Investigation on insulation requirement for motor driven by PWM converter

This paper present an investigation on over voltage occurring at motor terminals when motor is supplied with a PWM converter. When motor is operated on sinusoidal supply, then, during switching period voltage across motor phase becomes equal to supply line voltage and the maximum voltage across any of the coil obtained is 30% more than the rated value; however this is not producing large over voltages or high rate of rise of voltage. Voltage rise time when motor is supplied with PWM converter is very small and is of the order of micro second. In some cases the peak voltage at motor terminals reaches to a value which more than twice the value of dc link voltage and rated line to line voltage of the motor. The distribution of voltage during switching condition is not equal among the coils of a winding and hence turns. The voltage drop in coils near terminal is more than that of in other coils. The measured voltage drop across first coil from terminal of winding of four coils in series is varying 30 to 56% of the phase voltage against 25% of the phase voltage. During the transition period voltage across first coil may be 70% of the total voltage. This paper highlights the factors which affects the magnitude of over voltage and its propagation in motor winding.