Patent GB-794313-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: B66C13/26
Filed: 1956-07-02
Abstract: 794,313. Control of A.C. motors. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. July 2, 1956 [July 11, 1955], No. 20396/56. Addition to 721,084. Class 38 (3). An induction motor for driving a hoist is connected, as in the parent Specification, to saturable reactors Id, Iu adapted to impose a controlled phase unbalance on the polyphase motor terminal voltage. The vernier potential derived from a manually-adjustable induction regulator VI is now supplied to the control windings of the reactors independently and in parallel with the speed and weight-responsive potentials derived from the tachometer generator T and unit W respectively. When the master switch MS is in its zero position, rectifier RV supplies the input windings 35, 45 of the inductors Id, Iu, with a potential which tends to increase the impedance of inductor Id and decrease that of inductor Iu. The weightresponsive rectifier RW and tachometer T are connected in circuit with the windings 35, 45 and, with load on the hook, the potential output of the rectifier is such as to tend to increase the impedance of inductor Id and decrease that of inductor Iu. A contactor M is energized when the master switch is moved to its first lowering position so that the inductors are connected to lines L1 ... L4. A mechanical brake B is released, the field winding of the tachometer generator is excited, and a circuit is completed to prevent de-energization of the motor before the brake B is engaged. As the motor rotates, the potential developed by generator T has the effect of decreasing the lowering torque and increasing the raising torque so as to stabilize the speed. In the second and third lowering positions, portions of resistor R1V in circuit with rectifier RB are short-circuited and, in the third position, the rotor resistance is removed. If the load is so heavy as to tend to overhaul the motor, rectifier RW develops a potential which produces a net up-torque in the motor the magnitude of this torque being precisely controlled by the vernier VI. A hoist booster rectifier RH is energized through resistor R2V when the switch MS is moved to its first hoisting position, the tachometer generator supplying a potential which tends to reduce the up-torque and increase the down-torque. In the second and third positions of the master switch, the resistor R2V is reduced in value, the elements RV, RW and T are disconnected, and rectifier RH remains connected only to the input winding 45 of the inductor Iu. In the third hoisting position, the rotor resistance is removed in timed steps. The output of rectifier RW sets the up-torque in dependence upon the weight on the hook, and the vernier VI may then be adjusted to yield the desired up-torque. In another arrangement, the vernier potential source provides two separate components supplied respectively to the input windings of the two inductors. The vernier components may be impressed through a pair of magnetic amplifiers. In accordance with another feature, the contactor which short-circuits the rotor resistance of the motor during a late stage of the hoisting operation, also shunts the inductor Iu, the impedance of which is reduced when the motor is to have a hoisting torque. Specification 598,063 is referred to.