In Conventional Motor Control why it is necessary to put an Overload relay and a circuit breaker? and describe its purpose and function?

Overload relays protect a motor by sensing the current going to the motor, the relay will sense the termal signal from the motor.When current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor, The breaker will trip & isolate the supply from Motor.

Firstly we must difference between two type of protection Overcurrent protection and Overload protection, so:

Overcurrent protection:

 is protection against excessive currents or current beyond the acceptable current rating of equipment. It generally operates instantly. Short circuit is a type of overcurrent. Magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection.

Overload protection:

is a protection against a running overcurrent that would cause overheating of the protected equipment. Hence, an overload is also type of overcurrent. Overload protection typically operates on an inverse time curve where the tripping time becomes less as the current increases. Overload relays as well as “slow blow” fuses are commonly used to provide overload protection.

so the overload(for Overload protection  )  and circuit breaker(for  Overcurrent protection) complete each other.

By sensing the current going to the motor.

If there is over current the relay will open and the contactor coil will release to stop the motor to protect from damage

Circuit Breaker is to Interrupt the current flow.

Motor protection Circuit breaker is a protection corcuit drive to a motor.

When excessive current flows through the motor circuit, the relay opens due to increased motor temperature, relay temperature, or sensed overload current, depending on the relay type.