip detector (29, 28), respectively~
In operation, the circuit o Figure 2 functions in the
following manner. Under normal eperating conditions without
wheel slippage, short circuiting silicon control rectifiers
(SCRl, SCR2) are turned on and conducting. Conqequently,
; current will be allowed to flow through the secondary windings
`; (21A', 21B') causing the cores to be operated in a substantially
saturated condition so that essantially no power is tra~sferred
through the secondary windings (21A, 21B), xespectively, to the
; field windings o~ the traction motor~ Upon the occurrence o~
a wheel slippage on any of the traction motorsO for example
motor (Ml), the short circuiting silicon control rectifiers
connected across the additional secondary winding (21B') will
be turned off thereby open circuiting these winding~ As a
conRequence, current must ~low in winding (21B) due to the
action of Lenzls Law and will be coupled through the auxiliary
rectifier (23) to the series field winding (F1, F3) in the manner
, . ' .
,. . --10-- ,
-
. ' ' ' . ' , , ' ' :,, , ~
- 20 LC-368
`...... ;
described above with respect to Fiyure 1~ This results in
the introduction of sufficient auxiliary ield current to re-
duce the tractive effort of the slipping motor (~) un-~il the
slippage condition is corrected and the traction motor drive
system returns to a normal opercltion conditionO
In the arrangemenks shown in Figures 1 and 2, the
full KVA of the auxiliary power supply to the series field
i . . .
: windings of the æeries type DC traction motor~ is required in
the three main components, namelyO the short circuiting silicon
i~ 10 control rectifiers, the curr~nt transformer and the auxiliary
:
bridges and, thus, these components must be rated to handle the
power rec~ired. Providing a full three-phase system will
~` incrsase the number of individual components, but it would not
c result in changing the total KVA rating of the elements. To
. .
~; reduce the costs of the wheel slip correction system then, it
~i becomes necessary to reduce the K~A rating of any or all of the
i~` abo~e-listed main components of the ~heel correction system~
Figure 3 of the drawings illustrates a whe~l slip control
-~ system incorpo~ating many of the ~eatures of the ~ystems shown
; 20 in Figures 1 and 2, but which also allows a reduction in the
KVA rating of certain of the components~
In Figure 3, it will be seen that for a single axle
drive arrangement comprised of two series type DC traction
motors ~onnectecl in series across the main direct current
excitation supply terminals (16, 17), two curxent trans~ormers
(CTl, CTl'), are provided. The setup for additional axles
~, ,
would be the sameî however, for convenience and simplicity
o~ illustration, the arrangement for a second axle comprised
by traction motors (M2, M4) is shown only in block diagram
,
30 form.
In Figure 3, the current transformers (CTl~ CTl')
for each given axle are idsntical in construction and rating~
'
, .,~,.
, ~ .,
,~.
:
. ;, .
20-LC-368
IL27
The secondary windings (21A) of both current transformers
(CTl, C~ are connected so that they add in voltage9
Negl~cting for the moment the e~-fect of the additional secon-
dary windings (21A') on each of the respective core members
(18A, 18A'), the primary windin~s (19A, l9A') are wound in a
direction such that the ~lux ~rom the secondary windings (21A)
on each cora msmber opposes the flux ~rom the primary windings
~19A, l9A'), respectively~ Thucl, the ampere turns (NI) from
each of the windings (21A) on the respective core members
:~ 10 (18A, 18A') should be equal and opposite to the ampere turns
(~I) of the respective primary windings (19A, l9A').
The two additional secondary windings (21A') on the
respective cor~ members (18A, 18A') are connected in opposing
relationship so that the alternating current voltage from the
sum of the two windings (21A') is zero or substantially zero.
The voltage handlin~ requirem2nts ~or the control transistor
(Tl) therefore is only the voltage of the battery source ~B~
The arrangement is such that the control transistor
(Tl) is turned on by the wheel slip detector (28) only under
20 normal operating conditions where there is no wheel slippage,
and i8 cauqed to turn off upon the occurrence o~ a wheel slip
condition o either of the motors (Ml, M3). Under the normal
operating conditions with no wheel slippage, direct cuxrent
~low~ in the t~o additional secondary windings, (21A') on each
of the cores causing the resp~ctive core members (18A, 18Ai) to
be drivsn into saturation. For this to happen, the ampere turns
~I) supplied by control transistor (Tl), battery lBl) and the
windings ~21A') must be greater than the maximum ampere turns
.; :
.. (~I) from the supply conductor (12) and primary windings
. 30 (19A, l9A'). Upon the occurrence o a wheel slippage condition,
for example ~eries type traction motor ~Ml), control transistor
:. (T~ turned off by wheel slip detector (28)~ Remvval of the. :
~",'`
.
20-I,C-368
direct current flowing through the additional windings (21A')
allows the core members (18A, 18A') to be driven out of satur-
`~ ation by the primary windings tl9A, l9A') and to couple power
through the secondary windings (21A) on each core member.
This results in the production of an auxiliary current flow that
is supplied through the auxiliary rectifier (23) to the series
field windings (Fl, F3) in an aicling direction to thereby correct
' the wheel slippage condition.
In the arrangement of Figure 3, only the current trans-
former and auxiliary rectifier (23A) need carry the full KVA
: rating of the auxiliary current to be supplied to the field
windings of the series type DC traction motorsO The control
transistor (Tl), battery (Bl~ and associated windings (21A')
r need to carry only adequate current to assure saturation o~ the
, -
core members ~(18A) during normal operating conditions of the
traction motor drive system.
A preferred embodiment of the invention is shown in
Figure 4 o~ the drawings wherein no wheel slip detection and
control elements are required. The wheel slip correction
- 20 control system shown in Figure 4 provides to the series traction
motors a characteristic very close to that which would be
,' obtained by a shunt or separately excited traction motor. In
Figure 4, a complete three-phase system is disclosed for ex-
citing three seri2s type traction motors (~1~ M2, M3) which are
connected in three separate parallel paths across the output of
the main power rectifier (153 through the main direct current
.~
power supply terminals (16, 17~. The series connected field
windings of the respective series type DC traction motors are
shown at ~Fl, F2, F3). A three-phase alternator (ll) which is
driven by the prime mover such as a diesel engine of a diesel-
electric locomotive, supplies three-phase, variable voltage
alternating current power to the main power rectiier bank ~15)
,
-13-
: .
., .
20-LC~368
~L~48~Z7
through the conductors (12, 13, 14).
Three current transformers (CTl, CT2, CT3), one for
each traction motor, are provided. Each current transformer
has a three-phase connection ancl, for this purpose, is provided
with a three leg core member tl8A, 18B, 18C) with the respective
legs of the core members having a primary winding, such ~s
(19A, l9B, l9C) for core member (18A), connected in series cir-
cuit relationship in the three-phase connectlon to the main
power rectifier bank (15) provided by conductors (12, 13, 14).
Current transformers ~CT2, CTl) have corresponding primary
. :
winding connections, with all of the primary windings (19A, l9A',
;~ l9A'') for all three core members being connected in series
circuit relationship in the single phase connection comprised by
conductor (12), for example. The primary windings (19B, l9B',
l9B ") and the primary windings (lgC, l9C', l9C'') are similarly
connected in series circu~t relationship in their respective
phase supply conductors (13, 14~.
The secondary windings of the current transformers
- (CTl, CT2, CT3) are wound around corresponding legs of the
respective core members (18A, 18B, 18C) in tightly coupled
relationship with their respective corresponding primary
- windings. For example, the secondary winding (21A~ is wound
on a common leg of core member (18A) with primary winding (19A),
(21B) with (19B) and(~lC) with (19C), etcO Each set of secondary
: ::. , .
~s l windings of a respsctive current transEormer is connected in a
. ,
three-phase Y connection and, for this purpose, on terminal of
each secondary winding on a respective core member is connected
i in common with the corresponding terminal of the two remaining
secondary windings on the particular core member. The remaining
terminals of the secondary winding axe connected to an inter~
mediate tap point of one set of a pair of series connected
diode rectifiers, such as (Dl, D4) comprising part of a three-
... .
~ -14-
;:: ' . .. :
i 20-LC- 368
~L~4~ 7
phase auxiliary rectiEier bridge (31, 32, or 33) that is
connected in series circuit relationship between the respec-
tive series traction motors and their series connected field
windings~ For example, in the series path comprised by traction
motor armature ~Ml) and series field winding (Fl) the a~xiliary
three-phase bridge rectifier (31) is connected in series cir-
cuit relationship with it through a set of common, diagonally
opposed terminals indicated at (~ and B)~ The intermediate
tap points of sets of series connected diode rectifiers of
bridge rectifier 131), namely, the intermediate tap points of
(Dl', D4'3, (D2', D5') and (D3',D6') are connected through the
conductors (34, 35, 36) to the remaining three terminals of
the ~econdary windings (21C'', 21B'', 21A'') of the current
transformer (CT~), respectively. While in this arrangement,
these secondary windings of the current transformers are con-
nected in a three-phase Y connection, a delta connection could
be employed if desired. Further, it is, of course, possible to
fabricate the wheel slip control system of Figure 4 in a single
phase arrangement used to control slippage of a single traction
. .
motor in place of the three-phase arrangement shown, should it
be desired to do so.
Each current transformer ~CTl, CT2, CT3) is designed
so that it provides approximately a three to one current trans-
, formation ratio. That is to say, the secondary current flowing
;~ in each secondary winding i~ approximately one third of the
current flowing in the primary winding. In the system of
. .,
Figure 4, three series type traction motors are shown connected
- in three separate parallel connected paths with each parallel
path taking approximately one third of the total current supplied
~":
through the main power.rectifier (15) from alternator (11)~
It will be appxeciated, therefore, that because of the three to
one current transformation ratio the current capable of being
, 20-LC-368
27
supplied from any one of the current transformers (CTl, CT2,
CT3) through their respective auxiliary rectifier bridyes to
the respective series connected field windings (Fl, F2, F3)
will just about approximate the series field current normally
flowing in the field winding. If four traction motors were
used requiring four parallel, series connected paths, the
current ratio required in the current transformers would be
four to one, etc. In the case of the series parallel arrange-
ments shown in Figures 1-3 of the drawings, the current ratio
would correspond to the number of parallel paths or two in
the particular instances of Figures 1-3 systems. Additionally,
it should be noted that the number of current transformers
required is equal to the number of parallel paths employed in
the system~
In order to circulata auxiliary field current through
the respective series field windings (Fl, F2, F3) from their
associated auxiliary rectifier bridges (31, 32, 33), feedback
diodes (D7, D8, Dg) are connected in reverse polarity, parallel
circuit relationship across the series connectsd field windings
and their respective auxiliary rectifier bridges in the manner
shown in Figure 4. By reason of the inclusion of the feedback
diodes, should the terminal (B) o~ auxiliary rectifier bridge
(31), for example, being driven positive with re~pect to the
terminal (A), auxiliary current will be circulated through
the series connected field winding (Fl) back through the feed- -
,
back diode (D7) as will be described hereinafter.
In operation, the system of Figure 4 functions in the
following manner. For so long as the current flowing in the
armature of any given traction motor (for example, traction
motor (Ml))remains constant or substantially so, then the
current divides equally between the three legs of the asso-
ciated auxiliary rectifier bridge (31). This ~ame situation
~ '
. .~
-16-
. ~ .
: .
'','i''
. . .
20-LC-368
'`;`" ~ ~Lg~ lL27
will exist in the remaining two parallel paths compri~ed by
the traction motors (M2, M3) ancl their associated auxiliary
bridge rectifiers (32, 33) which, in the following example,
will be assumed to continue to drive in a normal manner
without slipping. A portion of the current flowing through
.
the auxiliary rectifier bridge (31) will flow through the
secondary windings (21A'', ~ls'', 21C'') of current transformer
(CTl) so as to satisfy the current ratio demanded by the trans-
former due to the current flowing in the respective associated
: 10 primary windings. For so long as this normal drive condition
~ exists, there will be only a small voltage generated at the
..:
secondary terminals of current transformers (CTl) which is -~
equal only to supplying the losses in the system, and littla
or no auxiliary current will be induced or supplied to the main
`~ power circuit through auxiliary rectifier (31) and field
.:..
winding (Fl~. This same condition will also pre~ail with
respect to the remaining two traction motors (M2, M3) and their
associated current transformers (CT2, CT3).
If it is assumed that a wheel slip occurs on the
traction motor (Ml) only, then the armature current of traction
motor (Ml) will drop suddenly due to the increased back EMF
generated by reason of the wheel slip. This sudden drop in the
value of the armature current also tends to cause the current
flowing through the auxiliary rectifier bridge (31) to drop.
The current transformer (CTl~, however, will develop whatever
. . .
voltage is necessary to maintain its current ratio dependent
upon of course the size of its core, number of turns in the
primary and secondary windings, the current flowing under nor~
mal design operating conditions, etc. In order to maintain
this current ratio, the voltage developed across the secondary
. ~
-- windings of (CTl) will drive terminal (B) of auxiliary recti-
.
fier bridge (31) positive with respect to terminal (A) and
':
. ~
-17-
.~,
., .
., .
; ~ 4 ~ 20-LC-368
.
current will circulate Erom terminal (s) through the field
winding (Fl) and back through feedback diode (D7) to
terminal (A) in order to complete the circuit. The circula-
tion of this auxiliary feedback current through the feedback
diode (D7) and through series ~ield winding (Fl) necessarily
will be sufficient to satisfy the current ratio demanded by
the current transformer (CTl) and results in maintaining the
current flowing through field winding (Fl) at a substantially
constant or only slightly lower value than that which pre~ailed
prior to the onset of the slipping condition. The response is
almost instantaneous, and does not require the use of a
separate wheel detector and associated control system such as
that employed in the previously described three embodiments of
the invention. The circuit, however, still functions to causs
the respective series type traction motors to exhibit shunt or
separately excited DC traction motor characteristics upon
occurrence of wheel slippage without the use of external control
circuitry.
From the foregoing description, it will be appre-
ciated that the invention provides a new and improved wheel
slip correction system for controlling slippaye of individual
ones of a plurality of series-type direct current traction
motors comprising a traction motor drive system, without re-
quiring that the tractive effort of non-slipping motors be
reduced. The improved wheel slip control system is capable of
use with relatively unstable, variable voltage power supply
sources of the kind available with diesel-electric locomotives
and like equipment. The wheel slip control system possesses all
.,
of the above-set forth characteristics, is reliable in opera-
tion, easily maintained and relatively inexpensive to manufac-
ture and install.
Having described several embodiments of a new and
-18-
; ' .
- 20-I,C-368
~q~4~ 7
; improved wheel slip correction syetem constructed in accor-
dance with the invention, it i9 believad obvious that other
modifications and variations of the invention will be suggested
-to those skilled in the art in 1he li,ght of the above teachings.
It is, therefore, to be under~tood that changes may be made in
~',, the particular embodiments of the invention described which
~'-', are within the full intended scope of the invention as defined
.,
1' by the appended claims.
''''''
..-
.
i~
., ~ .,
..
.'"
;.
, ~ .
~, ",
,.,~,.
'`
~;
. .
~':
:
''`'
`.
';
'`;
. .
19-
:,
, ~,
;
