When should I use Variable Speed Drives (VSDs) to control induction motors?

Answer by Hugh Falkner (Atkins)

The energy saving benefits of using a VSD are well documented, with the biggest energy saving seen on centrifugal pumps and fans, where the power cubed rule means that reducing the speed by just 20% can reduce the power consumed by up to 50%. 

The VSD itself has internal energy losses of typically 4%, but this is rarely significant compared to the energy savings that can be made.

Smaller savings can be made in constant torque loads such as positive displacement (screw or piston) compressors.

Other benefits of using a VSD can include:

  • Built in soft start capability.
  • Skipping of problem frequencies during start up of heavy loads.
  • Advanced motor monitoring and protection.
  • Tighter process control through using feedback or external speed demand signal.
  • Logging of energy use.

Not all motor systems will benefit from the use of a VSD, for example pumps with a high static head where if the speed is reduced too much the water will not reach the required height.

There are some technical issues that should be considered to avoid possible problems when using VSDs, but good suppliers will ensure that the installation works without difficulty:

  • The voltage waveform produced by the inverter will produce a sinewave that has a high (switching) frequency component.  This will induce additional losses in the motor laminations, equivalent typically to a reduction in the efficiency of the motor by half of an IE level.  So far there is no way for the specifier to know how a particular motor will respond to being driven by a VSD, but a new standard IEC60034-2-3 is being developed that will give a figure to characterise this.
  • The net effect on motor losses of using a VSD is that except in the rare circumstance that a VSD driven motor is working at full load, there will be a net reduction in motor losses and hence operating temperature
  • The high frequency distortion in the waveform can induce unwanted currents in the shaft.  These become a problem if they are able to circulate through the bearings, as the bearings will become pitted and may ultimately fail.  The use of insulated ceramic bearings will overcome this problem, and can be fitted in new motors on request.
  • The voltage waveform puts an additional stress on the motor insulation, which will shorten the lifetime of older motors.  A motor supplier can advise on whether a new motor is required; in cases where a new motor is needed the increase in efficiency of a new motor can help to offset the purchase price.
  • When using a VSD, the high frequency component of the output voltage will generate electro-magnetic interference that might compromise the performance of susceptible equipment nearby.  Screened cable should be used to avoid this problem.
  • Similarly the VSD will inject lower frequency harmonic interference into the mains supply, which is overcome by the use of filters supplied with the VSD.
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