GB616916A - Improvements in and relating to electric motor control systems - Google Patents

Abstract

616,916. Exciting dynamo-electric machines; automatic current control systems. HOLLYER, E. B., and METROPOLITAN - VICKERS ELECTRICAL CO., Ltd. Sept. 20, 1946, No. 28268. [Classes 35 and 38 (iv)] [Also in Group XXXVII] In a constant current system a voltage regulating means responsive to the motor armature voltage is connected with means controlling the motor excitation whereby to limit the maximum armature voltage of the motor irrespectively of the speed thereof, said regulating means comprising a control circuit responsive to the motor armature voltage for producing an excitation effect tending to reduce the motor excitation and acting in opposition to an excitation effect dependent on the setting of a control means, and said voltage regulating means being of a saturating kind whereby to provide regulation of the motor voltage only when the motor voltage is above a given value near the desired limiting voltage. An electric motor 1 is supplied from a constant or controlled current source and is provided with a field winding 2 energized from an exciter 3 having a main control winding 4 and an opposing field winding 5 excited in accordance with the armature voltage of the motor 1. The exciter 3 is heavily saturated so that, for armature voltages below a predetermined value, the motor field current is maintained substantially constant, but, when the strength of the winding 5 approaches that of the winding 4, the exciter voltage falls rapidly and the degree of motor field excitation is controlled to limit the armature voltage to a desired maximum. In an alternative arrangement, Fig. 3 (not shown), a vibrating-contact regulator may be employed, having, for example, a spring for affording a controlling input tending to produce motor excitation, and a coil energized in accordance with motor voltage acting on the regulator to reduce motor excitation. The regulator is designed so that for voltages below a critical value, the spring is not overcome, thereby maintaining maximum motor field, at higher voltages the regulator being operated to limit the motor voltage to a given value. As shown in Fig. 4, the motor armature 1 is supplied from a generator 20, a field winding 25 of which is energized by an exciter 22. The exciter 22 has a field winding 23 energized in accordance with the output current of the generator 20, and a field winding 21 energized from the exciter 3 in parallel with the motor field winding 2. The armature current of the motor is thereby maintained substantially proportional to its field strength, being reduced when the field excitation is reduced upon the motor voltage reaching the critical value referred to above. The motor is provided with a tachometer generator 26, which is preferably critically excited so as to come into operation when the motor speed reaches a high limiting value, to energize an exciter winding 27 and reduce the generator excitation and motor armature current. When two motors are supplied in parallel from the source, Fig. 5 (not shown), each motor has a field winding supplied by an exciter with an additional field winding connected in series with a corresponding winding of the other motor across resistors connected in series with the motor armatures. The additional exciter windings are thereby excited in accordance with the difference of the armature currents and are arranged to correct such difference. In Fig. 6 (not shown), each exciter has additional opposing windings each connected across a resistor in each separate motor circuit. This enables the motors to be supplied in a " Figure-8 " connection.