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Re: Inductance Issue HELP!!!!

Tesla List wrote:
> Original Poster: "BunnyKiller" <bigfoo39-at-telocity-dot-com>
> Tesla List wrote:
> > Original Poster: "Bert Hickman" <bert.hickman-at-aquila-dot-com>
> >
> > Tesla List wrote:
> > >
> > > Original Poster: "BunnyKiller" <bigfoo39-at-telocity-dot-com>
> > >
> > > Hey All .......
> > >
> Snipperzzz...
> Hi Bert ....
> the inductors are in series before the variacs and the amp measurements were
> taken between the variacs and the transformer ( pig). The measurement was
> as the coil was running.  Tank Cap is .109 uF -at-56KV rating, the pig is
> rated for
> 14400V / 240V  10KVA unit.
> The voltage output from the variacs to the pig ( while  coil is running )
> is 0 -
> 267V
> I do have about 500 uF's of motor run caps available to use if need be.
If i
> place these caps in what ever strategic position, would the phase shift
be enuf
> to solve the problem I am having ?
> I was thinking that if I ran the power to the variacs first than to the run
> caps
> and then to the inductors, the variacs would "see" less current, but ...
> wouldnt
> the inductors( if placed after the caps)  reshift the phase angle and the end
> results would be the same as ( or nearly the same as ) if i hadnt even
put the
> caps into the system ???
> one good thing about life is that learning never ends  :)
> Scot D

Hi Scot,

Since your ammeter is located between the variac output and the pig
while the ballast inductor is on the input-side of the variac, you will
indeed see many interesting effects as the variac is adjusted under
different load conditions. This is because the variac not only
transforms one voltage to another, it also transforms current since the
variac's output power is approximately equal to its input power. 

If the variac is operating mostly as a step-down autotransformer, and
the short-circuit input current to the variac is limited by the ballast
inductance, then the output current that is delivered from the variac
can be much higher than the mains input current. This effect becomes
very pronounced at low variac settings. A pig with the HV side
short-circuited will "look" like a virtual short-circuit on the LV side.
Now suppose your ballast limits incoming mains current to 30A, and your
variac is set to provide 10% of incoming voltage to the pig. In this
case, the output current from the variac (where you're measuring it)
will approach 300A! Fortunately, this is usually not a problem in normal
use since the pig's output voltage at this setting will not be
sufficient to fire the gaps. However, you'll still measure (and get)
about 2X the incoming mains current at a 50% variac setting. 

Compounding this are effects coming from the reflected reactances of the
tank cap (coming "through" the pig) and ballast inductor (variable,
coming "through" the variac). These reactances combine is a fashion
which tends to reduce the current limiting action of your ballast
inductor. The degree to which this occurs is also a function of ballast
inductance, tank cap size, pig transformation ratio, and your variac's
setting. You may encounter a variac setting that causes mains resonance
with the tank cap, excessive mains current draw, and potentially
destructive tank cap voltage levels. It will also cause loud buzzing,
growling, or thumping noises from the variac or pig. Based upon your
system's parameters and using a 10A ballast limit, you may encounter 60
Hz mains resonance at around a 40-45% variac setting. Once your gap
begins firing consistently, the "Q" of this resonant circuit is spoiled,
and paradoxically, you may actually see a decrease in current draw as
you increase the variac setting and also come off the resonance peak.

For now, rewire the ammeter so that it's on the input side of the
variac, or add another ammeter to monitor incoming mains current. Also,
set your ballast inductance for either the 33A or 43A settings to
prevent resonance effects around your variac's midpoint range. You
should now get more "rational" ammeter readings.

Safe coiling to you!

-- Bert -- 
Bert Hickman
Stoneridge Engineering