Thermal Protection of Motors?

Some motors have a temperature sensor in their windings, that can be wired through to a control unit in the starter that cuts off power to the starter contactor coil in the event of a predetermined temperature being exceeded.

Other ways of achieving over-temperature protection include setting an over-current trip to do the same thing, though this is less satisfactory as it presumes ambient conditions at the motor.

A bit addition to Mr 'PW.." anwer .

Its the RTD put inside winding to get the correct temperature of motor.

Overload relay(bi-metallic or electromagnetic/microprocessor etc) see the current and estimates the temperature rise from its inbuilt character curve. The trip setting of motor is changeable.

Normally the later one with bi-metallic relay one is followed mostly. But for higher rating motors say 90kW and above or MV motors RTD in winding is used.

BNDas, Kolkata, India

Any reasonable answer would require you to define what you mean by "motors". 1/10 HP 24VDC motors are protected very differently from 10MW 6.9kV motors. And that is assuming you even mean electric motors! (which I assumed because you posted in an Electrical Engineering forum)

A bit addition to Mr 'PW.." answer .

Its the RTD put inside winding to get the correct temperature of motor.

Overload relay(bi-metallic or electromagnetic/microprocessor etc) see the current and estimates the temperature rise from its inbuilt character curve. The trip setting of motor is changeable.

Normally the later one with bi-metallic relay is followed. But for higher rating motors, say 90kW and above or MV motors RTD in winding is used.

BNDas, Kolkata, India

If you use a starter (contactor) to run your motor, thermal heater coils are procured based on the full load rating of the motor (approximately 125%). That data is provided on the motor and you use the contactor information to get the correct size heater coils for your starter. Depending on the manufacturer, you will get either one or two for a single phase motor, or two or three for a three-phase motor. Heater coils and contactors are usually not interchangeable between different suppliers.