How to choose a Potentiometer

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Rick_B

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I have a 16" disc sander with a 1 HP 3 phase motor that I will be using a VFD to power. I want to include a potentiometer for speed control via the VFD control terminals. I "think" I want a 10K linear taper pot but was wondering what or how to determine the right resistance is correct for motor control.

Thanks
Rick
 
10K Ohms is the resistance.
 
You have to go to your VFD manual for this. There should be a voltage setting for your desired MAX speed and one for you MIN speed. This, along with the VFD circuitry, will allow you to select the correct pot (it all depends on how your VFD controls speed).
 
Usually a pot value of around 5K is specified- it depends somewhat on the amount of control current the VFD can supply.
Too high a pot value can pick up interference and too low can load down the VFD control current output.
 
For the most part, these are used as ratio setting, so the important circuit characteristic is not the absolute value of the pot. The circuit uses the pot as a voltage divider from 0% to 100% of rotation. Yes, use what the manual recommends, but half or 2X I predict will work fine also.

Remember, this is internet based advice. :)
 
The way these usually work is that without the potentiometer connected, there is an internal impedance circuit that sets a default set point, and these have quite high value, such that when one connects an external potentiometer of much lower impedance, it will take the higher current, and assert itself to dominate.

The manual should indicate suitable potentiometer values, and/or what current the electronics can supply. As example, suppose the reference voltage across the whole potentiometer is 10V, and the circuit could supply (say) 20mA.
If you chose (say) a 5K potentiometer, it would use 10V/5000ohms = 0.002A which is 2mA. That is then well within the reference supply capability.

A choice of anywhere between 5K and about 20K will normally work just fine. Do be sure that the potentiometer is linear type. Those made for audio volume control applications have a logarithmic law that would drive one nuts! You want the speed to be proportional to the knob rotation.
 
Thanks to all for the replies - if you look at the manual long enough you can find the one line spec potentiometer spec - for mine it is 10K 2 watt linear taper.

Rick
 
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