Thread: Anologue V digital potentiometer

One for the VFD specialists.

I have a remote anologue potentiometer for speed control on my woodlathe VFD.

Has anyone swapped it out for a digital and how did you do it?

Hi Richard,

Apologies for a technical answer !

An analog device uses a varying voltage, resistance or current to provide a constantly adjustable output value. In your case a potentiometer, which is a resistive device. The output of which would need to be digitised by chopping its output up into discrete steps, usually a multiple of 16, where each step represents a particular output value.

Lets assume a common value of 256 discrete steps. If we also assume that the potentiometer has a resistance value of 10,000 ohms (10K ohm), then each step will have a value of 10,000/256 = 390 ohms or 0.039K ohm.

If we assume that we use common digital logic, which runs at 5 volts is used then each discrete step only has 1 / 256 th of 5 volts output, ie 0.02 volts to play with, or 390 ohm per step.

You can use a higher logic voltage, sat 12 volts, which makes things easier but rapidly becomes non standard. Also your speed controller has to be able to either use the steps created directly or convert the steps back to an output that it understands. Usually a voltage.

It would be far easier to leave things as they are and simply count the number of revolutions of the controlled motor and display that.

I'm not saying it can't be done, just that it is not at all straight forward.

HTH.

Your maths is flawed, you lost it when you somehow jammed 256 39k steps into a 10k device..

I’ve had a bit to do with them at work, although not in a VFD control environment, we generally have VFD’s submissive to a PLC not a HMI.
In theory it shouldn’t be difficult, in practise it’ll probably be a right mind f… if my experience of swapping out some in a feedback a couple of months back was anything to go by. The voltages didn’t line up, the conversion steps weren’t precise enough because there weren’t anywhere near enough of them, and the thing was volatile so every time the power went out it had to be re-zeroed.

If you decide to have a crack feel free to get in touch if there’s anything more specific you want to know.

Honestly, the world would have had to have run out of analogue pots before I considered a digital on any of my VFDs, it makes a simple task somewhat complicated.

Originally Posted by racingtadpole

Your maths is flawed, you lost it when you somehow jammed 256 39k steps into a 10k device...

Hi Adam,

Yes you are right. I had got 'K' in my head and didn't put the decimal in the right place. Should have been 390 ohms.

Thanks for the correction.

Close but still no cigar, care to have another go?

Originally Posted by eskimo

One for the VFD specialists.

I have a remote anologue potentiometer for speed control on my woodlathe VFD.

Has anyone swapped it out for a digital and how did you do it?

Why would you want a digital potentiometer ? I may be wrong, but my understanding of a digital potentiometer is an electronic device that acts like an analog potentiometer, it's controlled by a digital signal either directly from a cpu such as arduino or possibly by a serial bus like I2C, I don't think it would be an easy add-on for your VFD. I've heard of them being fitted to CNC controllers so the controller can control spindle speed via a VFD


If you're looking for a speed control with click stops so you know spindle speed at each click stop then you could keep the current speed control and fit a tacho, you can buy these on Ebay, they do need a sensor to be fitted plus power supply, plus a box or housing for the readout and power supply.


If you don't want that, then it can be done with a single pole multiposition switch, common switches have up to 12 positions, up to 18 is less common and the HIFI enthusiasts use 24 position switches for volume controls, you just need to wire it up with a resistor chain - each click stop on the switch adds an extra resistor in much the same way as turning a potentiometer increase the resistance. Translating the click stops for a specific speed is very doable, one way could be to measure the resistance of the potentiometer at each speed setting you use, and then arranging the resistor chain to give those resistances at each click stop.

I'm throwing my vote in with the 'it can be done but..." crowd.

The analogue pot would be providing a proportional signal back to the VFD - for convenience sake, let's say it is a 0 to 5V signal as that is easiest to think about.

Going digital would mean an encoder that would then provide pulses to some components that would then have to use additional circuitry to turn that into a voltage level sitting between 0 and 5V. A pot will stay where it is set, so power up and you are good to go. An encoder does not have memory, so on start up it does not know where it is - either it needs to be calibrated or some more circuitry is needed to store the previous setting.

Starts getting complex really quickly.

Michael

Originally Posted by racingtadpole

Close but still no cigar, care to have another go?

No I'll pass My math is terrible...

Originally Posted by racingtadpole

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.Honestly, the world would have had to have run out of analogue pots before I considered a digital on any of my VFDs, it makes a simple task somewhat complicated.

Nailed it.

Originally Posted by familyguy

Why would you want a digital potentiometer ?

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because my potentiometer can only get down to 56hz. I was wondering if there was anything else which was more sensitive?

looks like its out of my depth...so I will just stick with the pot I have
thanks guys

I'm sure the bottom frequency of 56Hz is not set by the potentiometer but by the settings in VFD, there are settings in most common VFD's that set the upper and lower frequency limits, the potentiometer allows you to vary the frequency between those limits, I have both of my VFD's set for 25Hz as the minimum, have a look at your VFD manual and ask questions if you are unsure about which settings to change to get a lower bottom frequency, there are a lot of knowledgeable forum members here that can help. Beware of going too low, typically a lower speed is used for larger diameter turning, and in reducing the speed via a VFD you may lose torque just when you need it the most ie for large diameters, you may want to consider changing the gearing between the motor and spindle in conjunction with the frequency reduction.

Originally Posted by eskimo

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because my potentiometer can only get down to 56hz. I was wondering if there was anything else which was more sensitive?

That strongly suggests you have it wired incorrectly.

Yeah check the limits in the VFD, you’ll probably find the lower is set where the pot is currently bottoming.