How are built in thermal overload protector arranged Explain

How are built in thermal overload protector arranged? Explain briefly

Solution

On smaller motors LV winding motors, the thermal overload protector arranged or embedded in the overhang of the winding. On the medium voltage motors these are placed in between bottom and top coils in the slot portion of the core. Generally thermal overload protector consists of thermistors. These thermistors are very small semiconductor devices whose resistance changes rapidly with temperature. Since the motor consists of three windings so therefore three thermistors are inserted in the end-windings of the stator, one in each phase, and all of them are connected in series. The two thermistor terminals at the motor are connected to an electronic-amplifier-control unit in the starter, through which the tripping circuit of the starter is operated. The response of the thermistors to temperature change is extremely rapid, allowing this type of protection to be effective under all motor overload conditions.