- Variable Speed Drive
- Variable Frequency Drive
- Motor Failures
Fast voltage rise times of 1600 volts per microsecond can be typical as the motor lead length exceeds just a few hundred feet. Voltage rise time is referred to as dv/dt(change in voltage versus change in time). When the rise time is very fast the motor insulation system becomes stressed. Excessively high dv/dt can cause premature breakdown of standard motor insulation. Inverter duty motors typically have more phase-to-phase and slot insulation than standard duty motors (NEMA design B).
When motors fail due to insulation stress caused by high peak voltage and fast voltage rise times (high dv/dt) they have common symptoms. Most failures of these types occur in the first turn as either a phase-to-phase short or phase to stator short. The highest voltage is seen by the first turn of the winding and due to motor inductance and winding capacitance of the motor, the peak voltage and dv/dt decay rapidly as the voltage travels through the winding.
Normally, the turn to turn voltage in a motor is quite low because there are many turns in the winding. However, when the dv/dt is very high the voltage gradient between turns and between phase windings can be excessively high, resulting in premature breakdown of the motor insulation system and ultimately motor failure. This problem is most prevalent on higher system voltages (480 & 600 volts) because the peak terminal voltage experienced often exceeds the insulation breakdown voltage rating of the motor.