Unity power factor and efficiency control of a voltage source inverter-fed variable-speed induction motor drive

Variable speed AC drives with inverters contribute to the distortion of line side current waveforms and to poor power factors. They have inherent switching losses, and they also de-rate the motors due to harmonic losses. The scheme proposed here maintains the linearity of the modulation index throughout the speed range of the operation and the total losses within the permissible limit, and hence improves the de-rating of the motor. This is achieved by incorporating a boost converter in the conventional inverter-fed induction motor drives. This simple boost converter with a diode bridge rectifier enables unity-power-factor operation and improves the input current waveform. The magnitude of the DC bus voltage and proper selection of the switching pattern in the speed range, where the total loss exceeds the permissible loss-limit, is obtained by the control circuit. This unity-power-factor, efficiency-optimal control (UPFEOC) is achieved by using a personal computer. In this scheme, minimisation of the control hardware and application of a simple algorithm are implemented. The simulation results are validated with a prototype of an IGBT-based voltage-source inverter drive