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Re: VARIAC's -> parallel??

Tesla List wrote:
> 
> Original Poster: Terry Fritz <terryf-at-verinet-dot-com>
> 
> At 07:39 PM 11/12/98 EST, Jim Monte wrote in part:
> >
> >
> >How much does each variac weigh?  A 30A variac for 240V input will
> >probably weigh nearly 100 lbs, so if each array weighs around 500 lbs,
> >it should be possible to parallel connect them for a 6*30A = 180A -at-
> >240V variac.  That's over 40kVA, which would handle all but the
> >largest systems.  It they are in good condition, that is a really good
> >deal for $200!  Would you give more information about how to contact
> >the salvage yard?
> >
> >Jim Monte
> >
> 
>         My friend was talking to an engineer at a variac company the
other day.
> He warned against putting variacs in parallel.  Apparently, if the output
> voltage of each variac is a little different, there will be heavy current
> flow between the outputs due to the unbalanced voltage.  He had some
> special choke or something for $100 that would fix this problem and allow
> them to be put it parallel.  Anybody ever heard this before??
> 
>         We decided it was ok because we put the ballast resistors on each
leg of
> the three phase variac and then combined them.   So the resistors would
> balance the voltages out before they were joined.  The engineer's concern
> did seem very valid to us.
> 
> Something to ponder...
> 
>         Terry

Terry,

These are called paralleling chokes, and the commercial ones used for
variacs are actually small, high current, 1:1 transformers. These are
wired with the windings in series such that any voltage mismatch between
the pair of variacs is effectively "cancelled". If the voltage
difference between outputs of a pair of variacs is defined as V1-V2 =
Vx, the balanced 1:1 transformer circuit (below) will share this
difference at the output without causing large circulating currents.


                    + Vx/2  -
       V1  ----------OOOOOOO-----------  Vout = V1 - (V1-V2)/2
                     -------    |
                     -------    | 
       V2  ----------0000000----
                    - Vx/2  +

A less "elegant" way to handle this would be to use a small high current
single-winding reactor in the output leg of any variac to be paralleled
to the first variac. While this method will reduce the magnitude of
circulating currents, it does not eliminate them as in the above
circuit. 

-- Bert --