GB1319111A - Electric motor speed control - Google Patents

Abstract

1319111 Control of DC motors CHILES (PATENTS & SYSTEMS) Ltd 29 Oct 1971 [29 Oct 1970] 51572/70 Heading H2J A reversible motor may be driven at a variable speed in either direction of rotation by the control circuit shown, in which a unidirectional current corresponding to one or other direction is produced between AC supply terminals PT1, PT2 and motor terminals M1, M2 by firing of an appropriate one of thyristors SCR1, SCR2 whose current path is completed by a corresponding diode D3, D2. At the centre wiper position of a potentiometer R3, neither thyristor is fired and the motor is stationary. Variable resistors R4, R5 are adjustable to determine the instant in each half-cycle at which the thyristors conduct and thus provide adjustment of the "flat spot" in the centre of the potentiometer and compensation for differences between the thyristors and motors to be controlled. Automatic speed regulation at any particular setting of the wiper of potentiometer R3 is inherently provided by virtue of the fact that a variation in load on the motor varies its back EMF, which varies the trigger potential applied through members R1, R2, D1, R3 in a compensating manner, such that the firing point of the conducting thyristor is varied to restore the speed to its previous set value. The motor A is a universal motor having a field winding F supplied through a full wave rectifier bridge D4-D7. Alternatively the field winding F may be across the terminals M1, M2 and the armature A may be supplied through the rectifier bridge, Fig. 2 (not shown). Application of the circuit to control a seriesfield motor is also described. Fig. 3 (not shown). Alternatively the motor may have a permanent magnet or separately-excited field.