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Re: status report, questions, etc.

> 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 
transformation ratio.

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 
current loads.


Steven Roys (sroys-at-radiology.ab.umd.edu)