current measure with frequency drive

[Ashley Dawson]

Something Interesting to Note

For any of you who are interested, there is an interesting phenomonon that occurs when you try to measure the current on the output of a VSD/VFD that is driving a lightly loaded motor.
The measured current at the output of the drive can be significantly higher than the input current to the drive, even when the drive is operating at full output voltage(i.e. the increase in current is not just due to a decrease in voltage as in P=IV).

The reasoning behind this is explained in the Rockwell Knowlegdebase article A21151103

http://domino.automation.rockwell.c...591914DD639B16D385256C3200699826?OpenDocument

All of this means that in some circumstances, measuring current at the output of a VFD can be completely meaningless.
This caught me out once where we were trying to measure the current of a compressor at no load!

 

[jraef]

Many VFDs use a Hall Effect current sensor instead of a current transformer. The Hall Effect sensor works on both AC and DC current but a current transformer will only work on AC current. The drive current is measured in the DC Link between the converter and inverter sections of the drive and the AC current is calculated from the DC level.

I have found that the most accurate way to externally measure the motor voltage or current is with an analog meter. Digital meters, even True RMS ones, will not be accurate. Fluke claims that its new Model 87V is able to measure VFD outputs. To quote their advertising

"Until now, there hasn't been a multimeter able to accurately measure adjustable speed drives. But the new Fluke 87V is designed specifically to handle these complex signals."

In other words, they are admitting their older True RMS meters were not accurate. I have used a Fluke 87III for several years now and never trusted the readings when measuring drive outputs. At low voltage, I have seen the readings off by at least 50%. Don't throw out the Simpson yet.

Fluke 87V Features

The problem Fluke had was that in the past, they oversold the "True RMS" feature in the 87 and 787 meters, but if you looked carefully enough on the data sheet you would see that it was TRMS only on voltage, not current. Then people would use the current probe feature without realizing it wasn't giving them a correct reading. this new version supposedly fixes that problem. It also has additional filtering to reduce the interference caused by the fact that the VFD output conductors are unshielded at the point where you want to measure them.