vfd speed controlled by a thermistor

lesmar96

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May 2017
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Thermistors........Can someone explain how they work and how to control the speed of a vfd with them?

I am not familiar with thermistors, but a customer called me and wants to be able to control the speed of his fan with his thermistor. According to my online research, they evidently vary resistance according to a change in temperature?

Are thermistors 2-wire? It seems like they should be able to be wired as a 0-10V input?
 
Thanks.

So I have done more reading..........

It seems they come in two flavors NTC and PTC. I don't know which my customer has, so I will have to ask.
Also, is it correct that it is a bad idea or absolutely necessary to use a transmitter to connect these to a 0-10V input?
 
You aren't going to have success connecting a thermistor directly to a 0-10 VDC analog input module. A transmitter would be required.
 
Negative Temperature Coefficient - Lowers the resistance with the temperature rise.
Positive Temperature Coefficient - Raises the resistance with the temperature rise.

It's been nearly a decade since I worked with these, but back then PTC's were mostly used for thermal fuses and not really precise for control applications. You could use them, but the variation and precision were poor. I think this is no longer the case, but may be wrong.

To wire it to a 10V input you'd need something to do a voltage divider with your NTC. Using a transmitter that converts the signal is probably safer overall and easier to match with the VFD input without having to calculate voltage drops based on temperature.
 
Thanks!

This is making sense. Do you have a transmitter to recommend that you have had good experience with?
 
Thanks!

This is making sense. Do you have a transmitter to recommend that you have had good experience with?

I like the PR electronics ones that are field mounted. I think you may have to do a linearization on them for an NTC as they are not standardized like a PT100 or 1000.

A lot of VFD's have a potmeter input in them... you can use this input with your NTC and another resistance in series to create the same effect. But you'll have to calculate the series resistance to match the range you need on the input. This can work, but will give you a bit more headaches.
 
Yes non-linear is a problem, but if coefficient if the curve is small enough you can just use it in a linear manor.

Are you going to program this function in a plc?
 

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