Patent GB-909577-A (GB)
Title: Improvements in or relating to control systems for electric motors
Inventor: CROMPTON ERIC
IPC: H02P25/28
Filed: 1959-08-18
Abstract: 909,577. Control of A.C. motors. GENERAL ELECTRIC CO. Ltd. Aug. 17, 1960 [Aug. 18, 1959; May 3, 1960], Nos. 28194/59 and 15553/60. Class 38 (3). [Also in Group XXXV] An induction motor 10 has forward and reverse motoring contactors 14, 15, and a dynamic braking contactor 16, a liquid resistance 18 in the rotor circuit being adjusted by an hydraulic servomotor 20 associated with pilot valve 21. A winding 22 for the valve is energized from magnetic amplifier 23 having a control winding 25 which is energized in accordance with the rotor current and is opposed by winding 26. The voltage from tachometer generator 32 is compared with a speed-setting voltage from potential divider 33 with a slider 35 coupled to a driver's control handle and a reversing switch 36. The signals from the elements 32, 33 are compared by a pair of bi-stable magnetic amplifiers 37, 38 energized selectively in accordance with the sign of the speed error signal, amplifier 37 operating contactor 14 or 15 and amplifier 38 operating contactor 16. The torque of the motor is thus controlled to reduce the speed error, and is also controlled by amplifier 41 in cascade with amplifier 45. During dynamic braking operation, the stator excitation is controlled by amplifier 46 which in turn is energized in accordance with the output of amplifier 45. During both braking and driving operation of the motor, the resistor 18 is dependent upon the output of amplifier 23 whose control winding 26 is energized from amplifier 45. If the output of amplifier 41 is zero there is no direct-current excitation for braking and the resistor 18 is set to its maximum value, such output being dependent upon the signals applied to the control windings of the amplifier and being zero unless the resultant sum of the signals is positive. Control winding 42 is energized in accordance with the rate of change of speed whilst winding 43 is fed directly by the speed error signal, whereby transition between braking and driving operation cannot be effected unless the speed error signal and the resultant sum of the input signals are of the same sign. Control winding 44 receives a signal proportional to rotor current, and hence the torque to compensate for speed variations due to load changes. A resistor 52 in series with the winding 44 is adjusted by the control handle and is reduced as the handle is moved to increase the speed setting. In a modification, the torque control signal and a signal for controlling the direct-current excitation during braking are derived from a magnetic amplifier having a control winding fed from a pair of auto-excited push-pull amplifiers each of which has six control windings. The first two windings are energized in accordance with the rate of change of speed, the third windings being fed selectively with the speed error signal. A fourth winding has a rotor current-dependent signal and a fifth is used for biasing. The sixth windings are connected across the coils of the braking contactor and the driving contactor respectively. In this arrangement, the pushpull amplifiers are selectively locked out in dependence upon whether the motor is connected for driving or braking, Fig. 2 (not shown). Specification 717,253 is referred to.