Re: Power Load Balance Concern

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, September 23, 2001 11:52 AM
Subject: Power Load Balance Concern


> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Mddeming-at-aol-dot-com>
>
> Hi All!
>         I am in the process of upgrading my power supply to 4 15/60 NSTs
and
> already have a 120v input -at-30Amp,  model W30M Variac. Based on nameplate
> ratings, the NST bank  should draw about 33 Amps total (4X8.25). The
problem:
> since there is only one open slot left in my breaker box, I would like to
use a
> 40Amp breaker, but I have not seen single breakers for this kind of
current.
> Most are dual breakers -at- 240 V using both sides of the line to keep the
load
> balanced and prevent in-house brown-outs.

You can put in a dual breaker, and only use one side... (doesn't solve the
one free slot problem though).. You can also get "half width" breakers so
maybe you could get a double half width to replace 2 of your existing full
width breakers, freeing up 1 slot, so then, you can use a standard double
40A breaker.


 If I try to get a 4KVA
> step-down/isolation transformer so as to utilize both sides of the line, I
am
> looking at ~$350 plus S&H on 120 lbs.
> Questions:
> 1. Is running the Variac at +10% a real concern?
No, just watch the temp on the variac.    Why not use a switching panel to
switch in varying numbers of your NSTs or switch between series and
parallel?  I'd also make sure you've got PFC caps (no point in running extra
amps, just for the reactive load...) for those NSTs

> 2. Is placing this size load on one side of the panel going to create
in-house
> imbalance problems?
No..

> 3. Anyone got a cheap step-down 4KVA transformer?
You can always use banks of smaller transformers...  However, you should be
able to find standard "dry" transformers that are nominally set up for 480:
240 in this power range.  They have a pair 240V windings and a pair of 120V
windings.  You can wire anything from 240:240, 240:120, 480:120, 480:240,
etc. with these.  They are VERY common in factories and the like where they
are used to turn the 480V delta three phase into 120:0:120 (like your house)
for operating lightbulbs, office equipment, water coolers, etc.  $350 is
outrageously expensive for a used one of these.. Brand new in the Grainger
catalog, they are only $350.

What you do NOT want is an "isolation transformer", which is an entirely
different kind of beast, with very good isolation between primary and
secondary, copper foil shields on the windings, etc.


>
> Would appreciate response from any/all with experience in this area.
>
> Matt D.
> G3-1185
>
>
>
>