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Variable Speed Drives

A variable speed drive (VSD) is an electronic power controller that is able to adjust the electrical supply to an AC induction motor with a corresponding change in the motor’s speed and torque output. A VSD is also known as a ‘drive’, ‘frequency converter’, ‘adjustable speed drive’ or ‘inverter’. By installing this type of control a very close match between motor speed and the process requirements of the machine it is driving may be achieved.

VSD technology is now mature and enjoying widespread adoption and use with AC induction motors. They are extremely versatile and offer a high degree of motor control where motor speeds can be accurately varied from zero rpm through over 100% of the rated speed, whilst the torque is also adjusted to suit. Different options are available to suit a variety of applications. Basic VSD designs are used in simple applications such as fan and pump controls whereas more complex versions might be used for very precise speed and torque controls in applications such as multiple winders or material forming applications.

Sizes of VSD range from 0.18kW through to several MW; they are usually available as standalone devices and are connected to the motor’s electrical supply, however on some smaller motor designs, usually under 15kW the VSD may be built onto the motor and is available as an integrated motor-drive product. In many applications variable speed control can lead to a substantial reduction in energy use. The use of VSDs is particularly effective in fan and pump applications where they might be used to replace traditional methods of output regulation; here an exponential relationship exists between the machine speed (and output) and the energy used.

The benefits of VSDs:

  • VSDs have the potential to make energy savings and increase profitability in almost every sector of UK business
  • Using a VSD to slow down a fan or pump motor from 100% to 80% can save as much as 50% on energy use
  • Reducing fan speed not only reduces energy consumption but may also reduce noise and vibration
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