overload relay vs thermistor prodection

overload relay will trip if the current to the motor measured in the three power fases exceeds the adjusted value. Therefore it will protect your windings and also your motorcable for overload current.

A thermistor protection relay uses thermistors which are built inside the motor and measure the temperature of the motor windings. If the motor gets to hot, the thermistor relay will trip.

An overload relay must always be used, a thermistor relay is only usefull if you are using the motor in S6 mode (temporary overload).
In this mode the overload relay and cables are oversized and the thermistors will protect your motor.