Effects of switching frequency and motor speed on common mode voltage, common mode current and shaft voltage in PWM inverter-fed induction motors

High-frequency components and large amplitudes of the common mode voltage (CMV) at the motor neutral point generate high-frequency common mode current (CMC) to the ground path and induce voltage on the shaft, which eventually reduces the machine life. This paper presents the effect of switching frequency and motor speed on CMV, CMC and shaft voltage. Three parameters viz. common mode voltage peak factor, common mode current peak factor and shaft voltage peak factor are proposed to define the stresses generated at ground wall insulation and bearings when induction motor is operated with inverter. The results can be used for verifying the feasibility of the mitigation techniques for CMV and bearing currents at different switching frequencies and motor speed.

I. Introduction

Induction motors are the work horse of the industries. The energy consumption of all motors is approximately 70% of the total energy produced in the power system. Over past three decades, standard induction motors that have been designed to operate from fixed frequency sinusoidal power are being used with pulse width modulated (PWM) variable frequency drives (VFDs), also called as inverters in an increasing number. The application of PWM inverters has permitted the high performance and accurate control of electrical machines as well potential for energy saving. Insulated gate bipolar transistors (IGBTs) are the preferred power switching device in modern PWM inverters. Switching frequency is the rate at which the IGBTs fire to create PWM output. The PWM switching frequency from most drive companies usually ranges from 1 kHz to 20 kHz. The output voltage waveform of today's PWM drive is not a sine wave but, a series of square wave pulses that produce a reasonable approximation of sine wave current. PWM voltage waveforms have voltage spikes of large magnitudes with high slew rate, hence increase voltage stress in winding insulation [1], [2]. The frequency spectrum of these voltages reveals the presence of high frequency harmonics of non-negligible magnitude, thus increasing the thermal stress in the insulation [3]–[5]. The additional stresses so generated reduce the life of motors, hence leading to premature failure [6]. Various industrial surveys show that the additional stresses generated in the stator winding insulation are one of the leading root causes of 30%-40% of induction motor failures.