Re: In-Line 3-Phase Variac

To: tesla-at-pupman-dot-com
Subject: Re: 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: Sat, 21 Aug 1999 20:24:03 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

At 16:07 20/08/99 -0600, you wrote:
>Original Poster: "Choad" <choad69-at-geocities-dot-com>
>
>Mark,
>     What happens if you parallel the two outer windings and connect the
>middle winding in parallel also but reversing the leads. As H1-H2-H1 and
>H2-H1-H2. Seems to me that the flux lines should ge the oppisite directon
>for that winding which would be the same direction as the other two legs. If
>you just put voltage on the two outer windings in parallel won't you get the
>same voltage in an opposite phase on the middle winding? This idea is
>working in my head. So will someone please tell my why I'm wrong and this
>won't work. :)

Well,

I'm no engineer, which is why I asked the question in the first place, but ....

1.  Like most variacs there are a number of windings beyond the 'primary', 
so the output goes to about 110%.  Reversing the primary to the middle leg 
this would fail.

2.  If you imagined the case of just 2 legs on a rectangular core, this 
would effectively double the flux through the core and risk saturation.

3.  As I think Bert pointed-out, if we connect the 2 outer legs, then 
double the flux will be returning through the centre leg, so the dV across 
the windings will be *double* the supply dV at 180degree phase 
shift.  Connecting the supply to this would be like connecting the output 
of a 1:2 transformer back to the primary....... where there is smoke ...... 
!!!!!

The great advantage of this unit is its size (and cost!!) and heavy 
construction.  Two of the 3 windings will be ample for my task.

I'm going to do some test measurements under load today.  I'll post them to 
the list for those interested.

Cheers

Mark