Re: Current regulation for Variac

["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>

Tesla list wrote:

> Original poster: "Lau, Gary by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

> What you describe is a common problem with larger variacs.

	And any other largish iron core transformer.

> One is supposed to be using slow blow fuses, but I was tripping my
> circuit breaker in the garage and I wasn't aware of any slow-trip
> options there.

	They exist.  Whether they can be had to fit 'usual commercial
	breaker boxes' is another matter. 

> I ended up adding a PTC (Positive Temperature Coefficient) thermistor in
> series with the variac's input.  At room temperature, mine has a
> resistance of 1.0 Ohm to limit the starting current.  The unit rapidly
> heats up and achieves a .02 Ohm hot resistance under a full 20 Amp load.
> My unit is a Fenwal #ICL221R020, see
> http://www.fenwal-dot-com/catalog/prod.htp?GOTOPAGE=icl for specs.  I've had
> no tripping problems since using this, although I do often push the 20A
> max rating and have smoked one already.
 
> If you can't find the thermistor, you can add a 1 Ohm resistor in series
> with the variac input, and short out the resistor with a brief (<1 sec)
> time delay relay.

	agreed.	

> Original poster: "Tesla by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <tesla-at-tezla-dot-com>
 
> Hi all.
 
> I have a small question about variacs. I have a 3,5kVA variac
> (Uin=230V).  When i bring it online it sometimes (about 1/3 of
> the times) it draws so much current from the mains that my 10A
> fuse blows although i don't have any load,

	Ahhh, there IS a load... The variac itself.  see below...

> except the variac itself.  Is there some way to correct the power
> drain with say a choke

	Might do.  Might have same problem.
	Gary's suggestions, or a different fuse or beaker would do.

> or something to make it "turn on" smooth? This problem is really
>annoying.

	I learned the same lesson, years back.  Rewired the variac
	twice before deciding it was normal.

	What's happening (roughly) is:
		The inductance/impedance of the variac depends
		on the core.
		BUT, when first energized, it takes a finite amount of
		time to stabilize the mag field IN the core.  For that
		(brief) time, the variac (or any other transformer)
		looks like an air core coil.  MUCH lower inductance/
		impedance, so the momentary current is large.

	The magnitude of 'large' varies, depending on where in
	the AC sine the switch is closed.  If closed 'near zero'
	little current flows.  If closed near max, the breaker/fuse
	can trip.  A useful approximation is that the first surge can
	be up to 10X the rated full load current.  If a storage scope
	and current probe are handy, this can be readily checked.
	(It's not necessary to check it, just design around it as Gary
	advises.)

	best
	dwp