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How do variable-speed drives work?

Although variable-speed drives can differ in design, they all provide the same essential function - they change the fixed frequency and voltage of an electric motor to achieve the required speed for the process in hand.

But how exactly do variable-speed drives work? The most common drives are those applied to AC induction motors and their fundamental design consists of four elements:

1. Rectifier

This component transforms the incoming alternating current (AC) supply to direct current (DC) and can also control the direction of power flow. The rectifier is selected according to the performance required of the drive and its design will determine the effect of electrical harmonics on the incoming supply.

2. Intermediate circuit

The rectified DC supply is then conditioned in the intermediate circuit via a combination of inductors and capacitors. Most modern drives use a fixed-voltage DC link.

3. Inverter

The rectified and conditioned DC back is converted into AC variable frequency and voltage by the inverter. This is achieved by generating a high-frequency pulse width modulated signal of variable frequency and effective voltage. The semiconductor switches create the output and whilst there are different types available, the most common is the Insulated Gate Bipolar Transistor (IGBT). 

Variable-speed drives are often referred to simply as ‘inverters’ or ‘inverter drives’.

4. Control unit

This element manages the entire operation of the VSD. It monitors and controls the rectifier, the intermediate circuit and the inverter to produce the desired output when an external control signal is received.

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

Variable-Speed Drives

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