AC current measurement with a current transformer

Old textbooks seem to consider that the largest source of error will be from I-quadrature, dismissing I-inphase as almost a second order. I suspect that this may be because the burdens of that day would most likely be meters, which could be quite inductive.

AFAICS if the load is known to be resistive then errors in the current ratio will be mainly due to I-inphase, from the core loss.

I did have a chance to test this theory about 10-15 years ago, with a contract to investigate the cause of mfr'ing spread in precision 400Hz/500Arms CT's... was it due to turns-count or core problems (Ni-Fe cores).

Cores from various batches were carefully hand-wound with 1000-turn secondaries, and I made up a jig (out of tapped brass spacers) that centred a wound CT on an 8-turn primary. I had a milliWattmeter and access to a 100Arms precision current source, and ammeter, both with traceable calibration. Overall accuracy was estimated to be of the order of 0.03%.

We stimulated the secondaries at various voltages and measured the watts-loss. Then took the CTs up to the high current jig and measured the current-ratio.

There was good correlation between the current-ratio errors and the inphase currents calculated from the watts-loss at the same (internal) secondary voltage.

In fact the problem turned out to be a wider than promised spread in core loss, instead of 3-8mW some cores were up to 15mW (at 5Vrms/400Hz Vsec).

Unfortunately no, because watts-loss is a non-linear function with respect to the secondary voltage.

Probably academic anyway Fred, because I suspect that the OP should be measuring RMS, rather than averaging. Might do better to just go out and buy a LEM/HEME Hall Effect device, the version with a dc output proportional to the RMS of the primary current.

I didn't read the entire thread...someone may have mentioned this already...

beware , if the secondary is not loaded, it can develop a dangerous high voltage...

Mark

You mean, more than the existing ratio of bogus, misleading answers?

John

Yeah, about a week ago.

I say we should all boycott googlegroups, or give them really bogus, misleading answers, although I wouldn't advocate suggesting anything dangerous.

LOL!

Thanks! Rich

Certainly.

John

And more than once. :)

See! That's what _CONTEXT_ is for!!!!!!!!!!!!!

Cheers! Rich

Sunspots, poltergeists, UFO's, whatever. :-) I got lost in the thread, trying to figure out who was saying what, and why. :-(

Ed

I've already got a rule that ignores threads where the root message is posted via Google (with exceptions for a few notable people)

Right, it takes a Omron K3NX-VA1C or K3NX-AA1C to do the job without a rectifier. Your scheme should work fine, because the CT load is after the rectifier.

Yo Ho. If you had read the specifications for your 882gb you would know that all you need or want is just the CT. The process meter has facilities for AC current input.

Are you sure that what you said applies to the K3NX-AD1C model specifically?

Thanks Win. My answer to your another post, no, I have not yet fried other IGBTs.