They are thermal detectors made of semiconductor sensors that change their resistance abruptly when reaching a certain temperature. PTC is the positive temperature coefficient Gates B140 Hi-Power II Belts . The PTC type is a resistance thermistor that rises sharply to a well-defined temperature value, specified for each type. This severe variation in resistance interrupts the current in the PTC, triggering an output relay, turning off the main circuit. It can also be used for alarm or alarm and shutdown systems (two per phase). The NTC is the negative temperature coefficient. For the “NTC” thermistor, the opposite happens for PTC, but its application is not normal for electric motors, since the electronic control circuits available are usually for PTC (resistance increase).
The thermistor has a reduced size, no mechanical wear and has a faster response than other detectors, although they allow for continuous monitoring of the motor heating process. Thermistors with their respective electronic control circuits provide complete protection against overheating caused by lack of phase, overload, under or over voltages or frequent reversal or on-off operations. They have a low cost compared to the RTD type, however, they need a relay to control the operation of the alarm or operation.