Working Principle of the Thermal Overload Relay

On the heat produced by the excessive overload current, a thermal overload relay works. To trip the motor circuit, the heat produced by the overload current is used. For the protection of DC motors of lower output rating or low-voltage squirrel cage induction motors, these are used mostly.

In motor starter circuits, the motor can be prevented from drawing excessive current by the function of a thermal overload relay. This current is harmful to motor insulation.

It is connected either indirectly through current transformers or directly to motor lines. It stops the motor and de-energies the starter when excessive current is drawn.

Thermal Overload Relay Working Principle

It will be heated up gradually and draw more current from the line whenever the motor is overloaded. To protect the motor against sustained overloads, the overload relay is intended.

To mechanically operate the trip bar or make a contact with the trip circuit, the overload relay is installed on the motor control circuit. Thus in the event of excessive load, it shuts down the motor. The electronic thermal overload relay can also be preferred.

Bimetallic strips are present in it. To heat the bimetallic strips, the heat produced by the overload current is utilized.

The strip remains straight under normal operating conditions. But, the strip is heated and bent under the action of the fault current, and the relay contacts separate.  Thus, the motor control circuit gets de-energized.

By an adjuster, the force needed to bend the bimetallic strips can be adjusted. To operate at different overload currents, it can be adjusted In other words.

Short circuit protection is not offered by the fuji electronic thermal overload relay as it takes sufficient time to open the contacts. Therefore with fuses, this type of relay is used in conjunction to provide short circuit and overload protection to the circuit.

Inverse time characteristics are present in these relays i.e. current increases and the tripping time becomes less as the overload. In trip class, these are rated. The period it will take to operate in an overload condition is specified in the trip class. The most common classes are 30, 10, 5, and 20. Follow us on Facebook