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Re: current limiting and 220



I would guess your problem is a ground loop.  The midpoint of a transformer
connected line-to-line is not at the same potential as the center of a
three-phase system.  When in doubt, lift a connection between two points you
have labelled as *ground*, and *carefully* measure the voltage between them.

Gary Johnson

At 08:24 PM 2/7/99 -0700, you wrote:
>Original Poster: Matthew Wenger <m-wenger-at-uiuc.edu> 
>
>	I wanted to thank everyone for their help with current limiting questions!
> I also wanted to offer up my own experiences in case anyone needs
>information.  Oh yeah, and I have another question of my own!
>	I obtained a free welder transformer from a local welding supply company
>(if you ask for old, used welders they sometimes have some available, and
>apparently, the transformers don't usually go bad, it's often the fan or
>some other part you don't even need for current limiting).  I first shorted
>out the output leads from the welder transformer.  I then hooked up the
>primary to the wall (like plugging in a neon sign transformer and shorting
>out the secondary) in series with a current shunt (basically a resistor)
>that reads 100 mV across it when 50 amps are running through it.  I hooked
>up my voltmeter, screwed in the core as far as it would go (I got a
>transformer with a variable depth core) and plugged it in.  I then flipped
>the circuit breaker while my friend watched the voltmeter.  It worked, and
>the current turned out to be about 12.5 amps.  Since I was plugged into 220
>I calculate my power to be about 2.75 KVA.  Next, I hooked up my pole pig
>in series with the welder and set up a Jacob's ladder.  Very impressive!
>I'll probably pull out the core a little more and use about 3-3.5 KVA for
>my coil.
>	Unfortunately we ran into a problem.  We were working on hooking our
>variacs up to the 220 and it is not working so well.  We are trying to run
>our coil off of 220.  The lab where I work has 3-phase power and we are
>hooked up to two legs of the three phase.  We tried to hook up the 220 from
>the three phase into our Variacs (we put two in parallel to cut down the
>current running through each one).  When we turned on the power, they
>started buzzing and the fuse blew.  I know that three phase power is
>basically three 120 lines running 120 deg. out of phase with each other.
>As far as I can tell, a single 220 (like for our dryer at the apartment)
>and 220 from a three phase are about the same thing.  When I hook up across
>the 220 at home it reads 220 on the Voltmeter, when I hook across the
>neutral and either of the 220 leads I read 110, same for the neutral of the
>three phase and any of the other legs.  Anyway, I would like some
>information on transforming 220 using variacs.  We have checked out some
>books from the engineering library, but there's nothing like some first
>hand information from real coilers!  Any help you can provide would be
>really appreciated, especially since Engineering open house is a month
>away!  (F.Y.I. Engineering open house is an event at the U of I where all
>the engineers on campus show off cool research, projects, and basically let
>the public see how cool science, physics, and engineering can be.)  The
>Society of Inventors is working on the coil and we're trying to get done by
>March.  Wish us luck!
>
>    _/      _/
>   _/_/  _/_/
>  _/  _/  _/ _ _|_ _|_ |_  _            matthew wenger
> _/      _/ (_| |   |  | |(/_ |_|_|     m-wenger-at-uiuc.edu
>_/      _/    '
>
>