Amp Clamp for AC Drives

I am looking for a amp clamp to measure the output of my ac drives of various HP i was looking at this one http://us.fluke.com/Fluke/usen/Power-Quality-Tools/Single-Phase/Fluke-345.htm?PID=56070 which is the model 345 and is labeled as a power quality meter but it says it has a low pass filter.

Would this be a good choice? What brand / Model do some of you use for measuring drive outputs?

I would like to stay with fluke if possible. As long as it is under 10K cost is not an issue i just want the best clamp?

The only accurate method of reading drive output amps that I know of is the drive itself. I've used top-of-the-line Fluke and other test equipment with all kinds of claims of being "true rms" and "inverter power rated" and every one was hopelessly inaccurate. In addition, on 480V systems, you need equipment rated for 1600V which, I believe, is CAT IV.

All of that makes reading the drive display wonderfully cheap and convenient, it would seem!

What is it about the drives current rings that make them so accurate? Just curious.

Here it would be beneficial to read current at the motor terminal box (before the peckerhead)and kep an exy on the physical load at the same time and the drive itself is some distance away in a controls room with air conditioning.

That is our real need for a clamp. This would be 480 volt drives.

The Plc Kid said:

What is it about the drives current rings that make them so accurate? Just curious.

Here it would be beneficial to read current at the motor terminal box (before the peckerhead)and kep an exy on the physical load at the same time and the drive itself is some distance away in a controls room with air conditioning.

That is our real need for a clamp. This would be 480 volt drives.

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Kid I think the software in the drive is able to take the "choppiness or sawtooth" out of the current. I have used a shunt in series with one phase to display current on an oscope on small motor and drive - around 2 HP. It gives pretty good sine wave but it still has a small amount of sawtooth in the wave - guess 5 maybe 10%?

Am told by an instrumentation outfit that a Darsenell movement meter will read fairly true. So that may work out for you.

Best bet in my mind to show current in the control room
A run a signal line from the VFD or
B just take current reading from line feeding VFD and motor.

Maybe to prove to yourself read current in feed to VFD and read current out of VFD under similar conditions. Allow maybe 3 to 5% for losses in VFD. The problem with this approach is that the voltage is not sinusoidal but a pulse which is why the resulting sawtooth in motor current wave form on o scope.
Dan Bentler

Often the drive has a 4-20ma current output that can be set to indicate current or, better yet, torque. This ma signal can then be sent to whatever remote place to a meter to properly indicate load. And, it can be trusted!

In case someone is searching, the name from Dan's post is D'Arsonval. That is the galvanometer design most oftwen used in analog meters.

But just to keep this going, what is it about a drive that makes it's picture of current correct where a clamp-on ammeter won't be right? I agree with Dan that it is the discontinuous nature of the waveform. But the better digital clamp-ons will have a microprocessor in them just like a drive does. You would think VFD technology has been around long enough that someone at Fluke would have figured out how to make sense out of the choppy current waveform.

Keith

When I combine DickDV's comments about accuracy and using the drive's analog output. With what I'm presuming (almost always a bad thing) The PLC Kid wants. I'm presuming he wants a portable maintenance tool for monitoring the current remotely that's under $10,000.

I'm thinking a wireless transmitter unit like the Banner surecross networking units at the VFD and the receiver at the motor
http://www.bannerengineering.com/en-US/products/sub/431
and perhaps a redlion panel meter for the readout.

kamenges said:

You would think VFD technology has been around long enough that someone at Fluke would have figured out how to make sense out of the choppy current waveform.

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Dick DV might be able to confirm this: Perhaps there is enough difference between waveforms in say a ABB ACS800 using DTC and a plain jane scalar VFD. That it would require a different measuring technique. And if the meter had different settings. How would you know which one to set for which VFD?

Milldrone

you are correct i do need something that is portable and can be used from drive to drive. There are close to 400 drives setup like the described configuration here so portable is almost my only option.

While I am not an expert on drive internals and how they work, I am quite sure that motor amps is deduced from DC bus amps. Those are much easier to measure accurately.

PLC Kid's desire to have a portable measuring device is clear enough but two questions related to that occur to me. First, it must be that the motor is always closer or more convenient than the drive otherwise the keypad would do the measuring for you, and second, I fear that motor amps is being used as a load indicator when, in practice, except at full load, amps is not a very good load indicator. Slip speed is very accurate on DOL motors and torque is a direct load indicator on VFD motors.

My point is that, if motor loading is what is being measured, then even the accurate measure of amps is not a very accurate load indicator.

But, anyway, I'd be interested in what would be PLC Kid's best choice for a motor lead current measuring tool. Anybody got anything that works reasonably well?

DickDV said:

I fear that motor amps is being used as a load indicator when, in practice, except at full load, amps is not a very good load indicator.

My point is that, if motor loading is what is being measured, then even the accurate measure of amps is not a very accurate load indicator.

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That's why I suggested using the drive's analog out and hooked up to one of those surecross wireless transmitters, then wherever he needs an indicator have the surecross wireless reciever and a digital display. It would be a roll your own ammeter / load indicator.

Our local Fluke rep will allow us to borrow Fluke devices for evaluation as long as they have one in stock. You may want to ask this of your local rep and see what they say. If they say yes compare the Fluke against a given drive's current output and see how they compare. That is the only way you will really be able to convince yourself.

I have had power electronics guys tell me that the Tektroniks stuff is pretty good. The latest iteration appears to be the TPS2000. The TekScope 720 series is the one I have used in the past. But I don't know if it had meter outputs.

Keith