Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!samsung!think!mintaka!ogicse!blake!milton!whit
From: whit@milton.acs.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: X-formers saturation
Summary: Limited magnetizability of iron
Message-ID: <2439@milton.acs.washington.edu>
Date: 16 Mar 90 09:09:44 GMT
References: <2437@ultb.isc.rit.edu>
Reply-To: whit@milton.acs.washington.edu (John Whitmore)
Distribution: usa
Organization: University of Washington, Seattle
Lines: 34

In article <2437@ultb.isc.rit.edu> deb5584@ultb.isc.rit.edu (D.E. Baker ) writes:
>
>	Can someone tell me what happens when a transformer saturates ?
>Is it the primary,secondary,core...?  How do you tell when this happens?
>
	A transformer is a magnetic circuit.  The magnetic flux is
conducted by (usually) an iron transformer core, and the total flux
this core can conduct is its saturation limit.  When the limit
is reached, the iron has no further effect on the circuit, so your
transformer is now just two coils of wire.  Typically, the resistance
of the wire is low, and acts as a short-circuit; fuses pop.
	Saturation, then, occurs in the core.
	The primary is usually driving the current that causes
saturation (as in operating a transformer at excessively high input
voltage or excessively low input frequency), and the secondary
is carrying OPPOSING current.  Short-circuiting the secondary 
virtually guarantees the transformer will not saturate.  Opening
the secondary gets the transformer near to saturation.
	If you hear a transformer buzzing (generating acoustic
energy far from 60 Hz), saturation and the associated distortion
of the 60 Hz waveform should be suspected.  If the lights in
your vicinity dim, that is another clue.  When the core ceases
being a magnetic "conductor" at saturation, a large amount
of magnetic field leaks out of the iron; this can be detected
with a search coil or other magnetic field detector; it is not
unusual to see a transformer with a copper wrap around it so
that the leaking field encounters a short-circuited secondary
winding when saturation occurs.  
	Actually, some onset-of-saturation is normal for transformers;
there is no sharp occurrence that can be identified as saturation,
only a gradual lessening of permeability (think of this as
magnetic conductance and you won't be far wrong).

			John Whitmore