Re: Soft transformer turn on without a variac

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

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

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