GB1416681A - Electric motor control apparatus - Google Patents

Abstract

1416681 Firing circuits for thyristors PRINTED MOTORS Ltd 19 July 1972 [19 July 1971 9 Sept 1971] 33767/71 and 42132/71 Heading H3T [Also in Divisions G3 and H2] A triac controls a further triac in the supply circuit of a motor, Fig. 5 (not shown), and enables the further triac to supply the motor with a train of high power pulses in response to a single low power pulse. As shown in Fig. 8, the train is initiated by the application of the low power pulse to the primary winding of a transformer 101 which switches on a thyristor 102 controlling the charging of a capacitor 109 from a full-wave rectified supply 104 ... 108. When the capacitor voltage reaches a predetermined value, a diac 111 switches on to fire a triac 114 controlling the further triac, conduction of triac 114 assisting that of the diac to discharge the capacitor until the diac switches off. Thyristor 102 remains conductive through the action of members 109, 110 and the continued presence of the low power pulse, thus allowing the capacitor to recharge and again switch on the diac to fire triac 114. This operation is repeated until the end of the low power pulse, whereupon the thyristor 102 extinguishes. In the circuit of Fig. 6, high frequency output pulses are obtained from terminals 142, 143 by applying a high frequency A.C. supply to parallel-connected diode, resistor triac circuits 134, 136, 130 and 135, 137, 131, one or other of the triacs being fired by a positive or negative signal applied at terminal IN via diodes 132, 133. This signal is derived from the error signal of a process controller (see Division G3) and thus produces positive or negative half waves of the high frequency supply across the resistors 136, 137, which in turn control transistors 138, 139 to produce corresponding pulses at the terminals 142, 143 for application to a firing unit for thyristors controlling a D.C. electric motor, Fig. 7 (not shown).