Re: status report, questions, etc.
Subject: Re: status report, questions, etc.
From: "SROYS" <SROYS-at-radiology.ab.umd.edu>
Date: Wed, 15 Nov 1995 10:07:03 EDT
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> How does one use and calibrate a current transformer? More
> specifically, I know that you put one of the output (220V) leads through
> the hole, and then connect an ammeter across the output leads, but
> why would the current be limited? I thought non-current-limiting
> transformers would produce as much current as the load can draw.
> Since ammeters are low in resistance, wouldn't this short out the
> meter? Also, since there are 400 turns in the current transformer,
> would I get a 400:1 transformation in voltage? This would be 88kV.
> Also, how would I calibrate the current transformer? Should I
> just use ohms law and a big resistor or a few light bulbs?
Most (not all) CT's I've seen have the ratio somewhere on the unit. If
that's the case, you really shouldn't have to "calibrate" anything. The
transformation ratio is simply the current in to current out ratio (ex. - a
200:1 CT that you're running 50A through would give you 50A/200 =
250mA short-circuit current between the CT output terminals). In this
example, you would use a 250mA full-scale AC ammeter to monitor up
to 50A. You would then "calibrate" your ammeter by simply multiplying
the values on the faceplate by 200. I imagine that you could put a
known resistive load across the CT terminals to monitor the current
indirectly from the voltage (V=IR), but just using an ammeter would be
simpler and more direct (COMMENTS from anyone welcome?).
If I get a CT that doesn't have a ratio, or I just want to check it, I use a
space heater as a load and measure the load current and the CT output
current as I ramp up the input voltage with a variac (being careful not to
exceed the limits of my DVM). The ratio should be constant and you
simply divide the input current by the output current to get your
NOTE: By using an ammeter (which is essentially a short circuit)
across the CT terminals, you won't have any significant voltage across
the CT terminals. If you don't have a low impedence path (either an
ammeter or a short) across the terminals and the CT is energized, you
can get a substantial voltage across the terminals, especially with high
Steven Roys (sroys-at-radiology.ab.umd.edu)