Re: Variacs in parallel ?

To: tesla-at-pupman-dot-com
Subject: Re: Variacs in parallel ?
From: "Paul Eugene Kidwell" <tmb-at-ieee-dot-org> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Fri, 21 Apr 2000 12:02:02 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme
Hi Again!

I've been thinking about the 3 phase diagram that was posted a few
days ago. Here's the original...

> > For a 3-phase variac you'd need chokes between pairs in a tree
> > configuration:
> >
> >
> >   W1  ------------
> >                  O
> >                  o-----------
> >                  0          |
> >   W2  -------o----          |
> >              |              |
> >              |              o-----------  Vout
> >              |              |
> >              |              |
> >              -----          |
> >                  O          |
> >                  o-----------
> >                  0
> >   W3  ------------
> >

I'm planning on paralleling 3, 30 Amp Variacs, and I'm trying to
figure out exactly how to wind them. So (assuming a perfect world :)
 I started plugging in some current numbers...


                    Chokes
                       V
   W1  -30--------30----
                       O
                       o----45-----
                       0          |
              |---15----          |
              |                   |
              |                   |
   W2  -30----o                   o---90------  Vout
              |                   |
              |                   |
              |---15----          |
                       O          |
                       o----45-----
                       0
   W3  -30--------30----


You'll notice that the the chokes have 30 amps comming in on one
leg, and 15 comming in on the other. To me, this doesn't seem quite
right. Someone else suggested an arangment that looked like this...

                      Chokes
                        V
   W1  -30-o-------15----
           |            O
           |            o----30-----
           |            0          |
           |   |---15----          |
           |   |                   |
           |   |                   |
   W2  -30-----o                   |
           |   |                   |
           |   |                   |
           |   |---15----          |
           |            O          |
           |            o----30----o---90------  Vout
           |            0          |
           |   |---15----          |
           |   |                   |
           |   |                   |
   W3  -30-----o                   |
           |   |                   |
           |   |                   |
           |   |---15----          |
           |            O          |
           |            o----30-----
           |            0
           |-------15----


Here we're using 3 chokes, but the ends of the chokes are balanced,
and the current numbers are quite a bit lower. Low enough that I 
don't have to use 10 gage wire to wind them. I could get away with 
using 14 Gage wire, but I'll probably go with 12.

I've also managed to dig up a matched set of 3 solid steel cores.
(a couple of days ago, someone suggested solid cores rather than 
laminated ones. This seems right since there's little magnetic 
field generated in the core) So I'm all set to start winding. 

I'd *still* like some idea of how I can check to see if everything 
is working the way it should.

Later!
Paul