Thermistors consist of components made from semiconductor elements, in which their resistance varies significantly with temperature. PTC refers to the positive temperature coefficient and NTC, in the negative temperature coefficient.
Thermistors, together with their respective electronic control electronics, can provide complete protection against overheating caused by phase failure, overload or overvoltages and reversal operations. The PTC thermistor indicates increased resistance with increasing temperature and some can be characterized by this sudden rise, which makes them very useful in devices intended to protect against overheating.
In this way, the NTC thermistor presents the reduction of the resistance with the elevation of the temperature, normally not being used in electric motors. The variation in the resistance interrupts the current in the PTC, being able to activate an output device, responsible for turning off the main circuit. In such situations, two thermistors are required, which must be connected in series, per phase. The thermistors count on a smaller size, do not suffer mechanical wear and have a more responsive response to other detectors.