GB599696A - Improvements in or relating to alternating current electric motor control systems - Google Patents

Abstract

599,696. Control of A.C. motors. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Feb. 20, 1945, No. 4287. Convention date, March 9, 1944. [Class 38(iii)] [Also in Group XXXV] An impedance, possibly variable, connected in series with at least one phase of a multiphase motor, can reduce the motor torque, preferably to zero, by unbalancing the applied phase voltages, and further means-e.g. a transformer-can impose a voltage across the impedance for reversing the torque. In Fig. 1, the impedance is a saturable reactor SR connected between the supply and the terminal T2 of the motor M, and is variable by passing D.C. through coil 8. The transformer TR having its secondary winding connected across the reactor SR and its primary winding connected in series with a resistor R across the supply mains L1 and L2 can impose a voltage across the reactor SR. In Fig. 2, the supply voltages are represented by vectors L1L2, L2L3 and L3L1, the voltage across resistor R by ER, the primary voltage of transformer TR by EP and the voltage imposed on reactor SR by EX; the points T1 and T3, representing the potentials of terminals T1 and T3, coincide with points L1 and L3. The potential of terminal T2 is represented by a point which lies on vector L3L1 when reactor SR has maximum inductance, moves towards the point L2 as the coil 8 is energised and, when the transformer TR is effective, moves to a position such as T2 in Fig. 2 whereby the phase rotation and torque are reversed. Fig. 6 shows the application to the control of lowering of a load from the hoist drum D driven by motor M. A controller MC in its first hoisting position closes a relay CR to short-circuit the reactor SR, a contactor IH to close the motor circuit for hoisting, and a relay BR for energising the coil 2 (from transformer TR1 and rectifier R2) to take off the brake B. In subsequent positions the controller MC closes contactors 2A, 1A, 3A, 4A and 5A successively, to short-circuit the rotor resistance SRC. In its first lowering position, the controller MC again closes the relay BR and the hoisting contactor 1H, as well as contactors 2A and 3A andafter a time interval-contactors 4A and 5A. The reactor SR is now in circuit, with maximum inductance; the motor torque is zero. The control coil 8 is connectedthrough a rectifier R1-across the initially balanced bridge CB which includes the saturable reactors 5 and 6. As the loadfalling under its own weight-accelerates the motor M, a generator PG driven thereby-which could alternatively directly feed the coil 8-pre-magnetises reactors 5 and 6 to unbalance the bridge and so energises the coil 8 to reduce the reactance of the reactor SR; the motor M hence develops a counter torque in accordance with its speed. In subsequent lowering positions the controller MC re-opens contactors 4A, 5A and 3A successively, and, in its fourth position, re-opens the contactor 2A and closes, by contactor 2L, the secondary circuit (or, as in Fig. 1, the primary circuit) of transformer TR so that a small lowering torque is developed. Finally, the controller MC opens the contactors 2L and 1H, closes the lowering contactor 1L and contactors 1A-5A for developing full lowering torque. A relay 2CR can be closed by push PB (which is preferably on the controller handle and operable in the " off " and certain lowering positions) to disconnect the control coils from the generator PG and fully energise them from the transformer TR1 through rectifier R2, thereby increasing the counter torque, for accurate stopping. The same connection is made by a relay 3CR which is momentarily closed-to apply a retarding torque while the brake B is still held off-when the controller is returned to the " off " position, until it is opened by timing relay BT. The transformer TR2 and rectifier R3 supply the relay BT and also relays 4T and 5T which time the contactors 4A and 5A and of which the time of operation can be varied by control coils 86 and 96. During hoisting, the contactors 1H and the brake relay BR are subject to the limit switch LS. A no-volt relay ICR is included.