Using Corona Inception Voltage for Motor Evaluation

Failures of the interturn insulation system of induction motors fed by pulse-width modulated (PWM) variable speed controllers have plagued the industry since the introduction of fast-switching, IGBT-based drives. The majority of these failures are attributed to high voltage stresses imposed upon the motor by the drive. Several excellent articles have been written which explain the mechanism by which substantial over-voltages can be generated at the terminals of inverter-fed motors under certain conditions (see Fig. 1) [1]-[11]. The magnitude of these voltages are a result of overshoot at the drive's output, impedance mismatch between the drive and the motor, and the PWM algorithm used by the drive[10], [11]. Problems ensue primarily (although not exclusively) in applications which have long lead lengths between the drive and the motor. In worst-case situations, voltage spikes as high as 3 times the DC bus voltage of the drive have been reported [10]-[12], This can result in a peak value of 2150V if a 460V drive system is operating at a 10% over-voltage. This voltage can be as high as 2685V in a corresponding 575V system.