[Prev][Next][Index][Thread]

Variac Parallel Choke Questions

I have two 10A Powerstats that I plan to run in parallel.

Mechanicals: 

Regarding the common shaft, the list consensus (thanks folks) seems to be
that any material of the right size and sturdyness will suit, as long as
it's electrically isolated, so you don't form a closed electrical loop
around the core (thanks Terry).  I'm going to go with thick walled stainless
tubing, simply because I have it sitting here on my desk.  

Electricals:

Regarding the electrical connections, a search of the archives reveals that
some list members have had success just adjusting the rotor to the same
voltage, without any choke.  Any reports of failures or hot-running using
this kind of setup would certainly be appreciated, since this option
certainly is the easiest, if it works ok. 

However, other listees have stated that a "parallel choke" is desirable and
perhaps necessary to longevity of the equipment.   My problem is that I
don't understand the explanations of such that I have found in the archives,
and can't even tell if they are recommending the same thing, exactly.  (I
should mention that a large part of my new interest in coiling flows from my
desire to learn about elektrissity, and I tend to learn better if I have a
challenging and interesting project to throw myself into, despite the
obvious problems of getting in over my head.) I would be very happy to be
directed towards a drawing/schematic or more detailed instructions regarding
how to follow this procedure.

I've included some snippets of pertinent advice from the archives below, for
those interested.  If you've been on the list for many years and don't want
to re-read old material, please stop reading here.  My questions/comments
will be bracketed.

In March, 1997, Brent ____ wrote:

"When you parallel 2 or more variacs together, you really ought to use a
paralleling choke. Even though you might get the brushes aligned on the
exact same turn, there will still be slight variation in inductance and
such.  A paralleling choke works as a common-mode current choke. A current
imbalance in one variac is coupled to the other so that each variac ctually
'sees' it's share of the current. (The choke will create a slight voltage
imbalance on the input though this is how it keeps the currents split
equally.)"

In November, 1998, Bert Hickman wrote (replying to a thread regarding
commercial variac chokes):

">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."

[This is very helpful, or would be, I imagine, if I understood.  : )  Again,
any reports regarding success or, better, failure using either of these
methods wold be appreciated.  Moreover, any directions to inexpensive
sources of high current isolation transformers or single-winding reactors
would be likewise appreciated.]

Eric Forsman
Baltimore