Motor insulation breakdowns due to operation of frequency converters

This paper deals with transient overvoltages in frequency converter-fed AC motor drives. These overvoltages stress insulation system of fed motor and dramatically decrease motor lifetime. Exact model of drive system that enables fast design of protections was developed. Motor overvoltages, their influence on motor insulation and influence of inverter control on surges are explored. Obtained results are compared with existing standards. Recommended voltage capability of stator winding insulation is exceeded in many cases as is presented. A number of simulations and experiments were performed for real drives. As an illustrative example, tests performed on main drive of light rail vehicle recently developed in Skoda electric are presented.