GB1439643A - Control system for an ac induction machine - Google Patents

Abstract

1439643 Asynchronous motor control WESTINGHOUSE AIR BRAKE CO 9 Aug 1973 37738/73 Heading H2J Apparatus for controlling the operating point of an asynchronous motor, preferably for use with an electric railway locomotive, comprises a control loop consisting of an inverter 2 to provide 3-phase A.C. at 4 to the stator of motor 1, the rotor being connected to the output of the system and also to a tachometer 10 which provides signals indicative of rotor speed at 9 to an adder 8 for addition with a slip command signal 11 produced by manually operable regulator 19, the resulting signal at 7 being applied to inverter control logic 6 whereby a signal 5 is produced to control the frequency of the A.C. and hence of the rotating field applied to the motor stator, and also comprises means responsive to a deviation in the voltage at the input D.C. on lines 3 to the inverter 2 in excess of a predetermined amount for adjusting the slip control signal 11 independently of the regulator 19 to bring about a change in the operating point of the motor 1 to counteract the voltage deviation. The object of this is to enable inverter commutation to continue when breaks in the supply conductor are encountered by the locomotive, and to prevent protective trip circuitry from operating in the presence of transient surges. A capacitor bank 26 is charged by supply 3 and discharged by the motoring mode of motor 1. A transducer 25 is arranged to provide a constant reference on line 23 and a voltage on line 24 indicative of the capacitor voltage, which two signals are applied to network 22 comprising two operational amplifiers 27, 28 connected conventionally to provide a signal on line 20 as shown in the Figure if the voltage at line 24 increases above a predetermined limit, and a signal on line 21 if it decreases below a second limit. These signals are provided firstly to a fast acting control network 32 where by means of amplifier 31 and an exponential feedback arrangement, an output of exponential character is provided at 34. This is passed through differentiating network 35, 36 to a second amplifier which produces an output 16 which is proportional to the sum of both the value of the input and its rate of change. The signals at 20, 21 are also applied to network 41 to override the input from regulator 19 and provide an alternative output 13. The outputs 13, 16 are added at 12 to provide an alternative slip signal 11. Thus if the power at 3 is cut-off and the motor 1 is in a driving mode, rapid discharge at 26 occurs and a signal at 21 will be produced. The network 32 will react very quickly owing to its exponential and derivative character and provide a large negative signal at 11 to send the motor into a very strong regenerative mode. The capacitor will charge up again and the network 32 will loose influence relative to network 41 and a regenerative balance will be brought about until power at 3 is restored.