GB808117A - Improvements in or relating to control systems for electric motors - Google Patents

Abstract

808,117. Automatic speed control systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. July 5, 1957 [July 10, 1956], No. 21297/57. Class 38 (4). [Also in Group XXXVII] Induction motors Ml, M2 are mounted on a common shaft and both are energized for driving a load to a predetermined position, at which the motor M1 is disconnected from the A.C. source and dynamically braked for reducing the speed at which the load reaches a second position. Assuming that the load is at a lower position, push-button BMA is set in its " automatic " position, and push-button HPB is actuated, the motors Ml, M2 are energized and start to rotate, the rotor resistances of the motors being reduced in timed steps until maximum speed is reached. When the load reaches an intermediate position, a limit switch is actuated to increase the rotor resistance of motor M2 and to interrupt the supply of power to the motor M1 so that driving torque is now supplied by motor M2 only. A tachometer generator TK is coupled to the motor-shaft, and this machine is now connected through a differentiating transformer DT to main input windings 151 of magnetic amplifier DA. The tachometer generator is also connected to the input windings of magnetic amplifier SA. A braking exciter EB is connected to the motor Ml, the magnitude of the braking torque being determined by the degree of excitation of field winding 142 which is, in turn, dependent upon the outputs of the amplifiers DA, SA. The output of the amplifier DA is determined by the pattern winding 155 which is opposed by the windings 151 and 153, the winding 153 being supplied from the amplifier SA. Initially, the rate of deceleration is low so that the pattern winding predominates, the output from the amplifier SA being small since the speed is high. At first, therefore, the exciter EB is heavily excited and the braking torque is large. When the speed has been reduced to a landing value, the bias winding 163 of the amplifier SA becomes predominant whereby the excitation of the field winding 142 becomes dependent primarily upon the speed rather than the deceleration. The effect of the amplifier SA is to counteract variations of speed and to keep it at a suitable value for landing. When the landing- position is reached, limit switches are normally opened to de-energize brake solenoid SO and apply the brake BD, the motors being disconnected and the apparatus re-set for lowering operation. The operation of an overrun limit switch de-energizes the drive and applies the brake, the load being returned to its proper position by moving back-out switch BDR to its " lower " position and actuating a reset pushbutton BR. With the load at its top position, it may be returned under manual control by setting switch BMA in its " manual " position, and closing push-button BPB so that motor AM, driving the exciters EB, E1, is energized. A master controller, Fig. 1C (not shown), regulates the amount of resistance in the rotor circuits of the motors for determining the downward speed of the load. Excessive speed causes tachometer generator TK to energize relay OS whereby the rotor resistances are reduced to a low value so that the motors tend to operate at synchronous speed and to reduce the rate of descent of the load. A special low speed for inspecting the apparatus is obtained by setting the switch BMA in the " automatic " position and closing push-button BSS. The tachometer generator TK is connected directly to the winding 151 of magnetic amplifier DA, and push-button HPB or LPB is actuated, depending on the position of the load. Power is supplied to motor M2, the A.C. supply circuit to motor M1 being interrupted. In addition, the exciter EB is connected to motor M1. A speed-responsive signal is applied to winding 151 and a corresponding current is impressed on the field winding of the exciter, whereby the speed is maintained at a low magnitude.