Re: Soft transformer turn on without a variac

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

Original poster: robert & june heidlebaugh <rheidlebaugh-at-desertgate-dot-com> 

Try using a motor start relay. These have a low resistance in series to
reduce surge current.
     Robert   H
-- 


 > From: "Tesla list" <tesla-at-pupman-dot-com>
 > Date: Thu, 30 Oct 2003 06:26:07 -0700
 > To: tesla-at-pupman-dot-com
 > Subject: Re: Soft transformer turn on without a variac
 > Resent-From: tesla-at-pupman-dot-com
 > Resent-Date: Thu, 30 Oct 2003 06:33:27 -0700
 >
 > Original poster: davep <davep-at-quik-dot-com>
 >
 > Speaking as an EE, with some transformer test experience:
 > My understanding/observation:
 > Basic Physics:
 > It takes a (brief: milliseconds) period of time for
 > the mag field to 'penetrate' the core material.
 > Thus when initially powered up any transformer LOOKS
 > LIKE an air core coil: Very Low Impedance.  The effect
 > is not _noticed_ on small transformers as the
 > surge is small.  Rule of Thumb for large transformers
 > is: 'worst case' about 10x normal full load current.
 > The surge is independent of the _actual_ load on the
 > transformer.  (Until mag field can build up in the
 > core the load is 'invisible' to/from the primary.)
 >
 > The magnitude of the surge also varies with the (accidental)
 > phase of the closing of the main switch.  If closed at or near
 > a voltage peak, the surge current is maximum: 'full' voltage
 > across a (temporarily) 'air core' coil.
 >
 > I've never tried a zero crossing switch: solid state devices
 > around Tesla/10s of amps environments need to be well done
 > to work.  They should help.  I tend to think in terms of
 > resistors in series, with bypass relays, or whatever.
 >
 > In general transformer coil materials tend to be chosen to
 > have low magnetic remnance (tendency to store a field):
 > cuts hysteresis losses...
 >
 > (Curious Coincidence:
 > I first got a 'sharp education' in this effect powering
 > up unloaded, mid sized, variacs...  8)>>  Later
 > observed and measured with mid size power supplies.)
 > --
 > best
 > dwp
 >
 > ...the net of a million lies...
 > Vernor Vinge
 > There are Many Web Sites which Say Many Things.
 > -me
 > ...for he has read everything, and written nothing...
 > A J Raffles
 >
 >