Mechanism And Model Of Motor Overvoltage In ASD System

In an ASD (adjustable speed drive) system, the high dv/dt is produced when the power semi-conductor devices switch between turn-on and turn-off at a high frequency. The high dv/dt will cause a big impact to the inverter driving motor. On some occasions, the motor is installed far from the inverter, and the motor and the inverter are connected by a long cable, which results in the overvoltage and high frequency damped oscillation at the motor terminals. In a serious condition, the motor will break down the insulation performance of the motor or cable, and even burn out the motor, or burst the attached cable. So it is of important significance to study the mechanism and modeling of the ASD system.Based on the transmission line theory, this paper analyzed the propagation mechanism of PWM pulses in a transmission line, and presented an arithmetic circuitry of the ASD system to calculate the motor terminal voltage, and deduced the expression of the motor terminal voltage. It also analyzed the influence of the pulse rise time, cable features (length, inductance and capacitance), the motor characteristics, switch frequency, and the duty ratio on the motor voltage.This paper studied the amplitude, oscillation and damping features of the overvoltage, and the result shows that it is some specific frequency components of the PWM pulse which cause the motor terminal overvoltage. In final, the paper built an overvoltage model of the ASD system in Matlab/Simulink, and experiments were performed to prove accuracy of the overvoltage model.