A Generalized Method of Computer Simulation for Induction Motors with Stator Current Discontinuities and its Application to a Cycloconverter-Fed Drive

A digital computer method of analyzing a complex variable-frequency drive system comprising a three-phase induction motor and a phase-controlled thyristor cycloconverter is presented. A generalized set of performance equations in d-q axes representation is first developed for the induction motor with thyristor switchings in the source taking into account the discontinuities in the stator current possible due to opening of stator phases. Logic variables are used to represent thyristor switches, and the induction motor model thus developed is then combined with a phase-controlled cycloconverter model developed earlier for noncirculating current mode of operation. The combined model is then used in a digital computer program to obtain dynamic and steady-state performance of the system. Typical computed steady-state current waveforms using step-by-step integration method are then compared with the measured waveforms obtained on an experimental setup to check the validity of the analytical approach. The technique used is quite general and can be adopted for analyzing any other thyristor converter-induction motor system.