Re: Paralleling Choke for Paralleling a 3 phase variac???

To: tesla-at-pupman-dot-com
Subject: Re: Paralleling Choke for Paralleling a 3 phase variac???
From: "Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 08 Mar 2000 06:49:47 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
> 
> Original Poster: "Bunnykiller" <bigfoo39-at-idt-dot-net>
> 
> Big snipperzz...
> 
> >
> > I can take more acurate measurements when I get home if there is
interest. I
> > don't want to disassemble it, but I'll try to help anyone who wants to
> attempt
> > building one by providing info from mine.
> >
> > Travis
> 
> Hi Travis  ...
> 
> Yes by all means please release this info to us ... I have been going
> nuts trying to find a way to link my 2 Variacs ( 240V 28A ) together for
> 240V  56A to use on my piggie system...
> 
> Thanks
> 
> Scot D

Travis and all,

Here's a bit more information:
Commercial paralleling chokes are actually small, high current, 1:1
transformers. They are simply a single center-tapped high-current
winding wound on a silicon-steel core. The winding ends go to two variac
wipers, and the center tap is the combined output. Any slight voltage
mismatch between the pair of variacs is cleverly "cancelled" by the
transformer action, and there's virtually NO circulating current. 

If the voltage difference between outputs of a pair of variacs is
defined as Vx = V1-V2, the balanced 1:1 transformer circuit (below) will
share this difference. The trick is to make the windings robust enough
to handle the current, and the core large enough to handle the maximum
expected voltage difference between pairs of wipers on linked variacs
(typically only a few volts). 

                      Vout = [V1-(V1-V2)/2]
                                |
                                | 
To Wiper of          + Vx/2 -   |    + Vx/2 -          To Wiper of 
 Variac 1  <---------OOOOOOO----o----0000000--------->   Variac 2  
           V1        -----------------------        V2
                     ----------------------- 
                     -----------------------    
                          Common Core

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  ------------

A bit of heavy gauge wire or strapping and some transformer laminations
should be sufficient to do the job, and one heck of a lot less expensive
new paralleling chokes. 

A less "elegant" way to handle this would be to use a small high current
single-winding reactors in the output leg of each variac to be
paralleled (except possibly the first). Two chokes would be necessary to
connect three variacs together. While this method reduces the magnitude
of circulating currents, it does not eliminate them as in the first
circuit. However, each choke only needs to carry approximately the
output current from its variac plus a small circulating current. 

Hope this helps!

-- Bert --