Case Study #1

The Problem. Personnel from a Water Treatment Plant in Ontario, reported operational concerns regarding two 600 V, 50 HP pump motors in one of their pumping stations. Both motors are fed via PWM AC Drives. Within a year of operation, both pump motors were replaced due to premature insulation failure.

Summary. Instantaneous over-voltages in excess of 1.2 kV peak were measured at the motor terminals. The cable feeding the pumps consisted of a 3 conductor No. 6 AWG, with an approximate length of 95 feet. Furthermore, the motors being used were not rated for inverter duty.

Solution. The motors were replaced with inverter duty units. In addition, a properly sized dv/dt filter was installed at the output of the PWM AC Drive to reduce the voltage overshoots at the motor terminals.

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