Re: Soft transformer turn on without a variac

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Dave, all,

On 5 Nov 2003, at 8:01, Tesla list wrote:

 > Original poster: davep <davep-at-quik-dot-com>
 >
 >
 > >Ok, in light of information from the above links, I retract my
 > >suggestion to use a zero crossing switch.
 >          For turn on, I'd try it.
 >
 >
 > >It seemed like a good idea at the time (reasoning was
 >
 > >0 volts = zero amps at start up).
 >
 >          Exactly true.
 >          Has to be.
 >          The load _at_ _start_ _up_ is mostly resistive:
 >          the resistance of the windings.  The inductance is
 >          minimal untill the field builds and penetrates the
 >          core.  That latter takes time.
 >
 >          best
 >          dwp

My reading of the graphs shown at the website pointed to by the url
Bob Jones posted suggests that the real problem with a zero voltage
crossing turnon is that the flux in the core is forced to climb in a
unipolar fashion for half a cycle instead of just a quarter. There is
no instantaneous turn-on transient when switching at this particular
time, nor should there be since the applied voltage at turn-on is
zero with zero resultant current flow at that instant. Does this
sound like the correct story Bob?

Malcolm

 >
 >
 >