Motor PTC Thermistor Connection to a PowerFlex525

Can someone explain how a Powerflex 525 drive interprets the PTC Thermistor signal from a motor winding? How does one know what the levels are or how the levels are defined for a given motor?


i.e. Normal / Fault / High Temp



I am guessing for a PTC type....


High Temp = High Resistance

Fault = low Resistance
OK= Nominal Resistance


Where do I find these resistance values from? Are there standard Thermistors used by motor manufactures? Nothing in the motor manual. And finally, can I get the status of the Thermistor input tied to a PowerFlex 525 if only Ethernet/IP protocol is being used for communicating with the drive. (No discrete signals)


Thanks

As used in motor protection schemes, PTC thermistors are essentially a binary device; Go or No-Go, hardly different from an Over Load Relay as far as the VFD is concerned. The functional difference is that it is based on a DIRECT measurement of motor temperature, as opposed to an indirect calculation based on the amount of current flowing to it. It means that non-current based temperature influences are accounted for in the protection scheme. The threshold curve is very steep between cold and hot.


But PTC thermistor detection circuits need a reference voltage in order to detect that change in resistance, as opposed to being a dry contact in an OL relay. So when you program an Analog Input on the PF520 as a PTC Thermistor input, it is going to look at the return voltage on that input to see when it goes below the threshold; i.e. low voltage is the same as an open contact. The reason that the PF520 asks for references is because you can detect the point at which the motor has cooled off enough to run again.



It may not be in a basic motor manual for the motor, but the motor mfr is where you must get the resistance values. They have them, because they have to match them to the motor's thermal damage curve.

Thanks jaref,

I very much appreciate you taking the time to explain this. I read up on this subject matter quiet a bit and about motor protection in general. But I still don't have all the answers that I am looking for. While most of the Temperature/Resistance curves that I have come across for the PTC Thermistors are similar to what you have shown above, in my case, measured resistance values for PTC Thermistors for the three motors that I am working with are....

around 165, 172, and 190 ohms. (granted that these three motors are not identical and they range from 1 to 15 HP)


Soooo.....


By looking at the generic curve above and many others that I have come across, it implies that my motor temperatures are all well above 140 Degrees C mark based on the measured resistance of their PTC's. However, they are still sitting idle strapped on to the very same wooden shipping palette on which they were delivered to the customer. To boot, they are sitting inside a 72 Degree "F" room. I have reached out to the manufacturer only to be told to use a German manufactured signal conditioner that they recommend. That was no help as I could not track down any technical specifications (Temp/Resistance curves showing the tripping points) for it in English nor in German. Only a simple connection diagram to the conversion unit. Anyhow, search is still on. Thanks again for helping me to better understand this.

Cheers.

Usually the RRT (Rated Response Temperature) is selected to be at a resistance of 3300ohms, +-100ohms, with the reset point being below (I think) 1500ohms. These are IEC standard requirements for motor thermistor protection. That chart I picked was just meant to show the steep nature of the curve, sorry that I threw you off, I didn’t even look at the values on the axes of it.