Re: Arc Welders as Rheostats??

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Ed -

Tesla list wrote:

> Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<evp-at-pacbell-dot-net>
>
> Tesla list wrote:
> >
> > Original poster: "Barton B. Anderson by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> >
> > Bill -
> >
> > The variac is the ballast. Think of ballast as a current limiter. It can be
> > resistive,
> > inductive, or capacitive. Inductive current liminting is the norm where
> > typically variacs
> > or welders are used as they can be hooked up without modification.
> > Alternating current
> > flowing in an inductor causes inductive reactance which is an opposition to
> > current flow.
> > As the wiper is moved, the number of windings between legs changes,
> varying the
> > inductance, varying the inductive reactance, limiting available current the
> > load will
> > attempt to draw.
> >
> > Take care,
> > Bart
>
>         Not that simple by any means.  When the voltage across the portion of
> the winding being used exceeds the voltage in normal "variac" operation
> the core will saturate, and moving the brush only affects at what point
> in the cycle saturation will occur.  Of course, if the variac core is
> gapped appropriately, this won't happen but it's real tough to cut those
> wraps of thin core steel.
>
> Ed

Very true. I wasn't trying to go into what would happen when the core
saturates, but just give
a simple idea of the variacs use (consider the question that was asked).
Regarding the core
cutting. I've cut a couple variacs (laminated cores and solid composite -
they come in both
styles).

I used a gravity band saw. Lay it in position center of windings and let it
cut. Takes about 30
to 40 minutes on a 30A variac. I used ty-wraps to keep the laminations
together (there are
filings which find their way into the laminations, but not a big deal). On
both types, I
inserted a plastic shim expoxied into position. Rather easy IF you have the
right equipment.

Take care,
Bart