Variable frequency drive (VFD) is an electronic device that controls the speed of AC induction motors. Before we look at how this works and how it can be used, we should look at the history of motor controllers, and also how the induction motor itself works.

It has always been useful to control the speed of electric motors used in industry. Nearly every process that uses a motor will benefit from speed control. Not only is the process generally improved, but in many cases (particularly with pumps and fans) these is considerable energy saving.

Before electronic controllers were available, motors were controlled in various ways, for example by controlling the field current on a DC motor using a series of resistors, or by using other motors. However, when Thyristors, the first power semiconductors, became available in the 1950’s, it became possible to control the armature voltage, and therefore the speed, of a DC motor using phase control. These DC drives are still manufactured and in wide use today.

However, the challenge has always been to control the speed of the AC induction motor, also known as the Asynchronous motor. While a DC machine usually has two wound parts (the field and armature windings) as well as brushes and a commutator, the AC machine has a simple, fixed winding (the stator) and a rotor. The rotor usually consists of conductors formed by casting Aluminium or Copper in the iron core. There are no brushes or commutators. The machine is therefore cheaper, simpler and more reliable. It is little surprise that these machines make up the majority of motors used in the world’s industries. So how does it work, and why does it need a variable frequency drive?

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