Simulation of variable speed induction motor at constant stator current operation

Manufacturers of variable speed drive (VSD) systems are supplying custom built motors designed for optimum performance at 50 Hz for variable frequency applications. This is based on an assumption of a derating factor of about 20-25%, on account of additional harmonic losses generated by the nonsinusoidal input waveform to the induction motor. In this paper, reliable and integrated investigations of computer simulation of performance of a CSI fed high power, 4.1 kV/4 pole squirrel cage induction motor are presented. Analysis is based on the constant stator current mode of operation. Skin effect of rotor bars used in high power machine is considered. Elaborate methods are used to determine harmonic stray load losses. Performance results are generated when the induction machine is fed by stator currents of 0.8, 1.0, 1.5 PU at different stator base frequencies of 40, 50 and 60 Hz. Harmonic suppression filter is used to reduce the harmonic losses substantially. Motor losses, efficiency and other performance characteristics are generated and compared. Darting factor is computed and redesign considerations are discussed.