Temperature Sensors for VFD

Many VFDs now come ith the necessary circuitry to monitor the resistance change in a PTC thermistor, still the most common type of embedded thermal sensor used by motor mfrs. A PTC thermistor changes resistance rapidly as temperature exceeds a certain threshold, so they are chosen by the motor mfr such that the threshold corresponds with the thermal damage curve of the motor winding insulation. The ciruit needed to monitor this must place a small voltage on the circuit and monitor the drop across it to know when that resistance changes dramatically, indicating that the motor is over heating. A PTC thermistor is however not very linear, so it is not the best choice for READING an analog value to determine actual temperature, it's more like an overload relay, either on or off.

A KTY thermistor is a new type based on semiconductor material (silicon) that can be made to be more linear. So it needs the same circuitry to monitor it, but can more easily be used to provide an analog value that can represent actual temperature (with proper scaling). These are not yet very common in motors, they are more expensive than regular PTC resistors.

Thermo-Click is a trade name of a bimetal thermal switch made by Siemens embedded in the motor windings (you are likely reading this on a Siemens drive spec). Here in the US the predominant brand name is "Klixon" and the term is used a lot as if it is generic, but Kilxon is a trademark of Texas Intsruments so they do not allow competitors, or OEMs who use competitor versions, to use that name. Regardless of the name used, it is a simplistic bi-stable device that "clicks" to an open state when heated up above a threshold. So the exact same circuit that is monitoring resistance in the PTC or KTY devices can look for infinite resistance, the "click" to an open circuit, of that thermal device to know that the motor is over heated.

So bottom line, what your drive is really saying through all that jargon is, it has a resistance monitoring circuit that can be connected to external thermal sensors. However it cannot accurately monitor an RTD, because that takes a more sophisticated circuit. So rather than have to explain that, they chose instead to specifically list the types of devices it can monitor.