Motor Thermistor readings

I have a motor with 2 thermistors. One pair reads 1.0 ohm and the other 247 ohms

Does this indicate one of them is bad? Which one?

Also these connect to a power flex 755 drive and the prints show to wire the thermistors in series at the motor but we have not done that yet because of the weird readings.

Help Please.

https://en.wikipedia.org/wiki/Thermistor#

lots more information needed: positive coefficient? negative? What temperature are things at now? Do you have a manual for the thermistors which gives specifications?

They are in the motor with no documentation. It’s a Baldor motor

Do you have the part number for the Baldor motor?

This is wild conjecture based on a presumption but they both might be right. The thermistor that reads 1 Ohm would be more like what I am used to. It is in fact probably a series of three PTC thermistors internal to the motor windings that will go to a high resistance at a specific temperature. This is what you would see as overtemp "contacts" on may motors.

The second, higher resistance thermistor is likely an NTC thermistor that is intended to measure actual motor temperature.

Keith

kamenges said:

The second, higher resistance thermistor is likely an NTC thermistor that is intended to measure actual motor temperature.

Click to expand...

247 Ohms seems to be a standard PTC, for a single PTC it's limit is to be less than 250 Ohms, so maybe this is a series of two or three PTCs which is usual for a good 3-phase motor. You can't use this for exact temperature measurement, as there's only a jump in resistance at the designed temperature for the temperature class.

I've worked at a repair shop for electric motors for 10 years, but I've never seen PTC and thermistor contacts in the same motor. High quality motors for special applications had two PTCs for different temperatures (warning, shutdown), at least here in germany.

For Ex applications the tripping device for the PTC needs to check also for shortcuts.