Q&A

Supplying voltage over the design voltage of a motor will not increase motor torque or performance. European manufactured 3-phase motors are designed for 380V whereas the UK is supplying around 415V. The various problems associated with over voltage are as follows:

• Iron losses will always be higher in motors that are experiencing over voltage.
• In some cases, the motor draws excessive current and attempts to magnetise the iron core above its design capacity. This can lead to saturation of the iron core, above which additional losses are incurred due to eddy currents. These losses increase disproportionately with the rise in voltage over the design voltage.
• The drawing of excessive current will cause copper losses.
• Iron and copper energy losses are released as heat.
• As a motor is put under additional stress, its lifetime will decrease. 
• Small motors are more sensitive to over-voltage than large motors. 
• The effects of over-voltage are felt particularly on motors which are lightly loaded.  In reality, all motors are specified with a frame size that is large enough for the maximum load it will ever experience, with an additional margin.  So in effect all motors on real sites are lightly loaded, and there is always an opportunity to make savings by bringing the voltage down to a more appropriate level.

It should be noted that reducing the voltage slightly below the optimal operating point barely impacts on the efficiency. A typical motor will operate at near full efficiency down to 200V, without deviation from normal performance. The reduction in voltage will not affect the speed of the motor as this is a function of the frequency of the electricity and the number of poles on the motor. By keeping the voltage within the statutory band, no reduction in torque will be observed.