RE: Current regulation for Variac

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

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.  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.

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.

Regards, Gary Lau
MA, USA





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,
except the variac itself.
Is there some way to correct the powerdrain with say a choke or
something to
make it "turn on" smooth? This problem is really annoying.
Thought i hear with you guys first.
Any help is appreciated.

Thanks!

//Matt 

Matthias "Tesla" Andersson 
http://www.tezla-dot-com
tesla-at-tezla-dot-com
-"Why Am I Glowing In The Dark?"