Principle of operation of thermistors used for overload protection for motors

Thermistors are thermally sensitive resistors made of small pellets of semiconductor materials. They are embedded into the insulation of the three-phase windings of the stator windings at the manufacturing stage. 

They operate with a negative temperature coefficient of resistance. 

They are temperature sensors, which are used to monitor the winding temperature. 

The resistance is calibrated in terms of temperature in the meters which are pointer operated according to the current through the resistor placed in a bridge. 

They are connected so that if the winding temperature gets too high, the starter contactor will be tripped and stop the motor.

They are also mounted in the gaps created in the stator core to measure the temperature the stator iron core be more than the core temperature when the machine is loaded. The thermistors in the core is used to ac usually an alarm if the core temperature exceeds a set value.