Thursday, 20 March 2014

How exactly do single-phase and three-phase electric motors differ? Our latest jargon buster blog has all the information you need.

A quick look at our electric motors page will reveal that we supply them in single-phase and three-phase. But what is the difference? We’ll start by looking at three-phase motors.

Three-phase motors

Three-phase motors are powered by three alternating currents (of the same frequency) which reach their peak values at one third of a cycle from each other. This results in constant power transfer over each cycle of the current, making it possible to produce a rotating magnetic field in an electric motor.

This chart shows the sine waves of three-phase voltage:

The sine waves are separated by 120 electrical degrees because the current is generated by three separate sets of coils in an AC motor. These three sets of coils are mounted 120 degrees apart on the motor’s stator and the resulting rotating magnetic field means the motor is self-starting.

Single-phase motors

Both three-phase and single-phase motors can be powered from a three-phase supply, but this is not always available – such as in some rural areas. Where three-phase power cannot be accessed, a single-phase motor may be used.

A single-phase electric motor does not produce a revolving magnetic field – it can only create an alternating field. This means that it pulls first in one direction, then in the opposite. If hand-started, the momentum created will cause a single-phase motor to continue turning. However, hand-starting is not practical and therefore single-phase motors are fitted with a starting system, the two most common being permanent capacitor and capacitor start, capacitor run (we’ll look at these more in-depth in a future blog).

We hope this has cleared up the difference between single-phase and three-phase motors, but if you have any other questions then don’t hesitate to call us on 01621 868138 or email


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