GB913868A - Improvements in and relating to d.c. electric motor equipments - Google Patents

Abstract

913,868. Control of D.C. motors. ENGLISH ELECTRIC CO. Ltd. April 21, 1959 [April 22, 1958], No. 12814/58. Class 38 (3). [Also in Group XXXV] A separately-excited D.C. motor 10 has its armature connected to a supply transformer 12 through a grid-controlled converter 11, and, under motoring conditions, a winding 22a of a grid control unit is energized in accordance with the excess of a reference voltage from potentiometer 17 over a speed-dependent voltage from tachometer generator 19. The converter then acts as a rectifier and the motor operates at a stable speed. A reversal of the speed-error voltage reverses a control winding 16b of a bi-stable amplifier 16, so reversing the motor field excitation and the back E.M.F. As a result, polarized relays 29a, 29b operate to change-over the connections to a device 21 for operating the converter 11 as an inverter in accordance with the excess of the speeddependent voltage over the reference voltage, thus producing a braking current for the motor. A magnetic amplifier 15 has a control winding 15b which is arranged to reduce the energization of an exciter field winding 14a and hence the motor field excitation. The control winding 15b is supplied from magnetic amplifiers 35, 36, developing opposed voltages across resistors 37, 38. Control windings 35a, 36a are excited in accordance with the converter supply voltage, whilst windings 35b, 36b are supplied in accordance with the magnitude and sense of the motor field excitation. In addition, the energization of the windings 35c, 36c is dependent upon the magnitude and direction of the voltage developed across the motor armature. Under forward motoring conditions, with the converter operating as a rectifier, both of the amplifiers 35, 36 are biased so that the control winding 15b is de-energized. Any tendency for the motor voltage to increase above a predetermined value when the converter is operating as an inverter, however, increases the output of the amplifier 35 so that it exceeds the output of amplifier 36 and the winding 15b becomes energized in a sense tending to decrease the motor field excitation. In this manner, the motor voltage is prevented from substantially exceeding a value dependent upon the A.C. supply voltage. If this voltage falls below its normal value, a smaller motor voltage is sufficient to cause the amplifier 35 to energize the winding 15b and hence decrease the motor field excitation. Under braking conditions, when the motor is operating in the reverse sense, the amplifier 36 becomes effective to prevent the motor voltage from becoming excessive. In a modification, the amplifier 36 is omitted and the amplifier 35 has windings 35a, 35c only, a rectifier preventing energization of winding 35c except when the converter functions as an inverter. A reversing switch between the amplifier 35 and the winding 15b energizes the latter in a field-reducing sense for both directions of motor rotation. In another arrangement, an armature-reversing switch is controlled from the bi-stable amplifier 16, the motor field being excited uni-directionally from the amplifier 15. Specifications 892,231 and 894,583 are referred to.