RE: PT Ballast Question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Henry.Hurrass-at-aps-dot-com 

A couple years ago, we built a large Jacob's ladder using two GE
Hi-Butte 350 PT's hooked back-to-back. They're rated at 40,250 volts
each to ground (69kv phase to phase), 5000VA, and have two 115 volt
(secondary) windings. We connected the 115 volt secondaries in series
and, for current limiting, used three 500 foot spools of #10 THHN
stranded wire on each transformer, for a total of six spools. Each
transformer was then effectively limited to 25 amps each for a total
line current of about 50 amps; just over the transformers' thermal
rating. We used a powerful fan to try to keep the spools cool, but still
were forced to a "five minute on, 30 minute off" duty cycle. This baby
used a TON of electricity, weighed about a half a ton, and produced an
enormous power arc.

My suggestion: use a many spools of wire you need to get good results
without exceeding the VA limit of the transformer too much and keep an
eye on the "spool temperature", or, use a neon sign transformer. They
are designed as such that even a dead short won't hurt them. I once
measured the power consumed by a neon sign transformer and actually
found less watts consumption with the secondary shorted than with the
secondary open!

Good Luck,
Henry "Hankenstein" Woo

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Tuesday, October 12, 2004 11:04 PM
To: tesla-at-pupman-dot-com
Subject: Re: PT Ballast Question


Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi,

"Theoretically" doubling the current limiting reactance should decrease
to
current by 1/2...  So simply adding another roll of wire should get you
to
13 amps....  Things my get a little odd, but it is supposed to work...
Let
us know ;-))

Cheers,

          Terry

At 07:46 PM 10/12/2004, you wrote:
 >I recently picked up a nice GE 1.5kva 100:1 pt and
 >have a question or two about limiting the current it
 >pulls from the line.
 >
 >I set up a jacobs ladder using a 500ft spool of 12ga.
 >wire for ballast (as suggested by Terry Blake) and
 >found that it did indeed limit the current to around
 >26 amps.
 >
 >I'd like to reduce the current further (to around 12
 >amps or so) before I incorporate this transformer into
 >my coil.  Would it be feasible to add another 500ft
 >spool of wire in series with the first?  Any guesses
 >how much additional current limiting this might give
 >me?
 >
 >One other related question... If I were to get a MOT
 >or two to use as ballast, would this be more effective
 >than the wire spool method?  I'm keeping my eyes open
 >for an old arc welder to use as ballast, but in the meantime...
 >
 >All replies appreciated!
 >
 >Thanks,
 >Greg
 >
 >
 >