Re: [TCML] Variac Paralleling Transformers

[Bert Hickman <bert.hickman@xxxxxxxxxx>]

Hi Jon,

Jon Danniken wrote:
> I finally finished building my next control cabinet, built around three
> ganged variacs. I figured I would finish the cabinet before I tackled
> the issue of balancing the variacs, and that time is now.
>
> I came across a post detailing the "Hickman Method"
> (http://www.pupman.com/listarchives/2001/March/msg00469.html) of winding
> center-tapped transformer cores, which I will be utilizing with MOT
> cores. Note: I will be modifying his method by seperating and isolating
> each side of the center tap.
>
> My plan was to use three MOT cores, wired up in the standard method,
> shown as "B" in the following diagram:
> http://teslacoils.home.comcast.net/ParallelVariac.gif
>
> But then I came across topology "A" (from
> http://www.superiorelectric.com/PDF/z30pwst_instr_002105-006g.pdf),
> which uses only two balancing transformers.
>
> So here's my question: Would the method "A" actually work, and balance
> current equally from the three variacs? If so, is there any benefit from
> using three balancing transformers instead of just two?

The two-transformer circuit (A) very cleverly provides voltage balancing 
while eliminating one balancing transformer. Either approach should 
balance current evenly across the windings of the balancing 
transformers. This may be a bit easier to see in approach B versus A. 
The cost of approach A will be 2/3 that of approach B, and it may be 
easier to find two similarly-sized MOT cores than three.

BTW, you can also use simple house-wire wound around a stripped-down 
toroidal cores salvaged from 60 Hz current transformers. This may save 
some space, and it's what the folks from Superior Electric use on their 
balancing transformers. Inexpensive CT's can often be found at low cost 
on eBay or at ham fests.

> Also, do the balancing transformers need to have similar core areas, or
> does this not matter as long as they don't saturate?

You'll need to use more turns on the smaller core in order to handle 
worst-case voltage imbalances without saturation. Otherwise, it 
shouldn't matter.

> Thanks for any insight into this,
>
> Jon
>
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> http://www.pupman.com/mailman/listinfo/tesla

Please let us know how it worked out.

Bert
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