GB1245318A - Induction motor speed control arrangement - Google Patents

Abstract

1,245,318. Control of A.C. motors. STANDARD TELEPHONES & CABLES Ltd. 14 Oct., 1969, No. 50497/69. Heading H2J. A shaded-pole induction motor has a bifilar wound, centre-tapped stator winding W, a pair of electronic switches being connected to opposite ends of the winding and arranged to be alternatively conductive. One power terminal is connected to the centre tap whilst the other terminal is connected to both switches. Each switch may comprise a pair of parallel-connected, oppositely-poled thyristors or a single bidirectional thyristor. In one embodiment, a rectifier bridge is provided between the power terminals and the switches which each comprise a single thyristor, the bridge including two thyristors for phase control of the supply voltage. A firing circuit for the switching thyristors, which have their cathodes in common, comprises a unijunction transistor forming part of a free-running blocking oscillator, one base of which receives negative signals synchronized to the input frequency. A resistive capacitive network provides a trigger voltage for the transistor which breaks down to discharge the capacitor whereby a bi-stable circuit changes state. The latter circuit comprises a pair of transistors at whose collectors are developed two square wave firing signals displaced by 180 degrees, Fig. 3 (not shown). In an inverter arrangement with a smoothing capacitor, a trigger diode TD1 forms part of a free running oscillator. When capacitor C2 is charged to the break-over voltage of the diode TD1, a pulse is applied to a bi-stable circuit X2, X3 changing its state and so reversing the state of thyristors TH1, TH3. The voltage across the motor depends upon the voltage E, F, which also determines the motor frequency. Alternative firing circuits utilize R.C. circuits R4, C4, and R5, C5, and trigger diodes TD2, TD3. Thyristors TH1, TH3 are fired alternatively, the charge on capacitor C1 being reversed. Resistors may be introduced in series with each trigger diode, the resistors R4, R5, being each shunted by resistor-diode combinations. Resistors R4, R5, may be adjusted for equal conduction time of both thyristors TH1, TH3. A capacitor in parallel with the capacitor C5 prevents simultaneous firing when the motor is switched-on. Afterwards this capacitor is charged to reverse bias a diode which isolates it from the remainder of the circuit. The thvristors TH1, TH3, may be associated with auxiliary thyristors whereby the main thyristors receive gate drive for the whole 180- degree conduction period, Figs. 9, 10 (not shown).