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From: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
Newsgroups: sci.electronics
Subject: Re: Big transformers and circuit breakers
Message-ID: <1650@tekig7.MAP.TEK.COM>
Date: 26 Mar 91 00:57:56 GMT
References: <1991Mar23.024309.12334@spool.cs.wisc.edu> <1991Mar23.162927.21031@polaris.utu.fi>
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Reply-To: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
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In article <1991Mar23.024309.12334@spool.cs.wisc.edu> kolstad@jomby.cs.wisc.edu (Joel Kolstad) writes:

>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.

Not immediately.  It doesn't look like a short until the core saturates.
The only explaination I can think of for a core saturating *only* at
startup is that it comes so freaking close to saturation under normal
operating conditions that on the first halfcycle, the flux change that
usually starts out at -Phi/2 begins at zero instead, and heads for +Phi
instead of just +Phi/2.  Even this is very unlikely to cause problems,
since most 60Hz power transformers use metallic cores that saturate
gracefully (as opposed to ferrites, anyway). 

Other posters have suggested some nonsense about using thermistor inrush
limiting.  This might work, but it is a far, FAR less desirable solution
than having a properly designed transformer that won't saturate on startup
in the first place.  

Another bandaid fix would be to employ electronic switching to ensure that
the primary was only switched on at a supply voltage peak voltage (oddly,
the least stressful turn-on condition).

Brian Rhodefer