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Re: Question about ballasting and VA

At 03:42 PM 03/03/2000 -0800, you wrote:
>Ross,
>
>> The current limiting variac certainly doesn't behave as a big Tesla Coil
>> "dimmer".  There is a definite "sweet spot" on the variac that seems
>almost
>> magical
>
>This is very true. I'm not an engineer, but I play one on TV. I've run coils
>in
>the 10 - 20kVA range all over the US and in every case, the sweet spot
>you describe falls in a different place. And, as you describe, a little turn
>of
>the variable inductor EITHER DIRECTION will send the ammeter skyward.
>
>I believe the variable inductor is canceling or balancing other reactance
>in the primary circuit (capacitative, inductive & resistive) and not
>limiting
>current at all. A pole transformer doesn't spit out massive amounts of
>current at will, it has to be "asked". It seems to me that the pig just
>feeds
>the current demands of the load (bigger tank cap, bigger load - I use .1uf
>and .2uf and I notice the difference). No need to "limit" current at all...
>
>HOWEVER, this may only apply to salient pole synchronous spark gap
>coils... I don't know because that's the only type I use. The double "E"
>bunch can jump in here at any time ...
>
>Jeff Parisse
>www.teslacoil-dot-com
>

Hi Jeff and All,

In a reasonably designed (not all screwed up ;-)) coil, the various parts
have been more or less impedance matched to each other (whether you know
you did it or not ;-)).  The output arc impedance is matched to the
secondary system impedance.  The secondary is matched to the primary, the
primary is matched to the charging system....

When you vary the current limiting, you are either raising or lowering the
charging system's effective source impedance.  Since the secondary is
basically a fixed effective impedance at a given voltage and output arc
level, there is indeed a "best" current to supply.  Too little, and the
system starves for power and tries to suck more current.  Too much and the
gap just starts to cook off heat.  The impedance matching (almost SWR
matching) and such really does apply here.  Problem is, the system is
nonlinear and very complex so massive computer modeling is needed to figure
out exactly where to set the dial.  That's why all of us in the "double E
bunch" use MicroSim hour after hour and get big multiprocessor computers
with so much RAM an eight digit calculator can't add it all up...  

However, you "other guys" just turn the dial to the "sweet spot" to do the
some thing.  Hmmmmmm ;-)))

Cheers,

	Terry
	Genuine double E engineer
References: