Re: Assistance on AC current measurement


From: Dale LeDoux (
Date: Fri Sep 26 1997 - 23:36:45 EDT

At 23:31 9/26/97 -0500, you wrote:
>I apologize, for asking but I need some help.
>The test consists of applying an AC 60Hz voltage to a transformer and
>measuring the current. The current wave form through a 1 ohm resistor
>with the transformer is not a sine wave. The waveform does not contain
>any oscillation. The multi meter is reads true RMS up to a 5:1; RMS to
>average. I drew the waveform on graph paper. Next used calculus to
>determine the RMS value. (Only six sample points; where the line crossed
>the gradient.) The value calculated was within 3% with the value
>Have I missed anything?
I'm not exactly sure what you're trying to measure, but what you observed
is common to transformers. It is caused by the fact that as the current on
the primary winding collapses, the resultant magnetic field in the iron
core collapses slightly slower, with the effect causing a distortion in the
waveform of the output. #% sounds like a good figure. There are a number
of instruments on the market which will measure the voltage or current and
then give you the energy contained in each of the harmonics up to about the
31st. Fluke makes a nice on for a thousand bux. You might try a GOOD
electrical contractor who does power quality work and see if they might
help you.


Dale LeDoux - KD5QI
Bath Electrical Systems
Power Systems 480V to 230KV

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