An electric motor usually burns out because its winding insulation stays above the permitted temperature for too long; that heat is started by overload, a lost phase, voltage unbalance, moisture or blocked cooling. The way to prevent burnout is to close off these causes one by one and run the motor with correctly set protection.

Main causes of winding burnout

  • Continuous overload: If the machine demands more than the motor's rated power, current rises and heat builds up with every running hour.
  • Phase loss: If a fuse or contact stays open, the motor tries to run on two phases and the remaining windings overheat quickly. For details see the question on what happens to a motor left on two phases.
  • Unbalanced or low supply voltage: Even small differences between phases cause unequal currents in the windings.
  • Frequent starting: At each start the motor draws several times its rated current; on a machine that stops and starts constantly the winding has no time to cool.
  • Loss of cooling: A fan cover clogged with dust, a broken fan or an airless cabinet stops the airflow over the frame.
  • Moisture and dirt: In a motor that has stood idle for a long time the insulation absorbs moisture and can break down at the first start.

What to do to prevent it

  1. Have the overload relay set to the rated current on the motor nameplate; protection set by guesswork does not delay burnout.
  2. Have a phase monitoring relay added to the panel; it takes the motor off the line as soon as a phase is lost.
  3. For frequent starts or inverter duty, request a PTC/PT100 temperature sensor embedded in the winding; on HEM Motor motors this is an order option.
  4. Clean the fan cover and frame fins regularly and leave space around the motor for air to circulate.
  5. Before commissioning a motor that has stood for a long time, have a qualified electrician test its insulation.

The roles of the protective devices are explained in our article on overload relay and fuse selection. If a burnt motor needs replacing, the three-phase electric motors page shows models from 0.25 to 355 kW. Photograph the old motor's nameplate and terminal box and send them on WhatsApp; we will find the equivalent that matches power, speed and mounting dimensions together.