Here are the reasons why:

1. Over Heating

  • The cooler the motor operates, the longer its expected life…
  • A 10°C reduction in operating temperature typically doubles the motors lifetime.

Excessive starts are a major cause of over-heating. During a non-VSD start-up a motor typically sees between 6 to 8 times its rated current.

  • This increases the thermal status of the motor, increases thermal stress on the windings and can cause failure.
  • PTC Thermistors are a common protection method to protect against over-heating.
  • Thermistors have a positive temperature coefficient meaning that the resistance is increasing rapidly around the trip temperature. Connected to a thermistor relay this will trip preventing over-heating. Normal operating conditions will not cause this to happen.

How do I tell if the windings on a motor are under thermal stress?
Look for darkened areas on the motor windings, these marks are signs of over-heating.

2. Over Loading

  • Motor windings can fail due to over-loading at the motor shaft which causes excessive heat build-up and failure.
  • Fit thermal thermistor protection to guard against failure.
  • A thermal overload relay is a common protection method used to protect against over-load. It is a bimetallic strip that bends when over-loaded due to heat build-up.
  • Normal operating currents will not cause this to happen.


  • Connect thermistors to a thermistor relay in the motor control package and set the current overload limits to the rating plate FLC.
  • Follow manufacturer manuals for correct installation and limits for your motor.

We provide 24/7 Technical Support to engineers, our dedicated Support line is available 8am – 5pm every day, manned by one of our fully qualified engineers.

If you need further assistance on how to prevent your motor from failing, call us, day or night, on 0330 9000 247.