power metering accuracy, induction motors

Hi, I have a feeling this has been posted before, but it doesn't show in my current postings, as don't the FAQs. Anyway, here goes:
Can anyone give me an idea on the stability to be expected in determining the power input to a 3 phase induction motor from a current measurement of one phase, appropriately linearised and calibrated. Ie. the current measurement is logged against the output of an accurate, temporarily installed polyphase power meter at several working points, then the curve between the current test points is interpolated and used to provide an ongoing measurement of power. Assume the CT is accurate and the supply voltage steady. I'm after variable factors within the drive that would affect the relationship, and by how much.
Drives are 3.3KV 500KW, standard squirrel cage induction. Incidentally, to hopefully prevent spurious responses, this relates to a real industrial problem, not homework. TIA
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my
to
Your assumptions will kill you in the real world. Even slight voltage unbalance will result in large current unbalance. Believe me, the voltage unbalance WILL change over time. Therefore, my answer is "punt". If you want accurate measurement for a three phase motor, you will need three currents. Or you can use an electronic polyphase meter to supply the data you need. SATEC (http://www.oksatec.com /)makes some inexpensive ones ($300 or so for the PM130). They are not revenue certified, but they are accurate enough (.5% of full scale). Using one current will give you errors as high as 19% for relatively common levels of voltage unbalance.
Charles Perry P.E.
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Honeywell and JCI, Metasys both have used this method. When we installed 3 phase utility grade equipment we found the errors as small as 5% and as large as 40%. If your really interested in accuracy you will need 3 phase current and voltage measurements. If you want a ball park or county accuracy then do as you suggest. Have fun with your project
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