Re: Using a variac for current control

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 9/9/02 11:15:50 PM Eastern Daylight Time, tesla-at-pupman-dot-com
writes:


>
> My question is; do I have to make a cut in the toroid of the current variac?
> Mr Couture makes no mention of this operation.  Is it necessary?


Terry,

Whether you can get away with not cutting the variac depends on
a number of factors.  It would seem that two important factors
are, (1) the rating of the variac, and maybe even more importantly, (2)
how many turns of the variac will be in use.  If not enough turns
are used, the variac core will saturate, and the system will be
inefficient.  I've heard of folks getting good results if they are
able to use 1/2 or so of the variac turns.  In the one time when
I tried using a variac as a current limiter, I used a 20 amp
variac for a coil that drew only 5 amps.  Still, the system became
very inefficient, drawing 1200 watts from the wall compared to
600 watts using my homemade ballast, for the same spark
length.  I figure this inefficiency occured because I was using
only about 10 turns of the variac to get the inductance I needed,
and the variac core was probably saturating.  In any event,
I simply went back to using the homemade variac.  In some
cases where folks claim a good result with an uncut variac,
it's interesting to wonder if they did a comparison with some
other type of ballast.  It could be they are losing some efficiency.

Some variac cores if cut, will tend to spring apart.  This may
be more likely with newer variacs which tend to have a strip-
wound core.  Some old variacs use flat disc lamination cores.
These will not spring apart.  

John