Harmonic torques in two-phase induction motors using six-step drives

The use of six-step drives for two-phase induction motors supplied from controlled d.c.-a.c. inverters promises a new lease of life for a.c. machines in position control applications. Unfortunately the steady-state static torque produced when both windings are simultaneously controlled by six-step waveforms in quadrature is reduced from that predicted by a fundamental analysis due to the presence of third harmonic in the waveform. There is also a significant oscillatory component of torque present with a frequency of four times line frequency. Although these effects have been observed in the past, no attempt has been made previously to provide a coherent analytical explanation. This paper presents such an explanation using a Fourier analysis in conjunction with the generally accepted motor model.