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AUTOMATIC 1'LASHOVER PR~OTECTIC?N
FOR LOCOMOTNE TRACTION MOTORS
8~~
This invention relates generally to electrlcsl propulsion systems for
traction vshiclas (such as diesel-electric lo~a~tfvu) equipped with direct
curxeat traction motors, and it relates more par~i~utarly to improved
means for protecting such a sv~tetn from serious damage in the event of a
flashover on the oommutator of such a motor.
In a modern diesel-electric locomotive, a thermal prime mover
(typically a 16-cylinder turbocharged diesel engine) is used to drive an
electricsl transmission comprising a synchronous generator that supplies
electric currant to a plurality of direct current (d-c) traction motors whose
rotors are drivingly coupled through speed-redudng gearing to the
respective rile-wrheel sets of the locomotive. The generator typically
comprises a main 3-phase traction alternator, the rotor of which is
mechanically coupled to the output aha~ of the engine. l~hen excitation
current is supplied to field windings on the rotating rotor, alternating
voltages are generated in the 3-phase armature windings on the stator of
the alternator. These volt are rectised and applied to the armature
and/or field vrindiags of the d~c traction moton.
In normal motoring operation, the propulsion system of a diesel
electric locomotive is so controlled as to establish a balanced steady- state
condition vrbenrin the engine-driven alternator prodaces,~ for each discrete
position of a throttle handle, a substantially constant, optimum amount of
electrical poorer for the traction motors. In practiw suitable means qre
provided for overriding normal operation of the propulsion controls end
reducing engine load is response to certain ab~rormal conditions, such as
loss of wheel adhesion or a load escesding the popsr apabdlity of the engine
at whatever engine speed the throttle is commanding. This response,
generally referred to as deration, reduces traction pourer, tisereby helping
the locomotive recover from such temporary conditions and/or preventing
serious damage to the engine.
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In addition, the propulsion c~n~ol system conventionally includes
means for limiting or reducing alternator output voltage as necessary to
keep the magnitude of this voltage and the magnitude of load current from
respectively a:seeding predetermined Cafe ma~cimum levels or limits.
Current Umit is effective when the locomotive is accelerating from rest. At
low locomotive tpesds, the traction motor armatures are rotating slowly, so
their back EMF is low. A low alternator voltage can now produce
maximum motor current which in turn producer the high tractive effort
required for acceleration. On the other hand, the alternator voltage
IO magnitude matt be held constant at its ma~cimum level whenever
locomotive speed it high. At high speeds the traction motor armatures are
rotating rapidly and have s. high back , and the alternator voltage must
then be high to produce the required load current.
In an electric propulsion system, all of the power components
(alternator, rectifier, traction motors, and their interconnecting contactors
and cables) need to be well insulated to avoid harmful short circuits
between the electrically energized parts of these components and ground.
The insulation has to withstand very hare!? conditions on a locomotive,
including constant vibration, frequent mechanical shocks, infrequent
maintenance, occasional electrical overloads, a wide range of arubient
temperatures, and an atmosphere that can be very wet and/or dirty. If the
insulation of a component were damaged, or if its dielectric strength
deteriorates, or if moisture or an accumulation of dirt were to provide a
relatively low resistance path through or on the surface of the insulation,
then undesirably high leakage current can flow between the couiponent
and the locomotive frame which is at ground potential. Such an insulation
breakdown can be accompanied by ionization discharges or flashovers. The
discharge will start before the voltage level reaches its ultimate breakdown
value. The dirtier and wetter the insulation, the lower the discharge
starting voltage relative to the actual breakdown value. Without proper
detection and timely protection, there is a real danger that an initially
harmless electrical discharge will soon grow or propagate to an extent that
causes serious or irreparable damage to the insulation system and possibly
to the equipment itself. ,
It is conventional practice to provide ground fault protection for
locomotive propulsion systems. Such protective systems typically respond
to the detection of ground leakage current by overriding the normal
propulsion controls and reducing traction power if and when the
magnitude of such current exceeds a pecmassibie limik which depends on
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the magnitude of motor curreht. Sea U.8. patent No. 4,608,619
and
Canadian patent No. 1,266,117. Such systems have not been
wholly
successful in preventing damaging flashoven on the commutators
of the
traction motors.
In d-c traction motors, carbon brushes rubbing on commutator
bars
are utilised to provide current to armature windings of the
motor. This
current establishes a magnetic field in the armature and
corresponding
magnetic poles. The magnetic poles created in the armature
interact with
magnetic poles in field windings of the motor to produce
torque in the
machine. The magnetic poles in the held windings of the motor
are
eBtabHshed by means of direct current flowing through these
windings.
The motor includes a plurality of commutator ban equally
spaced around
one end of the armature, each of the commutator bets being
connected to
selected windings of the armature for establishing the magnetic
poles. As
adjacent comaautator ban periodically pass under the carbon
brushes; the
armature coils connected thereto are momentarily short circuited.
Since
the coils associated with the short circuited commutator
bars are displaced
from each other, they will be passing through magnetic fluz
fields created
by the magnetic poles of the field windings which are of
different
magnitudes. Accordingly, a potential difference will eiist
between the two
commutator bars. In the design of ap ideal machine the brushes
are
located between field poles at a point where Quz created
by the field poles
passes through aero in its reversal between adjacent poles
of opposite
magnetic polarity. This ideal point shifts with chang.~s
in armature
current since the total fluz is the sum of field flue and
armature flux.
Typically, a commutating pole or interpole is put between
adjacent field
poles, each commutating pole having a wig which is serially
connected
in the armature current path so that the fluz generated by
the
commutating pole is proportional to armature current. This
method
generally serves to minimize changes in the interpole ilus
thus allowing
the brush to transfer current between commutator bars without
an undue
amount of electrical arcing.
For motors that ass subject to heavy onrloads, rapidly changing
loads,
operation with weak main fields, defective braehes, brush
bounce, or rough
commutators, there is a possibility that the mutating pole
action may be
insufficient, and a simple sparking at the brushes may become
a major
. arc. For szsmple, at the instant an armature coil is located
at the peak of a
badly distorted Duz wave, the coil voltage may be high enough
to break
down the air between the adjacent commutator bars to which
the coil is
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connected and result in flashover, or arcing, between these bars. Arcing
between commutator segments may quickly bridge adjacent brush holders
or spread to the grounded flash ring that usually surrounds the commuta-
tor of a d-c traction motor, thereby short circuiting the output lines of the
traction alternator. While such flashovers are relatively rare, if one occurs
it will usually happen when the locomotive is traveling at a high speed.
Many ditl'erent systems are disclosed in the relevant prior art for
automatically detecting and recovering from flashover conditions. See for
example U.S. patent No. 4,112,475 - Stitt and Williamson. To minimize or
avoid serious damage to the traction motor and associated parts of the
propulsion system when a flashover occurs, it is desirable to extinguish the
flashover before the current being supplied to the faulted motor has time to
attain its maximum available short-circuit magnitude. By very rapidly
reducing or interrupting such current as soon as the flashover can be
detected, the amount of electrical energy in the faulted motor circuit will be
kept low enough to prevent permanent damage to the commutator bars,
brush holders, and flash ring. This desired high speed flashover protection
cannot be obtaiaed by opening the electrical contactor that connects the
faulted motor to the rectified output of the alternator, because the opening
action of a conventional contactor is too slow and by the time the contactor
tips start to separate the fault current magnitude could be so high as to
cause undesirable arcing or welding of such tips. The deration function of
the propulsion controls cannot be relied on to reduce the initial surge of
current that the traction alternator supplies to the faulted motor, because
the relevant time constants of the controls and of the alternator field
excitation circuit introduce a finite , delay between the occurrence of a
flashover and the response of the alternator.
,~umm~nr~»..In~ntis~u
A general objective of the present invention is to provide improved
flashover protection means for locomotive traction motors.
Another objective is to provide flashover protection characterized by its
very fast response to the detection of a flashover coadition in a d-c traction
motor and by its effective suppression of such a condition before current
supplied to the faulted motor can rise to its ma~dmum available magnitude.
A more specific objective of the invention is to provide, for a locomotive
propulsion system, flashover protection means that rapidly extinguishes a
flashover in a d-c traction motor and that electrically disconnects the
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faulted u~otor from the traction alternator for a su~cient interval to allow
the Dashed commutator to heal.
Another specific objective is to provide, for a diesel-electric locomotive
propulsion system, flashover protection means that is operative rapidly to
5 extinguish flashovers in the d-c traction motors but is not operative in the
event of short circuits caused by failed diodes is the electric power
rectifier
bridge of the propulsion system.
A further objective ie the provision, is flashover protection means
utilizing a high-speed, solid-state controllable electric valve, of means for
coordinating the turn off and turn on operations of such valve in a manner
that permits simplification of the saubber circuit shunting the valve.
The improved flashover protection means is useful in a traction vehicle
propulsion system comprising a controllable source of electric power for
energizing a plurality of d-c traction motors each having armature and
field windings and a commutator subject to flashovers. The power source
comprises a 3~phase synchronous generator having armature and field
windings and a rotor driven by a prime mover on board the vehicle. The
generator field windings are connected to a controllable source of
unidirectional excitation current that includes means for varying the
magnitude of such current as a function of the value of a variable control
signal. Suitable ex~tatioa control means normally determines the value of
this control signal in response to selected input signals, including a
reference signal the value of which normally depends on the power setting
of the vehicle throttle (or brake handle).
The 3-phase armature windings on the stator of the synchronous
generator are connected to the traction motor commutators by means of an
uncontrolled electric power rectifier (comprising a plurality of pairs of
power diodes sad associated electrical fuses) and a plurality of electrical
contactors. Operating means is provided for causing each contactor to
change between closed and opened poHitions, thereby connecting (or
disconnecting) the respective motors to the rectifier as desired. A family of
feedback signals respectively representative of the magnitudes of armature
currents in the traction motors is derived by suitable current sensing
means. A flaahover on the commutator of aay one of the motors causes an
abnormally large increase in the armature current magnitude of that
motor.
In carrying out the invention in one form, a high-speed, solid-state
controllable electric valve ie connected between the excitation current
source and the generator field windings. This valve has alternative first
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and second states. In its first state the valve has negligible resistance and
freely conducts ezcitation current, whereas in the second state it has a
resistance of very high ohmic value that resembles an open circuit and
e~'ectively decouples the excitation current source from the generator field
windings. The controllable valve is suitably constructed to change states
very quickly, and means is provided for changing it from first to second
states in response to a fault signal being produced by tlashover detecting
means whenever a flashover occurs in any of the traction motors. As soon
as the valve changes to its second, open-circuit state, ezcitation current in
the generator field windings rapidly decays toward sero, and the output
voltage of the generator is correspondingly decreased to rapidly extinguish
the flashover before the armature current in the faulted motor can rise to
the maximum available short-circuit magnitude. In other words,
whenever a llashover occurs the initial current surge is desirably limited by
quickly disconnecting the generator Reld from the source of excitation
current.
In one aspect of the invention, the aforesaid oontrollabie electric valve
comprises a gate turnoff thyristor poled to conduct generator field excitation
current when in a turned-on state (corresponding to said first state) and
et'fectively blocking such current when in a turned-off state (corresponding
to said second state). This thyristor is shunted by electrical impedance
means that preferably comprises the parallel combination of a non-linear
resistor and a snubber capacitor which limit the rate of change of voltage
across the GTO thyristor when changing states.
In anothes aspect of the invention, the afor~sid flashover detecting
means comprises means responsive to the current feedback signals for
producing the fault signal if the magnitude of armature current in any of
said traction motors ezceeds a predetermined threshold which is higher
than the magnitude of motor armature current under all normal
conditions.
In a different aspect of the invention, when a fault signal is produced
by the Qashover detecting means, the control means for the generator
ezcitatioa current source temporarily imposes a control signal value
corresponding to zero ezcitation current. At the same time, the operating
means for the traction motor contactors opens each of these contactors but
not before t~ magnitude of armature current in the associated motor is
below a predetermined level to ensure that the contactors will open safely.
Because the controllable electric valve in the generator ezcitation current
path quickly decoupled the excitation source from the generator field as
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summarized above, the magnitude of current in the faulted motor
decreases very rapidly from its initial surge to ouch safe level.
Preferably, the tlashover protection means includes additional means
operative after the oontactors have been opened is response to a flashover to
cause the aforesaid operating means to reclose all of the contactors except
the ones) assoaated with the faulted motor(s), as determined by means for
identifying any traction motor in which armature current has a magnitude
exceeding the aforesaid predetermined threshold. . The operating means is
prevented from reclosing the latter contactor(t), following the initial
production of the fault signal by the flaahover detecting means, for an
interval aui~ciently long to allow the Qashed commutator to be cleaned by
the brushes ring over the commutator aurfaca.
In still another aspect of the invention, GTO gating means is provided
for supplying the aforesaid gate turnoff thyriator with alternative turn-on
I S and turnoff signals as Selectively controlled by associated logic means.
The
logic means iS arranged to perform Several different functions. It includes
means for providing a command signal having a first state if there is no
fault signal and if certain other conditions are normal and having a second
state otherwise. Whenever the command signal is in its first 'state, the
turn-on signal is produced and has a certain minimum duration. Means
including a timer produces the turnoff' signal while the command signal is
in its second state and no turn-on signal is being produced. The timer is
arranged to prevent the coaimsnd signal, aftes changing from fi rst to
second States, from returning to its first state for a predetermined period
long enough to allow the traction motor contactors to open and reclose as
summarised above. This also provides time for the capacitor in the
aforesaid impedance means to discharge through the parallel resistor,
thereby avoiding the need to use conventional means in series with the
capacitor for limiting its discharge current when the GTO tt, ristor is
returned to a turned on state by the next turn-on signal.
Preferably the logic means also includes means responsive to the
magnitude of generator excitation current for preventing the command
signal from changing from first to second states if such magnitude exceeds
an abnormally high level which is higher than the masimum excitation
current typically attained in response to a flashover. This will inhibit the
production of a turnoff signal in the event ea~tadon current is greater than
such level, as will be the case if a diode in the power rectifier fails to
block
reverse current (in which event it is desirable to let the generator output
current rise to a magnitude high enough to blow the fuse associated with
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the failed diode). Consequently, the gate turnoff t6yristor need not be
capable of turning off current greater than the aforesaid high level.
The invention will be better understood and its many objectives and
advantages will be more fully appreciated from the following description
taken in conjunction with the accompanying drawings.
~,Descrintion of ~~y~gg
FIG. 1 is a block diagram of an electrical propulsion system for a
locomotive, including a thermal prime mover (such as a diesel engine), a
synchronous generator, an electric power rectifier, a plurality of traction
motors, a controllable source of excitation current, and a controller;
FIG. 2A is a schematic diagram of oae of the d-c traction motors
represented by simple blocks in FIG. 1;
FIG. 2B is a family of load saturation curves of a typical synchronous
generator, showing the relationship between output voltage and current for
various magnitudes of excitation current;
FIG. 3 is an expanded block diagram of certain parts of the controller
that cooperate with the generator excitation source to implement the
present invention;
FIG. 4 is a schematic circuit diagram of the Qashover detecting means
shown as a single block in FIG. 3;
FIG. 5 is an expanded diagram of the generator excitation current
source shown as a single block in FIGS. 1 and 3;
FIG. 6 is an expanded block diagram of the GTO control means shown
as a single block in FIG. 5;
FIG. 7 is a functional block diagram of the logic means represented by
a single block in FIG. 6; and
FIG. 8 is a Bow chart that explains the presently preferred manner of
implementing the system response function represented by a single block in
FIG. 3.
The propulsion system shown in FIG. 1 includes a variable-speed
prime mover 11 mechanically coupled to the rotor of a dynamoelectric
machine 12 comprising a 3-phase alternating current (a-c) synchronous
generator, also referred to as the main traction alternator. The main
alternator 12 has a set of three star-connected armature windings on its
stator. In operation, it generates 3-phase voltages in these windings, which
voltages are applied to a-c input terminals of at least one 3-phase, double-
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way uncontrolled power rectifier bridge 13. In a conventional manner, the
bridge 13 is formed by a plurality of pairs of power diodes, two or three such
pairs being assoaated with each of the three different phases of the main
alternator 12. The diodes in each pair are serially connected between
relatively positioe and negative direct current (d~c) outRut terminals of the
rectifier bridge, and their junction is connected via a protective fuse (not
shown) to the respectively associated a-c input terminal of the bridge. The
output of the bridge 13 is electrically coupled, via a d-c bus 14 and a
plurality
of individual electrical coatactors 15C, 16C, in energizing relationship to a
plurality of parallel-connected, adjustable apaed d-c traction motors, only
two of which ( 18, 16) are shown in FIG. 1. Prune mover 11, alternator 12,
and rectifier 13 are suitably mounted on the platform of a self propelled
traction vehicle which typically is a 4-rile or 6-axle diesel-electric
locomotive. Ths locomotive platform is in turn supported on two trucks (not
shown), each having two or more axle-wheel sets. A separate traction
motor is hung on each axle, and its rotor is mechanically coupled via
conventional gearing in driving relationship to the associated axle-wheel
set. Suitable current sensing means are used to provide a family of current
feedback signals I1, I2, etc. that are respectively representative of the
magnitudes of the motor armature currents.
The first traction motor 15 is shown in FIG. 2A and is typical of the
others. On the cylindrical rotor of this motor there are a plurality of
armature windings that respectively terminate at different bars or
segments of a conventional commutator 15A with which non-rotating
carbon brushes 158 are in sliding contact. A grounded flash ring 15R is
positioned around the commutator in spaced relation thereto. The motor
has field windings 15F on its stator, and during the motoring or propulsion
mode of operation these windings are electrically connected in series with
the armature as is shown in FIG. 2A. The direction of armature rotation,
and hence the direction in which the locomotive is propelled, depends on
the relative direction of field current and can be reversed by changing the
contact position of a conventional bistable electromechanical reverser (not
shown) connected in series with the field windings 15F. For dynamically
braking or retarding the locomotive the armature windings of each traction
motor are disconnected from the power rectifier 13 and reconnected to a
conventional fan-blown dynamic braking resistor grid (not shown), and the
field windings of all of the motors are reconnected in series with each other
for energization by the rectified output of the main alternator 12 As can be
seen in FIG. 2A, the current feedback signal I1 is provided by a suitable
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current sensor ib8 connected in aeries with the armature windings of the
traction motor 15. h is therefore representative of the magnitude of current
in the series~connected armature and field windings of this motor when
operating is a motoring mode.
5 The main alternator 12 and the power rectifier 13 serve as a
controllable source of electric power for the respective traction motors. The
magnitude of output voltage (or current) of this source ie determined and
varied by the amount of ezdtation currant supplied to field windings 12F on
the rotor of the main alternator. There field windings are connected for
10 energisation to the output of a suitable sonrc~ iT of regulated excitation
current IF. In !he illustrated embodiment of the invention, the connection
between the H~ windings 12F and the esdtation current source 1? include
a contact 12C of a conventional elechromechanical Reld switch The field
switch has oontsol means 12D for moving it to a first or normal state, in
which the contact 12C is cloned and freely conducts excitation current, and
for causing thin switch to change between ib first state and a second or
alternative state, in which the contact 12C is open and excitation current is
effectively interrupted. In practice the control means 12D comprises an
electromagnetic coil and an associated actuating mechanism that will
move the field switch to its normal state end hold it there only if this coil
is
energized.
Preferably the ezcitatian current source 1? comprises a 3-phase
controlled rectifier bridge the input terminals 18 of which receive
alternating voltages from a prime mover~driven auxiliary alternator that
can actually comprise an auxiliary set of 3-phase armature windings on
the same frame as the main alternator 12. The source 17 is labeled "Field
Regulator" in FIG. 1. It includes conventional means for varying the
magnitude of direct current IF supplied to the alternator field 12F (and
hence the output of the alternator 12) ae necessary to minimize any
difference between the value of a variable control signal VC on an input line
19 and a feedbsclc signal which during motoring is representative of the
average magnitude V of the rectified output voltage of the main alternator
12. The latter voltage magnitude is a lcnowa fua~ction of the magnitude of
ezcitation curr~t in the field windings 12F and the magnitude of output
current is the armature windings of the main alternator, respectively, and
it also varies with the speed of the prime moves 11. It is sensed by a
conventional voltage sensing module connected across the d-c output
terminals of the power rectifier. The curves in FIG. 2B illustrate
ezemplary relatianahips between V and the average magnitude of load
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20LC01583A
current at the output terminals of the power rectifier 13 as supplied by a
typical alternator 12 driven at constant speed (e.g., 1050 RPM) by the prime
mover 11 and a:ated by field current IF of various different magnitudes
which are labeled on the respective curves.
A current detecting module 22 of relatively low resistance (e.g.,
approximately ten ohms) is connected between the neutral S of the
alternator armature windings and the grounded chassis or frame of the
locomotive, as indicated in FIt3. 1. The module 22 provides on an output
line 29 a feedback signal reprssentative of the magnitude (IGND) of ground
leakage current in the electric propulsion system. It will be apparent that
I(IND is a measure of current bowing, via the module 22, between the
neutral S and any ground fault in the armature windings of the main
alternator 12, is the power rectifier 1S, or in the elet~tric load circuit
that is
connected to the power rectifier. The latter circuit includes the field
windings of the traction motors 16, 16, etc. and, in the motoring mode of
operation, the motor armature windings as well.
The prime mover 11 that drives the alternator field 12F is a thermal or
internal-combustion engine or equivalent. Oa a dies~~-electric locomotive,
the motive power is typically provided by a high-horsepower, turbocharged,
16-cylinder diesel engine. Such an engine has a fuel system 24 chat
includes a pair of fuel pump racks for controlling how much fuel oil (lows
into each cylinder each time an associated fuel injector is actuated by a
corresponding fuel cam on the engine camshafts. The position of each fuel
rack, and hence the quantity of fuel supplied to the engine, is controiled by
an output piston of an engine speed governor system 25 to which both racks
are linked. The governor regulates engine speed by automatically
displacing the racks, within predetermined limits, in a direction and by an
amount that m'usimizes any difference between actual and desired speeds of
the engine crankahaR. The desired speed is set by a variable speed call
signal received from an associated controller 26, which signal is herein
called the speed command signal or the speed call signal. An engine speed
signal (RPM) indicates the actual rotational speed of the engine crankshaft
and kenos of the alternator field.
The speed command signal for the engine governor system 25 and the
a:citation control signal VC for the alternator field current sourcs 1? are
provided by the controller 28. In a normal motoring or propulsion mode of
operation, the values of these signals ate determined by the position of a
handle of a manually operated throttle 2? to which the controller 26 is
electrically coupled. A locomotive throttle conventionally has eight power
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positions or notches (N), plus idle and shutdown. N1 corresponds to a
minimum desired engine speed (power), while N8 corresponds to
maximum speed and full power. With the throttle in its idle position, the
controller 26 is operative to impose on the control signal VC a value
corresponding to IF=S, and no traction power is produced by the main
alternator 12. When dynamic braking of a moving locomotive is desired,
the operator mares the throttle handle to its idle p~ition and manipulates
an interlocking handle of a companion brake control device 28 so that the
main controller 26 is now supplied with a variable "brake call" signal that
will determine the value of the alternator euitatlon control signal VC. (In
the braking mode, a feedback signal which is representative of the
magnitude of the current being supplied to the traction motor Reld
windings from the rectified output of the main altesnatQS 19 will be supplied
to the alternator excitation source 17 and there subtracted from the control
signal on line 19 to determine the difference or error signal to which the
source 17 responds.) In a consist of two or more locomotives, only the lead
unit is usually attended, and the controller onboard each trail unit will
receive, over tramlines, encoded signals that indicate the throttle position
or brake call selected by the operator in the lead unit.
For each power level of the engine there is a corresponding desired
load. The controller 26 is suitably arranged to translate the notch
information from the throttle 2? into a reference signal value substantially
equal to the value that the voltage feedback signal V will have when the
traction powei matches the called-for power, and eo long as the alternator
output voltage and load current are both within predetermined limits the
control signal VC on the input line 19 of the excitation current source 17 is
varied as necessary to obtain this desired load. For this purpose, and for
the purpose of deration (i.e., unloading the engine) andlor limiting engine
speed in the event of certain abnormal conditioaa, it is necessary to supply
the controller 28 with information about variow operating conditions and
parametess of the propulsion system, including the engine.
As is illustrated in FIG. 1, the controller 28 receives the above-
mentioned engine speed signal RPM, voltage feedback signal V, and
current feedback signals I1, I2, etc. which are representative, respectively,
of the magnitudes of current in the armature windings of the individual
traction motors. It also receives a load control signal issued by the governor
system 25 if the engine cannot develop the power demanded and still
maintain the called-for speed. (The load control signal is effective, when
issued, to reduce the power reference value in the controller 26 so as to
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weaken the alternator field until a new balance point is reached.) Additional
data
supplied to the controller 26 include: "VOLT MAX" and "CUR MAX" data that
establish absolute maximum limits for the alternator output voltage and
current,
respectively; "CRANK" data indicating whether or not an engine starting (i.e.,
s cranking) routine is being ezecuted; and relevant Inputs from other selected
sources,
as represented by the block labeled "OTAER." The alternator excitation aoarce
17
and the controller communicate witb each other via a multi~ine aerial data
link or bus
21. The controller 26 also commnnicatea with the control means 12D that is
operative, when energized in reapouae to a "clone" command from the
contro~ler, to
i o move the held switch contact 12C to its cloned position in which it la
held by the
energized control means, and it commnnicatea with "CONTACTOR DRIVERS"
(block 29) which are suitably constructed and arranged to actuate the
individual
traction motor contactors 15C, 16C, etc. Typically the contactor drivers 29
are
pneumatic mechanisms controlled by associated electromagnetic valves which in
turn
is are controlled, selectively or in unison, by commands from the controller
26.
L~or the purpose of responding to ground faults in the propulsion
system, the controller 26 is supplied, via the output line 23 of the current
detecting
module 22, with the aforesaid feedback signal whose value varies with the
magnitude ~GND of ground leakage current. If this signal indicates that IGND
is
2o abnormally high, the controller automatically executes certain protective
functions
and, at the same time, sends appropriate messages or alarm signals to a
display
module 30 in the cab of the locomotive. Preferably the ground fault protective
functions implemented by the controller 26 are the same as or equivalent to
those
disclosed in the previously cited prior art Canadian patent 1,266,117 granted
on
2s L:ebruary 20, 1990, and assigned to General Electric Company. In summary,
the
referenced protection is effective to modify the value of the control signal
VC on
line 19 when ground leakage current is abnormally high so that: (1) It the
ground
current magnitude la in a range between a predetermined deration threshold
level
and a predetermined maximum permissible Ilmit, the magnitude of alternator
field
3o current IF is reduced and consequently the power output of the main
alternator
12 is reduced to a fraction of its normally desired amount, which fraction
varies
inversely with the magnitude of ground current in excess of the deration
threshold
level, and (2) the power output is restricted to zero for at least a minimum
CA 02365474 2002-O1-10
20LC01583A
14
interval of time if the ground current magnitude increases above its maximum
limit.
In the preferred embodiment of the present invention, the controller 26
comprises a microcomputer. Persons skilled in the art will understand that a
microcomputer is actually a coordinated system of commercially available
s components and associated electrical circuits and elements that can be
programmed
to~ perform a variety of desired functions. In a typical microcomputer, a
central
processing unit (CPU) executes an operating program stored in an erasable sad
electrically reprogrammable read only memory (EPROM) which also stores tables
and data utilized in the program. Contained within the CPU are conventional
io counters, registers, accumulators, fiiptlops (flags), etc., along with a
precision
oscillator which provides a high-frequency clock signal. The microcomputer
also
includes a random access memory (RAM) into which data may be temporarily
stored
and from which data may be read at various address locations determined by the
program stored in the EPROM. These components are interconnected by
appropriate
i s address, data, and control buses. In one practical embodiment of the
invention, an
Intel 8086 microprocessor is used.
The controller 26 is programmed to produce, in the motoring mode of
operation, a control signal value on the line 19 that varies as necessary to
zero any
error between the value of the alternator voltage feedback signal V and a
reference
Zo value that normally depends on the throttle position selected by the
locomotive
operator and the traction power output of the main alternator. The presently
preferred manner is which this is accomplished is disclosed in U.S. patent
4,634,887-
Balch et al, issued .ianuary 6, 1987, and assigned to General Electric
Company. In
order to implement an electrical braking mode of operation, the controller 26
is
zs programmed to vary the value of the control signal VC as necessary to zero
any error
between a motor armature current feedback value and a reference value that
normally depends on the dynamic brake position selected by the locomotive
operator.
In accordance with the present invention, the above-described propulsion
system includes means for protecting the traction motors from flashovers. The
desired
3 o fiashover protection is implemented by the controller 26 in cooperation
with the main
alternator excitation current source 17. The parts of the controller that are
involved
in flashover protection are shown in simplified form in FIG. 3 where the block
32
CA 02365474 2002-O1-10
20LC01583A
represents suitable means for detecting the occurrence of a flashover on the
commutator of any one of the d-c traction motors 15, 16, etc.
The detecting means 32 receives the family of traction motor current
feedback signals I1, I2, etc. and the ground leakage current (IGND)
5 feedback signal on line 23. It is operative to produce a fault signal on an
output line 33 (labeled "FLASH" in FIGS. 3~7) whenever a Qashover occurs,
as indicated by an abnormal rise in the magnitude of at least one current
feedback signal in the event either (1) the magnitude of armature current in
any traction motor exceeds a predetermined threshold which is higher
10 than the magnitude of armature current under all normal conditions, or
(2) the magnitude of IGND exceeds another threshold (e.g., 2.5 amperes)
which is higher than the mazianum permissible limit of leakage current
above which the above-mentioned ground fault protective function clamps
the control signal VC to its zero traction power value. The threshold
15 magnitude of motor armature current is preferably nearly twice the
maximum current that each traction motor will normally conduct; in one
practical application of the invention, a threshold magnitude of 3,000
amperes has been selected. In order to respond as quickly as possible to the
occurrence of a tlashoves, the detection function is preferably performed by
means of analog circuitry rather than by the microcomputer.
The presently preferred embodiment of the tlashover detection means
32 is shown in FIG. 4 and will now be described. The motor armature
current feedback signals I1, I2, etc. are respectively supplied to first
inputs
of an array of comparators 35, 36, etc. The second inputs of the same
comparators are connected in common to suitable means 37 for deriving a
bias signal of predetermined coaatant magnitude Kl corresponding to the
aforesaid high threshold magnitude of motor current. 'The outputs of these
comparstors are respectively coupled through diodes 38, 39, etc. to a line 40
which in turn is connected through a buffer 41 $nd an..:.her diode 42 to the
base of a PNP traasiator 43. The emitter of the transistor 43 is connected via
a diode 44 and a resistor 45 to a control voltage bus (+) of relatively
positive
constant potential, and a resistor 46 is connected between the transistor
base and the junction of the diode 44 and resistor 46. The collector of the
transistor 43 is connected via a resistor 47 to a reference potential bus
represented in FIG. 4 by a circled minus symbol, and it is also connected
via a resistor 48 to the output Line 33 of the llashover detector. Normally,
none of the feedback signals I1, I2, etc. has a magnitude exceeding K1, all
of the comparators 35, 36, etc. have high outputs, the diodes 38, 39, etc. are
reverse biased (i.e., non-conducting) and the signal on the line 40 is high,
CA 02365474 2002-O1-10
20LC01583A
16
the transistor 43 is turned off, there is no current in resistor 47, the
potential of the translator's collector (and also of the line 33) ie low or
zero
with respect to the reference potential, and no fault signal is being
outputted by thin detector. However, if and when any one (or more) of the
motor current feedback signals rises above K1, the output of the associated
comparator will switch to a low state which causes the signal on line 40 to
be low and the diode 49 to conduct, thus forward biasing the emitter-base
junction of the transistor 43 which now turns on and conducts 1 current
through its collector resistor 4T, thereby raising the collector potential and
produdng a hldr fault signal oa tiu output line 98.
As can be seen in FIG. 4, the current feed~tdc signal on line 23,
representing the magnitude of ground lea><age current IGND in the
armature windings of the traction alternator 12, is anpplied to one input of
an additional comparator 51, the other input of which is connected to
suitable mesas 63 for deriving another bias signal of predetermined
constant magnitude K2 corresponding to the aforesaid high threshold
magnitude of IGND. The output of comparator 61 is coupled through a
diode 54 to a line 66 which in turn is connected through a buffer 57 and a
diode 58 to the base of the transistor 43. Normally the magnitude of the
ground current feedback signal dose not esceed KZ, the comparator 51 has a
high output, the diode 54 is reverse biased (i.e., non-conducting), and the
signal oa the line 58 is high. However, if and when the magnitude of this
feedback signal rises above K2, the output of comparator 51 is switched to a
low state which causes the signal on line 66 to be low and the diode 58 to
conduct, thereby turning on the translator 43 and producing a high fault
signal on the output line 33. In effect, the diodes 42 and 58 form an "OR"
logic circuit which enables the detector to produce a fault signal in response
to an abnormal magnitude increase of either the ground leakage current in
the alternator armature windings or the armature current in any one of
the traction motors, such increase being caused is either case by a
»lashover an a motor commutator.
As is shown in FIG. 4, the ground leakage current feedback signal on
line 23 is also supplied to summing meaty 69 where another signal on a
line 61 is subtracted therefrom. The signal on line 61 has a predetermined
constant magnitude K3 corresponding to the deration threshold level of
IGND (e.g., approrianately 0.6 ampere). If IGND is higher than this level,
the resultant value from the summing means 59 ~ activates a deration
program 62. As ie fully disclosed in the previously cited Canadian patent
1,266,117, the deration program 62 modifies the value of the control signal
CA 02365474 2002-O1-10
20LC01583A
17
VC on the line 19 (see FIG. 1) in a manner that reduces the magnitude of
alternator field current so that the power output of the alternator 12 is
reduced to a fraction of its normally desired amount, which fraction is
inversely proportional to the magnitude of leakage current in excess of the
deration threshold level, and equals zero if the leakage current magnitude
exceeds its maamum permissible li~ait (e.g., approximately one amps- J.
Note that K2 is higher than the magnitude of the feedback signal on line l3
when the last-mentioned limit is reached.
Returning to FIG. 3, the fault signal that the detecting means 32
produces an the output line 33 whenever a flashover occurs is supplied to
the alternator a=citation current source 1? via the data link 21. In
accordance with the present invention, the excitation source 1? is provided
with a solid-state controllable electric valve in the path of the alternator
field
current for quickly decoupling this source from the alternator field
windings 12F in response to a fault signal being produced, whereupon the
magnitude of excitation current in the alternator field is rapidly reduced
toward zero and the output voltage of the main alternator 12 is
correspondingly decreased. The organization, operation, and advantages of
this part of the ilashover protection means will now be described in more
'20 detail with reference to FIG. 5 which illustrates the presently preferred
embodiment of the excitation current source 17. The illustrated source 1?
comprises a 3-phase double-way rectifier bridge 64 formed by the
iterconnection of six controllable, unidirectional electric valves or
thyristors having gates which respectively receive periodic firing or turn-on
signals from conventional control means 85 shown as a block labeled
"thyristor bridge control," such firing signals being synchronized with 3-
phase alternating voltages that are applied to three a-c input lines 18 of the
bridge 64. The latter voltages are obtained from au~dliary windings of the
alternator 12, whereby their frequency and amplitude will vary with the
rotational speed (RPM) of the prime mover. Typically the input voltage
magnitude is in a range from approximately 30 volts rms at idle speed to 68
volts tins at full speed. In order to achieve the desired alternator field
regulation as previously described, the control means 85 ie operative to
advance or to retsrd the timing of the firing signals as a function of any
error between the control signal VC on line 19 and the feedback signal
representative of the alternator output voltage V.
Aa is shown in FIG. 5, the negative d-c output terminal N of the
rectifier bridge 64 is connected directly to one end of the 5eld windings 12F
of the main alternator, and the relatively positive output terminal P of this
CA 02365474 2002-O1-10
20LC01583A
18
bridge is oonaected to the other end of the field 12F by means of a line 66,
the
normally closed contact 12C of the alternator field switch, and a line 67.
The Seld 12F and the contact 12C are shunted by a voltage limiting resistor
68 of relatively small ohmic value (e.g., two ohms), in series with a
bipolarity voltage breakover device 69 having a positive terminal connected
to the line 66 and a neggtive terminal connected to the line 67. The
breakover device 69 in its normal state provides a very high resistance and
is essentially an open circuit. However, it is suitably constructed and
arranged .to switch abruptly to. a negligible resistance state if either the
potential of line 87 is negative and exceeds a first predatesmined breakover
level with respect to output terminal N of the bridge 64 (e.g.; .1,000 volts)
or
the potential of line 86 is relatively positive and exceeds a second breakover
level which can equal or differ from the first breakover level. So long as the
device 69 is in the latter state, any excitation current in the field 12F will
circulate or "free wheel" through the 2-ohm resistor 68.
Controllable circuit means 70 is connected in series with the field
switch contact 12C between the positive output terminal P of the bridge 64
and the line 66. In the illustrated embodiment of the invention, the circuit
means ?0 comprises a high-speed, solid-state contrnllable electric valve 71
connected in parallel circuit relationship with electrical impedance means
which in turn comprises a snubber capacitor 72 in parallel with a non-
linear resistance element 73. Preferably, as is indicated in FIG. 5, the valve
?1 is a gate turnoff device (GTO). Alternatively, this valve could comprise a
power transistor or a conventional combination of a silicon controlled
rectifier and external commutation means.
The GTO device ?1, usually referred to as a GTO thyristor, is a multi-
layer semiconductor designed to freely conduct "forward" load current ti.e.,
current flowing into its anode and out of its cathode) when its gate electrode
is triggered by a suitable turn-on or firing signal and to effectively block
such current otter a negative turn-off signal is alternatively applied to the
same gate. In one application of the invention, this thyristor is rated to
conduct steady state unidirectional load current of 4b0 amperes when
turned on sad to withstand a forward voltage in excess of 1,600 volts when
turned off, such thyristor being capable of suaxssfislly turning off current
as high as 1,200 amperes in response to a turaofl' signal of suitable
magnitude. The GTO ?1 is poled to conduct excitation current when turned
on. To protect the GTO thyristor from damage in the event of a voltage
surge of reverse polarity, it is shunted by an inversely poled diode 74 which
could alternatively be embodied in the GTO structure if desired.
CA 02365474 2002-O1-10
19
20LC01583A
Whenever the GTO thyristor or valve T1 is in a turned on state, it
presents negligible resistance to forward load current. Now excitation
current can freely flow from terminal P thsough the arcuit means 70, the
contact 12C (assumed closed), and the main alternator field 12F to the
terminal N. But in its turned off state the valve resistance has such a high
ohmic value se to block or interrupt forward load current. To change the
GTO valve 71 between these two alternative states, suitable control means
80, labeled "GTO Control," is assoaiat~ tlmrewith. The control means 80 is
operative in response to receipt of a fault signal on the line 33 to change
the
GTO valve ?1 from first to second states sad et the same time to change s
normally high ("1") "status" signal on an output line 81 to a low ("0") state.
The control means 80 is also operative in response to receipt of an "enable"
signal on an input Line 82 to change the GTO valve T1 from second to first
states. A current sensor 83 in the line 68 is coupled to the control means 80
to provide a feedback signal representative of the magnitude of excitation
current (IF) being supplied to the alternator field 12F.
The non-linear resistance element T3 in the circuit means 70 is
commonly called a "varistor." It is made of suitable malarial (e.g., silicon
carbide) that decreases in ohmic value as the magnitude of applied voltage
increases, whereby current through the element varies as a power n of
such voltage. Typically n is greater than 3. A useful form of varistor is
known as s Thyrite disk. In one application of the invention, the illustrated
element 73 comprises an assembly of two Thyrite disks in parallel. Its
resistance (at 25°C) is approximately 1,100 ohms when there is 150
volts
across the element and decreases to less than two ohms if the applied
voltage increases to 1,600 volts.
When a flashover occurs and the GTO valve ? 1 is turned off by the
control means 80 in response to the fault signal produced on the line 33 by
the flashover detecting means 32, alternator field excitation current is
rapidly transferred ("commutated") from the valve ? 1 to the parallel
impedance means 72, ?3 where it encounters a high resistance. The
capacitor T2 limits the rate of change of voltage across the GTO valve. Being
non-linear, the resistance of the varistor T3 decreases as the voltage across
the tusned-off valve ?1 rises and limits the ma~omnm level of this voltage to
a safe value. , The turn off process of the GTO valve ? 1 actually hae three
stages. Once a GTO turnoff signal is applied to the gate of this valve, there
is a brief delay (known as the "storage" time) before forward load current
begins to decrease. Then, during a very short interval known as the "fall"
time, current decreases rapidly to a very low magnitude. The turnoff time
CA 02365474 2002-O1-10
20 ZOLC01583A
of valve ?1 (e.g., approximately 15 microseconds) is the sum of the storage
and fall times. However, the turnoff' signal must not be removed before the
end of a longer interval (known as the "tail" time) which is required for the
valve to recover fully its ability to withstand off state voltage without
prematurely reverting to a turned on state. During the .latter interval the
valve will continue to conduct a relatively small amount of forward load
current (known ae "tail" current) as its resistance increases and the
voltage across it rises At the conclusion of this process, there is no load
current in the GTO valve ? 1, the o~ stets voltage applied to the main
electrodes of this valve is the same as the voltage across the d-c output
terminals P,N of the rectit3er bridge 64, (e.g., approsia~ately 80 volts), and
the bridge 64 is effectively deooupled from the altdrnator field windings 12F.
During the valve turn ofl' process summarised above, the high
impedance of the varistor ?3 is inserted in the path of alternator field
excitation current. This immediately reduces the magnitude of current in
the alternator field. The rapidly changing current induces a high voltage
in the field windings 12F. This voltage soon attains the negative breakover
level of the breakover device 89, whereupon the latter device will switch to
its negligible resistance state, thereby conaecNng the 2-ohm resistor 68
across the field 12F and permitting a portion of the held current to circulate
through the resistor 68 as Aeld current continues to decrease. The re-
duction in field current magnitude causes a much larger current reduction
in the armature windings of the main alternator 12, and the alternator
output voltage and current rapidly decrease. FIG. 2B demonstrates that the
alternator output current decrease, per ampere of held current reduction,
varies from approrimately five amperes to nearly 18 amperes, depending
on the magnitude of alternator output voltage V. The advantageous results
of quickly inserting impedance in the excitation current path and
decoupling the rectifier bridge 64 from the alternator field 12F will be
better
understood from the following explanation of the alternator s response to
flashovers.
The masn alternator 12 is a high-reactance, salient-pole synchronous
machine without damping or amortisseur windings. If the load circuit
connected to the output terminals of the armature windings of this
machine were short-circuited. by a flashover, the amplitude of armature
current would tend to increase abruptly to a peak much higher than
normal and then to decay with time. The initial current increase in the
armature windings produces magnetomotive force (MMF) that almost
directly opposes the Held MMF, whereby tending to demagnetize or weaken
CA 02365474 2002-O1-10
20LC01583A
21
the resultant magnetic field in the stator-rotor air gap of the machine. The
demagnetizing MMF induces extra current is the Held 12F so that the total
ilux linkages will remain constant. The control means 85 for the controlled
rectifier bridge 64 in the excitation current source 1? responds to the
resulting change is output voltage V by initiating corrective action, but its
response time is too slow and the bridge 64 has insutFcient voltage to
prevent this Held current increase. So long ea the excitation current source
1? remains unchanged, the initial peak magnitude of short circuit current
is determined by the transient reactance of the alternator (more precisely,
the direct axis transient reactance) and the rea~ctea~ in the current path
between the alternator armature windings and the traction motor whose
commutator flashed. The time constant of the ensuing current decay is
determined by the electrical inductance and resistance in the path of the
excitation current. As soon as the above-described GTO valve ?1 starts
turning off and the resistance in the latter path is etI'ectively increased,
this
time constant becomes significantly smaller and a:citation current will
very rapidly decay toward zero. In effect, the reactance of the alternator
rapidly increases from its initial relatively low transient value (which is no
more than approximately 3096 of the machine's steady state synchronous
reactance) to the value of its synchronous reactance, and the armature
current magnitude is decreased correspondingly. If the excitation current
source is quickly decoupled from the field 12F as described, the output
current of the alternator 12 wiil begin to decrease from its initial surge
before reaching the maximum available short-circuit magnitude. In one
application of the invention, peak short circuit current to a faulted motor
has been limited to approximately 18,000 amperes in a propulsion system
capable of supplying 28,000 amperes or more without this improved
flashover protection means, and the electrical energy in the faulted motor
circuit has been limited to about 2596 of what it would otherwise be.
As previously described, the GTO valve ?1 is the circuit means ?0 is
changed hstweaa its on and off states by the control means 80 in response to
the fault signal on line 33 and the enable signal on line 82. The presently
preferred emba~diment of the control means 80 is illustrated in FIG. 6 which
will now be described. It comprises suitable gat3ag means 85 for supplying
the gate electrale of the valve ? 1 with eithes a podtivs current that e8'ects
turn-on of this device, or a relatively negative current that effects turn-aff
of
the same device. To supply the turn-on signal, a suitable source of positive
potential of. approzimately 5 volts with respect to the cathode potential of
valve ?1 is connected to the gate of the GTO valve ?1 through a resistor 86 of
CA 02365474 2002-O1-10
20LC01583A
22
low ohmic value (e.g., 0.5 ohm), a first controllable solid~atate switch 87,
and a line 88. Preferably the switch 87 is a conventional power field effect
transistor (FET). A capacitor 90 is connected between the high side of this
switch and the cathode of the GTO valve 71. When the state of the first
switch 87 is chaogad from non-conducting to conducting, the +5 volts turn-
on signal source is immediately applied to the GTO gate electrode.
Preferably this source includes electric energy storing means (e.g., a
capacitor of relatively high capaatance value per pre~charged to a higher
level of voltage, for ezample 13 volts) that rapidly discharges when the
switch 87 starts conducting so that an initial pulse of turn-on energy i s
supplied to the gate-cathode junction of t3T0 91, attes which the switch 87
continues conducting the required holding current frown the +5 volts
source. To supply the turnoff signal far the GTO valve 71, its gate is
connected to a control voltage terminal having a negative potential of
appro>annately 13 volts with respect to the GTO cathode through the line 88
and a second controllable solid-state switch 89 which preferably comprises
a parallel array of two or more FETs arranged to turn on and off in unison.
The second switch 89 is shunted by a resistor 91 and also by a circuit 92
comprising another resistor in series with a capacitor. When the second
switch 89 changes to its conducting (turaed~on) state, the latter capacitor
will discharge through thin switch, thereby reversing the direction of
current in the line 88. Current in the gate~csthode junction of the GTO
valve will change rapidly (e.g., approzimately 40 amps per microsecond)
from its positive holding current magnitude (e.g., +6 amperes) to a peak
negative magnitude (e.g., approzimately 100 amperes or more, depending
on the magnitude of load current being commutated) that restores the
valve's ability to block forward load current. Once the GTO valve turns off
and the tail time aspires, it will remain off until another turn-on signal is
Bpplied to its gate, and the resistance of its gate-cathode junction will
limit
the negative gate current to a trivial magnitude. The gals potential of the
GTO valve is slightly above cathode potential when this device is on, and is
nearly the asma ss the potential of the -13 volts terminal when the valve 71
is off sad the second switch 89 is turned oa. .
The conducting states of the two switches 87 and 89 are selectively
controlled by associated logic means 93 so that only one switch is
conducting (turn~eid oa) at say time. A first output line 94 of the logic
means
93 is coupled through an amplifier 95 to the control terminal of the first
switch 87, and a second output line 96 is coupled through a duplicate
amplifier 95 to the control terminal of the second switch 89. The logic
CA 02365474 2002-O1-10
20LC01583A
23
means 93 is supplied with both the fault signal on the line 33 and the enable
signal on line 82. The lines 33 end 82 are respectively connected to two
diR'erent inputs of the logic means through suitable optical couplers 98.
The logic means 93 is also supplied with a feedback signal representative of
the -sagnitude of current IF in line 66 of the excitation current path (as
sensed by the current sensor 83 in FIG. b), and it is coupled to the second
switch 89 via s line 99 which supplies it with a feedback value that is a
- measure of the voltage magnitude across the switch 89. In a manner that
is explained hereinafter, the logic means 89 responds to its various input
signals by providing the following alternative combinations of signals on its
respective output lines 84 and 96: (1) the output signal on the line 94 has a
high or "ON" state that causes the first switch 8? to conduct, thereby
changing the controllable GTO valve ?1 to its turnedon state, while the
signal on the line 96 is concurrently low so a~ to bias the second switch 89
to
its non-conducting state; or (2) the output signal on the line 96 has a high
or
"OFF" state that causes the second switch 89 to conduct, thereby changing
the GTO valve to its turned-off state, while the signal on the line 94 is
concurrently low so as to bias the &rst switch 8? to its non-conducting state;
or (3) the signals are low on both of the output lines 94 and 96.
The presently preferred embodiment of the logic means 93 employs a
plurality of dual input analog logic circuits suitably interconnected and
arranged to perform the various functions that will now be described with
reference to FIG. ?. For the sake of convenience, the individual logic
circuits have been shown symbolically in a somewhat simplified form in
FIG. ? and are heseiaatter called "units." One type of unit has a high (" 1")
output state only when its first input is high and its other input is low
("0"),_
the latter input being referred to as a "not" input. In practice. this same
function could be performed by other equivalent logic circuits, such as the
combination of a conventional AND logic ascuit with a polarity inverter
ahead of its second input, or the combination of ~ conventional NOR logic
circuit with a polasity inverter ahead of its first input.
As is shown in FIG. 7, the fault signal is coupled to the "not" input of a
&rst unit 101. The first input of the unit 101 is oomected to a line 102 on
which a normally high signal is provided, a~ the output of this unit is
normally in a hid state because its not input is low in the absence of a fault
signal. The output of the first unit 101 is connected to tha first input of a
second unit 103, and the not input of the latter unit is connected to the
output of a third unit 104 which normally has a low output state. The
output of the second unit 103, which normally is in a high state, is
CA 02365474 2002-O1-10
24 20LC01583A
connected via a line 105 to the first input of an AND logic unit 106. The
enable signal is coupled to the second input of the latter unit. As will be
e~cplained in more detail hereinafter, ao long a the control means 12D is
energized to close the contact 12C of the alternator field switch, the enable
signal is high; otherwise, the second input of the AND unit 106 is low.
Assuming the field switch contact 12C is dosed (its~normal :fate), the
normal output state of the AND logic unit 108 is high. The output of this
unit is coupled through an OR logic unit 10? to the first input of another
unit 108 whose not input is connected to a lice 109 on which there is
normally no high signal. Consequently. the output state of the logic unit
108 is normally high. The latter unit provides a command signal "OND" on
a line 110 which is connected to its output,. This has first and second
alternative states; its first state is high sad coiacidas with the normally
high output state of unit 108, whereas its second state is low and coincides
with a low output state of unit 108.
Between the output line 110 of the unit 108 sad the first output line 94 of
the logic means 83 there is en OR logic unit 111 having two inputs. The
first input of this unit is connected directly to tlu, line 110, whereby the
unit
111 is effective to produce a high signal oa the output line 94 (i.e., the
aforesaid ON state for turning on the GTO valve ? i) concurrently with a
high command signal OND on the line 110. The second input of the unit 111
is also connected to tha line 110 but through a first timer 112 labeled
"dwell"
in FIG. ?. The timer 112 is a conventional "one~ehot" time delay circuit, the
first output of which is normally low but wiU change to a temporary high
state as soy as the signal applied to the input of the circuit changes from
low to high, will then remain high for a fised interval of time even if the
input signal changes sooner from high to low, and will automatically
return to its noarmel low state at the end of such interval even if the input
signal remains high. The purpose of the dwell timer 112 is to ensure that
any turn-oa signal applied to the gate of the GTO valve ? 1 has at least a
desired minimum duration (e.g., 65 microseconds) is order to allow the
parallel capadtor ?3 to discharge fully each time the state of the GTO ~ a l v
a
?i is changed from off to on. Therefors this timer is suitably adjusted so
that the interval of its one-shot high output state equals such minimum
duration.
The output line lI0 of the unit 108 is also corrected to a not input of
another unit 114, and a second, normally high output of the first timer i 12
is connected to the first input of the latter unit. Consequently, the output
state of the unit 114 is the opposite of the state of the signal on the first
CA 02365474 2002-O1-10
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output line 94. Aa csa be seen in FIG. T, as output line 116 of the unit 114
is
oonaected to the second output line 96 of the logic means 9S by means of an
OR. logic unit 116 having two inpub, one input being connected through a
second timer 11? (labeled "min off ) to the line 115 and the other input being
connected via a line 118 to the output of yet another unit 119 tlas Srst input
of
which is conned~sd to the line 116 and the not input of which is connected to
a line 120 on which a high dgaal is provided when the GTO valve is on but
not when the OTO valve is off under normal conditions. So long as the
coaoasand signal OND on the Hae 110 is high, the dgnal on line ilb is low
and therefore t6s output state of the OIi logic unit 1i6 is low. Hut when the
command signal changes to a low state is r~apoase to either one of ~a high
signals at the inputs of the AND logic unit 106 being removed, aj would
occur if a high fault dual were applied to the not input of the first unit
101,
the signal oa output line 94 changes state from high to low and the signal
on line 115 goes high (but not before the dwell timer 119 has returned its
first and second outputs to their respectively low and high normal states).
The min-off timer 11T, which is essentially the same as the one-shot dwell
timer 112, responds to the low-to-high transition of the latter signal by
temporarily supplying a high signal to the OIi unit 116 which consequently
produces a high signal on the second output line 98 (i.e., the aforesaid OFF
state for turning o~ the GTO vslve ?1). Once produced in this manner, the
high signal on line 98 will remain high for a desired minimum interval
measured from the moment of time when the signal on line 115 changed
from low to high states. 'The timer 11? is suitably adjusted to ensure that
the duration of any turnoff signal applied to the gate of the valve 71 will,
equal or exceed the sum of the aforesaid turnoff and tail times of this valve.
The fins 116 is also connected through a third timer 12Z (labeled "ofF
pulse") to the lime 109. Ths off-pulse timer 1?.9 is siaaisar to the dwell
timer
119, and it is ~erative to prevent the command signal OND, after changing
from high to low states, from resuming its high state for a predetermined
period (e.g., approsimately ooe second) in order to delay the nett GTO turn-
on signal. Tbia delay period serves two purposes. It ensures that the GTO
valve 91 whey turned off can not be returned to its on state before certain
system r~pome functions are completed, as will soon be described with
reference to FI(I. 8. It also allows the snubba~ capscitor ?2 to discharge,
through the parallel varistor 93, to the relatively low level (e.g., approx-
imately 60 volts) of the output voltage of the rectifier bridge 6~ before the
valve ?i is retmrned to an on state. Note that the capacitor 72 is charged to
a
high vdtage (e.g., 1600 volts or more) as alternator excitation current IF is
CA 02365474 2002-O1-10
26
20LC01583A
commutated from the GTO valve ?1 to the impedance means 72, 73 during
the turn off process of this valve (see the earlier description of FIG. 5).
Because of the off pulse delay, however, there is ample time for the
capacitor to discharge such high voltage before the nezt GTO turn-on signal
is produced, and the relatively small residual charge will not cause
damage or untoward switching losses when the valve ie again turned on.
Consequently, the GTO snubber circuit is aimpli8ed and its cast and size
are minimized by omitting a conventional current limiting resistor
(shunted by a diode poled to conduct charging currant) is series with the
capacitor ?2.
In operation, the off pulse timer 122 sha~wn in FIG. T has a normally
low output, but sa soon as the signal on line 118 ch8agea from low to high,
its output changes to a high state and thereafter automatically returns to
normal at the end of the predetermined delay period. This temporary high
output is coupled by the line 109 to the not input of the unit 108, and
consequently the output state of the latter unit remains low concurrently
with the high output of the timer 122. It is also coupled by the line 109 to
the
first input of the third unit 104, the not input of which is connected to the
line 105 and has the same state as the output of the second unit 103. It will
be apparent that normally the output state of the unit 104 is low and the
signal on the line 105 is the same as the high signal being applied to the
first input of the unit 103, but once a high fault signal is applied to the
not
input of the first unit 101, the signal on the line 105 changes from high to
low states and remains in a low state so long as the output of the off pulse
timer 122 is high.
The line 108 is oo~cted to the first input of another unit 124. The not
input of the unit 124 is connected to a line 126 on which normally there is no
high signal, and therefore the output of this unit is normally in a high
state. The latter output serves as the aforesaid status signal which is
provided on the output line 81 of the GTO control means 80. It is high
whenever the status of the GTO valve 71 is "good" (i.e., when the GTO valve
is turned on and its gate is not shorted). But as soon as a fault signal is
supplied to the logic means 93, the signal on the line 108 and consequently
the output of the wait 124 will change from high to low, thereby indicating a
"bad" status (i.s., the GTO valve is ofd. Bad states is maintained by the
above-described unit 104 for the period that the output of the off pulse timer
122 is high.
As will soon become apparent (see the description of FIG. 8), the
controller 26 includes means responsive to a high-to-low change of the
CA 02365474 2002-O1-10
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27
status signal for immediately de-energizing the alternator field switch
control means 12D, thereby causing the switch cont$ct 12C to open and the
enable signal supplied to the logic means 93 to change from high to low
states. Subsequently the control means 12D is re-energized, thereby
returning the contact 12C to its normal, closed position and restoring the
high state of the enable signal. Throughout the interval that the enable
signal is low, the output of the AND logic unit 106 is law, the command
signal OND on the line 110 remains low, and no GTO turn-on signal can be
produced. But as soon as the high state of the enable signal is restored
(assuming that the high fault signal was previously removed and that . the
output of the off pulse timer 122 has returned to its. normal, low state), the
output of unit 106 changes from low to high, the command signal OND
returns to its high state, the signal on line 115 goes low, the signal on
output line 96 changes states from high to low thereby terminating the GTO
turnoff signal, and concurrently the signal on output line 94 changes from
low to high theseby causing the GTO gating means 85 to apply a turn-on
signal to the gate of the GTO valve ?1.
In the event the GTO valve ?1 has a shorted gate, this device will fail to
turn off when a turnoff signal is applied to its gate. The logic means 93
includes means for detecting this abnormal event and for protecting the
GTO cantrnl means 80 from resulting damage. The latter means preferably
incorporates the shorted gate detection and protection features that are
disclosed and claimed in copending U.S. patent application (20-LC-1571)
Hled concurrently herewith for R. B. Bailey and H. J. Brown and assigned
to General Electric Company. As is. illustrated in FIG. ?, it comprises the
min-off timer 11?, the logic unit 119, end suitable means 12? for comparing
the feedback value on line 99 (i.e., the actual volts across the turnoff
switch
89 in FIG. 8) ~rith a predetermined, relatively small reference magnitude .
K4 (e.g., appro»nately 0.9 volt). K4 is selected to be equal to the voltage
developed across the turnoff switch 89 whenever this switch is in its
conducting state and negative current in the gate of the GTO valve 71 equals
a certain high threshold magnitude (e.g., approximately 100 amps) that is
normally e:perieneed only during the turnoff and tail times of a successful
turn off process of this valve, and it is much less than the volts across the
switch 89 when biased to its non-conducting state. The comparing means
12? has a high output state so long as the feedback value on line 99 is
greater than H4, which is true if either the turnoff switch 89 is turned on
and conducting high current or both the turnoff switch and the GTO valve
are turned off but is not true if the turnoff switch 89 is turned on and not
CA 02365474 2002-O1-10
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2g
conducting appreciable current. In the latter event, the comparing means
output has a low state. The output state of the comparing means 127 is
conveyed by means of the line 120 to the not input of the logic unit 119. So
long es this input ie low, the output of the latter unit (i.e., the signal on
line
118) will track the high and low states of the signal on the line 1I5;
otherwise the signal on line 118 is low.
In operation, as soon as the signal oa line 115 changes from low to
high in response to a high fault signal being supplied to the logic means 93,
the output of t66 min-off timer 117 changes from ib normal low state to a
high state which subsists until the end of the aforementioned minimum
interval. This temporary high output is coupled by the OR logic unit 116 to
the second output line 98 of the logic means 93, thereby turning on the
turnoff switch 89 which conducts the negative gate current required to turn
off the GTO valve 71 as previously described. When the switch 89 initially
turns on, the voltage across it collapses to a negligible magnitude and the
signal on the output line 120 of the comparing means 12? changes from
high to low. Concurrently, the unit 119 causes the signal on line 118 to
change from low to high. Thereafter the negative GTO gate current in the
switch 89 rapidly increases and soon a:needs the, threshold magnitude at
which the feedback value on line 99 equals K4, whereupon the signal on line
120 returns to its former high state and the signal on line 118 returns to its
former low state. During a narmal GTO turnotl' process, negative gate
turnoff current subsides from a high peak value to a trivial magnitude, and
the volts across the turnoff switch 88 will decrease to less than K4 before
the
end of the tail time of the valve 71. As such voltage decreases below K4, the
signal on line 120 again changes to its low state and the the unit 119
concurrently changes the signal oa line 118 to its high state. The latter
signal is coupled by the OR. logic unit 116 to the second output line 96,
thereby sustaining the OFF state of the output signal on this line after the
output of the mia-off timer 117 automatically ravarts to its low state at the
sad of the minimum interval.
However, if the gate of the GTO valve ?1 were shorted, negative gate
current would not subside during the attempts turn-off process. In this
event, the volt across the switch 89 would nat dea~ease below K4, the signal
oe line 118 would remain low, and the signal on ontpnt line 96 would return
to a low state as soon as the output of the mia-off timer 117 returns to i is -
normal, low state at the end of the minimuia turnot~' interval. The
resulting low signal on the output line 96 will bias the switch 89 to its off
state, thereby removing the low-resistance patb that this switch would
CA 02365474 2002-O1-10
29
20LC01583A
otherwise provide, if it were not turned off, fronn the gate of the GTO valve
to
the -13 volts -control voltage terminal (see FIG. 8). Interrupting negative
current in the shorted gate of the valve ?1 will protect the switch 89 from
damage caused by continu;ng to conduct high current aRer the minimum
turnoff internal expires. To ensure proper operation of the shorted gate
detector, the minimum turnoff' interval is approximately 80 microseconds.
The detection of a shorted gate also causes a low-to-high state change
of the signal oa the line 135 of the logic means 93. As is shown in FIG. 7,
the latter signal is provided by a logic unit 128 whose Srst input is
connected
to the line 115 and whose not input is oonaectsd to the lice 118. The signal
on line ll8 would be low if a shorted GTO gate were detected after the above-
described low~to-high state change oa line 118. Ia this abnormal event, the
signal on line 125 is high gad the output state of the unit 1?4 must be low,
whereby a bad status will continue to be indicated after aspiration of the
delay period provided by the previously described off pulse timer 122.
The logic means 99 includes additional means for preventing the
command signal OND on the line 110 from changing from high to low
states if the magnitude of alternator escitatioa current IF exceeds a
predetermined abnormally high level (e.g., 1200 amps) which is higher
than the ma~dmum ezcitatioa current typically attained in response to a
Qashover, thereby inhibiting the production of a GTO turnoff signal in the
event IF is greater than that level. Preferably, as is shoarn in FIG. 7, the
alternator excitation current feedback signal is supplied to a first input of
a
comparator 130. The other input of this comparator is connected to suitable
means 131 for deriving a Dins signal of predetermined constant magnitude
I~5 corresponding to the aforesaid high level of IF which ie well above (e.g.,
2.5 times higher thaw) the highut magidtude of IF during normal full-load
operation of the propulsion system, but not above the magnitude of IF
observed when a Qashover occurs. The output of the comparator 130 is
normally high but switches to a low state if the value of the excitation
current feedback signal exo~ds Kb. This output is conveyed via the line 102
to the first input of the first unit 101, and it is also conveyed to a not
input of
yet another unit 193 whose &rat input is connected to the line 110. The
output of the unit 133 is e~oupled to one of the two inputs ~ the OR logic
unit
10?. So long as IF has not attained the predetermined high level, the logic
mmuns 93 operate as previously explained. But if IF' exceeds this level, the
output of compasatar 180 changes from high to low state, the signal on the
line 105 is changed from high to low thereby causing a corresponding
change of the status signal, and the logic unit 132 produces a high output
CA 02365474 2002-O1-10
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which overrides the AND logic unit 106 and "seals in" the high state of the
command signal OND oa line 110. So long ae OND is high, no GTO turnoff
signal will be produced by the GTO gating means 85. This feature of the
invention serves two useful purposes. If a diode in the power rectifier
5 bridge 13 fails to block reveres current, the resulting abort circuit at the
output of the main alternator 12 will cause IF to exceed Ii;5. In this event
it
is desirable to let the alternator output current rise to a magnitude high
enough to blow the protective fuse associated with the failed diode so as to
isolate the failed diode from the source of propulsion power. To avoid
10 interfering with this desired response, the additional means 130-32
prevents turn off of the GTO valve T1. The additional means also enables
the commutation ability of the GTO valve (i.e., the masimum current that
can be successfully turned o0' by this valve) to be less than the highest
possible magnitude of IF, whereby the cost and size of this valve are
15 minimized.
Having described the presently preferred embodiment of the alternator
excitation current source 1? as it is shows in FIGS. 5-7, the remainder of
the improved Qashover protection means will now be described with
reference again to FIG. 3. The statue signal oa the output line 81 of the
20 source 1~ is coupled via the data link 21 to the controller 28. As soon as
the
normally low signal on the output line 33 of the Daahover detecting means
32 goes high due to a Qashover oaurring on the commutator of one or more
of the traction motors 15, 16, etc., the GTO control means 80 in the
excitation curnnt source 17 simultaneously applies a turnoff signal to the
25 GTO valve ?1 and removes the normally high status signal on the line 81.
This high-to-low change of the status signal initiates two functions in the
controller. The first function, represented in FIG. 3 by a block 140 which is
supplied with the family of motor armature current feedback signals I1, I2,
etc., identi5es any traction motor in which the magnitude of armature
30 current a:seeds a predetermined high threshold whenever the status
signal changes from high to low. The latter threshold (e.g., approximately
2,000 amps) is greater than the ma~aimum magnitude of armature current
under all normal conditions. The identifying function 140 is suitably
programmed to read the asagnitudes of the current feedback signals, to
compare each one with a value ~rresponding to the aforesaid threshold,
and to store the identification number ("gX") of nay traction motors) whose
current is higher than such threshold. Motor ~X is presumed to be
experiencing a flashover. The identification of the faulted motor is
available on an output line 141 of the block 140.
CA 02365474 2002-O1-10
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31
The other function initiated by a status change is represented in FIG. 3
by a block 142 labeled "system response function." It is suitably arranged to
command the following actions in immediate response to any high-to-low
change of the status signal on line 81: the speed call signal for the engine
governor system 2b is changed to its idle value; the power reference value in
the excitation control means of the controller 28 is reset to zero, thereby
temporarily imposing a value corresponding to IF=0 on the control signal
VC; a flashover message is entered in the display module 30, and the
identification of the faulted motors) is logged; an "open" command is
transmitted via a fins 143 to the Held switch control manna 12D so as to de-
energiae 12D which enables the operating mechanism of the contact 12C to
move this contact from its normal, closed position to its alternative, open
position; contactor opening commands are issued for all of the motor
contacto» 15C,16C, etc.; each of these opening commands is transmitted to
the operating means 29 of the motor contacto» as soon as armature
current in the corresponding motor lus decreased to a p»deteranined level
that can be safely interrupted by the associated oontactor without untoward
arcing or welding (but no later than five seconds after the opening
commands are issued); and a "flash timer" is activated. As a result of
these actions, the firing signals for the controlled rectifier bridge 64 in
the
excitation source 1~ are retarded so that the output voltage of this bridge is
soon reduced to zero, the Beld switch contact 12C in the a:citation current
path is opened (although the alternator field 12F may continue to be excited
by residual current circulating through the resistor 68 and the breakover
device 69), sad all of the traction moto» are disconnected from the d-c bus
14 of the propulsion system. ~fthenever a flashover is detected, the quick
response of the solid-state controllable valve 71 in the excitation current
path will cause the alternator output current to dec»sae very rapidly from
its initial surge as previously explained. Consequently, the respective
motor currents decrease rapidly, and the time delay between issuing and
transmitting contactor opening commands is relatively short. Note that
when opening commands are received by the control means 12D and the
operating means 29, respectively, the contact tile of the field switch and
motor oontacto» will not separate immediately due to inherent time delays
(e.g., approzimately 100 milliseconds) is the operation of these electro-
mechanical devices. By the time the contactor in series with the faulted
motor is open, the flashover is extinguished and the fault signal on the
output line 33 of the flashover detector 32 is removed. The aforesaid enable
signal, which is supplied via the line 82 from the field switch control means
CA 02365474 2002-O1-10
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32
12D to the control means 80 in the excitation current source 17, will have a
low state so Iong as the field switch contact 12C is open.
After the actions described in the preceding paragraph are completed,
the system response function 142 will coa~anand several additional actions:
contactor closing commands are transmitted to the operating means 29 of
the motor contacto» so as to reclose all of the contactora 15C, 16C, etc.
except the ousts) assoaated with the faulted traction motors) (i.e., motor
#X) as identified by the previously described function 140; a "close"
command is transmitted via the line 148 to the field switch control means
12D so as to energise 12D and thereby cause it to return the contact 12C to
its
closed position; and the engine speed call signal is pernritted to return to a
value determined by the position of the throttle 89. As soon as the control
means 12D receives the close signal on line 143, the enable signal on line 82
changes from low to high states, and the GTO control means 80 in the
excitation source 1? automatically responds to such state change by
producing a GTO turn-on signal that causes the GTO valve 71 to return to a
conducting state as previously dascribed. However, this turn-on signal
cannot be produced sooner than approximately one second (the delay period
provided by the off pulse timer 122 in the logic aneans 93) after the
preceding
GTO turnoff signal was initiated by a fault signal from the flashover
detector 32. The high state of the status signal on line 81 is automatically
restored upon the expiration of this one-second delay period (assuming the
flashover detector 32 is not producing a fault signal at that time). As a
result of removing the idle value restriction on the speed call signal,
recloaing the Held switch contact 12C, and turning on the GTO valve ? 1, the
alternator a:citation current will ramp up to a desired steady-state
magnitude, and the electric power that the main alternator 12 now
reapplies to the unfaulted traction moto» will increase smoothly to
whatever level is determined by the throttle position. After a delay
determined by the flashover timer in the system response function 142, the
operating means 28 is permitted to reclose the contactor associated with
motor #X, such »closure actually occurring the nest time the throttle
handle is moved through its idle position. If the locomotive speed is
relatively high (e.g.. 60 miles per hour or more) when a ffashover occurs, as
is usually true, the delay time is so computed as to obtain a certain number
of commutator revolutions, whereby the flashed commutator will be
cleaned by the brushes riding over its surface as the rotor of the de-
energized motos #X continues to be turned by the locomotive axle to which it
CA 02365474 2002-O1-10
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33
is coupled. At lower speeds the delay time bas a predetermined maudmum
length (e.g., 15 minutes).
Although the system response function could be implemented in a
variety of ditl'ereat ways to obtain the results summarized above, the
presently preferred way is to program the controller 26 to execute the
routine that is illustrated in FIG. 8. This routine is repeated once every ten
milliseconds. It begins with an inquiry 151 to determine whether or not the
statue signal oa line 81 has changed from high to low. If not, the routine
proceeds to a second inqu~y 158 to determine whether or not a flash timer
is active. If the saswer is a~rmative, the nest and Bmal step 153 in this
routine is to decrement the flash timer by one. Otherwise the routine
proceeds from inquiry 188 to the step 168 by way of an additional step 154
that removes any constraint that may be preventing radosure of the motor
contactor associated with a previously faulted traction motor ttX. After
such constraint has been removed, such contactor can be reclosed by the
operating means 29 whenever commanded by the controller 26.
If the answer to the first inquiry 151 crate aI$rmative, the FIG. 8
routine would proceed from this inquiry to the 6na1 step 153 by way of a
series of steps 180-68 that will now be described. In step 160 a flashover
counter is incremented by one. The asst step 161 is to change the engine
speed call signal to its idle value, to reset the power reference value to
zero,
to initialize other variables in the ezcitation control, and to issue opening
commands for the field switch control means 12D and the motor contactor
operating means 29. (Note that the relevant time constants of the engine
fuel system, the alternator field excitation circuit, and their respective
controls are .such that the alternator output power responds relatively
slovrly to the ezecntion of step 161, too slowly to be relied on to keep the
initial
surge of current in the faulted motor from attaining a potentially damaging
magnitude.) Step 161 ie followed by step 188 in which the identity of the
faulted traction motors) is fetched from the fi:action 140 (FIG. 3) and then
entered in tha display module 30. This same information is used in step 163
to impose a reclosing constraint on the contactor(a) associated with such
motor(s).
In the nest step 184, the FIG. 8 routiat ooanputes a cerrtaia initial count
corresponding to a time delay that is the lessee of 15 minutes or of 900
divided by the actual locomotive speed in units of miles per hour. Then in
strap 165 the flash timer is activated by loading a register of the micro
computer with the initial count found in the previous step. The flash timer
remains active only so long as the count in this register does not reach zero.
CA 02365474 2002-O1-10
20LC01~83A
34
The initial couat is su~ciently large so that the oonat stored in the
register.
when decremented at the rate of 100 per second, will reach zero upon
ezpiration of the aforesaid maximum length of time (e.g., 15 minutes) or
sooner if the locomotive speed was greater than 60 mph when the initial
count was com~putad. The cart step 166 is esecutsd as soon as position
sensors in the coatactos operating means 29 indicate that all of the motor
contactors ibC, 16C, etc. haw opened in respoase to the opening commands
issued is step 181. It removes the idle valve roan from the speed call
signsl, iasues a' closing command to the field switch ooatrol mesas 12D,
and issues commaads to the operatiag aneans 29' for closing all of the
contactoss iSC, 16C, etc. a:cept the ones) a~aodated with , the faulted
motors) gX whose reclosiag is prevented so long as the constraiat imposed
in step 163 is active. The last-mentioned constraint is active uatil removed
by the esacution of step 154.
While a preferred embodimeat of the inv~tion has been shown and
described by way of e:mnple, many modifications will uadoubtedly occur to
persona skilled in the art. For example, the conventional field switch 12C,
12D could be omitted and the valve 71 could be suitably controlled to perfona
all of its usual functions. In addition, the thyriator bridge 64 in the
alternator a:citation current source 1T could be replaced with a diode
rectifier bridge, in which case the tiTO valve T1 would be controlled to
operate normally as a switching regulator element ao as to regulate the
average magnitude of alternator held current as desired. The concluding
claims are therefore intended to cover all such modifications that fall
within the tsue spirit cad scope of the invention.
35
