An Equivalent-Circuit Model for Phase-Back Voltage Control of AC Machines

The use of a simple series reactance model to represent the fundamental-component behavior of phase-back silicon-controlled rectifier (SCR) or silicon bidirectional thyristor (Triac) switches controlling an ac machine is explored. A conceptual model of the phase-back switch as a harmonic power converter is developed to explain the reactance model and its limitations. Several sample applications, which are compared to ``exact´´ state space computer solutions, are presented to illustrate the accuracy and limitations of the method. The ratio of harmonic power to fundamental-component VA at the switch is used to develop an impedance ratio criterion to determine the applicability of the model to specific cases.