Thermal Overload Relay Working Principle

A thermal overload relay works on the heat produced by the excessive overload current. The heat produced by the overload current is utilized to trip the motor circuit. These are mostly used for protection of low-voltage squirrel cage induction motors or DC motors of lower output rating.
 
thermal overload relay working principle
 
The function of a thermal overload relay, used in motor starter circuits is to prevent the motor from drawing excessive current which is harmful to motor insulation.
 
It is connected either directly to motor lines or indirectly through current transformers. It de-energies the starter and stops the motor when excessive current is drawn.
 
Whenever the motor is overloaded, it will draw more current from the line and will be heated up gradually. The overload relay is intended to protect the motor against sustained overloads.
thermal overload relay working
 
The overload relay is installed on motor control circuit to make a contact in the trip circuit or mechanically operate the trip bar thus shutting down the motor in the event of excessive load.
 
It consists bimetallic strips. The heat produced by the overload current is utilized to heat the bimetallic strips.
 
Under normal operating condition the strip remains straight but under the action of fault current the strip is heated and bent and the relay contacts get separated which de-energizes the motor control circuit.
 
thermal overload relay
 
The force required to bend the bimetallic strips can be adjusted by an adjuster. In other words, it can be adjusted to operate at different overload currents.
 
The thermal overload relay does not provide short circuit protection as it takes sufficient time to open the contacts. Therefore, this type of relay is used in conjunction with fuses to provide overload and short circuit protection to the circuit.
 
These relays have inverse time characteristics i.e. the tripping time becomes less as the over load and hence current increases. These are rated in trip class. The trip class specifies the period of time it will take to operate in an overload condition. The most common classes are 5, 10, 20 & 30. Class 30, 20, 10 and 5 overload relays will trip within 30, 20, 10 and 5 seconds respectively at 600% of motor full load amps.
 

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