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Re: [TCML] Variac

At 02:41 2008-10-29, you wrote:

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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.


Although this is a nice description it would be nice with a schematic as I am a little curious of how to wire the A1 and A2 connections(the relay winding) of the contactor
in this setup.

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