Transformer current limiting

To: richard.quick-at-slug-dot-org
Subject: Transformer current limiting
From: Esondrmn-at-aol-dot-com
Date: Thu, 13 Apr 1995 13:08:54 -0400
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Richard,

I fired up my new pole transformer last night.  It seems that the inside tap
is set at 14,400 volts.  I apply 10 volts and get 600 out.  60:1 turns ratio.

I tried several experiments to limit the current into the primary.  Right now
all I have is two 120 v 10 amp variacs which I'm using in series.  My goal is
to limit the primary current to 10 amps without dropping too much voltage
across the ballast.  10amps in at 220 volts would give me 2.2kva.  This
should be about 14kv at 160ma.  Using my old neon sign transformers with the
secondaries shorted as ballast, I was able to limit the primary current to
15amps at 50% on the variacs but I had about 100volts dropped across the
neons.  I set the pole transformer up as a jacobs ladder so I would have some
current draw on the secondary.  I never did get any more than a pitiful small
spark - like a 12kv 30 ma neon.  At first I tried both neons in parallel and
then in series with the line side of the variac.  I slowly turned up the
variacs and when the jacobs ladder finally fired, it drew 45amps and burned a
rough spot in my variacs.  My later testing was done with the ballast between
the variacs and the pole pig.


I'm sure at this point that I will need to go buy that used 240v 40amp variac
that I've had my eye on.  The big problem now seems to be how to control the
primary current.  I have Henry Transtrom's book Electricity at High Pressures
and Frequencies on order at the library but I don't know when it will come
in.  Is it realistic to expect to limit the primary current to 10 or 20 amps
with only 10 or 20 volts dropped across the ballast?  Do I need to use an arc
welder?  I have one at work that I could take home and try.

This is a 5kva transformer.  With 240v input, that means I need to limit the
primary current to 21 amps.  Assuming the ballast will drop at least 20
volts, then I can input 23 amps at 220 volts for 5kva.  Will it hurt this
transformer if I allow the primary current to go up to 30 or 40 amps?

Thanks,  Ed Sonderman