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From: pierson@cimnet.enet.dec.com (dave pierson)
Newsgroups: sci.electronics
Subject: Re: Big transformers and circuit breakers
Summary: not saturation, as i understand it...
Keywords: inrush, transformer
Message-ID: <4224@ryn.mro4.dec.com>
Date: 28 Mar 91 00:59:45 GMT
Sender: guest@ryn.mro4.dec.com
Distribution: na
Organization: Digital Equipment Corporation
Lines: 21

In article <1991Mar23.024309.12334@spool.cs.wisc.edu>
kolstad@jomby.cs.wisc.edu (Joel Kolstad) concerning a transformer tripping
a breaker:
>>immediately trips (even with no load on the secondary!) since the lack of
>>a magenetic field in the toroid causes the primary windings to look like
>>a short.
	I think he's right.  the usual explantion is that it takes a finite
	amount of time for the field to build up (penetrate) the core.  During
	this time, the apparent inductance (reactance) setting the surge
	changes in value from that of the windings alone (essentially air core,
	very small) to the steady state value.  Its not uncommon to see a 10X
	peak on the first half cycle (though it may take a few tries to capture
	it.)  The "best" fix is a breaker with a different delay curve, though
	i gather that's not practical, here.
thanks
dave pierson			|the facts, as accurately as i can manage,
Digital Equipment Corporation	|the opinions, my own.
600 Nickerson Rd
Marlboro, Mass
01752				pierson@cimnet.enet.dec.com
"He has read everything, and, to his credit, written nothing."  A J Raffles