Re: variac amperage control??

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Oxandale, Terry by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <Toxandale-at-SPP-dot-org>
> 
> Bill
> 
> Rather that using the variac "across" the source (both legs) to control
> voltage, you instead use the variac in "series" with only one leg. To
> connect, you only use two wires (vs 4 wires for voltage control). Your
> source (wall) wire will go to one side of the winding (1 or 5, I
> believe) for current "in", and the other wire to your load goes to the
> brush (3, I believe) for current "out" of your variac (thus only two
> wires are connected to the variac) and into the xfmr. Then the current
> goes through your transformer and returns back to the source
> (wall)directly. Depending on which side of the winding you connect the
> souce to, will determine whether the current increases from nothing to
> fry, or decreases from fry to nothing with the same movement of the
> control knob. This should be verified and tested with some resistance
> and/or fuse (or even a variac across the circuit to keep voltage to a
> minimum) in the circuit to make sure your working the knob the correct
> way. Otherwise you could smoke something real quick when you flip the
> switch to energize the circuit. Also, the reactance in not linear, so
> the last 10%-20% of current rise will ramp up very very quickly, so be
> deliberate with the adjustments.
> 
> (un)Terry


	The last sentence is critical.  What is really happening is that the
core of the variac is saturating and it is acting as a saturable reactor
with varying number of turns, NOT a linear reactor at all.  Cutting a
thin slot through the core would probably cure the saturation and turn
it into a linear reactor, but have never sacrificed a variac to find
out.  Cutting through that silicon steel ribbon would be a pretty tough
proposition, but think some have reported it.

	As for the variac saturation, think that is the explanation for some of
the "bumping" and other strange phenomena reported here.  Suppose, for
example, that the whole circuit consisting of the power transformer and
its secondary loading presents a capacitive reactance to the line and
variac.  As the inductance is reduced resonance is possible.  If
reasonance is approached the current will begin to increase, the voltage
across the variac will increase forcing it into saturation, and the
current will abruptly jump up.  Only speculating, since I haven't done
it.

	What I have observed is the following:

	Playing with a 15 kV, 60 ma transformer with 0.006 ufd capacitor across
it I increased the line voltage from zero with a variac and measured the
secondary voltage.  At an input voltage around 20 the output suddenly
jumped up to almost 15 kV and the line current went way up.  This was a
repeatable phenomenon and must have been due to the fact that at that
particular voltage the inductance of the transformer had changed to
produce series resonance.  Luckily I had started out with the VM set to
30 kV scale, so didn't fry it. Non-linear magnetic circuits are strange
to the uninformed such as I!

Ed