Motor Protection with Thermistor


Thread Starter


I have about 2 dozen motors (3 Phase) with built-in thermistors(PTC). A telemechanique motor protection relay is used to interface
thermistor with Motor starter.

One of the motors tripped with indication for shorted thermistor. When i measured thermistor resistance (when motor is cool) it was about
175 Ohms. I checked a healthy motor it was 245 ohms. The catalogue for protection relay recomends to use 1000 ohms resistor to replace the thermistor if thermal trip function is not used.

Can somebody tell me the upper and lower limits of resistance for a healthy thermistor .

Jaroslaw Klak


I have never used Telemecanique thermistor overload relays, but for
equivalent Moeller EMT6-DB relay these thresholds are:
tripping for R>3600Ohm
reset for R<1600Ohm

For Siemens 3RN1* thermistor overload relays R<20Ohm = 'PTC short-circuit', but I don't remember exact values for overtemperature tripping
and reset.

I hope this helps
Jarek Klak