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Re: [TCML] Variac Paralleling Transformers



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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Please let us know how it worked out.

Bert
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