EMI in Modern AC Motor Drive Systems

In this paper, several aspects of high frequency related issues of modern AC motor drive systems, such as common mode voltage, shaft voltage and resultant bearing current and leakage currents, have been discussed. Conducted emission is a major problem in modern motor drives that produce undesirable effects on electronic devices. In modern power electronic systems, increasing power density and decreasing cost and size of system are market requirements. Switching losses, harmonics and EMI are the key factors which should be considered at the beginning stage of a design to optimise a drive system. Introduction Nowadays, more than 60 percent of the world’s energy is used to drive electric motors. Due to growing requirements of speed control, pulse width modulated inverters are used in adjustable speed drives. Rapid developments in semiconductor technology have increased the switching speed and frequency of power switches dramatically. In a motor drive system, a voltage source converter with hard switches generates high dv/dt, which causes leakage currents due to stray capacitances in an electric motor. As shown in Fig.1, a modern power electronic drive consists of a filter, a rectifier, a DC link capacitor, an inverter and an AC motor. Many small capacitive couplings exist in the motor drive systems which may be neglected at low frequency analysis but the conditions are completely different at high frequencies. Electromagnetic Interference (EMI) is a major problem in recent motor drives that produces undesirable effects on electronic devices. In modern power electronic systems, increasing power density and decreasing cost and size of a system are market requirements. Switching losses, harmonics and EMI are the key factors which should be considered at the beginning stage of a design to optimise a drive system. Common mode voltage creates shaft voltage through electrostatic couplings between a rotor and stator windings and the rotor and a frame which can cause bearing currents when the shaft voltage exceeds a breakdown voltage level of the bearing grease. An increase in the carrier frequency of voltage-source Pulse Width Modulated (PWM) inverters based on high-speed switching devices has improved operating characteristics of the inverters. High speed switching can generate the following serious problems due to high dv/dt: Ground current escaping to earth through stray capacitors • inside a motor Conducted and radiated noises • Shaft voltage and bearing current • Models of parasitic couplings and high frequency components for an inverter fed induction motor drive system are investigated to determine suitable models to predict