static regelgeraet for implementation of repetitive - and ausschaltungen a load to a direct current source is used, and lastkreis and source are affected

摘要

1,227,138. Control of D.C. motors. SEVCON ENG. Ltd. 7 June, 1968 [14 June, 1967], No. 27413/67. Heading H2J. In a static switching controller of the type wherein a thyristor repetitively connects and disconnects a load and a D.C. source and wherein the thyristor is switched off by a commutating capacitor forwardly charged from the source, the mean current is limited by reducing the ratio of the conducting to non-conducting periods of the thyristor in response to the current passing therethrough at the instant of commutation, this limitation occurring if a voltage dependent on the current exceeds a predetermined value which in turn is dependent on the inductance of the source/load circuit. Fig. 1 shows the application of the invention to the control of a D.C. series traction motor 2 by a main thyristor 1, a commutating capacitor 5, a turn-off thyristor 6 and a reversal thyristor 7. The voltage across the thyristor 1 is sensed by a diodecapacitor-resistor circuit 20, 21, 22 which controls, via a Zener diode 24 and transistor 25, an oscillator 9 for firing the thyristors 1, 7 at a rate dependent on the setting of an adjustable resistor 15. If upon commutation of thyristor 1, the voltage across circuit members 20, 21 exceeds a predetermined value, the Zener diode 24 breaks down to make transistor 25 conductive and so hold the relaxation oscillator 9 in a quiescent condition until the voltage across the capacitor 21 has fallen to a value enabling members 24, 25 to revert to their original conditions. Thus the firing of thyristors 1, 7 is delayed to an extent dependent on the amplitude of the transient voltage occurring on commutation of thyristor 1. The circuit comprising members 20 ... 25 may be modified, Fig. 2 (not shown), to make the firing delay more directly dependent on the amplitude of the transient voltage and hence on the current through thyristor 1. In another embodiment, Fig. 3 (not shown), the load current is sensed by a current transformer whose output controls a relaxation oscillator for firing thyristors 1, 7 in a manner similar to that already described. The current transformer may be replaced by an impedance connected in the load circuit and supplying an A.C. amplifier. The embodiment shown in Fig. 4 uses the load current on commutation to control the firing of the turn-off thyristor 6, the arrangement being such that the voltage of the capacitor 5 is increased by the transient voltage and, if the latter exceeds a predetermined value, causes breakdown of a Zener diode 105 to advance the subsequent firing of the thyristor 6 and hence the subsequent commutation of the thyristor 1. The embodiment of Fig. 3 (not shown) may be modified to control the thyristor 6 to achieve the effect of the embodiment of Fig. 4.