Motor Overload Protection

Overload protection is installed in the main control circuit or power line of motor to protect from damage due to mechanical malfunction overload conditions when it is running.

The effect of an overload is an excessive rise in temperature in the motor windings due to current higher than full load current.

Properly sized overload protection disconnects the motor from the power supply when the heat generated in the motor circuit or windings approaches a damaging level for any reason.

What is motor overload protection?

The larger the overload, the more quickly the temperature will increase to a point that is damaging to the insulation and lubrication of the motor.

The over current device will not react to this low level overload. The motor overload device prevents this type of problem from severely damaging the motor and also provide protection for the circuit conductors since it is rated for the same or less current as the conductors.

Basic Operating Principle

  • The eutectic alloy in the heater element is a material that goes from a solid to liquid state without going through an intermediate putty stage.
  • When the motor current exceeds the rated value, the temperature will rise to a point where the alloy melts; the ratchet wheel is then free to rotate, and the contact pawl moves upward under spring pressure allowing the control circuit contacts to open.
  • After the heater element cools, the ratchet wheel will again be held stationary and the overload contacts can be reset.

Designation of overload protection

Overload devices are designed to allow high currents to flow briefly in the motor to allow for:

  • Typical motor starting currents of 6 to 8 times normal running current when starting.
  • Short duration overloads such as a slug of product going through a system.

If the motor inlets and outlets are covered by a blanket of lint or if a bearing should begin to lock, excessive heating of the motor windings will “overload” the motors insulation which could damage the motor.


For more detail information,please refer to ELECTRIC MOTOR HANDBOOKS :


  1. Will an OL relay trip during frequent start/stop of motors?.

  2. For setting OL relay how SF influences and how to set for a SF of 1.15 or 1.25?. Similarly according to insulation class of winding how should be adjust the setting of OL relay?.

    • SF of 1.15 or 1.25 the overload setting should be 125% of the FLA rating on the name plate. If it is over 40 degree C then it would be 115% of the FLA.

  3. zekent montegrande says

    hey. can i ask about the advantage and disadvantage between the electrical and mechanical interlocking in motors and control system? plz…

  4. Mutomb says

    Dear Sirs I have an installation of submersible motor which pump water in a tank at 100to 200 meters away. I would like to advise me what protection I can put to Protect the motor and switch off the motor when tank is full.
    What will be the best protection against lightning?

    • you can use high level switch or electrode level switch or any kind or level switch to control your pump stop or run condition…
      For typical motor protection : you can use Overload relay or can run your pump with VFD or inverter :D
      Protection for lightning : you can used surge protection or lightning protection..many types of protection available for your application :D

  5. It’s enormous that you are getting ideas from this post as well
    as from our argument made at this time.

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