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Power Factor Question

by krw
Tags: factor, power
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krw
#1
Jun19-11, 11:41 AM
P: 5
Hi everyone! This is my first post here, and I will start with a question:

I used the wattmeter-voltmeter-ammeter method to measure both the original power factor of a compressor motor and its power factor after installing PFC caps. The values, respectively, were 0.78 and 0.91. I am 100% confident in these values because they correspond with an independent analysis someone else did.

Just for the hell of it, I also used an oscilloscope to take voltage and current waveforms of each phase to see how they differed. What I found was that for the corrected load, the current and voltage waveforms were 163 degrees out of phase rather than 17 degrees out of phase. For the uncorrected load, the current and voltage waveforms were 146 degrees out of phase rather than 34 degrees out of phase.

Essentially, the current waveform was just inverted from what I expected to see. Can anyone explain why this might be happening?
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uart
#2
Jun19-11, 12:27 PM
Sci Advisor
P: 2,751
Quote Quote by krw View Post
Hi everyone! This is my first post here, and I will start with a question:

I used the wattmeter-voltmeter-ammeter method to measure both the original power factor of a compressor motor and its power factor after installing PFC caps. The values, respectively, were 0.78 and 0.91. I am 100% confident in these values because they correspond with an independent analysis someone else did.

Just for the hell of it, I also used an oscilloscope to take voltage and current waveforms of each phase to see how they differed. What I found was that for the corrected load, the current and voltage waveforms were 163 degrees out of phase rather than 17 degrees out of phase. For the uncorrected load, the current and voltage waveforms were 146 degrees out of phase rather than 34 degrees out of phase.

Essentially, the current waveform was just inverted from what I expected to see. Can anyone explain why this might be happening?
You've got a CT winding inverted. (Guess of course, but without more info what can we do. It's a measurement error but you'd have to detail you're complete measurement set up before anyone can tell you for sure what's wrong.)
Antiphon
#3
Jun19-11, 01:06 PM
P: 1,781
Yes. When you have an extra 180 degrees, that's backwards polarity in one of the measurement probes.


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