In single-phase and three-phase ac circuits the current passes through zero and thereby changes its polarity after each half-wave of the mains cycle. If one introduces a semiconductor switch into a single-phase ac circuits, it must be designed to carry current in both directions; for instance it could consist of two thyristors in antiparallel (see Fig. 2.2). In such a semiconductor the current begins to flow as soon as the thyristors are triggered. The current half-cycles of different polarity are carried alternately by the two thyristors. So that a continuous alternating current can flow the thyristor taking over the current must be retriggered after each current half-cycle. If the thyristor is not triggered, the alternating current is extinguished as it passes naturally through zero.

Semiconductor Switches and Power Controllers for AC

In Wechsel- und Drehstromkreisen geht der Strom nach jeder Halbschwingung der Netzperiode durch Null und wechselt dabei seine Polaritat Fugt man in einem Wechselstromkreis einen Halbleiterschalter ein, so mus dieser fur Stromfuhrung in beiden Richtungen ausgelegt sein, beispielsweise aus zwei gegensinnig parallelen Thyristoren bestehen (s Bild 22) In einem solchen Halbleiterschalter beginnt der Strom zu fliesen, sobald die Thyristoren gezundet werden Die Stromhalb Schwingungen unterschiedlicher Polaritat werden abwechselnd von den beiden Thyristoren gefuhrt Damit ein kontinuierlicher Wechselstrom fliesen kann, mus nach jeder Stromhalb Schwingung der stromubernehmende Thyristor neu gezundet werden Wird der nachste Thyristor nicht gezundet, so erlischt der Wechselstrom im naturlichen Nulldurchgang

Halbleiterschalter und -steller

A novel antiparallel-connected thyristor scheme for feeding highly inductive loads, such as the field of dc machines, alternators, is described. A simplified analysis of the power scheme is described and the choice of the circuit element values based on this analysis is also detailed. Transient analysis of the power circuit is carried out using a general-purpose power electronic converter simulation program, named ATOSEC I, and is compared with experimental waveforms. The suitability of the proposed scheme is demonstrated in the case of a reversible dc drive based on the familiar Ward-Leonard scheme. The performance specifications of an experimental reversible drive scheme are given.

Gerald Lemire

Papers

Antiparallel-Connected Thyristor Scheme Suitable for Feeding Highly Inductive Reversing Load