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Controlling a VFD
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Controlling a VFD

by 762spr on Thu Aug 15, 2019 7:25 pm

Hello I'm working on retrofitting an old industrial bandsaw I got for my shop. The motors are all 3 phase, but I got some Variable Frequency Drives so I can use 1 phase power. Then it occurred to me to just ditch the whole 90's era control panel with all of the 3ph control circuitry and relays and instead do more modern controls and interface.

Toggling the VFD's on/off and interfacing with the limit switches should be fairly straightforward, but I would also like the arduino to control the speed so I can just enter the desired SFM of the blade and it will adjust the RPM accordingly.

The VFD's accept several types of input for speed control. 0-5v, 0-10v, and 0-20mA. A quick search turned up pwm to voltage and pwm to current boards for fairly cheap on amazon, but I'd much rather get everything from a preferred vendor like adafruit (and also not have to wait two months for a poorly made board straight from china to arrive and then not work!) I didn't find any similar boards on here, but there are some logic level shifters, would that work?

Something like https://www.adafruit.com/product/1875

Maybe a https://www.adafruit.com/product/935 in conjunction with the logic level shifter?

OR could I use the 5v out from the VFD and a digital pot like this https://www.adafruit.com/product/4286 into the 0-5v input?

Thoughts on the best approach? Thanks!

762spr
 
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Joined: Thu Dec 20, 2018 4:23 am

Re: Controlling a VFD

by zener on Fri Aug 16, 2019 4:48 pm

Obviously, the 0-5V input option to the VFD would be the closest to what an Arduino based system would want.

Either the DAC or the digital pot would work, with 5V power supply. You would not need the level shifter. They would read 3.3V signals OK. The drive will make switching noise, so use a shielded cable on your 5V control line, and put some capacitance on that, like a 1uF ceramic or film cap. Small electrolytic would probably be OK also.

Make sure your VFD's can handle 1 phase power mains. Some can and some can't. Even the ones that do usually have a derating factor when using them that way.

And of course, if your motor goes the wrong direction, swap 2 leads on the OUTPUT of the drive.

Good luck

zener
 
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Re: Controlling a VFD

by 762spr on Mon Aug 19, 2019 1:22 am

Thanks for the reply!

Yes I know the VFD should be good to go on 1 phase I've already tested it with the drive motor. I think I'll go with the digital pot for simplicity sake, and that's a good point on the noise, thanks!

One other thing, There is a terminal labeled 5v frequency out on the VFD. I'm assuming it is 0-5v (it says max 5v) depending on the actual frequency of the vfd so that I could double check that is is close to the commanded frequency. Would a logic lavel shifter work well here to convert to 0-3.3v so I can monitor "closed loop" monitor this pin?

Thanks again!

762spr
 
Posts: 9
Joined: Thu Dec 20, 2018 4:23 am

Re: Controlling a VFD

by zener on Mon Aug 19, 2019 12:23 pm

If that is a 0-5V analog output, then you can read it with an analog input. But if you have a 3.3V CPU then you need to "squash" the 0-5V signal down to 0-3.3V. The simplest way to do that is with a voltage divider (lots of info on the web about that). You would want a total resistance that is: high enough that it doesn't load down the output (requires some experimentation), and, is low enough that it isn't too sensitive to noise. So I would guess something in the neighborhood of 5-10K. Again I would recommend shielded cable. You could run your in and out analog signals through a single shielded cable with 3 conductors: Gnd/Com, In, Out

zener
 
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Re: Controlling a VFD

by 762spr on Mon Aug 19, 2019 3:54 pm

Yes, I believe it is 0-5v analog. I'll do some more testing on the vfd today. I'm about to order some components as well!

Just so I'm sure I'm tracking this correctly. The level shifters would be more for a digital signal vs an analog one where you would want a voltage divider? Or is it just a matter of the voltage divider being cheaper/simpler to construct?

Thanks for all the help! I know this is pretty basic stuff, but while I can handle mechanical things and programming easy, electronics are a bit of a dark art to me!

762spr
 
Posts: 9
Joined: Thu Dec 20, 2018 4:23 am

Re: Controlling a VFD

by zener on Mon Aug 19, 2019 7:16 pm

The level shifters are just for digital. What that would likely do to your analog signal is just cut off everything above 3.3V. So 0-3.3 might go through OK but then it would never get above 3.3V. What you want is a ratiometric scaling. So, for example, you put a 1.7K in series with a 3.3K (5K total) (notice the 3.3 and 5 there.....) with the 1.7K "on top" and the 3.3K below, then you hook your 5V signal to the "top" and ground at the bottom and pick off the 3.3V scaled signal in the middle. Classic voltage divider.

https://en.wikipedia.org/wiki/Voltage_divider

zener
 
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Re: Controlling a VFD

by 762spr on Tue Aug 20, 2019 4:41 pm

Awesome! Thanks so much for all the help and the great explanations!

762spr
 
Posts: 9
Joined: Thu Dec 20, 2018 4:23 am

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