ANALYSIS OF MULTI INVERTER FED INDUCTION MOTOR

Abstract

Variable speed AC Induction motors powered by switching power converters are inevitable as they offer an easy way to regulate both frequency and magnitude of the voltage and current applied to the motor. As a result, much higher efficiency and performance can be achieved by these motor drives. A multi inverter system based on phase shifted triangular carrier wave technique using closed loop v/f control is proposed here. In multi inverter topology several voltage source pulse width modulation (PWM) inverters using common sinusoidal modulating signals and phase shifted triangular carrier are operated in tandem with their outputs coupled through current sharing inductors; with an objective of increasing the overall rating , reducing the harmonic content, increasing the reliability with phase redundant operation under various fault conditions. Also the effective carrier frequency for N inverters connected in parallel increases to N. ff , which in turn reducing the total harmonic distortion of current and voltage considerably. However one of the shortcomings of the asymmetric regular sampling PWM technique is the limitation placed on the fundamental voltage that cannot exceed 50% of the input dc voltage at unity • depth of modulation (M). So, one method of increasing the output voltage is achieved by introducing a certain amount of third harmonic to the phase voltage waveform. The line amplitude of the fundamental component then increases without need for over modulation. Scalar control or v/f control, which requires the magnitude and frequency of the voltage applied to the stator of a motor maintain a constant ratio, is widely used for induction motor drives due to its simplicity and low cost.