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How does a variable-speed drive control electric motor speed?

Variable-speed drives are invaluable devices that allow users to control the speed of an electric motor, meaning the energy efficiency of all manner of processes – from conveyor belts in manufacturing to pumps in wastewater processing – can be optimised and electricity consumption kept to a minimum.

So how do AC variable-speed drives work? Leading global automation company ABB has produced a handy guide which we’ve summarised here.

  1. 1. Incoming three-phase alternating current (AC) is converted to direct current (DC) via a rectifier.
  2. 2. Capacitors ‘clean’ the DC power by smoothing the electrical wave.
  3. 3. The drive calculates the voltage and current required by the motor then uses an inverter to produce it in AC power.

The ingenuity of a drive lies in its capacity to constantly monitor the process and make adjustments to frequency and voltage as the motor’s demand changes. Without a variable-speed drive, the electric motor runs at full speed throughout the process and manual speed control such as throttling must be used. This is clearly inefficient and also causes maximum wear and tear to components which could be minimised with the use of a variable-speed drive.

Gibbons is an ABB authorised value provider, a prestigious position which allows us to supply, install, commission and service ABB drives for clients across all sectors. Plus, our expertise in electric motors makes for a winning combination as we can provide tailor-made solutions for automated systems.

If you have any questions relating to ABB variable-speed drives, call Drives Manager Alan Roberts on 07966468430 or email

Variable-Speed Drives, Electric Motors

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