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Accuracy of Fluke PQ analyzers for VFD output

Accuracy of Fluke PQ analyzers for VFD output

Accuracy of Fluke PQ analyzers for VFD output

I did a VFD motor testing at the VFD supplier's factory. We used a Fluke 434 PQ analyzer to measure drive output voltage, current, KW, frequency etc.

PQ analyzer showed output was 400 V while the drive's own panel output meter showed only 370 V. Fluke DMM also showed 400 V output.

The KW, Amps, frequency readings from PQ analyzer were matching with the drive's panel meter.

Why are the PQA and DMM showing higher voltage than the drive's panel meter but the KW, Amp, Hz are matching?

I understand these external DMM / PQA are not designed PWM output of the drive. If so, how to externally measure these output voltages accurately which match the drive's panel meter?


RE: Accuracy of Fluke PQ analyzers for VFD output

Some guesses;
The VFD used an algorithm related to the algorithm that controls the PWM for a given voltage. The algorithm may not account for reflected waves from the motor.
The Fluke may not have a high enough frequency response to cope with the square waves of the PWM.
Suggestion; Try testing at differing base PWM frequencies.
Try another meter make. Gunnar was having some issues with Fluke's algorithms a few years back.

"Why not the best?"
Jimmy Carter

RE: Accuracy of Fluke PQ analyzers for VFD output

Thanks Bill. I used another make PQA and it too showed the same 400 V. The drive manufacturer also used his Fluke PQA which also showed 400 V. The end client is questioning this discrepancy between the panel meter volts and the external meters volts despite our and drive manufacturer's explanation about drives, carrier frequencies, fixed sampling rate of PQA etc.


RE: Accuracy of Fluke PQ analyzers for VFD output

The VFD might simply be taking the bus voltage measurement and calculating the PWM duty cycle to produce the desired output voltage and also have some error in it's method.

RE: Accuracy of Fluke PQ analyzers for VFD output

Thanks Lionel.

So, which voltage readings are taken are to be true and accurate, the drive's HMI or the external PQ analyzers?

And how the KW and Amps readings of PQ analyzers matched that of the drive's HMI, but not the voltage? The drive HMI did not have the option to show the motor PF. So, not sure how it calculated the KW.


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