In-Line 3-Phase Variac

To: tesla-at-pupman-dot-com
Subject: In-Line 3-Phase Variac
From: Mark Finnis <mefinnis-at-medicine.adelaide.edu.au> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Mon, 16 Aug 1999 22:43:20 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Hi Folks,

Help needed from the engineers/professionals (Bert, Malcolm etc.)......

You may recall I found a nice 3-phase variac, complete with control motor 
etc and posted a picture of this to the list several weeks back.  See:

	http://www.adelaide-dot-net.au/~mefinnis/images/variac_cf.jpg

Now I have since been thinking about this and I have some concern.  Most 
3-phase variacs are simply 3 ganged single phase units on a common shaft 
.... easy !

This unit is 3 linear windings, one on each leg of a double-E core.  I had 
originally planned to use the three windings (15A each) in parallel for a 
45A variac - very conservatively looking at the size of the cores and wire 
gauge.  However ....

1.  Trying to visualize the core magnetic fluxes, if I simply parallel the 
3 windings there will be no continuous path for the flux to follow.

	Am I thinking correctly here ??
	Or has my lack of knowledge brought me unstuck again !
	(alternatively would this simply act as a form of current limiting)

2.  Thinking further, if I used just the 2 outside legs in parallel, then 
the flux could take the center leg - providing the core area is sufficient 
(which I suspect it would be).  Is this an adequate solution, assuming my 
concerns are correct in point 1 ?

3.  Taking the mental process another step.  Could I construct a current 
limited variac by running the outside 2 legs in parallel, then short the 
center-leg to the wiper ? (which could then be independently variable)

Only problem with the idea in (3) is the wipers are mounted on a common bar 
which is motor controlled.  Separating the center wiper would be messy, but 
not impossible.

All help and ideas appreciated.

Many thanks

mark