Re: $10 7.2kv pot trans

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: $10 7.2kv pot trans
> Date: Saturday, July 31, 1999 9:31 AM
> Original Poster: Doug Brunner <dabrunner-at-earthlink-dot-net> 
> I know potential transformers are generally overbuilt, but do they
> generally have extra iron in their cores? I'm wondering if this PT could
> be run at 240V in/14.4kV out.

Nope.. sorry... First off, the PT is really designed to measure high
voltages, so the design is for the HV to be the input and the output to
hook up to a standard 120VAC meter movement, or a watt hour meter, or a
relay. In this application, there is very little power drawn from the unit,
so they typically aren't rated with a high VA rating.  However, because
most of the bulk (and material cost) in a PT is in the insulation, it
doesn't cost much more to make a PT rated at say 1 kVA ,than at 250 VA. 
Furthermore, if you make it a bit higher rated, then, you can run standard
110V appliances and other useful stuff (like lights) at the place you are
measuring the voltage.

The fairly large bulk, in comparison to the VA rating, means that you can
overload them fairly successfully, because there is a large mass to suck up
the excess heat in short duty cycle operation. Also, just like pole
transformers, they are designed to work over a wide temperature range out
in the hot burning sun, so there is a fair overdesign.  Not only that, PT's
aren't used in such large quantities as pole transformers, so there isn't
as much economic incentive to reduce the price.  The little transformers in
a typical "wall wart" is a good example of a transformer designed to use
the absolute minimum quantities of everything, where they'll trade off a
bit of extra heat dissipation due to saturation and winding resistance for
saving a few tenths of a cent in iron cost and smaller copper wire.

A PT will also be designed to handle a wider variation of voltage than a
typical power transformer (after all, that is what it is designed to do,
measure the variations of the voltage), and to not have much saturation,
which would distort the waveform being measured. This all said, the cross
section of the core of a PT (which determines the turns/volt) will be
chosen so that it doesn't saturate, but, they're not going to put twice the
core material in (which is what would be required to run at 240V).   

You can find out pretty easily with a variac.  Hook up an ammeter in series
with the transformer, no load.  As you increase the voltage, you'll find a
point where the current starts to rise much more quickly than the voltage
does. That's the core saturation point.

Typical NST's start to saturate around 140VRMS, I've runs some good quality
power transformers up to 150-160V, but no 110V transformer I've ever
measured has made it to 200V without saturating. (the aforementioned wall
warts sometimes start to saturate at 90-100V).

>             --Doug Brunner
>                     <dabrunner-at-earthlink-dot-net>
> Tesla List wrote:
> > Original Poster: "Aron" <kc5uto-at-wt-dot-net>
> >
> > 7.2kv, large potential transformer. E-bay address below.
> >
> > http://cgi.ebay-dot-com/aw-cgi/eBayISAPI.dll?ViewItem&item=137404860
> >
> > Aron