duplicate 4-20ma signal to multiple VFD

lesmar96 · May 22, 2017

Hello,

My customer is asking me to control the frequency reference of 8 VFDs , all off of one potentiometer. That way he can turn one knob and all the VFD adjust together. Is there any reason I cannot put all the drives in series through a 4-20ma loop? I will use a 24VDC power supply to power the loop and use a resistor to convert it to 4-20ma. Are there any downsides to this type of setup?

JesperMP · May 22, 2017

Dont fiddle with a DIY resistance-to-mA converter. Get a proper signal conditioner.
Then loop the 4-20mA signal to all VFDs.
You have to check the input resistance of the analog input of each VFDs. The total resistance must not be more than what the signal conditioner can drive - typically 600 ohm. If that is a problem, you can split up into several 4-20mA loops.

dmargineau · May 22, 2017

...or, use the potentiometer as a PLC Analog Input and the eight VFDs connected to eight Analog Outputs of said PLC.

The value of the Input reference signal could be easily multiplied into eight identical values to be generated as follower Analog Outputs speed references.

mendonsy · May 22, 2017

I don't know of any 4-20ma signal splitters with 8 outputs, but here is one with 4 outputs. Two of these would solve your problem and keep the drives isolated.
https://www.asi-ez.com/member/~ASI451141.asp?

danw · May 22, 2017

wrong thread

lesmar96 · May 22, 2017

Yes, I was not able to find a signal splitter with that many outputs. The deal is this is just temporary and I was hoping to quick set something up. Please explain the issue with using putting them in series. Would it cause control issues because the VFDs are not totally isolated from each other?

Pete.S. · May 22, 2017

lesmar96 said:
Yes, I was not able to find a signal splitter with that many outputs. The deal is this is just temporary and I was hoping to quick set something up. Please explain the issue with using putting them in series. Would it cause control issues because the VFDs are not totally isolated from each other?

You gotta know your ohms law. Having a potentiometer feed a 4-20mA loop is tricky. And if you put 8 of them in series the voltage needed is 8 times as high.

So for instance normally you have 5V or so to get 20mA. With 8 of them you need 5 x 8 = 40 Volts to get 20mA at each input.

What kind of VFD is it? Doesn't it have a 0-10V input? Most have.
Also doesn't the VFD have a 4-20mA output? Because then you could daisy-chain them. Take the the output of one VFD and feed it to the input of the next one.

The proper way of doing this of course is to connect the pot to a 0-10V input on the PLC and then run the speed from one or several 4-20mA PLC outputs.

.

keithkyll · May 22, 2017

Is he expecting all to have the exact same frequency?
You might have a bit of error with analog.

My first choice would be a small HMI, PLC, and RS485. You can daisy chain the RS485 to each VFD.

If you must have a pot and don't need feedback, then use an Analog CLICK and RS485.

JesperMP · May 22, 2017

The analog 4-20 mA inputs on VFDs are usually isolated, for exactly this reason, to be able to connect several in series.
However, you must check with your particular VFD.

lesmar96 · May 22, 2017

These are Fuji Electric Eco series. They have 0-10v inputs as well as analog outputs. I like your various suggestions. I had not thought of voltage drop in a 4-20ma loop, but that is a very good consideration because doing the calculations, I would not be able to get full speed out of all 8 drives with only a 24vdc power supply.

I might go to using the analog output of the drive and daisychain them that way. Any other suggestions would be welcome.

danw · May 22, 2017

Run all the voltage inputs in parallel from a resistor across the mA output.

If you only have 4-20mA use a 500 ohm resistor and the voltage is 2-10Vdc.

If you can get a 0-20mA output, the same 500 ohm load creates a 0-10Vdc drop that you can parallel to all the inputs.

Yes, there's some loading so the signal might not reach full scale, but it's quick and dirty.

diat150 · May 22, 2017

lesmar96 said:
These are Fuji Electric Eco series. They have 0-10v inputs as well as analog outputs. I like your various suggestions. I had not thought of voltage drop in a 4-20ma loop, but that is a very good consideration because doing the calculations, I would not be able to get full speed out of all 8 drives with only a 24vdc power supply.

I might go to using the analog output of the drive and daisychain them that way. Any other suggestions would be welcome.

seems like using the 0-10 volt input would be better.

mellis · May 22, 2017

If all the drives are located close together (like in the same panel), then paralleling the 0-10vdc inputs as danw described will work well.

duplicate 4-20ma signal to multiple VFD

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