Re: Potential transformer

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 09:34 AM 10/24/2005, you wrote:
> Original poster: "Mike" <mike.marcum@xxxxxxxxxxxx>
>
> Probably 2-3x rated kVA for a few min. with cool downs in between. I 
> have a 3 kVA PT I ran at 7kVA for 2-3 min at a time in a jacob's 
> ladder and it just got warm on the outside (probably hotter in the 
> middle) and it still works after 7 years. It'd be less for 15 
> min.(almost considered continuous duty, 1x). If the ballast is the 
> problem (I use an old neon bombarder core converted into an 
> saturable reactor) switch to a roll of 10 awg magnet wire. The heat 
> radiates faster without pvc insulation on it.


One can probably get the manufacturer's datasheet with the 
temperature limits, and then measure the temperature under 
overload.  The usual way to do this is to measure the DC resistance 
of the copper winding, and use the temperature coefficient of the 
copper to turn resistance into temperature.

This way you're measuring the actual temperature in the middle, as 
opposed to the surface temperature.

You can also put a known power into it and measure the temperature vs 
time profiles to determine the thermal time constant.  Any decent 
electric motor handbook will tell you how to turn on off duty cycle 
into equivalent continuous duty, given on and off time durations and 
amount of overload. (I'm being a bit lazy here, because I don't 
remember the exact equation off the top of my head, and my motor book 
isn't here.)

I'd imagine that the limit for the wire is 80C or 105C or something like that.




> Mike
>
>
>
> > Original poster: "Ken May" <kens.coil@xxxxxxxxxxx>
> >
> > What is a good output for a potential transformer and not hurt it?
> > Running 450 feet of 10awg for current limit and I'm geting 400ma to 500ma..
> > Also with 450 feet 12awg getting 240ma but the wire is getting too hot in
> > 15min...
> > What else is there ????