A Variable Low-Frequency Inverter Using Thyristors

An inverter using thyristors, commutated by small inductance and capacitance only, and having an output frequency range suitable for motor speed control, is developed. The circuit operates as the combination of a high-frequency inverter and a cycloconverter in a single stage. As a high-frequency inverter its commutation requirements are very simple. As a cycloconverter it holds its output frequency independent of variations in load and is controlled entirely by the gating circuits. The output has high-frequency ripples characteristic of the cycloconverter, and the inverter operates with a wide range of resistive inductive loads. The inverter circuit is analyzed, and the limitations on the load circuit are discussed. Characteristics of an experimental inverter built to work in the frequency range of 1 to 70 Hz are presented. Certain problems that are to be solved before applying this inverter for control of induction motor speed are also indicated.