Adjustable Speed AC Motor Drives-Applications Problems

Abstract

Adjustable speed AC drives have become the preferred choice in many industrial applications where controlled speed is required. At the same time, the maturing of the technology and the availability of fast and efficient solid state power semiconductor switches (IGBTs) has resulted in voltage source, PWM controlled inverters becoming a standard configuration in the power range to 500kW. While high frequency PWM control represents the most advanced drive concept, when inappropriately applied, it generates side effects, some which have been recognized only recently. This paper presents a comprehensive coverage of application issues of PWM inverter controlled ac motor drives which include damage to motor insulation due to reflected voltages caused by long motor leads, electromagnetic interference, the mechanism of motor bearing failures due to electrostatic discharge leakage currents to ground, and poor total harmonic distortion (THD) of utility line currents. Following a description of the problems, several practical solutions are also presented and analyzed. Finally, line harmonic problems caused by the input diode rectifier are also described and some low cost solutions selected to meet IEEE 519-1992 are presented.