Inside a Variable Speed Drive (VSD)

We should be aware of possible side effects caused by high pulsing frequency when applying a VFD to an existing motor. These negative effects include additional heat, audible noise, and vibration. PWM circuitry, which causes a high rate of voltage rise of the carrier frequency, can cause insulation breakdown of the end turns of motor windings as well as feeder cable insulation.

The losses in an induction motor are as follows:

  • Iron losses: Predominantly hysteresis and eddy current losses in the stator, rotor iron losses are generally negligible due to the very low frequency in the rotor under normal operation and practically pure sine wave supply
  • Windage and friction losses
  • Stray losses
  • I2R losses in rotor and stator


Take note

  • It is important to match a VFD to a motor.
  • There are three common types of VFD – Current Source Inverter (CSI), Voltage Source Inverter (VSI) and Pulse Width Modulated (PWM) Inverter).
  • Explore how best to correct current problems and always select inverter duty-rated motors.


Published as:

About VFDs

By H du Preez, H du Preez & Associates

Electricity+Control, July 2012 (pgs 16 – 20)