Correct application of VSDs and motors

By Johan van Niekerk, ZEST WEG Group


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Published in:

Energy Efficiency Made Simple Vol III: Energy and enviroFiciency

November 2012  Chapter 4: Pages 44 – 48

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The questions frequently asked by users regarding Variable Speed Drives (VSDs) and motors include:

  • By how much does a VSD reduce motor efficiency?
  • What is the effect on motor efficiency at different speeds?
  • Are the energy savings more than the increased losses?
  • Is it worth using a high efficiency motor on a VSD if one considers the reduction in efficiency caused by the VSD?


While it is understood that users seek clarity, it is impossible to give single answers to these questions because there are many variables and complexities.

Power consumption, and therefore costs, can demonstrably and effectively be significantly reduced if VSDs and high efficiency motors are correctly selected and appropriately applied. A realistic approach with correct engineering is required if optimum results are to be achieved. Four factors contribute:

  • Use of ac VSDs
  • Correct VSD and motor selection
  • Practical considerations for energy savings
  • Different levels of motor efficiency


Take note

  • There are real energy saving opportunities – with some negative aspects - when using VSDs.
  • It is essential to understand the drawbacks because ac motors are the single largest electrical power load.
  • Regulatory standard - IEC 60034-30:2008 (locally SANS/IEC 60034-30:2008) defines efficiency standards for motors from 0,75 to 375 kW, 2, 4 and 6 poles, 50 Hz and 60 Hz.