CONTROL APPARATUS FOR ELECTRIC CARS PROPELLED BY INDUCTION MOTOR

Abstract: Power is supplied from a DC electric car line (1) through an L-C filter (3) to a PWM inverter (4). The three-phase AC output from the inverter (4) is applied to an induction motor (5) for driving an electric car. The output frequency of the inverter (4) is determined by adding or substrating the slip frequency to or from the speed frequency of car. On the other hand, the output voltage of the inverter (4) is controlled by PWM so that the deviation as a result of comparing the current command to motor and the actual motor current is reduced to zero. In this case, an electric oscillation of current flowing from the DC electric car line (1) to the filter (3) is detected by a CT (13) with an air gap, and the electric oscillation is suppressed by adjusting the output voltage or frequency of the inverter.

Description: CONTROL APPARATUS FOR ELECTRIC CARS PROPELLED BY INDUCTION MOTOR This invention relates to an improvement of a control apparatus for an electric car propelled by an induction motor in which electric power is supplied from DC power supply through an invertor to an induction motor so as to drive the rolling stock. In electric trains, DC motors have been used as traction motors for many years, and use of induction motors has been discussed from old times because induc-tion motors need no maintenance, are easy to make in small-size and have excellent adhesion characteristics. Recently, a power converter capable of operating induction motors at a high efficiency has been developed with an advance of semiconductor technology, and at last an electric car driven by an induction motor has been realized. As the power converter, there are used an inverter in a DC electric railway, and the combination of a converter and an inverter or a frequency converter in an AC electric railway. In either case, use of an inverter is advantageous. The inverter is able to derive polyphase (usually three-phase) alternating current of variable frequency and variable voltage from a DC power supply (also including the output of the converter). In order to operate induction motors at high efficiency, it is advantageous to use a pulse width modulation inverter which causes little waveform distortion. The output frequency of this inverter is controlled to be a frequency obtained by adding the slip frequency of an induction motor to the speed of a car upon power running, and to be a frequency obtained by subtracting a slip frequency from the speed of a car upon regenerative braking. The output voltage is desirably controlled to be proportional to the output frequency, and to do so, two control systems can be considered. First, the voltage is adjusted by a current-constant control system, and as a result the output voltage of the inverter is approximately proportional to the output frequency. Second, a voltage control system arranged to produce a voltage proportional to the frequency is used to maintain the motor current approximately constant. In order to apply such control to all the speed ranges of a car, the output frequency of inverter is changed from several Hz (corresponding to the slip frequency) to a hundred and several tens of Hz. Therefore, the AC component of such a frequency is flowed in a DC current path between the DC power supply and the inverter. This AC component is transmitted directly on the aerial wiring, causing induction interference in a DC electric railway, and it may cause the operation of the converter to be unstable in an AC electric railway. For these reasons, a filter circuit is inserted between the DC power source and the inverter, and this filter circuit is arranged to have its resonant frequency lower than 50 to 60 Hz in order to prevent a track circuit relay for the operation of a crossing alarm or the like from malfunctioning. However, the resonant frequency of the filter circuit can not practically be reduced to a value lower than the lowest frequency (several Hz) of the inverter from the consideration of the size of its reactor and capacitor. Thus, if the resonant frequency of the filter circuit is selected to be 20 Hz, the filter circuit resonates to 20 Hz when the operation frequency of inverter changes past this frequency upon low-speed running of car, causing an oscillation in DC input current to the inverter, the voltage across the filter capacitor and the current from DC power supply. This electrical oscillation causes the commutation of inverter to fail the torque of induction motor to oscillate, and the ride in the car to be uncomfortable. It is an object of this invention to provide a control apparatus capable of suppressing the oscillation in DC current and so forth without increasing the size of the filter circuit, thereby stabilizing the operation of the inverter and making the ride in the car comfortable. According to one aspect of this invention, detection is made of the oscillating change of electrical quantity in an electrical circuit between the DC power supply and the induction motor, and the detected output is used to adjust the output voltage of the inverter. In other words, if the DC current flowing into the filter circuit from the DC power supply tends to increase, having an oscillation upon, for example, power running, due to the resonance of the filter circuit, this tendency is detected and the increase of the voltage across the filter due to the increase of current is suppressed by decreasing the inverter voltage as viewed from the filter (DC power supply side). If the DC current decreases having an oscillation, the inverter voltage is increased thereby suppressing the decrease of the voltage across the capacitor due to the decrease of current. Upon regenerative braking, the inverter voltage as viewed from the power supply side is increased and decreased against the oscillating increase and decrease of current flowing to the power supply side. As a consequence, the electric oscillation is suppressed, and thus the trouble caused thereby is eliminated. According to another aspect of the invention, detection is made of the oscillating change of electrical quantity in an electrical circuit between the DC power supply and the induction motor, and the output frequency of the inverter is adjusted in accordance with the output. In the induction motor, it is desired to maintain the ratio of voltage to frequency, v/f to be constant, and a control system for this purpose is constructed as set forth above. However, the output voltage from the inverter has a maximum level which is low as compared with the output frequency associated therewith. When the output voltage reaches its maximum value, the oscillation can no longer be suppressed by the adjustment of the output voltage, but the output frequency of the inverter can be increased and decreased yet. If the output frequency of the inverter is increased and decreased at a given speed, the motor current can be increased and decreased respectively and also the above oscillation can be suppressed. Particularly in the case of the PWM inverter, it is advantageous to suppress the electrical oscillation by adjusting the output frequency. In other words, the PWM inverter has a stepwise voltage change when the output voltage is increased to the maximum as described later. If the filter circuit resonates to the stepwise voltage change, the voltage from the PWM inverter in the maximum output voltage condition can not be adjusted continuously, and therefore it is ineffective to suppress the oscillation. However, the electrical oscillation can be suppressed by adjusting the operating frequency of the inverter. The above feature and other features of this invention will become more readily understood from the following exemplary description taken with the accompanying drawings, wherein: Fig. 1 is a circuit diagram of one embodiment of the control apparatus for an electric car propelled by an induction motor according to this invention; Fig. 2 is a graph of the phase and gain characteristic curves of a band-pass filter circuit; Fig. 3 is a waveform diagram of voltage and current at each part useful for explaining the oscillation suppressing operation according to this invention; Fig. 4 is a graph of the current-frequency characteristic curves of the induction motor; Fig. 5 is a graph of the pulse number and output voltage-frequency characteristic curves of the pulse width modulation inverter; Fig. 6 is a waveform diagram useful for explaining the operation of three-pulse modulation mode; ; Fig. 7 is a waveform diagram-useful for explaining the operation of one-pulse modulation mode; Fig. 8 is a circuit diagram of another embodiment of the control apparatus for induction motor driven electric car according to the invention; and Fig. 9 is a graph of the current characteristic curves of motor useful for explaining the oscillation suppressing operation in Fig. 8. Fig. 1 shows an arrangement of one embodiment of the control apparatus for an electric car propelled by an induction motor according to this invention. A current collected by a pantograph 2 from a DC power supply or power line 1 is supplied through a filter circuit 3 to a three-phase inverter 4. The L-C filter circuit 3 is consisted of a reactor 31 and a capacitor 32. The inverter 4 is an inverter chiefly having gate-turn off thyristors (GTO) 41 to 46 capable of pulse width modulation (PWM), or a general type thyristor inverter. The output of the inverter 4, or a three-phase AC of variable frequency and variable voltage is applied to a three-phase induction motor 5. The motor 5 drives the electric car and also allows a pulse generator 6 to rotate to generate a speed frequency signal fn corresponding to the speed of the electric car. The electric car has a slip-frequency fs settingup device 7, the slip-frequency fs from which is applied to an adding-and-subtracting device 8, to which the speed frequency signal fn is also applied. Consequently, upon power running, the device 8 calculates f = fn + fs, and upon regeneration, it calculates f = fn - fs so as to generate an inverter operation frequency command f. The electric car also has a motor current command T p setting-up device 9, the output Ip from which is compared with an output 1M from a motor current detector 10 by a comparator 11, thus a duty ratio command y being produced from the comparator 11. The inverter operation frequency command f and duty ratio (also called the modulation degree corresponding to the output voltage) command y thus produced are applied to a PWM- or simpleinverter control apparatus 12. The PWM inverter control apparatus 12 is well known, for example, in Hitachi Review, Vol. 61, No. 5 (1979-5), p. 27 to 32, particularly in the modulator section and gate processing signal circuit in Fig. 6 at p. 30. Thus, the induction motor 5 can be controlled in its frequency and voltage so that the motor current 1M follows the set-up value Ip with the set-up slip frequency fs. Of course, as described in the Hitachi Review, a control system can also be provided which controls the voltage to vary as the frequency, or the current control system can be replaced by this voltage control system. In such control system, an electric oscillation 15 caused as described above. Thus, the oscillating variation of the line current is detected and applied to the comparator 11 of the current control system to adjust the inverter output voltage. In other words, a current transformer 13 with an air gap can detect an AC component of the line current. The output of the transformer 13 is negatively fed back to the comparator 11 within the current control system, through a phase shift circuit 14 and a band-pass filter circuit 15. The phase shift circuit 14 is formed of a resistor 141 and a choke coil 142 connected in series. The voltage across the choke coil 142 results from delaying the output voltage of the transformer 13 by 90 degrees. The output of the current transformer 13 with an air gap can be regarded as a constant current source, and thus the voltage drop eR across the resistor 141 is given by eR =iR where i represents the current, R the resistance value (Q) of the resistor 141, and L the inductance (H) of the choke coil 142. If the internal resistance of the choke coil is such smaller than the resistance R, the voltage eL across the choke coil 142 is given by di eL =L dt and therefore it leads the eR by 90". If this voltage is inverted in its polarity, the output by the current transformer 13 can be delayed by 90 . The band-pass filter circuit 15 is formed of an operational amplifier 151, resistors 152, 153 and 154 and capacitors 155 and 156. The gain and phase characteristics of the band-pass filter circuit 15, as shown in Fig. 2, provides a high gain and in-phase input and output at around the resonant frequency, fr of the filter circuit 3. Fig. 3 shows the voltage and current waveform of each part, assuming that the output of the band-pass filter circuit 15 is not applied to the comparator 11. The oscillation suppressing operation will be described with reference to Fig. 3. When the voltage across the capacitor 32 oscillates as shown in Fig. 3a, the current in the reactor 31, or the line current leads the voltage across the capacitor 32 by about 900 as shown in Fig. 3b. The oscillating portion of the line current (AC component) is detected by the current transformer 13 with an air gap as shown in Fig. 3C. The output of the current transformer 13 is applied to the phase shift circuit 14, which then produces an output delayed about 900 with respect to the input, as shown in Fig. 3d. This output is applied to the band-pass filter circuit 15 where it is amplified because the band-pass filter circuit 15 has a high gain at the resonant frequency of the filter 3. If the output of the band-pass filter circuit 15 is positively fed back to the comparator 11 (upon power running), the tendency of the voltage across the capacitor 32 to increase having an oscillation will be suppressed by the increase of the duty ratio y, so that the inverter voltage when viewed from the power supply side is decreased. Thus, the voltage across the capacitor 32 is decreased by the increase of discharge current to the inverter 4 and thus the oscillating decrease and fade away. As a result, the oscillating phenomenon as shown in Fig. 3 does not occur. Upon regenerative braking, since the line current is reversed in its direction, the output of the band-pass filter circuit 15 is negatively fed back to the comparator 11. Fig. 4 shows the speed vs. current characteristics of the motor to which reference is made in explaining the oscillation suppressing operation. It is assumed that the car is running at the frequency fl, or at point a on the current characteristic curve I1 at a certain duty ratio y1. At this time, if the oscillation of the voltage across the capacitor 32 tends to increase, the duty ratio y is increased with the result that the operating point moves to the current characteristic curve 12, or from point a to b. Thus, the motor current increases to suppress the rise of the voltage across the capacitor 32. The PWM inverter compares a command for the operation frequency, for example, a sine wave and a carrier, for example, a triangle wave to perform pulse width modulation. The frequency ratio of the triangle wave to sine wave is changed in response to the operating frequency of the inverter. Fig. 5 shows the frequency ratio as the number of pulses n/Hz. The output voltage E from the inverter (proportional to the duty ratio y) is changed stepwise with the operation frequency of the inverter. This will be described with reference to Figs. 6 and 7 which are useful for explaining the PWM operation. The PWM operation itself is known and thus will not be described in detail. Fig. 6 shows the maximum voltage condition in the PWM operation with a triangle wave of triple the frequency of the sine wave. That is, the GTO or thyrister has a commutation period; if an extinction signal is produced at tl and then a retiring signal is produced before t2 after twin, commutation fails. Thus, the maximum output voltage in the three-pulses state is much lower than the inverter rating voltage. In order to increase the inverter voltage to larger than that value, it is absolutely necessary to decrease the triangle wave stepwise to a single-pulse condition, or rating voltage condition as shown in Fig. 7. The result is that as shown in Fig. 5, the inverter output voltage E is increased stepwise. The filter circuit 3 resonates to the stepwise increase of voltage to induce an electrical oscillation as set forth above. However, the output voltage from the inverter can no longer be adjusted continuously and this can not be suppressed by the apparatus of Fig. 1. Fig. 8 shows another embodiment of this invention which is capable of suppressing such electric oscillation. The output of the band-pass filter circuit 15 is fed back to the adding-or-subtracting device 8 within the frequency control system. That is, in Fig. 9, it is assumed that the car is running at point c on the current characteristic curve I1 with the inverter operation frequency fl and set-up slip frequency 5l. Symbol f, a represents the frequency corresponding to the speed of the car. Here, the tendency of the voltage across the capacitor 32 to increase having an oscillation is detected from the increase of the line current, and the operation frequency is increased to f2. Thus, since the speed of the car is not changed, the operating point shifts to point d and thus the motor current is increased, so that the discharge current from the capacitor 32 is increased without adjusting the output voltage from the inverter 4, thus suppressing the oscillating increase of the voltage. It will readily be understood that the tendency of the voltage across the capacitor 32 to decrease having an oscillation can be suppressed. In addition, upon regenerative braking, the output of the band-pass filter circuit 15 is of course negatively fed back to the device 8. Thus, the electrical oscillation is suppressed beforehand and as a result, current induced interference, commutation failure of the inverter and uncomfortableness in riding in car can be prevented which are caused by the electrical oscillation. While in the above embodiments the case of the DC electric car is illustrated, this invention can similarly be applied to the AC electric car in which case a converter is only required as a DC power supply for the inverter 4. In addition, the electrical quantity for use in the detection of oscillating variation may be a DC input current to the inverter, a voltage across capacitor, or a motor current. However, these quantities include a ripple, and thus the effect of this ripple must be prevented. The band-pass filter circuit is not always necessary for detecting the oscillating change from the line current. Moreover, although it is desirable that the comparison, adding and subtracting operations are performed in digital way by a computer, they can be performed in analog way. In this sense, the speed frequency and slip frequency of course include an analog signal.

Claims: WHAT IS CLAIMED IS: 1. A control apparatus for an electric car driven by an induction motor comprising: an inverter (4) supplied with electric power from a DC power supply (1) through a filter circuit (3); induction motors (5) for traction energized by said inverter (4); means (6) for generating a speed frequency corresponding to a car speed; means (7) for setting up a slip frequency corresponding to the slip of said induction motor (5); frequency control means (8) for adding or subtracting said slip frequency to or from said speed frequency to control the output frequncy of said inverter (4); voltage control means for controlling the output voltage of said inverter (4); means for detecting an oscillating variation of electric quantity in a circuit between said DC power supply (1) and said induction motor (5); and means for adjusting the output voltage from said inverter (4) in accordance with the output of said oscillating variation detecting means. 2. A control apparatus according to Claim 1, wherein said voltage control means comprises motor current command means (9), motor current detecting means (10), means (11) for comparing said current command and said motor current detected signal, and means (12) for adjusting the duty ratio of said inverter in accordance with the output of said comparing means and said output voltage adjusting means utilizes said duty ratio adjusting means (12). 3. A control apparatus according to Claim 1, wherein said inverter (4) is formed of phase arms (41) (46) each having a forced commutation ability, and said voltage control means is formed of means for controlling said inverter by pulse width modulation. 4. A control apparatus according to Claim 1, wherein said oscillating variation detecting means is formed of means for detecting an AC component included in a DC current between said DC power supply (1) and said filter circuit (3). 5. A control apparatus according to Claim 1, wherein said oscillating variation detecting means comprises a current transformer (13) with an air gap, a series circuit of a resistor (141) and a choke coil (142) for permitting the output current from the transformer to be flowed therethrough and circuit means (15) for taking out the voltage from said choke coil (142). 6. A control apparatus according to Claim 1, wherein said oscillating variation detecting means comprises a band-pass filter circuit for tuning to the resonant frequency of said filter circuit (3). 7. A control apparatus for an electric car driven by an induction motor comprising: an inverter (4) supplied with electric power from a DC power supply (1) through a filter circuit (3); induction motors (5) for traction energized by said inverter (4); means (6) for generating a speed frequency corresponding to a car speed; means (7) for setting up a slip frequency corresponding to the slip of said induction motor (5); frequency control means (8) for adding or subtracting said slip frequency to or from said speed frequency to control the output frequency of said inverter (4); voltage control means for controlling the output voltage from said inverter (4); means for detecting an oscillating variation of electric quantity in a circuit between said DC power supply (1) and said induction motor (5); and means for adjusting the output frequency of said inverter (4) in accordance with the output of said oscillating variation detecting means. 8. A control apparatus according to Claim 7, wherein said voltage control means comprises motor current command means (9), motor current detecting means (10), means (11) for comparing said current command and said motor current detected signal and means (12) for adjusting the duty ratio of said inverter (4) in accordance with the output of said comparing means (11), and said output voltage adjusting means utilizes said duty ratio controlling means (12). 9. A control apparatus according to Claim 7, wherein said inverter (4) comprises arms (41)-(46) each having a forced commutation ability, and said voltage control means comprises means for controlling said inverter (4) by pulse width modulation. 10. A control apparatus according to Claim 7, wherein said oscillating variation detecting means is formed of means for detecting an AC component included in a DC current between said DC power supply (1) and said filter circuit (3). 11. A control apparatus according to Claim 7, wherein said oscillating variation detecting means is formed of a current transformer (13) with an air gap, a series circuit of a resistor (141) and a choke coil (142) for permitting the output current therefrom to be flowed therethrough, and circuit mens (15) for taking out a voltage from said choke coil (142). 12. A control apparatus according to Claim 7, wherein said oscillating variation detecting means comprises a band-pass filter circuit for tuning to the resonant frequency of said filter circuit (3).

Inventors: IBAMOTO, MASAHIKO, SUZUKI, KATSUAKI, TANAMACHI, TOKUNOSUKE, TERUNUMA, MATSUHIRO, TSUBOI, TAKASHI
Applicants: HITACHI, LTD.
Application year: 1982
Main classification: H02P5
IPC codes: B60L9, H02P27, B60L3, H02P23