Re: Soft transformer turn on without a variac

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

Original poster: "Crow Leader" <tesla-at-lists.symmetric-dot-net> 

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, October 29, 2003 7:45 PM
Subject: RE: Soft transformer turn on without a variac


 > Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com>
 >
 > I don't believe that zero crossing switching will solve the problem.  I
 > think the problem is that the variac core retains a magnetic bias, so if
 > the initial turn-on phase is going the "wrong" way, the current will be
 > high regardless of turn on timing and variac loading.

I don't agree with this. The magnetic field in a transformer core is always
flipping directions, the constantly changing AC input forces it to, and we
don't get huge inrush currents when the AC wave crosses zero, or the voltage
starts to drop on its way back to zero in an already running transformer.

Another way to look at it, I don't see how a biased core can store or absorb
more energy than it takes to run normally. A transformer is "off" 120 times
a second at 60Hz line frequency anyways.

Still, say a core was magnetized the "wrong way" and needed twice the amp
seconds to be reset to zero and then charge the "correct" polarity (and
magnetize the other direction). You've doubled the current you drew for
1/240th of a second from what it normally uses. That's not that much, and a
fairly short period of time. A zero crossing power switch sounds like it
would work perfectly. Twice the rated current for 1/4 cycle is still less
than you can get plugging in a transformer in an outlet at random parts of
the AC cycle.

KEN