Re: Soft transformer turn on without a variac

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

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>
 >
 > Hi Ed,
 >
 > Would this be true even if the variac output was set to zero when first
 > turned on?
 >
 > Gerry R
 >
 >  >
 >  > A large current surge when the variac is connected to the line is quite
 >  > normal, depending on the phase at which the switch or relay closes.  The
 >  > bigger the variac the lower its resistance and the bigger the surge.
 >  >
 >  > Ed

	Yes.  Only depends on applying voltage to the "primary" winding, in
this case the entire winding.  My AC Machinery textbook is at work, but
this was a standard transient problem we had to work in school.  As I
remember it (it's almost 60 years since I've thought about it) the
applied voltage can be considered as the sum of a DC component equal to
the instantaneous voltage at turnon and the AC voltage.*

   The DC causes a transient current to flow which decays at the L/R time
constant of the winding and can be quite large.  Normally the line
impedance is so low that all that happens is that "the lights blink".
This isn't quite the same effect as happens when you turn on a large
motor and large currents are drawn to provide the torque necessary to
bring the rotor up to speed.  That current surge is independent of
starting voltage.

*Antonio" Any comment?

Ed