Failure of VSD voltage output waveforms on motors

Photo: Cephas Mutize, Sasol/Stellenbosch University
Photo: Cephas Mutize, Sasol/Stellenbosch University

Current waveforms are important because they determine the effect of the Variable Speed Drive (VSD) current on the supply, and the VSD output current is the primary factor that affects thermal aspects of the driven motor. While the latter current harmonics are crucial because they determine the increased operating temperature of the motor -the high frequency components of the VSD voltage waveform stress most sections of the motor insulation to extents that can cause infant failures in serious cases, and even medium- to long-term failures in the case of over-voltage VSD effects (as opposed to purely high-frequency effects).

Naturally – understanding and hence mitigating these high frequency effects can lead to greatly improved motor long-term reliability – or even reduce infant failures – of VSD driven motors. Reliability of the latter has undoubtedly improved drastically over the last decade – but there is still a lack of appreciation for why this is the case, and there are still failures related to misapplication – especially in the case of VSD retrofits to existing motors.


Take Note

  • ‘Good judgement comes from experience – but a lot of that comes from bad judgement’.
  • While VSD current waveforms are important for both the supply and the motor, often the output voltage waveform is not considered.
  • Motor insulation is affected by voltage – and hence the voltage waveform must be understood.

Published as:

A closer look at VSD voltage output waveforms
… and their failure effects on motors

By R Melaia, LH Marthinusen

Electricity+Control, August 2012 (pgs 22 – 26)