Petro Min Engineers were approached by a client who had been experiencing multiple AC induction motor winding failures on their on-site AC Variable Speed Drive systems. Several attempts had been made by third party consultants to determine the root cause.
The client arranged for a string test to be performed at their Perth facilities (witnessed by the mine site owners) to replicate the on-site system, including the following:
- Passive line filter,
- 1kV PWM VSD
- Factory supplied VSD output dV/dt filter
- Various motor cable types and lengths
- Vent fan and motor assembly.
- Additional test VSD output dV/dt filter
- VSD output Sin filter.
After a day of testing, Petro Min engineers were able to determine that the root cause of the motor insulation failures was due to ringing in the PWM waveform exceeding the motor rating. The issue had been compounded by the following:
- Insufficient sizing of factory-supplied dV/dt filter.
- Previous third party consultants had used incorrect instrumentation unable to detect the ringing phenomena.
Testing was performed with various dV/dt filters and with a sinusoidal filter which reduced the ringing effect, to within the motor datasheet requirements. The following waveforms were obtained which demonstrate the ringing effect measured at the motor terminals with the factory fitted dV/dt filter, with increased dV/dt reactance and with a sinusoidal filter.