In the UK the nominal voltage is 400 volts 3 phase 50 Hz. Normally on electric motor nameplates we see either 220-240/380-415 volts on smaller motors or 380-415/660-720 volts on larger sizes.

Small motors tend to be used with DOL starting because the starting current is not an issue. By having a motor which is 230/400 volts delta/star we can use on either voltage or use a single phase input inverter which has a 230 volt 3 phase output to the motor.

Larger motors can be started DOL but the high starting current can be an issue so traditionally a star/delta starter is used and this needs to be a 400/690 volts delta/star design.

In many cases we are now seeing the use of soft starters and inverters which inherently reduce the starting current to more manageable levels.

There is no standard which says where this changeover is but it is usually about 4kW and motors whether large or small can be manufactured for either configuration.

The following should be taken into consideration when looking for a replacement Motor
  • What is the supply voltage the motor will operate on?
  • What method of starting do I require?
If it’s a replacement motor don’t assume this will be the same connection as the original. Always check the nameplate for the connection method before wiring up.

When replacing an Electric Motor the most important thing is that the Terminal Box is connected correctly for the required voltage before starting to put the Motor into operation.

Below is a diagram of the Star/Delta connections. Star Connection for 400Volts for Standard IEC Motors 0.09Kw up to 3Kw and 690Volts from 4Kw Motors upwards. Delta Connection for 230Volts for Standard IEC Motors 0.09Kw up to 3Kw and 400volts from 4Kw Motors upwards.

STAR & DELTA connection of 

Three-phase Squirrel-cage Electric Motors


To change direction of rotation swap any two Life (L1, L2, L3) inlet leads

Grantham Electrical can help you find a solution for most requested voltages and frequencies. Alternatively we can offer a rewind or get a new motor for your specific requirements.
Contact: nigel.skeith@granelect for further information.
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