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Heat: A major cause of electric motor failure

We’ve been rewinding and repairing electric motors for 45 years, so there isn’t much we haven’t seen when it comes to the causes of motor failure. Heat, from a variety of sources, is among the most common reasons for an electric motor to stop working. Here we discover why and how this happens.

Why does heat cause motor failure?

Before we look at the primary reasons for motors being exposed to potentially damaging temperatures, we should ask why overheating leads to failure. It’s actually very straightforward:

  • The windings within a motor are coated with a special insulating varnish or lacquer
  • When the coil becomes too hot, the insulation melts
  • This causes the windings to short circuit and therefore the motor to stop operating.

Each time a motor’s windings rise 10°C above the design operating temperature, the life of the insulation is halved – even during a short period of overheating.

Now we’ll look at the three main reasons for motors being exposed to excess heat:

Unsuitable for application

If a motor is too small for the application it’s being used for, or if it has the wrong starting current (also known as inrush current) and torque characteristics, it will operate hotter than its rated temperature. It’s vital that a motor is matched to its connected load, whether that’s a pump, HVAC system or humidifier.

Hot environment

When a motor is operated in an environment with a high ambient temperature, heat cannot be conducted from it as quickly as its design allows for. Therefore a motor located in a poorly-ventilated area or near a heat source could be exposed to temperatures high enough to cause damage.

Ambient temperature shouldn’t be the only consideration – a motor with plugged air passages or blocked cooling fans and vanes will quickly overheat.


If a motor is enclosed - in a protective housing, for instance – then the ambient temperature is limited to that space only and may exceed the rated level. Temperatures in enclosed areas should be monitored and if they are excessive then ventilation added.

If you’d like to know more about our electric motor rewinds and repair service, give us a call on 01621 868138 or email

Electric Motors

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