MXPA96001938A

MXPA96001938A - Automatic overvoltage protection for unalternator in a delocomot propulsion system

Info

Publication number: MXPA96001938A
Application number: MXPA/A/1996/001938A
Authority: MX
Inventors: Robert Cooke Philip; Andrew Laukaitis Joseph
Original assignee: General Electric Company
Priority date: 1994-09-23
Filing date: 1995-09-19
Publication date: 1998-01-01

Abstract

The present invention relates to a short-circuited diode protection system for a vehicle propulsion system that includes at least one electric traction motor, a synchronous generator having an armature and coils and field coils, a controllable current source of excitation connected to the field coils, and electric power conditioning means interconnecting the coils of the armature with the traction motor, the energy conditioning means including a bridge rectifier circuit to convert the alternating current from the coils of the armature to direct current on a pair of busbars of relatively positive and negative output, the system comprising: sensor means for producing a gate signal in response to a reflected alternating current voltage appearing on field coils in excess of a quantity previously selected, at least one electric control valve solid state connected in a parallel circuit configuration with the field coils, this valve being normally non-conductive and which is switchable to a conducting state by said gate signal applied to a gate terminal thereof, and coupling means the gate signal to the valve to limit the voltage on the cam coil

Description

AUTOMATIC OVERVOLTAGE PROTECTION FOR AN ALTERNATOR IN A LOCOMOTIVE PROPULSION SYSTEM BACKGROUND OF THE INVENTION The present invention relates generally to electric propulsion systems for traction vehicles (such as diesel-electric locomotives) equipped with either direct current or alternating current traction motors, and relates more particularly to improved elements to protect this system from serious damage in the case of an overvoltage reflected on a field coil of a synchronous generator as a result of a cut-off diode in an energy rectifier circuit coupled with a generator output. In a modern diesel-electric locomotive, a primary thermal engine (typically a 16-cylinder turbocharged diesel engine (to drive an electrical transmission comprising a synchronous generator that supplies electrical power to a plurality of electric traction motors whose rotors is used is used. they are coupled in an impulsive manner through speed reducing gears with the respective axle-wheel series of the locomotive.The generator typically comprises a three-phase main traction alternator, whose rotor is mechanically coupled with the engine output shaft When an excitation current is supplied to the field coils of the rotating rotor, alternating voltages are generated in the coils of the three-phase armature on the stator of the alternator.These voltages are rectified and applied to the armature and / or to the armatures. Field coils of direct current traction motors, or inverted to current and apply to AC motors. In normal motorized operation, the propulsion system of a diesel-electric locomotive is controlled in such a way that a balanced condition of continuous state is established where the alternator driven by the engine produces, for each discrete position of an accelerator handle , a substantially constant optimum amount of electrical energy for the traction motors. In practice, suitable elements are provided to cancel the normal operation of the propulsion controls and reduce the load of the engine in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capacity of the wheel. engine at any engine speed that is driving the accelerator. This response, generally referred to as deaeration, reduces the traction power, thus helping the locomotive to recover from these temporary conditions, and / or preventing serious damage to the engine. In addition, the propulsion control system conventionally includes elements to limit or reduce the output voltage of the alternator as necessary to prevent the magnitude of this voltage and the magnitude of the load current from exceeding maximum safety levels or limits, respectively. previously determined. The current limit is effective when the locomotive is accelerating from rest. At locomotive low speeds, the traction motor armatures are spinning slowly, so that their return EMF is low. A low alternator voltage can now produce a maximum motor current, which in turn produces the high tensile stress required for acceleration. On the other hand, the voltage magnitude of the alternator must be kept constant and at its maximum level as long as the speed of the locomotive is high. At high speeds, the traction motor armatures are spinning rapidly and have a high EMF back, and then the alternator voltage must be high to produce the required load current. In an electric propulsion system, all power components (alternator, rectifier, traction motors, and their contacts and interconnecting cables) need to be well insulated to avoid dangerous short circuits between the electrically energized parts of these components and the ground. The insulation has to withstand very difficult conditions in a locomotive, including constant vibration, frequent mechanical shocks, infrequent maintenance, occasional electrical overloads, a wide range of ambient temperatures, and an atmosphere that can be very humid and / or dry. If the insulation of a component is damaged, or if its dielectric strength deteriorates, or if moisture or an accumulation of dirt provides a relatively low resistance path through, or above, the insulation surface, then a current can flow Undesirably high leakage between the component and the structure of the locomotive that is on the ground potential. This breakdown of the insulation may be accompanied by ionization discharges or disruptive discharges. The discharge will start before the voltage level reaches its final cutoff value. The dirtier and wetter the insulation, the lower the initial discharge voltage will be in relation to the actual cutoff value. Without proper detection and timely protection, there is a real danger that an initially harmless electric shock will soon grow or spread to a degree that causes serious or irreparable damage to the insulation system and possibly to the equipment itself. It is a conventional practice to provide ground short circuit protection for locomotive propulsion systems. These protection systems typically respond to the detection of ground leakage current by overriding normal propulsion controls and by reducing tensile power if and when the magnitude of this current exceeds a permissible limit that depends on the magnitude of the current. motor current. See U.S. Patent Number 4,608,619 and Canadian Patent Number 1,266,117. These systems have not been completely successful in preventing harmful disruptive discharges on the switches of traction motors. In direct current traction motors, carbon brushes are used that rub the commutator bars to provide current to the coils of the motor armature. This current establishes a magnetic field in the armor and corresponding magnetic poles. The magnetic poles created in the armature interact with the magnetic poles of the motor field coils to produce a torque in the machine. The magnetic poles of the field coils of the motor are established by means of the direct current flowing through these coils. The motor includes a plurality of specially spaced commutator bars around one end of the armature, each of the commutator bars connecting to select coils of the armature to establish the magnetic poles. As the adjacent commutator bars pass periodically below the carbon brushes, the coils of the armature connected thereto momentarily short circuit. Since the coils associated with the short circuit switch rods move from one to the other, they will be passing through the magnetic flux fields created by the magnetic poles of the field coils that are of different magnitudes. In accordance with the above, there will be a potential difference between the two switch bars. In the design of an ideal machine, the brushes are located between the field poles at a point where the flux created by the field poles passes through zero in its inversion between the adjacent poles of the opposite magnetic polarity. This ideal point changes with changes in armature current, since the total flux is the sum of the field flux and the flux of the armature. Typically, a switching or interpole pole is placed between the adjacent field poles, each switching pole having a coil which is connected in series in the current path of the armature, such that the flow generated by the switching pole is proportional to the current of the armor. This method generally serves to minimize changes in the interpol flow, thus allowing the brush to transfer current between the commutator bars without an undue amount of electric arc. For motors that are subject to heavy overloads, rapidly changing loads, operation with weak main fields, defective brushes, brush bounce, or coarse switches, there is a possibility that the switching pole action is insufficient, and a simple spark in the brushes it can become a larger arc. For example, at the instant when an armature coil is located at the peak of a highly distorted flow wave, the coil voltage may be high enough to cut the air between the adjacent commutator bars to which the coil is connected. coil, and result in a disruptive discharge, or arcing, between these bars. The arcing between the segments of the commutator can quickly bridge the adjacent brush clamps, or it can extend to the earth discharge ring that normally surrounds the commutator of a direct current traction motor, thereby short-circuiting the output lines of the traction alternator. Although these disruptive discharges are relatively rare, if one occurs, it will usually happen when the locomotive is traveling at a high speed. Many different systems are described in the prior art pertinent to automatically detect and recover from the disruptive discharge conditions. See, for example, U.S. Patent Number 4,112,475 to Stitt and Illiamson. To minimize or avoid serious damage to the traction motor and the associated parts of the propulsion system when a disruptive discharge occurs, it is desirable to extinguish the spark gap before the current being supplied to the failed motor has time to reach its maximum short circuit magnitude available. By very rapidly reducing or stopping this current as soon as the flashover can be detected, the amount of electrical energy in the failed motor circuit will be kept low enough to prevent permanent damage to the commutator bars, to the fasteners brush, and the discharge ring. This desired high-speed spark-gap protection can not be obtained by opening the electrical contact that connects the failed motor with the rectified output of the alternator, because the opening action of a conventional contact is too slow, and for the time when the tips of the contact begin to separate, the magnitude of the short circuit current could be so high that it will cause an undesirable arc or welding of these points. The deaeration function of the propulsion controls can not be relied upon to reduce the initial surge of current supplied by the traction alternator to the failed motor, because the relevant time constants of the controls and the field excitation circuit of the alternator introduce a finite delay between the presentation of a disruptive discharge and the response of the alternator. Although AC motors do not present the problem of direct current motors, the power system for AC motors may exhibit a condition, commonly referred to as a "trip through", which has the same characteristics. harmful from a disruptive discharge. In a typical alternating current traction motor system, the power output of the traction alternator is supplied to a rectifier circuit that converts the alternating current output of the alternator to direct current. This direct current energy is then inverted by a solid-state inverter, in a frequency-controlled AC power to be applied to the AC motor. The speed of the AC motor is controlled by the frequency of the supplied AC power. The inverter is conventionally configured to provide three-phase AC power, and includes a plurality of controllable rectifiers, such as silicon controlled rectifiers (SCR) or gate deactivation thyristors (GTO). Each phase has at least two of these devices connected in series between the relatively positive and relatively negative direct current energy bus bars extending from the rectifier circuit. During the motorization operation, one of the devices of a phase is always deactivated, while the other device is driving. If both devices were driving simultaneously, the devices would short-circuit through the output busbars of the rectifier. This condition is referred to as a tripped shot, and can result in currents that are of the same magnitude as those that occur during a disruptive discharge. Several short circuits can contribute to a tripping condition traversed. For example, a device may simply fail to switch to off before another device begins to drive. Most commonly, a device initially fails to a short circuit condition, and the second device in series with it enters the conduction, resulting in a short circuit between the direct current energy bus bars. As with the disruptive discharge fault, the propulsion system deaeration function can not respond fast enough to prevent damage to the power system. U.S. Patent Nos. 5,168,416, and 5,245,495, describe a form of spark arresting protection circuit for a direct current electric traction motor utilizing a series of connected solid state switching devices to disconnect the coil from field of the alternator of its energy source when detecting a surge of high current characteristic of a disruptive discharge. A drawback of this system is that the series switching device, for example, a gate deactivator, must be sized to carry the field current of the alternator during the normal operation of the system, in addition, the serial device requires cooling with forced air to prevent overheating and its voltage level is high due to the continuous current it must carry. As described above, the three-phase synchronous generator of a locomotive propulsion system develops an output voltage that is a function of the rotations per minute of its rotor's arrow and direct current voltage and the current applied to its coils of field The three-phase output is converted into direct current energy by a three-phase complete bridge rectifier connected to the coils of the generator armature. This rectifier contains fuses that function as protective devices to protect the alternator from overvoltages caused by the short circuit of a device in the rectifier. The devices are typically solid state diodes and fail to a "short circuit" condition. In a direct current electric traction motor system, the direct current energy is directly coupled to the traction motors. In an AC motor system, direct current energy is applied to an inverter and inverted to a frequency controlled power. Both alternating current and direct current locomotives require protection for rectifier short circuit faults, and this protection has usually been provided by fuses. "Fuses are often a maintenance problem, as they last only about 3.27 years in the most severe conditions of the locomotive (for example, pulling coal up a steep slope, - that is, low speed, maximum power, and higher rectifier output currents.) When a fuse blows, the locomotive has to operate at reduced power or at no power (depending on whether it is a direct current or alternating current locomotive). In accordance with the above, it is desirable to provide a protection system that does not use fuses.

SUMMARY OF THE INVENTION A general objective of the present invention is to provide better protection for a locomotive propulsion system that experiences a failed rectifier and an output circuit of an energy alternator.

A more specific objective of the present invention is to provide a cut-off diode protection system, which protects an alternator or a synchronous generator from a locomotive propulsion system, from an overvoltage condition caused by the alternating current voltages reflected on the field coil of the generator as a result of a diode cut off in the output rectifier connected to the generator armature. In one form, the cut-off diode protection system of the present invention is described in an application for a propulsion system of a traction vehicle, including at least one electric traction motor, a synchronous generator having armature and field coils , a controllable source of excitation current connected to the field coils, and electric power conditioning elements that interconnect the coils of the armature with the traction motor. The power conditioning elements include a bridge rectifier circuit that converts the alternating current from the coils of the armature into direct current on a pair of relatively positive and negative output busbars. The protection system includes detection elements to produce a false signal in response to a reflected alternating current voltage that appears on the field coils in excess of a previously selected quantity, the magnitude being selected to limit the voltage reflected on the field coils. of the alternator to a sufficiently low value to prevent damage to the coils due to insulation cut-off. The system further includes at least one controllable solid-state electric valve connected in a parallel circuit configuration with the field coils, the valve being normally non-conductive, and being able to be switched to a conducting state by a gate signal applied to a terminal of gate. An element that responds to false signals supplies the gate signal to the valve to limit the voltage on the field coil by entering the valve to the line when the alternating current voltage reflected to the field coil exceeds the amount previously selected The preferably cut diode protection system includes a second controllable solid state electrical valve connected in parallel with the at least one first valve, and configured in such a way that the valves are connected in a reverse conduction relationship. In this way, the system can detect either a positive voltage or a negative voltage on the field coil, and energize an appropriate valve of the valves to short circuit the voltage regardless of polarity. Preferably, each of the valves comprises a silicon control rectifier or SCR. The system also includes a thyristor connected in parallel with the field coil to dissipate the transient voltages that are presented on the field coil that are smaller than the previously selected quantity. In one embodiment, the sensing element comprises a solid state rectifier circuit coupled with the field coil and adapted to produce a direct current voltage output proportional to the alternating current voltage component on the field coils, due to the short circuit of the rectifier on the output of the generator. A trigger circuit produces a trigger pulse when the direct current voltage exceeds a previously selected value, and the trigger pulse is applied to a silicon control rectifier gate signal generator circuit to produce gate signals for the duration of time of the trigger impulse. The trigger circuit may comprise a monostable multivibrator connected to a comparator, the comparator having input terminals connected to the output of the rectifier circuit of the field coil. The multivibrator generates a trigger pulse having a predetermined duration of time, such as, for example, one second. The pulse output of the multivibrator is then applied to the gate pulse circuit of the silicon control rectifier to cause a gate pulse generation towards the 16 silicon control rectifiers for the duration of the pulse output from the multivibrator The excitation current source for the field coils of the alternator includes a current detector (IF) that operates a current limit protection built into the system controller, and causes a return phase of the trigger signals of the gate to the silicon control rectifier in the excitation current source, to reduce the current supplied to the short circuit formed by the silicon control rectifiers through the field coil. This ensures that the excitation current source is not damaged during the time that the silicon control rectifiers are coupled in parallel with the field coil to reduce the voltage applied to the coil. In still another embodiment, the cut-off diode protection system is incorporated into a circuit with a short-circuit current detection system that detects an overcurrent condition caused by a trip through an inverter system in an alternating current locomotive, or a Disruptive discharge of a direct current motor in a direct current energized traction vehicle. The short circuit current system includes an element connected in circuit with the field coil of the synchronous generator to switch the controllable source of excitation current on when a short circuit current condition is detected. Subsequently, elements in circuit are selectively connected to the field coil to dissipate the energy in the field coil subsequent to the commutation of the series of the excitation source. The system includes an element to inhibit the operation of the element that responds to the short circuit signal when detecting the short circuit current, in such a way that the silicon control rectifiers of the cut diode protection system do not enter into conduction during a period of time when the short-circuit current in the field coil is dissipating. The invention further includes the method for protecting a synchronous generator from a reflected overvoltage condition, by coupling a rectifier circuit through the field coil to develop an output voltage representing an alternating current voltage on the field coil , which detects when the AC voltage exceeds a previously selected maximum amount in the generation of a false signal in response to the same, and the short-circuit of the field coil in response to the false signal. The method also includes the step of making a short circuit in the field coil by connecting a solid state controllable electrical switch in parallel with the field coil, and putting the switch to the conduction for a predetermined duration of time . In the manner in which the switch comprises a pair of inversely connected silicon control rectifiers, the gate gate step preferably comprises applying gate pulses to the gate terminals of the silicon control rectifiers for the predetermined duration of time. of the short circuit signal. The method of the invention further includes the steps of supervising the coils of the armature by an overcurrent condition, connecting the switching element through the field coils in response to a supervised overcurrent., and inhibit the application of the gate impulses to the silicon control rectifiers during the overcurrent condition.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, reference can be made to the following detailed description taken in conjunction with the accompanying drawings, in which: Figure 1 is a block diagram of an electric propulsion system for a locomotive, which includes a thermal primary engine (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.

Figure 2A is a schematic diagram of one of the direct current pull motors represented by simple blocks in Figure 1. Figure 2B is a family of load saturation curves of a typical synchronous generator, showing the relationship between the Output voltage and current for different magnitudes of excitation current. Figure 3 is an expanded block diagram of certain parts of the controller, cooperating with the excitation source of the generator to implement the present invention. Figure 4 is a schematic circuit diagram of the spark gap detection element shown as a single block in Figure 3. Figure 5 is an expanded diagram of the generator excitation current source shown as a single block in the Figures 1 and 3 for a direct current electric traction motor. Figure 5A is an expanded diagram of the excitation current source of the generator of Figure 1 and 3 for an electric AC traction motor. Figure 6 is an expanded block diagram of the silicon control rectifier control of Figure 5. Figure 7 illustrates a typical voltage waveform on the field coil of the generator during the operation of the current protection system. short circuit. Figure 8 is a simplified illustration of a propulsion system for a locomotive driven by a direct current electric motor. Figure 9 is a simplified illustration of a propulsion system for a locomotive driven by an electric alternating current motor. Figure 10 is a schematic of a simplified propulsion system incorporating cut rectifier diode protection. Figure 11 illustrates other elements of the protection circuit of Figure 10. Figure 12 is a flow chart explaining the operation of the short circuit current protection system.

DETAILED DESCRIPTION OF THE INVENTION The propulsion system shown in Figure 1 includes a variable speed primary motor 11 mechanically coupled with the rotor of a dynamo-electric machine 12 comprising a synchronous three-phase alternating current (ac) generator, also referred to as the main traction alternator. The main alternator 12 has a series of three armature coils connected in Y on its stator. In the operation, it generates three-phase voltages in these coils, whose voltages are applied to the alternating current input terminals of at least one full-wave, three-phase uncontrolled energy rectifying bridge 13. In a conventional manner, the bridge 13 is formed by a plurality of pairs of power diodes, two or three of these pairs being associated with each of the three different phases of the main alternator 12. The diodes of each pair are connected in series between the direct current output terminals ( dc) relatively positive and negative of the bridge rectifier, and its connection is connected by means of a protection fuse (not shown) with the respective alternating current input terminal of the bridge respectively associated. The output of the bridge 13 is electrically coupled, by means of a direct current busbar 14 and a plurality of individual electrical contacts 15C, 16C, in an energizing relationship, with a plurality of adjustable speed direct current traction motors., connected in parallel, only two of which (15, 16) are shown in Figure 1. The primary motor 11, the alternator 12, and the rectifier 13, are properly mounted on the platform of a self-propelled traction vehicle. which is typically a four-axle or six-axle diesel-electric locomotive. The platform of the locomotive in turn is supported on two trucks (not shown), each having two or more series of axles-wheels. A separate traction motor is hung on each axis, and its rotor is mechanically coupled by means of a conventional gear in an impulse relation, with the associated axle-wheel series. Suitable current sensing elements are employed to provide a family of current feedback signals II, 12, et cetera, which are respectively representative of the magnitudes of the armature currents of the motor. The first traction motor 15 is shown in the Figure 2A and is typical of the others. On the cylindrical rotor of this motor, there is a plurality of armature coils terminating respectively in different bars or segments of a conventional commutator 15A, with which the non-rotating carbon brushes 15B are in sliding contact. An earth discharge ring 15R is placed around the switch in a separate relation thereto. The motor has the field coils 15F on its stator, and during the motorization or propulsion operation mode, these coils are electrically connected in series with the armature, as shown in Figure 2A. The direction of rotation of the armature, and therefore the direction in which the locomotive is propelled, depends on the relative direction of the field current, and can be reversed by changing the contact position of a conventional bistable electromechanical inverter (not shown) connected in series with field coils 15F. To brake or dynamically retard the locomotive, the coils of the armature of each traction motor of the power rectifier 13 are disconnected, and reconnected to a conventional ventilated dynamic braking resistor grid (not shown), and turned over to connect the field coils of all the motors in series with each other for energization by the rectified output of the main alternator 12. As can be seen in Figure 2A, the current feedback signal II is provided by an appropriate current sensor 15S connected in series with the coils of the armature of the traction motor 15. Accordingly, this is representative of the magnitude of current in the armature connected in series and the field coils of this motor when operating in a motorization mode. The main alternator 12 and the power rectifier 13 serve as a controllable source of electrical power for the respective traction motors. The magnitude of the output (or current) voltage of this source is determined and varied by the amount of excitation current supplied to the field coils 12F on the rotor of the main alternator. These field coils are connected for energization to the output of a suitable source 17 of regulated excitation current IF. In the illustrated embodiment of the invention, the connection between the field coils 12F and the excitation current source 17 includes a contact 12C of a conventional electromechanical field switch. The field switch has the control element 12D to move it to a first normal state, where the contact 12C closes and conducts the excitation current freely, and to cause this switch to switch between its first state and a second alternative state, wherein the contact 12C opens and effectively the excitation current is interrupted. In practice, the control element 12D comprises an electromagnetic coil and an associated actuator mechanism that will move the field switch to its normal state, and will keep it there only if this coil is energized. Preferably, the excitation current source 17 comprises a rectifier bridge controlled by three phases whose input terminals 18 receive alternating voltages from an auxiliary alternator driven by the primary motor, which may actually comprise an auxiliary series of three phase armature coils. on the same structure as the main alternator 12. The source 17 is labeled with "field regulator" in Figure 1. It includes conventional elements to vary the magnitude of the direct current IF supplied to the field of the alternator 12F (and consequently, the alternator output 12) as necessary to minimize any difference between the value of a variable control signal VC on an output line 19, and a feedback output that, during motorization, is representative of the average magnitude V of the rectified output of the main alternator 12. The last voltage magnitude is a known function of the mag excitation current in field coils 12F, and the magnitude of the output current in the coils of the armature of the main alternator, respectively, and also varies with the speed of the primary motor ll. It is detected by a conventional voltage sensing module connected through the direct current output terminals of the power rectifier. The curves of Figure 2B illustrate exemplary relationships between V and the average magnitude of the load current at the output terminals of the power rectifier 13, as supplied by a typical alternator 12 driven at a constant speed (eg, 1050). revolutions per minute) by the primary motor 11, and excited by the IF field current of several different magnitudes which are labeled on the respective curves. A current sensing module 22 of a relatively low resistance (e.g., about 10 ohms) is connected between the neutral S of the coils of the alternator frame and the chassis to ground or structure of the locomotive, as indicated in Figure 1 The module 22 provides, on an output line 23, a feedback signal representing the magnitude (IGND) of the leakage current to ground in the electric propulsion system. It will be seen that IGND is a measure of the current flowing through the module 22 between the neutral S and any earth faults in the coils of the armature of the main alternator 12, in the power rectifier 13, or in the electric charge circuit that is connected to the power rectifier. The last circuit includes the field coils of the traction motors 15, 16, and so on, and the motor operation mode, also the coils of the motor armature. The primary motor 11 driving the field of the alternator 12F is an internal or thermal combustion engine, or equivalent. In a diesel-electric locomotive, the driving power is typically provided by a turbocharged, high-horsepower 16-cylinder diesel engine. This engine has a fuel system 24 that includes a pair of fuel pumping grilles to control how much oil-fuel flows into each cylinder each time an associated fuel injector is actuated by a corresponding fuel cam on the camshafts of the engine. motor. The position of each fuel grid, and consequently, the amount of fuel supplied to the engine, is controlled by an output piston of a motor speed regulator system 25 with which both grids are linked. The regulator regulates the speed of the engine by automatically moving the grids, within predetermined limits, in one direction and by an amount that minimizes any difference between the actual and desired speeds of the engine crankshaft. The desired speed is established by a variable speed reminder signal received from an associated controller 26, the signal of which is referred to herein as the speed command signal or the speed reminder signal. An engine speed signal (RPM) indicates the actual rotation speed of the engine crankshaft, and consequently, of the alternator field. The speed command signal for the motor control system 25, and the excitation control signal VC for the field current source of the alternator 17, are provided by the controller 26. In a normal motorization or propulsion operation mode , the values of these signals are determined by the position of a handle of a manually operated accelerator 27 with which the controller 26 is electrically coupled. A locomotive accelerator conventionally has eight power positions or notches (N), plus the running in Empty and deactivation. NI corresponds to a minimum desired engine speed (power), while N8 corresponds to a maximum speed and a full power. With the accelerator in its idle position, the controller 26 operates to impose on the control signal VC a value corresponding to IF = 0, and no pulling power is produced by the main alternator 12. When a dynamic braking of a locomotive in motion, the operator moves the throttle handle to its idle position, and manipulates an interagrip handle of a companion brake control device 28 such that a signal is now supplied to the main controller 26 variable "brake reminder" that will determine the value of the excitation control signal of the alternator VC. (In the braking mode, a feedback signal will be provided which represents the magnitude of the current being supplied to the field coils of the traction motor from the rectified output of the main alternator 12, to the excitation source of the alternator 17 , and will be subtracted from the control signal on line 19 to determine the difference or error signal to which the source 17) responds. In a group of two or more locomotives, normally only the front unit is assisted, and the controller on board each rail unit will receive, on the train lines, coded signals indicating the position of the accelerator or the selected brake reminder by the operator in the front unit.

For each energy level of the motor, there is a corresponding desired load. The controller 26 is suitably arranged to translate the information from the notch from the accelerator 27 to a reference signal value substantially equal to the value that the voltage feedback signal V will have when the pulling power matches the requested power, and always that the alternator output voltage and load current are both within the previously determined limits, the control signal VC is varied over the input line 19 of the excitation current source 17 as necessary to obtain this desired load . For this purpose, and for the purpose of de-aeration (i.e., engine discharge) and / or to limit engine speed in the case of certain abnormal conditions, it is necessary to supply controller 26 with information about different conditions and parameters. operating the propulsion system, including the engine. As illustrated in Figure 1, the controller 26 receives the revolutions per minute of the aforementioned motor speed signal, the voltage feedback signal V, and the current feedback signals II, 12, et cetera, which are representative , respectively, of the current magnitudes in the coils of the armature of the individual traction motors. It also receives a load control signal emitted by the regulator system 25 if the engine can not develop the demanded power and still maintain the requested speed. (The load control signal is effective, when issued, to reduce the power reference value in the controller 26, to weaken the alternator field until a new equilibrium point is reached). Additional data supplied to controller 26 includes: "MAXIMUM VOLTAGE" and "M XIMA CURRENT" data that establish the absolute maximum limits for the alternator output voltage and current, respectively; the "CRANKSHAFT" data indicating whether or not a start-up routine (ie start-up operation) of the engine is running; and the relevant entries from other selected sources, as represented by the block labeled "OTHERS". The excitation source of the alternator 17 and the controller communicate with each other by means of a multi-line serial data link or busbar 21. The controller 26 also communicates with the control element 12D which is operative, when they are energized in response to a "close" command from the controller, to move in contact with the field switch 12C to its closed position, where it is maintained by the energized control element, and communicates with the "CONTACT IMPULSORS" (block 29) that are properly constructed and configured to drive the individual traction motor contacts 15C, 16C, et cetera. Typically, the contact impellers 29 are pneumatic mechanisms controlled by associated electromagnetic values which in turn are controlled, selectively or in unison, by commands from the controller 26. For the purpose of responding to earth faults in the propulsion system, the controller 26 is supplied, by means of the output line 23 of the current detection module 22, with the aforementioned feedback signal, whose value varies with the magnitude IGND of the earth leakage current. If this signal indicates that IGND is abnormally high, the controller automatically executes certain protective functions, and at the same time, sends appropriate alarm messages or signals to a visual display module 30 in the cab of the locomotive. Preferably, the ground fault protective functions implemented by controller 26 are the same as, or equivalent to, those described in previously cited Canadian Patent No. 1,266,117 issued February 20, 1990, and assigned to General Electric Company, and the description of that patent is expressly incorporated herein by reference. In summary, the referenced protection is effective to modify the value of the control signal VC on line 19 when the earth leakage current is abnormally high, such that (1) if the magnitude of the earth current is in a scale between a previously determined deaeration threshold level and a previously determined maximum allowable limit, the field current magnitude of the IF alternator is reduced, and consequently, the power output of the main alternator 12 is reduced to a fraction of its amount normally desired, the fraction of which varies inversely with the magnitude of the earth current in excess of the deaeration threshold level, and (2) the power output is restricted to zero for at least a minimum time interval if the current magnitude of the land is increased above its maximum limit. In the preferred embodiment of the present invention, the controller 26 comprises a microcomputer. Those skilled in the art will understand that a microcomputer is really a coordinated system of commercially available components and electrical circuits and associated 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 and electrically reprogrammable read only memory (EPROM) that also stores tables and data used in the program. Contents inside the central processing unit, there are counters, recorders, accumulators, flipflops (flags), etc., conventional, together with a precision oscillator that provides a high frequency clock signal. The microcomputer also includes a direct access memory (RAM) in which the data can be temporarily stored, and from which the data can be read at different address locations determined by the program stored in the read only electrically reprogrammable memory. These components are interconnected through appropriate address, data, and control busbars. In a practical embodiment of the invention, an Intel 8086 microprocessor is used. The controller 26 is programmed to produce, in the motor operation mode, a control signal value on the line 19 which varies as necessary to zero any error between the value of the voltage feedback signal of the alternator V, and a reference 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 in which this is done is described in U.S. Patent Number 4,634,887 to Balch et al., Issued January 6, 1987, and assigned to General Electric Company, the disclosure of which is expressly incorporated herein by reference. the present as a reference. In order to implement an electric braking operation mode, the controller 26 is programmed to vary the value of the control signal VC as necessary to zero any errors between a current feedback value of the motor armature and a reference value that normally depends on the dynamic braking position selected by the locomotive operator. In accordance with the present invention, the propulsion system described above includes an element for protecting the traction motors from the spark discharges. The desired spark gap protection is implemented by the controller 26 in cooperation with the excitation current source 17 of the main alternator. The parts of the control that are involved in the flashover protection are shown in a simplified form in Figure 3, wherein the block 32 represents a suitable element for detecting the presentation of a disruptive discharge on the commutator of any of the direct current traction motors 15, 16, and so on. The sensing element 32 receives the family of current feedback signals from the traction motor II, 12, etc., and the feedback signal from the earth leakage current (IGND) on the line 23. It operates to produce a signal of short circuit on an output line 33 (labeled "SHORT CIRCUIT" in Figures 3 to 7) whenever a disruptive discharge occurs, as indicated by an abnormal elevation in the magnitude of at least one current feedback signal in the case that (1) the magnitude of the armature current in any traction motor exceeds a previously determined threshold that is higher than the magnitude of the armature current under all normal conditions, or (2) the IGND magnitude exceeds another threshold (for example, 2.5 amperes) that is higher than the maximum allowable leakage current limit above which the ground fault protection function previously men The control signal VC is attached to its zero tensile power value. The threshold value of the armature current of the motor is preferably almost double the maximum current that each traction motor will normally drive; In a practical application of the invention, a threshold amount of 3,000 amperes has been selected. In order to respond as quickly as possible to the presentation of a disruptive discharge, the preference detection function is performed by means of analog circuits rather than by means of the microcomputer. In Figure 4 the currently preferred mode of the short circuit current sensing element 32 is shown, and will now be described. In a propulsion system of a direct current traction motor, the current feedback signals of the motor armature II, 12, etc., are respectively supplied to the first inputs of an array of comparators 35, 36, and so on. In a propulsion system of an AC motor, the signals II, 12 ... IN are derived from the current sensors coupled in circuit with the inverters that supply a variable frequency energy to the motors. The second inputs of the same comparators are connected in common with an appropriate element 37 to derive a bias signal of a previously determined constant quantity K1 corresponding to the aforementioned high threshold magnitude of the motor current. The outputs of these comparators are coupled respectively through the diodes 38, 39, etc., with a line 40 which in turn is connected through a buffer zone 41, and another diode 42 with the base of a PNP transistor 43. The emitter of transistor 43 is connected by means of a diode 44 and a resistor 45 with a control voltage bus (+) of a relatively positive constant potential, and a resistor 46 is connected between the base of the transistor and the junction of the diode 44 and the resistor 45. The collector of the transistor 43 is connected by means of a resistor 47 with a reference potential busbar represented in Figure 4 by a minus symbol enclosed in a circle, and also connected by means of a resistor 48 with the output line 33 of the spark gap detector. Normally, none of the feedback signals II, 12, etc. has a magnitude exceeding Kl, all the comparators 35, 36, etc. have high outputs, the diodes 38, 39, etc. are inversely biased (ie, they do not conduct) , and the signal on line 40 is high, transistor 43 is turned off, there is no current in resistor 47, the collector potential of the transistor (and also on line 33) is low or zero with respect to the reference potential , and this detector is not producing any short circuit signal. However, if "when any (or more) of the feedback signals from the motor current rise above Kl, 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 42 conducts, thus polarizing forward the junction of the emitter-base of transistor 43, which is now energized and conducts current through its collector resistor 47, thereby raising the collector potential and producing a short-circuit signal high on the output line 33. As can be seen in Figure 4, the current feedback signal on line 23, which represents the magnitude of the earth leakage current IGND in the coils of the armature of the traction alternator 12, is supplied to an input of an additional comparator 51, whose other input is connected to a suitable element 53 to derive another bias signal of a previously determined constant quantity K2 ue corresponds to the aforementioned high threshold magnitude of the IGND. The output of the comparator 51 is coupled through a diode 54 with a line 56, which in turn is connected through a buffer zone 57 and a diode 58 with the base of the transistor 43. Typically, the magnitude of the feedback signal of the ground current does not exceed K2, the comparator 51 has a high output, the diode 54 is reverse polarized (ie, it does not conduct), and the signal on the line 56 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 56 to be low and diode 58 to drive, activating this way the transistor 43 and producing a high short circuit signal on the output line 33. In effect, the diodes 42 and 58 form an "OR" (or) logic circuit that makes it possible for the detector to produce a short circuit signal in response to an abnormal magnitude increase of either the earth leakage current in the coils of the alternator armature, or the current of the armature in any of the traction motors, this increase being caused in any case by a spark gap over an engine switch. As shown in Figure 4, the feedback signal of the earth leakage current on the line 23 is also supplied to the summing element 59, where another signal on a line 61 is subtracted therefrom. The signal on the line 61 has a previously determined constant quantity K3 and corresponds to the IGND deaeration threshold level (for example, approximately 0.5 amperes). If IGND is higher than this level, the resulting value of the summing element 59 activates a deaeration program 62. As fully described in Canadian Patent Number 1,266,117 cited above, the deaeration program 62 modifies the value of the control signal VC on line 19 (see Figure 1) in a manner that reduces the magnitude of the field current of the alternator, such that the energy output of the alternator 12 is reduced to a fraction of its normally desired amount, the fraction of which is inversely proportional to the magnitude of the leakage current in excess of the deaeration threshold level, and is equal to zero if the magnitude of the leakage current exceeds its maximum allowable limit (for example, approximately one ampere). Note that K2 is higher than the magnitude of the feedback signal on line 23 when this last mentioned limit is reached.

Returning to Figure 3, the short-circuit signal produced by the sensing element 32 on the output line 33, whenever a disruptive discharge occurs, is supplied to the excitation current source of the alternator 17 via the power link. 21. In accordance with the present invention, the excitation source 17 is provided with a controllable solid-state electric valve and a series-connected capacitor coupled in a parallel circuit configuration with the field coils of the alternator 12F to rapidly switch to the field current source is deactivated, and the magnitude of the field excitation current is quickly reduced when the valve enters conduction by a short circuit current signal, thereby correspondingly decreasing the magnitude of the output voltage of the main alternator 12 The organization, operation, and advantages of this part of the discharge protection element to disruptive will now be described in greater detail with reference to Figure 5, which illustrates the currently preferred embodiment of the excitation current source 17 for a direct current electric traction motor system. The illustrated source 17 comprises a two-way, three-phase rectifier bridge 64, formed by interconnecting six controllable unidirectional electric valves or thyristors having gates that respectively receive the periodic trip or deactivation signals from the conventional control element 65 shown as a block labeled with "thyristor bridge control", these trigger signals are synchronized with the three-phase alternating voltages that are applied to three AC input lines 18 of bridge 64. The last voltages are obtained from the auxiliary coils of the alternator 12, whereby its frequency and amplitude will vary with the rotational speed (RPM) of the primary motor. Typically, the magnitude of the input voltage is on a scale of approximately 30 volts rms at idle speed, up to 68 volts rms at full speed. In order to achieve the field regulation of the desired alternator, as described above, the control element 65 operates to advance or delay the time of the trigger signals as a function of any error between the control signal VC on the line 19 and the feedback signal representative of the output voltage of the alternator V. As shown in Figure 5, the negative direct-current output terminal N of the rectifier bridge 64 is directly connected to one end of the field coils 12F of the main alternator, and the relatively positive output terminal P of this bridge is connected to the other end of field 12F by means of a line 66, the normally closed contact 12C of the alternator field switch, and a line 67. The field 12F and the contact 12C are derived by a voltage limiting resistor 68 of a relatively small ohmic value (e.g., two ohms), in series with a two polarity voltage cut 69 having a positive terminal connected to line 66, and a negative terminal connected to line 67. Cutting device 69, in its normal state, provides a very high resistance, and is essentially a circuit open. However, it is properly constructed and configured to abruptly switch to a negligible resistance state if either the potential of line 67 is negative and exceeds a previously determined first cut level with respect to the N output terminal of the bridge 64 (for example, 800 volts), or the potential of line 66 is relatively positive and exceeds a second level of cut that can be equal to or different from the first cut level. Whenever the device 69 is in the last state, any excitation current in the field 12F will circulate or "spin freely" through the two ohm resistor 68. The field excitation system of Figure 5 is particularly suited for a system of propulsion for a locomotive using direct current electric traction motors, and includes a controllable circuit element 70 connected in parallel with the field coil 12F, and with the bridge of the thyristor 64, between the negative direct current output terminal N and the relatively positive output terminal P. In the illustrative embodiment of the invention, the circuit element 70 comprises a relatively high speed controllable electric valve 71 connected in the vicinity with a relatively large capacitor, for example, 420 μF, between the terminals N and P. The valve 71 is preferably a rectifier of silicon control. Whenever a short circuit current is detected as evidenced by an abnormally high value of current II, 12, etc., towards one of the traction motors, the circuit of Figure 4 generates a short circuit current signal, SHORT CIRCUIT, which is coupled with the control of the silicon control rectifier 80. The control of the silicon control rectifier 80 operates in response to a SHORT CIRCUIT signal to give gate to the silicon control rectifier 71 to enter conduction, and at the same time to change a normally high STATUS signal ("1") on an output line 81, to a low state ("0"). The control of the silicon control rectifier 80 further includes an element for charging the capacitor 72 to a relatively high voltage with respect to the bridge output voltage of the rectifier 64. In an exemplary embodiment, the normal voltage from the bridge 64 measured at through the field coil 12F is on the scale of approximately 30 to 40 volts, while the voltage to which the capacitor 72 is charged is approximately 440 vdc. A more detailed description of the control of the silicon control rectifier 80 with respect to FIG. 6 is given below. When the silicon control rectifier 71 is fed to the conductor, the capacitor 72 is coupled in parallel with the coil. 12F field between rectifier terminals N and P. Capacitor 72, charged to a voltage much higher than the rectifier output, reverse polarizes the silicon control rectifiers in rectifier 64, and becomes the current source for the current to the field coil 12F. In an exemplary system it has been found that the rectifier 64 can be switched to off within microseconds with the voltage on the capacitor 72 going through an inverse polarity of about 800 volts within one to two milliseconds. When the reverse voltage reaches approximately 800 volts, the cutting device 69 is triggered towards the conducting coupling resistor 68 in parallel with the field coil 12F. The current in the field coil 12F quickly decays to zero by dissipating in the resistor 68, thereby reducing the alternator energy output to zero within the same time interval. An example voltage waveform for field coil 12F is shown in Figure 7. The voltage from time t0 to t! is a conventional phase-controlled output voltage from the bridge rectifier of the silicon control rectifier 64 having an average direct current value of about 30 voltioe. At time t ,, a short circuit current is detected, and gate is given to the silicon control rectifier 71 to enter conduction. The voltage across the coil 12F immediately jumps up to the value of the voltage on the capacitor 72, in this example, a value of 440 volts. At the same time, the gate signals to the silicon control rectifiers at 64 are deactivated for 2 seconds. The current to the coil 12F is transferred concurrently from the rectifier 64 to the capacitor 72, thereby switching the silicon control rectifiers in the rectifier 64 to a non-conductive state. Current flows through coil 12F and capacitor 72, charging the capacitor 72 in an inverse polarity. When the voltage across the capacitor 72 (and the coil 12F) reaches a sufficient magnitude to trigger the cut-off device 69, the resistor 68 couples in parallel with the coil 12F, thus providing a low impedance to absorb the energy in the coil 12F, in such a way that its current rapidly decays to zero. In the example, the device 69 fires at approximately -800 volts at time t2, approximately 1 to 2 milliseconds after the short circuit is detected. The current in coil 12F decays to approximately zero within approximately 168 milliseconds. From time tj to time t2, the circuit operates in an underdamped mode, and then, at time t2, switches haeta to an overdamped mode. The reduction in the magnitude of the field current causes a much larger current reduction in the armature coils of the main alternator 12, and decreases the alternator output voltage and current rapidly. Figure 2B shows that the decrease in the alternator output current, per ampere of the field current reduction, varies from about 5 amperes to about 15 amperes, depending on the magnitude of the alternator output voltage V. The results In order to quickly derive the field coil with a low impedance and to uncouple the rectifier bridge 64 from the field of the alternator 12F, it will be better understood from the following explanation of the response of the alternator to the spark arresters. The main alternator 12 is a synchronous, high-reactance pole-synchronous machine without damping coils. If the charging circuit connected to the output terminals of the coils of this machine's armature were shorted by a flashover, the current amplitude of the armature would tend to increase abruptly to a much higher peak than would otherwise be expected. normal, and then to decay over time. The initial current increase in the coils of the armature produces a magnetomotive force (MMF) that is almost directly opposed to the field's magnetomotive force, thus tending to demagnetize or weaken the resulting magnetic field in the stator air gap. rotor of the machine. The magnetomotive force that is being demagnetized induces an extra current in the field 12F, so that the total flow links will remain constant. The control element 65 for the controlled rectifier bridge 64 in the excitation current source 17, responds to the resulting change in the output voltage V, initiating the corrective action, but its response time is too slow, and the bridge 64 has insufficient voltage to prevent this field current from increasing. As long as the excitation current source 17 remains unchanged, the initial peak magnitude of the short-circuit current is determined by the transient reactance of the alternator (more precisely, the transient reactance of the direct axis) and the reactance in the path of the current between the coils of the alternator armature and the traction motor whose switch had a spark gap. The time constant of the next current decay is determined by the electrical inductance and the resistance in the path of the excitation current. As soon as the above-described cutting device 69 starts to conduct, the effective resistance in parallel with the coil 12F is greatly reduced (the resistor 68 has a value of about 2 ohms in one mode), and this time-keeper comes to eer. eignificantly smaller, and the excitation current will decay very rapidly towards zero, since the available current (energy) from the capacitor 72 is very small. In effect, the reactance of the alternator increases rapidly from its initial relatively low transient value (which is not greater than about 30 percent of the synchronous reactance of continuous machine setting) to the value of the synchronous reactance., and in a corresponding manner, the magnitude of the current of the armature decreases. If the excitation current source is rapidly decoupled from the field 12F as described, the output current of the alternator 12 will begin to decrease from its initial surge before reaching the maximum amount available in short circuit. In one application of the invention, the peak short circuit current for a failed motor has been limited to approximately 18,000 amperes in a propulsion system capable of delivering 60,000 to 70,000 amps or more without this improved spark arresting protection element, and The electrical energy in the failed motor circuit has been limited to approximately 25 percent of what it would otherwise be. The valve of the silicon control rectifier 71 in the circuit element 70 goes into respire conduction to a short circuit signal, such as a high current signal detected by the circuit of Figure 4. Referring to Figure 6, the gate signal to the valve 71 is shown as being generated in response to the SHORT CIRCUIT signal of Figure 4 by the control of the silicon control rectifier 80. The SHORT CIRCUIT signal is coupled with a monostable multivibrator diepoeitive (commonly referred to as a "one shot" multivibrator 85 through an optical isolator 89. The device 90 produces an output pulse having a predetermined duration of ti mpo, for example, 45 microseconds.An example device 90 is a hardware device programmable type 4538 CMOS The output pulse is supplied to a silicon control rectifier gate impulse 91, which conditions the signal to to a suitable form for applying to the gate electrode of the valve 71 to give gate to the valve 71 to enter conduction. The output pulse of the device 90 is also coupled with another one-shot multivibrator device 92, which may also be an integrated circuit of type 4538 CMOS. The device 92 produces a BLOCK output pulse of a slightly longer duration, for example, 2 seconds, and is used to block or prevent the application of the trigger pulses to the rectifier 64 during the period of time immediately afterwards. a short circuit current detection. The output pulse from the device 91 is coupled to the driver of the drive seventh 26, which controls the application of the gate or trip pulses to the rectifier 64. The control of the silicon control rectifier 80 also includes an apparatus In one form, the charging apparatus may comprise a conventional battery charger 93 connected to receive the battery voltage (typically 45 to 90 volts), and to stagger the battery voltage. battery voltage to a higher value, eg, the aforementioned 440 volts, to charge the capacitor 72. The voltage on the capacitor 72 can also be monitored to provide a STATUS signal indicating whether the protection circuit is operating or not. The STATUS signal may be merely an alarm, or it may be used to deenergize the alternator power circuit. In Figure 6, the voltage conditioning circuit 94 is coupled to the capacitor 72 to reduce the detected voltage to a suitable logic level to be applied to a voltage comparator 95. The comparator 95 provides a one-way logic signal, for example, a logical 1, if the voltage on the capacitor 72 exceeds a minimum voltage, for example, of 389 volts, and provides a logical signal of another direction, for example, a logical zero, if the capacitor voltage is less than or equal to Minimum voltage of the example voltage of 389 volts. An optical aperture 96 is used to guide the voltage overvoltage circuit of the controller 26. The circuit of FIG. 5 is preferably used in a propulsion system utilizing direct current electric traction motors., which conventionally includes the voltage cutoff device 69 and a series resistor 68. In a propulsion system using AC electric motors, the transient eupreation circuit comprising the cut-off device 69 is not commonly used. and the series resistor 68. In accordance with the foregoing, it is necessary to provide another energy dissipation circuit to absorb the reactive energy from the coil 12F and the capacitor 72. In FIG. 5A, this function is provided for a propulsive system. alternating current by means of a diode 73 and a resistor connected in series 74. The diode 73 has a cathode terminal connected to the junction intermediate capacitor 72 and the valve 71, while the resistor 74 connects the anode terminal of the diode 73 with a opposite terminal of the capacitor 72. In the operation of the system of Figure 5A, the detection of a short-circuit current caused by a Tripped fault trip, triggers the valve of the silicon control rectifier 71 in the same manner as in Figure 5, and the load on the capacitor 72 again switches the rectifier 64 to deactivated by supplying current to the field coil 12F . As current flows through coil 12F and capacitor 72, it starts charging capacitor 72 towards an inverse polarity. When the voltage across the capacitor 72 reaches a slightly negative value, eg, about -1 volts, the diode 73 becomes forward biased, thereby coupling the resistor 74 in the circuit with the coil 12F. Resistor 74 has a low value, for example, of about 0.65 ohms, and provides a low impedance discharge path to rapidly dissipate energy from coil 12F. The characteristic response of the system of Figure 5A has a characteristic with that shown in Figure 7 for Figure 5, and the final result is the same, that is, the field current is interrupted rapidly, so that the current of Crossed shot remains at a non-destructive level. For the circuit of Figure 5A, the characteristic exhibits approximately the same applied peak voltage to switch rectifier 64 to deactivated, but has a higher peak negative voltage and seems to decay exponentially with some AC resonance superimposed on the waveform which is declining. For example, the peak short-circuit current, for a worst-case condition, is limited to approximately 18,000 amperes for direct-current locomotives, while the peak current without the short-circuit current protection of Figures 5 or 5A, It can be as high as 40,000 amps. In addition, for the AC locomotives, the time duration and the energy content of the short-circuit current are significantly reduced by the short circuit current protection circuit. Referring to Figure 8, which is a simplified representation of Figure 1, and Figure 9, which illustrates a corresponding propulsion system using AC electric traction motors, the three-phase stator circuit of the alternator powers the a three-phase complete bridge rectifier 13. This rectifier also contains F-fuses that function as protective devices, that is, protect the alternator from overvoltages, and open fuses when the rectifier fails (a rectifier usually fails short). The direct current voltage and the current from the rectifier 13 are used to energize the direct current traction motors 15, 16, etc. in a "direct current locomotive", and the inverters 96, 97, et cetera, which then energize the Squirrel cage induction motors alternating current 15A, 16A in an "alternating current locomotive". Both types of locomotives (direct current and alternating current) require the fuses of the main rectifier or another type of protection scheme. Fuses are often a maintenance problem, since they only last approximately 3.27 years in the most severe conditions of the locomotive (for example, pulling coal down a steep slope, that is, at low speed, with maximum power, and in the higher output rectifier currents). When a fuse blows, the locomotive must operate at reduced power or at no power (depending on whether it is a direct current or alternating current locomotive). It has been found that power fuses can be removed by holding in place of them the field currents resulting in the gates of the alternator field during a cut-off diode fault. More specifically, a diode cut in rectifier 13 (without fuses) can be modeled by an almost-current source that creates an alternating current signal having its source from the alternator field. The only way to prevent high voltages and the cutoff of the field insulation resulting from the rotor circuit, is to provide a very low impedance path for these bidirectional currents, until the motor speed is reduced to low idle speed (speed lower), and the direct current voltage and current of the rotors are removed. Figure 10 illustrates an electronic device 100 comprising anti-parallel silicon control rectifiers that fire when an included detection control card 99 determines a valid cut-off diode condition in locomotives where the fuses are removed. The detection scheme is connected to the field circuit of the alternator, as well as the antiparallel silicon control rectifiers. When these silicon control rectifiers fire, they provide a very low impedance path for the reflected alternating current field currents, until the system controller can reduce the motor speed to low idle speed. The end result is that the only time a locomotive fails due to a fault in the rectifier circuit is when there is a real cut diode, since the annoying faults of the fuses are now eliminated. The present invention allows to remove the fuses of the rectifier in the direct current locomotives, and allows to design the locomotive of alternating current without fusiblee. As mentioned above, fuses can have a short life span. This high failure rate is multiplied by the fact that, in a 6-axle locomotive (6 direct current traction motors) there is a total of 18 fueiblee. In a 4-axle locomotive (4 direct current traction motors) there are 12 fuses. For the case of 18 fuses, and each fuse fails in 3.27 years, this leads to an average of the worst case of a failure every 2 months and 5 days. In practice, of course, most locomotives will not have this severe. However, the fuses are far from obtaining a life of 20 years. When a rectifying diode in the power rectifier 13 fails short, and there are no fuses in the rectifier assembly, an alternating current is induced to the field circuit of the traction alternator 12. This current is actually a source of near-current , that is, if a short is placed across the field 12F, the current will flow at a frequency that is the same as the stator frequency. This frequency is related to the speed of the motor, which is mechanically coupled to the alternator. If a short is not provided, the current produces a severe overvoltage over the field, but is limited by the characteristics of isolation cut in the field. This current source can have a peak value of 3,000 amps when it is cut. If these currents were not allowed to flow through a path of very low impedance, the resulting eo-voltages would break the insulation in the rotor circuit as mentioned above, and could cause permanent mechanical and electrical damage to the alternator and support systems electrical and mechanical circuits. The present invention provides a protection element for establishing this low impedance path whenever a diode cut in the rectifier circuit 13 creates an alternating current reflected in the field coil of the alternator 12F. In a preferred way, the protection element comprises a pair of inversely polarized silicon control rectifiers, connected in parallel, connected in shunt with the field coil 12F. Referring to Figures 10 and 11, the silicon control rectifiers 101 and 102 are connected in parallel through the field coil 12F. A safety circuit comprising the resistor connected in series 103 and the capacitor 104 is also connected through the coil 12F, together with a thyristor 105. The safety circuit limits the rate of change of voltage over the control rectifiers of silicon during the transition from one state to another. The voltage across the coil 12F is detected by a rectifier circuit 106 connected to the coil 12F. The rectified output of the rectifier 106 is coupled with a bilateral trigger circuit 107 that produces an output signal, such as a transition from logical 0 to logical 1, provided that the rectified voltage exceeds a previously selected amount, for example, 830 volts. The signal from the circuit 107 is coupled with a one-shot multivibrator 108, such as a type 4538, which produces a pulse output signal of a previously selected duration, for example, one second. The circuit 107 preferably includes an optical isolator between it and the multivibrator 108. The pulse output signal from the multivibrator 108 is applied to a conventional silicon control rectifier gate circuit 109, which provides gate signals to the rectifiers of silicon control 101 and 102 during the time duration of the multivibrator pulse output signal. In response to the gate signals, whenever one of the silicon control rectifiers 101, 102 is forward biased, conduction will start and a short circuit current path will be established in parallel with the coil 12F. As will be appreciated, the rectifier circuit 106 detects the alternating current reflected in the field coil 12F when a cut-off diode creates an alternating current path in the alternator output circuit. Thyristor 105 is used to dissipate the transient energy if the reflected AC voltage does not reach the previously selected trigger, ie + 830 volts. The pulse output signal from the multivibrator 108 is also supplied as a cut-off diode STATUS (SD) signal to the system controller 26. The seventh controller can be programmed to take different corrective actions in response to a cut-off diode detection . In one example, the controller 26 records the short circuit and then allows a restart of the power system. If a second contiguous short circuit is then detected, the controller secures the system off-line. At the same time, the engine speed is reduced to low idle speed, that is, the lowest speed of a working diesel engine. When the engine speed is being reduced, the cut-off diode detection circuit continues to operate as necessary to prevent damage to the alternator field. The cut-off diode detection can be used in the electric drive motor propulsion sevenmae, both alternating current and direct current. It is also used in combination with the detection of short circuit current, either by a disruptive discharge (direct current system) or by a crossed trip (alternating current system). When combined with short-circuit current protection, it is desirable to disable the cut-off diode circuit when a short-circuit current is detected. More particularly, if the silicon control rectifiers 101, 102 were tripped, during a short circuit current condition, the capacitor 72 would be shorted, and would be disabled to switch the rectifier .64 off. The cut-off diode circuit is disabled by coupling the SHORT CIRCUIT signal from the short circuit current sensing circuit (Figures 5, 5A) to a one-shot multivibrator 110, which may be a 4538 CMOS type device. The device 110 produces a pulse output of a fixed duration, for example, 200 milliseconds, which makes a transition from a logical 1 to a level of logical 0. The pulse output is coupled through a diode 111 with the signal line 112 coupled between the trigger circuit 107 and the multivibrator 108. This signal holds the line 112 to a low value and prevents triggering of the multivibrator 108. In general, The protection systems operate essentially in the same way, whether the propulsion system is for direct current traction motors or for AC traction motors. One area of difference is in the corrective action that must be taken. In the case of a disruptive discharge of a direct current motor, the motor can "compose" itself after working for some time without power. In accordance with the above, the controller 26 will normally block the energy to a motor with a flashover during a selected time, for example, 25 kilometers, and then reapply power. In the case of a trip through, the controlled rectifier has failed and the controller 26 can block the operation of the associated inverter. Typically, each AC traction motor is coupled with a separate inverter, and this blocking merely disables one motor. Having described the currently preferred embodiment of the excitation current source of the alternator 17, as shown in Figures 5 to 7, the rest of the improved short circuit current protection elements will now be described with reference again to Figure 3 The state signal on the output line 81 of the source 17 is coupled via the data link 21 to the controller 26. As soon as the signal normally falls on the output line 33 of the short-circuit sensing element. circuit 32 becomes high due to a disruptive discharge that is present on the switch of 1 or more of the traction motors 15, 16, etc., or a trip shot through one of the inverters 96, 97, etc., the control element deactivation gate 80 in the excitation current source 17 simultaneously applies an activation signal to the gate deactivation valve 71, and removes the status signal normally high on line 81. This change from high to low of the status signal initiates two functions in the controller. The first function, represented in Figure 3 by a block 140, which is supplied with the current feedback signal family of the motor armature II, 12, etc., identifies any traction motor where the magnitude of the current of the motor the armature exceeds a previously determined high threshold, provided that the status signal changes from high to low, or that the inverter experiences a similar current. The last threshold (for example, of approximately 3,000 amperes) is greater than the maximum magnitude of the armature or inverter current under all normal conditions. The identification function 140 is suitably programmed to read the magnitudes of the current feedback signals, to compare each one with a value corresponding to the aforementioned threshold, and to store the identification number ("#X") of any motor of the current. traction (or inverter) whose current is higher than that threshold. It is presumed that the #X motor is experiencing a disruptive discharge. The identification of the failed motor is available on an output line 141 of block 140. The other function initiated by a change in state is represented in FIG. 3 by a block 142 labeled "system response function". This is properly configured to order the following actions in a direct current locomotive, in immediate response to any change from high to low of the state signal on line 81; the speed reminder signal for the seventh of the motor regulator 25 is changed to its idling speed, - the energy reference value in the excitation control element of the controller 26 is reset to zero, thus imposing temporarily a value corresponding to IF = 0 on the control signal VC; a disruptive discharge message is inserted into the visual display module 30, and the identification of the failed engines is recorded; an "open" command is transmitted via a line 143 to the field switch control element 12D to de-energize 12D, which makes it possible for the contact operating mechanism 12C to move this contact from its closed normal position to its open alternative position; contact opening commands are issued for all motor contacts 15C, 16C, etcetera; each of these opening commands is transmitted to the opening element 29 of the motor contacts as soon as the current of the corresponding motor armature has decreased to a predetermined level which can be safely interrupted by the associated contact without an arc. or welding (but not after 5 seconds after the opening commands are issued); and a "disruptive discharge timer" is activated. As a result of these actions, the tripping signals for the controlled rectifier bridge 64 in the excitation source 17 are delayed, so that the output voltage of this bridge is soon reduced to zero, the switch contact is opened. field 12C in the excitation current path (although the field of alternator 12F can continue to be energized by the residual current flowing through resistor 68 and cutting device 69), and all traction motors in the busbar are switched off direct current 14 of the propulsion system. Whenever a disruptive discharge is detected, the rapid response of the solid state controllable valve 71 in the excitation current path will cause the output current of the alternator to decrease very rapidly from its initial surge, as explained above. As a result, the respective motor currents decrease rapidly, and the time delay between the issuance and transmission of the contact opening commands is relatively short. Note that when the control element 12D and the operating element 29, respectively, receive the opening commands by the control element, the contact points of the field switch and the motor contacts will not be separated immediately due to the time delays. inherent (for example, approximately 180 milliseconds) in the operation of these electromechanical devices. For the time when the serial contact with the failed motor is opened, the flashover is extinguished, and the short-circuit signal is removed on the output line 33 of the flashover detector 32. The aforementioned enabling signal, which is supplied via line 82 from the control element of the field switch 12D to the control element 80 in the excitation current source 17, will have a low state whenever the contact of the field switch 12C is open. In the case of an alternating current locomotive, essentially all the steps listed above are duplicated, except that the message identifies a trip through an inverter rather than a disruptive discharge of the motor, and the system blocks the inverter from another operation. After the actions described in the previous paragraph are completed, the system 142 response function will order several additional actions; contact closure commands are transmitted to the operating element 29 of the motor contacts, to re-close all the contacts 15C, 16C, etcetera, except those associated with the failed traction motors (ie the #X motor), as was identified by the function described above 140; to transmit a "close" command via line 143 to the field switch control element 12D, to energize it 12D, and thereby return contact 12C to its closed position; and the motor speed reminder signal is allowed to return to a value determined by the position of the accelerator 27. As soon as the control element 12D receives the close signal on the line 143, the enable signal on the line 82 It changes from low to high states. As a result of removing the idle speed restriction on the speed reminder signal, and again closing the contact of the field breaker 12C, the excitation current of the alternator will rise to a desired continuous state magnitude, and Now, the electrical energy that reapplies the main alternator 12 to the non-failed traction motors, will increase smoothly up to any level determined by the position of the accelerator. After a delay determined by the disruption timer in the response function of the system 142, the operating element 29 is allowed to close again the contact associated with the motor #X, actually presenting this reclosing the next time the motor is moved. Accelerator handle through its idle position. If the speed of the locomotive is relatively high (for example, 96 kilometers per hour or greater) when a disruptive discharge occurs, as is normally true, the delay time is calculated in such a way that a certain number of revolutions of the commutator is obtained. , by means of which, the switch will be cleaned with a flashover by means of the brushes that run over its surface as the rotor of the de-energized motor #X continues to rotate through the axis of the locomotive with which it is coupled. Although the system response function could be implemented in a variety of different ways to obtain the results summarized above, the currently preferred way is to program the controller 26 to execute the routine illustrated in Figure 12. This routine is repeated once every 10 milliseconds. Start with a question 151 to determine if the status signal on line 81 has changed or not from high to low. If not, the routine for a direct-current engine locomotive proceeds to a second question 152 to determine whether or not the flashover is active. For an AC motor locomotive, the routine ends at this point. If the answer is affirmative, the next final step 153 in this routine is to decrease the disruptive discharge timer by one. Otherwise, the routine proceeds from question 152 to step 153 by means of an additional step 154 that removes any limitation that may be preventing the motor contact associated with a previously failed traction motor #X from closing again. After this limitation has been removed, the contact can be closed again by operating element 29 provided it is commanded by the controller 26. If the answer to the first question 151 was affirmative, the routine of Figure 12 would proceed from eeta question haeta the final step 153 by means of a series of steps 160-66 that will now be described. In step 160, a disruptive discharge counter is incremented by one. The next step 161 is to change the engine speed reminder signal to its idling speed, to reset the energy reference value to zero, to initialize other variables in the excitation control, and to issue opening commands to the control element of the field switch 12D and the operating element of the motor contact 29. (Note that the relevant time constants of the engine fuel system, the field exciter circuit of the alternator, and their respective controls, are such that the output power of the alternator responds relatively slowly to the execution of step 161, too slow to rely on to prevent the initial surge of current in the failed motor from reaching a potentially damaging magnitude). Step 161 is followed by step 162, where the identification of failed traction motors or failed inverters is sought from function 140 (FIG. 3), and then entered into visual display module 30. This same information is used in step 163 to impose a reclosure limitation on the contacts associated with those motors. In the next step 164, the routine in Figure 8 calculates a certain initial count that corresponds to a time delay that is less than 15 minutes or 900 divided by the real speed of the locomotive in units of 1,609 kilometers per hour. Then, in step 165, the disruptive discharge timer is activated, charging a recorder of the microcomputer with the initial count found in the previous step. The disruptive discharge timer remains active only when the account of this recorder does not reach zero. The initial count is large enough for the account stored in the recorder, when it decreases at the rate of 100 per second, reach zero at the end of the aforementioned maximum time duration (for example, 15 minutes), or sooner if the The locomotive's speed was greater than 96 kilometers per hour when the initial count was calculated. The next step 166 is executed as soon as the position sensors in the contact operating element 29 indicate that all the contacts of the motor 15C, 16C, and so on have been opened in response to the opening commands issued in step 161. Remove the restricting the idle speed value of the speed reminder signal, issuing a close command to the field switch control element 12D, and issuing commands to operating elements 29 to close all contacts 15C, 16C, etc., except those that are associated with the failed engines X whose reclosure is prevented whenever the limitation imposed in step 163. is active. The last mentioned limitation is active until it is removed by executing step 154. Although a mode has been shown and described Preferred of the invention, by way of example, undoubtedly the people in this field will think of many modifications. For example, the conventional field switch 12C, 12D could be omitted, and the valve 71 could be adequately controlled to perform all its usual functions. In addition, the bridge of the thyristor 64 in the excitation current source of the alternator 17 could be replaced by a diode rectifier bridge, in which case, the valve of the silicon control rectifier 71 would be controlled to operate normally as a regulating element. of switching, to regulate the average magnitude of the field current of the alternator, as desired. Accordingly, it is intended that the conclusive claims cover all modifications that fall within the true spirit and scope of the invention.

Claims

1. A cut-off diode protection system for a propulsion system of a traction vehicle, including at least one electric traction motor, a synchronous generator having an armature and field coils, a controllable source of excitation current connected to the field coils, and an electrical power conditioning element that interconnects the coils of the armature with the traction motor, including the power conditioning element a bridge rectifier circuit to convert the alternating current from the armature coils into direct current on a pair of relatively positive and negative output busbars, the system comprising: a sensing element for producing a short-circuit signal in response to a reflected alternating current voltage appearing on field coils in excess of a previously selected quantity; at least one controllable electric valve of solid state connected in a parallel circuit configuration with the field coils, this valve being normally non-conductive, and being able to be switched to a conductive state by means of a gate signal applied to a gate terminal thereof , - and an element that responds to the short circuit signal to supply the gate signal to the valve to limit the voltage on the field coil.

2. The cut-off diode protection system of claim 1, and including a second controllable solid-state electrical valve connected in parallel with the at least one valve, these valves being unidirectional devices, and coupling in an inverse relationship.

3. The cut-off diode protection system of claim 2, wherein each of the valves comprises a silicon control rectifier.

4. The cut-off diode protection system of claim 3 and including a thyristor connected in parallel with the field coil to dissipate transient voltages less than the previously selected magnitude.

The cut-off diode protection system of claim 1, wherein the sensing element comprises a solid state rectifier circuit coupled with the field coils and adapted to produce a direct current voltage output proportional to a voltage component. of alternating current on the field coils, a trigger circuit to produce a trigger pulse when the direct current voltage exceeds a previously selected value, and an element that responds to the trigger pulse to generate the gate signal for a duration of time previously determined.

The cut-off diode protection system of claim 5, wherein the element for producing the gate signal comprises a monostable multivibrator that reeposes the trigger pulse to generate a signal of the previously determined time duration, and a signal generator gate that responds to the multivibrator signal, to supply gate signals to the valve for the duration of the multivibrator signal time.

The cut-off diode protection system of claim 1, and including: an element for detecting a short-circuit current in the coils of the armature; an element connected in circuit with the field coil for switching to the controllable source of excitation current, when detecting the short circuit current; an element that can be connected in a selectable way in circuit with the field coil to dissipate the energy in it, subsequent to the switching of the excitation source; and an element to inhibit the operation of the element that responds to the short circuit signal, by detecting the short circuit current.

8. A method to protect a synchronous generator from a reflected condition in a system where the generator has field coils connected to a controllable excitation current source, and armature coils connected to a bridge rectifier to supply direct current electrical power (de), the reflected overvoltage being caused by a short circuit condition in the rectifier, which causes an alternating current to appear reflected on the field coils , the method comprising the steps of: coupling a rectifier circuit through the field coil to develop an output voltage representing an alternating current voltage on the field coil; detect when the AC voltage exceeds a previously selected maximum quantity, and generate a short-circuit signal in response to the same, and short-circuit the field coil in response to the short-circuit signal. The method of claim 8, wherein the step of shorting includes the steps of: connecting a solid state controllable electrical switch in parallel with the field coils, and giving the switch gate to enter into conduction by a previously determined duration of time. The method of claim 9, wherein the switch comprises a pair of inversely connected silicon control rectifiers, and the gate step comprises applying gate pulses to the gate terminals of the silicon control rectifiers by the pre-determined duration of time. The method of claim 10, and including the steps of: monitoring the coils of the armature by an overcurrent condition; connecting the commutator element through the field coils in response to a supervised overcurrent; and inhibit the application of the gate pulses to the silicon control rectifiers during the overcurrent condition. The method of claim 11, wherein the bridge rectifier is coupled to supply direct current electrical power to at least one solid state inverter, the inverter having a plurality of controllable electrical valves to convert direct current energy to AC power at a selected frequency, and wherein the step of monitoring includes the step of detecting the overcurrent through the inverter from a cut valve. The method of claim 10, and including the step of coupling a safety circuit in parallel with the silicon control rectifiers. The method of claim 10, and including the step of dissipating the transient voltage on the field through a thyristor coupled in parallel with the field coils.