RE: Does a variac with no load dissipate power?

["Lau, Gary" <Gary.Lau-at-compaq-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

It is common to have problems with a variac tripping one's circuit breaker
when it is initially turned on.  This will happen regardless of whether of
not a load is connected, what the variac dial is set to, and whether the
load is power-factor corrected.  The reason for this initial current surge
is that the variac's core will retain a magnetic bias depending on what
phase the power was when it was last turned off.  And depending upon the
phase of the power when it is turned on the next time, the core may
saturate, generating a huge breaker-tripping current spike.

The solution is to either switch to a slow-trip breaker (I myself was not
able to find such a thing at Home Depot), or limit the startup surge
current.  I corrected my problem with a PTC thermistor surge limiter.
Alternately you can just insert a very small value resistor in series with
the input line, then short it out with a relay or switch shortly after
turning on the power.

Gary Lau
Waltham, MA USA

>but on the other hand my smaller coil is equiped with a 25 A 120 V variac
>with about 125mF of PFC ...
>when I turn on the switch to this system  when the variac is above 75% it
>will blow the 20A breaker...
>obviously there is a decient current draw to the variac setup...
>
>I havent checked but I wonder if the PFC on the smaller coil is responcible
>for the large initial current draw...
>
>Scot D