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Re: [TCML] Variac


It is the inrush on the Variac when turned on.
Variacs, depending on many factors, may turn
on without a whimper, or may have a severe
turn on transient when energized
(as much as 10X FLA).  The start up
transient includes magnetic saturation, coil 
resonance (or ferroresonance), and higher
order harmonics which in general can wreck
havoc on the circuit feeding the variac.

The solution is what is called in trade a "step start"
relay.  Install a resistor on order of 10 ohms, 25W
in series with variac.  Install a motor contactor or
BEEFY 120VAC power relay in circuit;  wire the coil 
in parallel with 120V winding of variac
and wire a NO (normally open) contact of this relay
across (in parallel with) the surge limiting resistor.

When you turn on now, inrush is limited to ~12A, voltage
builds based on L/R decrement across variac until
sufficient voltage is across variac to allow relay to
pull in, bridging the surge limiting resistor and
providing full power to variac.  I would
also install a 20A 250V ABC ceramic cartridge fuse
in series with variac; available at Radio Shack.
The fuse will limit the heartburn, hate, and nashing of
teeth and maintenance being PO'd about
resetting the breaker...AGAIN...  ;^D.

Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA

 -------------- Original message ----------------------
From: jhowson4@xxxxxxxxxxx
> Ok this is attempt 3 my e-mail keeps saying that there was a sending error. so 
> if 3 of the same message were in fact sent i am sorry.
> Ok so i think that this so called surge is the culprit since at the time of the 
> breaker being blown I was trying to figure out where a weird sound was coming 
> from and this entailed turing it on and off.  learing from my mistakes i won't 
> be doing that again... ever...
> How much current is the norm if nothing is plugged into it? If the unit is rated 
> for 22.5 amps why is it not pulling that much power all the time?
> How does starting the variac with it turned up help.  I understand what you want 
> me to do but I am curious as to why it works.
> Even if the surge current was the culprit i want to be sure before I plug it 
> back in and the people start asking why stuff blew.
> Thanks, 
> Jay. H
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