VFD speed control possible?

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cyberbackpacker
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Joined: Wed May 20, 2009 10:41 am

VFD speed control possible?

Post by cyberbackpacker »

I've been racking my brain, and thought I'd bring this to a more experienced group.

I am building a new panel wanting to eliminate all physical buttons/switches as much as possible and depend on the UI of the BCS.

My problem; I run two 3-phase pumps in my system, run by VFDs (variable frequency drives). The drives have remote speed control via a POT that measures 0-10v. (See pg. 11 here : http://www.automationdirect.com/static/ ... 1m/ch2.pdf and pg.8 here http://www.automationdirect.com/static/ ... 1m/ch3.pdf for more details).

Do you think there is a way using the BCS that I could replicate a "virtual" POT to control the motor speeds, or am I pretty much stuck with a physical POT?

Thanks for your time.
pngaudioguy
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Re: VFD speed control possible?

Post by pngaudioguy »

A quick Google search turned up several hits for "pwm to voltage" to convert PWM output to analog voltage output. There's a pretty simple one based on a 5V arduino supply which is the same as what the BCS outputs are. You'd have to add some circuitry to scale it to 0-10V, but might give you some ideas?

http://provideyourown.com/2011/analogwr ... o-voltage/
cyberbackpacker
Posts: 27
Joined: Wed May 20, 2009 10:41 am

Re: VFD speed control possible?

Post by cyberbackpacker »

Thanks for that lead, I will research that some more. I didn't think that the potentionometer control was basically PWM. That opens up more areas for me to look into. Thanks again.

-Kevin
pngaudioguy
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Re: VFD speed control possible?

Post by pngaudioguy »

From reading those pages of your documentation, it looks like you can also switch it to a 0 to +10V input instead of potentiometer, which is what got me thinking along those lines. A potentiometer will be a variable resistance, not variable voltage. I was thinking outside of the box provided ;).
cyberbackpacker
Posts: 27
Joined: Wed May 20, 2009 10:41 am

Re: VFD speed control possible?

Post by cyberbackpacker »

Mostly for my own records and ability to find it later, but also to keep for posterity anyone else who might go down this road...

From doing some basic initial reading of that article, I found the following comment:

"If your dial gauge measures up to 5 volts, just the resistor and capacitor given should work fine. If your gauge requires a higher voltage, you will need to drive the gate/base of a transistor hooked up to a voltage source matching the gauge, and then filter that PWM signal as before."

So that basically sounds like what I will need to do WRT to the 10Vdc input on the Automation Direct GS1 that I am using to drive my pump motors.
clearwaterbrewer
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Re: VFD speed control possible?

Post by clearwaterbrewer »

the issue with the PWM to voltage converter is that the PWM implemented in the BCS is really really slow... like 10hz... some need 500Hz minimum... I have another thread on 4-20ma in the roadmap category showing what i got but have not tried yet...
pngaudioguy
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Re: VFD speed control possible?

Post by pngaudioguy »

There's a calculator on this page (linked from the other one I mentioned above) that allows you to calculate the results of different filtering combinations, and graphs the output over time, etc. It also allows you to change the frequency of the PWM signal as one of the variables. I input 10Hz for PWM frequency, and using a different calculator on their site determined that I wanted a 0.1Hz cut-off frequency to get a fairly stable voltage ripple. This results in a resistance value of 7.5k ohm and 220 uF capacitance (both standard values.) You can adjust the % duty cycle to see the resulting voltage output characteristics. Pretty nifty little tool if you're trying to come up with a LPF conversion circuit!

http://sim.okawa-denshi.jp/en/PWMtool.php

Yes, it's in English.
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