Principles and problems of building dc electric drives with symistor control

Reversible symistor (triac) converters are considered for d.c. electric drives. The main problem is the much lower reliability of symmetric thyristors than that of plain ones, owing to the possibility of spontaneous reversal in the absence of a resolution signal from the switching logic when the device is controlled by unipolar pulses or upon reversal of voltage polarity across the power terminals when the device is controlled by bipolar signals. Spontaneous symistor reversal can be prevented either by limiting the reverse dv/dt to the permissible commutation level or by delaying application of the reverse voltage till after recovery. The latter condition is achieved by means of saturable reactors. The authors propose using a polyphase laminated reactor core, with alternating solid and slit laminations, so as to combine these delaying reactors and the switching reactors with common windings. Two methods of controlling symistors were developed which not only improve their reliability but also reduce the size of such dual reactors involving, respectively, monitoring the point where the counter-emf wave intersects the voltage wave across the switched-off rectifier or monitoring the voltage polarity across the switched-off symistor.