Re: Questions about Variacs.

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

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

> I have searched all over the web and exhausted every scrap of info I
> have found and I still can't seem to figure it out, so here goes!!

> The only thing I can do with it is run a leg/center tap

> configuration: 1 & 3
 	Depends on what it is to be used for.  If variable

	inductor, for Ballast, yep.

	If variable voltage supply, cf at bottom.


>>From what I found the popular consensus seems to be is Input on 1 & 2
> and Output on 1 & 3. All I do in this configuration is pop breakers. 
 	cf below


>        ____ 1
>       (
>  5 ___(
>       (
>       (
>       (<--- 3
>       (
>       (
>       (
>  2 ___(
>       (
>       (____ 4
> 
> The total 1-4 resistance is .6 ohms.
 
> Can someone clue me in as to what's going on here??
	Sure.

	How BIG is this variac?
	How BIG is the breaker?
	What load is connected?

	That said:
	It is routine for 'large' (10A nominal, full load)
	Variacs to pop 15A breakers, with no
	load (or with any load).  Need a slower breaker.
	(may have to specially acquired....)
	(MIGHT use a larger breaker: this presents a
	safety risk as the breaker may not trip when/if
	needed.)  Or a variety of other current limiting
	schemes.

	When the current first hits ANY large transformer
	(or: any transformer) the first half cycle can be
	up to 10x the normal.  For small transformers
	this surge is small enough to not affect anything.
	For this size (and up) the effect is noticable.

	Why (non rigorously):
	The normal current is set by LOAD and IMPEDANCE
	(reactance plus resistance) of winding.
	Reactance is set by presence of core.  But. When first
	energized it takes a measurable amount of time
	for the mag field to 'penetrate' the core, so the
	reactance looks LOW, thus the current is HIGH.

BTW:
	One could also put line to 1 and 4, take output
	from 3 and 4, and a range of other possibilities,
	if using as a variable voltage source.

	best
	dwp

...the net of a million lies...
	Vernor Vinge
There are Many Web Sites which Say Many Things.
	-me