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Thermistor vs RTD for VFD driven motor

Thermistor vs RTD for VFD driven motor

Thermistor vs RTD for VFD driven motor

Why Thermistor are preferred by users over RTD for VFD driven motor winding temperature measurement.

RE: Thermistor vs RTD for VFD driven motor

I never see an RTD sensor that had to have an analog input to a PLC. Thermistor sensors usually have a normally open or closed contact to when the over temperature is reached that contact goes to the opposite state. So any 24vdc or 120vac card can be used with a Thermistor.

RE: Thermistor vs RTD for VFD driven motor

Because the difference in resistance between a hot thermistor and a cool one is so marked, you can use them with the same trip circuit regardless of whether you just use one on its own, put three in series, or put half a dozen in series. Being able to put three in series has obvious benefits.


RE: Thermistor vs RTD for VFD driven motor

Many LV VFDS will have an input designed for a thermistor. I have never seen a LV VFD with a RTD input.
Generally, a motor will not be equipped with RTDs unless the NP HP > 250hp.
I would have to assume that installed-cost is why thermistors are preferred over RTDs.

ps ControlsDude: A Thermistor is an analog device, and not a discrete device. The discrete device, often used for motor over-temperature protection, is often called a Klixon.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Thermistor vs RTD for VFD driven motor

Klixon, thats a new one on me. Thanks!
When an electrician next time refers it as a thermistor, I will tell them that name. I am sure they will just cuss under their breath at me.

RE: Thermistor vs RTD for VFD driven motor

A Kilxon is not a thermistor, although it tends to act in a similar way, which I think was GGs point. "Klixon" was a brand name of a very simplistic thermal cutout device, often embedded into small motors, going back to the 1920s I think. The first versions (still in use today) were a simple convex bi-metal disk that made contact with an electrical point. As current flowed through, the disk would heat up at at the prescribed temperature, generally relating to about 140% current, the little disk would swell and "pop" or click into being concave, thus no longer making contact with the point, interrupting current flow. that original design has morphed into dozens of alternatives, but the basic principal remains the same. Klixon was, for a while, absorbed into Texas Instruments, but maybe a decade or more ago was spun off and is now part of a company called "Sensata".

A Thermistor is a "Thermal Resistor and looks like any other resistor, but it is different in that it has a somewhat sharp "knee point" in which as temperature changes, the resistance changes rapidly. you can have thermistors with a Positive Temperature Coefficient of resistance (PTC), in which the resistant RISES as temperature RISES, or a Negative temperature Coefficient of resistance (NTC), in which resistance DROPS as temperature RISES. Generally motor protection thermistors are PTC, meaning as the motor heats up to a threshold point, the resistance rapidly changes from low to high, creating a voltage drop on a circuit going through it. So it is very easy to make a circuit to monitor that voltage drop. If you then have that circuit in something, like lets say a VFD or an OL relay, then if you want to use that input for something else, opening a dry contact has basically the same effect, so your input can easily serve dual purposes.

An RTD (Resistance Temperature Detector) is an ANALOG device, meaning the resistance changes in a somewhat linear fashion with temperature, so you can use that to track temperature CHANGES over time. That can be a lot more useful in an intelligent protection system to avoid shutdowns, compared to a thermistor that is going to act more like the "idiot light" telling you your car engine over heated, which by the time it comes on means it's already too late. The problem with using an RTD is that it then requires an ANALOG input into something with enough intelligence to not only track the values, but be able to adjust for any non-linear aspects of it, however minor. So RTDs require more of an investment in the device that will monitor them.

Some high performance VFDs have options to add analog input cards calibrated to accept RTDs, but many even low end VFDs will offer a Thermistor Input, either separately, or one of the basic Digital Inputs will be capable of accepting a thermistor. that's because in areas of the world outside of North America, using an Explosion Proof motor on a VFD REQUIRES the VFD to accept a thermistor input.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Thermistor vs RTD for VFD driven motor

Nice summary Jeff. lps

"Why not the best?"
Jimmy Carter

RE: Thermistor vs RTD for VFD driven motor

Fixed it, thanks Compositepro. Old bad habit stemming from the first time I ran across the term and someone incorrectly told me that, now I am stuck on it even though I know the correct definition...

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Thermistor vs RTD for VFD driven motor

thanks again! info like this is gold in your pocket. Especially the part about use in explosion proof motors as a requirement outside of NA.

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