GB1418573A - Ac motor and control circuit - Google Patents

Abstract

1418573 Control of electric motors H FORSSELL 19 April 1973 [21 April 1972] 18920/73 Heading H2J A polyphase A.C. motor M is energized by a control circuit comprising a static converter 1 which includes controlled rectifiers commutated in dependence on the amplitude of flux in the motor. A step switch 3, which may be a ring counter, receives inputs from a pulse generator 2 and provides outputs for commutating the converter rectifiers in the desired sequence. The step switch 3 is stepped on by a digital output discriminator 7 whenever the amplitude of flux in any direction in the motor exceeds a reference value set by a potentiometer 6. The flux sensor 4 may comprise Hall plates but in a preferred embodiment comprises a circuit producing an output indicative of the growth of magnetizing current in the motor and hence of the growth of flux. This circuit comprises reactances 11, Fig. 2, so chosen that the growth of current in them corresponds to the growth of current in the motor windings, the reactances being connected to the supply voltage through resistances 10 and current transformers 9 which compensate for the resistive drop in the motor windings. The growth of current in reactances 11 is monitored by current transformers 12 feeding a diode bridge 13 which in turn feeds to the summation device 5 the signal indicative of flux amplitude. The converter 1 is also controlled by a current regulator 17 connected to a diode bridge 16 supplied from additional windings in current transformers 9. Speed and/or torque control may be achieved by altering resistances 14, 15 associated with reactances 11 and current transformers 12, or by altering the setting of potentiometer 6, or by altering the output of a device 40 which may be used to feed a speed indicative signal from tachogenerator T to the summation device 5. Alternatively, current regulator 17 may be controlled by a device 41 which compares the tachogenerator output with a reference value set by potentiometer 42. Variation of the potentiometer setting alters the motor speed. The control circuit may also be operated to provide regenerative braking.