The indication is the rating plate that is permanently attached to the motor. Set the overload trip setting to the full load current value for the voltage applied, according to the information on the plate.

Indication can also be had remotely by using the auxiliary contact(s) on the overload trip device.

if motor is overload than what will happen

Either the trip will operate or smoke will come out of the motor. The smell of burning varnish and paint is also a good indication that a motor is overloaded.

ok one more question a 20kw motor can run in place of 30 kw if yes than how and if no then why...

What is the mechanical load attached to it? If a 20kW motor is to be deemed suitable for the application, why was a 30kW one fitted originally?

Maybe they had one in their junk box under the bench .

Oh, that it should stay there.....

NO, you can't do that.

If you hook a 30kw motor up where you had a 20 kw motor, you'll have 10 kw of electricity running out onto the floor.

If ther LOAD connected to the 30kW motor was only requiring 20kW or less, then it would be OK. If the LOAD connected to the 30kW motor requires 21kW or more, then it will overload and fail.

Why do you ask? Maybe if we understood why you are wanting to do this we can give you more guidance.

Motor applications have everything to do with the connected LOAD. But sometimes, as indicated in the tongue-in-cheek responses above, people will use a larger motor because that is all that was available at the time. However, you CANNOT assume that, you have to work based on the assumption that the 30kW motor was used because they NEEDED a 30kW motor. So unless there has been some DOCUMENTED installation of a larger unit in an emergency, or there has been a DOCUMENTED change in the load profile, then you should not attempt it.

But if this is a homework assignment for a student, begone with you, cheater...

The most common protection consists of an overload relay. The passage of current through this relay heats some elements in it; if they get too hot, they open the circuit to the motor contactor.

Indication in advance can be given by an ammeter.

Select the recommended type (I/t curve)of OC relay suitable for the type of load.