A novel hybrid synchronous PWM inverter based IFO drive system

Synchronous Sine Triangle Pulse Width Modulated (SPWM) inverter operating an induction motor ensures lower harmonic losses and better drive system performance. Conventionally, this synchronism is realized by linear frequency controller which multiplies the fundamental frequency with a suitable number to attain the value of carrier frequency, thus linearly increasing the carrier frequency as the motor speed increases.This work is based on the offline analysis of motor current data and selects the optimal carrier frequency for various operating speeds. This optimal carrier frequencies data results into a second degree polynomial which is then incorporated in the drive system controller. The proposed polynomial dynamically adjusts the carrier frequency with the changing motor speed which results in lower total harmonic distortion and lesser overall drive system losses as compared to the linear frequency controller. This work also suggests a novel strategy for switching the inverter mode of operation from sine triangle to square wave when the machine speed reaches above the base speed. The proposed methodology for mode switching is realized by adjusting the carrier frequency of the triangular waveform to zero which makes this transition seamless and does not cause any torque ripples. The so designed drive system with synchronous SPWM and mode switching from SPWM to square wave and vice versa is simulated and its performance is experimentally validated on a 175 Watt prototype induction machine.