Overload Relay – Types & Tripping | Overload Protection

What is Overload Relay? Types of Overload Relay

Almost all the industry rely on electric motors for production. An electric motor is an integral device in an industry that converts electrical energy into mechanical energy. The electric motor may be powered by AC or DC sources. Some of the examples such as blowers, compressors, conveyors, drums, pumps, crushers, etc. are running with the help of electric motors in the industry.


What is an induction motor?

An induction motor is one of the most common types of AC electric motor that is used in the commercial and industrial environment. An induction motor works on the principle of a rotating magnetic field.

You can have a look at it, what is an induction motor? and how does it work? how does rotating magnetic-field work in an AC motor?


What are overload protection and overload relay?

An electric motor draws some amount of current while running. Each motor has a full load current as per its power rating. In normal condition, the motor draws a current that is under the full load current.

But when the motor draws a current beyond its full load current then this condition is referred to as an overload.

This may cause overheating from the windings of the motor and may damage the windings. It is important to protect the motor from this overload condition.

Overload relay is a device that is used to detect overload conditions. Overload relay protects the motor, motor power circuit, and motor power circuit components from excess heating and overloading.

Overload relay is the part of the motor starter, It continuously monitors the current flowing through the circuit. When the amount of current is greater than the specified limit for a certain period of time, then the overload relay will trip.

This will cut off the power from the motor and de-energize the contactor. The overload relay can be reset manually, some overload relay can be reset automatically after a certain period of time.

overload protection


Types of Overload-Relays

The most commonly used overload relays are thermal overload relay and electronics overload relay.

Thermal overload relay

A thermal overload relay works on the thermal expansion property of the metal with temperature in a bimetallic strip.

A bimetallic strip is made up of two different metals with different temperature co-efficient. So when this bimetallic strip gets heated up it bends. The following video illustrates that how the bimetallic switch works?

Video Credit – Elektronaut

The bimetallic strip gets heated up by the excess current flowing through the circuit and activates the trip contact. That breaks the power supply by de-energizing the main contactor.

We can find the components of the thermal overload relay in the below diagram.

Overload Relay


Electronics overload relay

Electronics overload relays are advanced than thermal overload relays. It uses a current transformer or temperature sensors to sense the amount of current flowing through the circuit. It uses microprocessor-based protection technology.

Electronics overload relays are more accurate than thermal overload relay. It has some additional protection such as,

  • Under-voltage and over-voltage
  • Phase-loss
  • Current Unbalance
  • Ground Fault

overload relay



Overload relay is a protective device that is used to protect the motor from overcurrent, phase-loss, current imbalance. Overload relay can be used with the combination of contactors. Overload protection is necessary to increase the life of the motor.


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You can read more articles about Electrical and you can also find books that boost your knowledge in the field of Instrumentation

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