# Current Transducer



## JRaef (Mar 23, 2009)

micromind said:


> Does anyone know of a current transducer that can accurately convert amps to 4-20ma. on the load side of a VFD?
> 
> I've installed all sorts of these on sine-wave lines, but never a VFD.
> 
> ...


Veris Industries makes a 4-20ma transducer that they say is designed for the output of a VFD. I can't vouch for this particular unit, but I've used other Hawkeye transducers and they are great, just never on the load side of a VFD.

http://www.veris.com/Category/Current-spcMonitoring/Variable-spcFrequency-spcSensor.aspx

N-K technologies makes the claim as well with one of their products. Again, never used it so I can't vouch for them.
http://nktechnologies.com/current-transducers/at-atr-current-transducers.html

But why is output current so important to you? In a VFD it's always relative to speed/frequency, so what purpose do you think it is going to serve you to know the motor current?


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## Big John (May 23, 2010)

We had a project where they tried to get CT pickups on VFDs for an automation upgrade and it failed miserably, despite using true-RMS CTs: Even at near 60Hz they weren't reading true.

When you say speed pickup, do you need an actual Hz reading or could you make do with an RPM reading?

I'm thinking grab current from the VFD and put a tachometer in the field.

-John


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## micromind (Aug 11, 2007)

This plant is loosely based on another one I work on, it's sort of a one-of-a-kind experiment that actually worked. 

The blower in question is the weak spot, it was made a bit larger than the previous plant, but in a nutshell, we want to be able to run it pretty close to its current limit at times. 

The 4-20 speed command is based on a derivative of most of the rest of the process, and it depending on other parts of the plant, it can easily push even a huge blower past its limits. 

I need to use the speed as well as current as a basis for other parts of the process. 

Plus, if the blower O/Ls, it takes about 2 hours to start the plant back up. 

I thought of a tach, maybe I'll go that route. They're not too hard to attach to the back of a NEMA frame motor. 

I've used Varis transducers a few times; had good luck with them. I guess I should have looked at their site a bit more carefully. 

Lol.

I could program a digital signal for an O/L prewarning, but an analog signal would be more useful.

P.S. The main reason I need current is because the load on the blower is not constant. It varies with pressure across it and even worse, density of the gas through it. I seriously doubt I could derive motor load from the rest of the process.


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## MDShunk (Jan 7, 2007)

Does the drive have an Ethernet, device net, or similar option? You can get all of these measurements via a network connection on most drives nowadays.


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## JRaef (Mar 23, 2009)

MDShunk said:


> Does the drive have an Ethernet, device net, or similar option? You can get all of these measurements via a network connection on most drives nowadays.


I was ASSuming that if it was a pre-Fuji GE drive, it was not likely going to have that kind of capability. But now that I re-read it, I'm not so sure.

Micromind, did you mean it is a NEW GE drive, as in POST-Fuji? If it is, that must be an AF-6 or AF-600. In that case, it will have comms available and MDShunk's suggestion was a good one. It might be really easy to pull the right data out of the drive rather than try to measure it externally. Even a cheap micro PLC can probably do it for what you would spend on tachs and transducers.

A newer Vector capable drive will also be fully capable of torque or current limit, i.e. you set the limit, it responds by automatically reducing the speed (and thus the load) to maintain that limit. Might be a useful way to approach this.


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## MDShunk (Jan 7, 2007)

I'm not trying to re-design your process, but I am curious about something... why not just use a 4-20 vacuum transducer to drive the whole process? I'm assuming, perhaps wrongly, that the vacuum pressure is what your process variable is. Sorta wondering why all the other feedback. Safety? For a small drive like that, I'd be half-inclined to replace the drive with a model that can communicate directly with the PLC via whatever network is in place, and pull all that data right from the drive. Less points of failure, at least, and maybe a wash cost-wise.


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## John Valdes (May 17, 2007)

Some drives have 4-20 ma and several other output configurable in relation to speed, current or frequency. I also agree the drive should have the capability to use the output current to produce most any output signal you choose.

Should the drive not have this capability. There are converters that can take most any signal and make it what you want.


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## John Valdes (May 17, 2007)

How about this. You can take a configurable drive output and make your 4-20ma signal.

http://www.directindustry.com/prod/ditel/signal-converters-31116-591887.html


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## micromind (Aug 11, 2007)

It's an AF600. I couldn't remember until JReaf mentioned it. 

I hadn't thought of using ethernet. I don't remember, but I'd bet it has one. And I'm pretty sure the PLC has one too. 

Not only would that provide any info I would need, but I could also command speed and enable it as well. 

That would free up both a digital and an analog for some other thing they'll add later. 

If I can't use the ethernet, I'm going to give the Veris transducer a shot. 

Thanks for the help!


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## MDShunk (Jan 7, 2007)

micromind said:


> It's an AF600. I couldn't remember until JReaf mentioned it.
> 
> I hadn't thought of using ethernet. I don't remember, but I'd bet it has one. And I'm pretty sure the PLC has one too.
> 
> ...


I bet that 99 out of the last 100 drives I've wired up (where a PLC is part of the system) have nothing but the line and load side wiring, plus the EtherNet/IP connection. Makes things soooo much easier. Far, far less wires to run, and gets you all the I/O you'd ever want or need. Worst case, the only extra wiring might be for an e-stop or process stop to hit the enable input.


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