Tomm,
That's a good example of it and shows how much higher the flux density
is at startup where the density is past the knee into saturation. I'm
not sure how many cycles this happens in, most likely by the design of
the transformer itself. I've never really set down and run the numbers
on any, just knew it was there. A lot of the relay manufacturers will
have ratings for switching transformers, how to size them for surge
current, and by the KVA of the transformer.
Best,
Will Matney
Tomm Aldridge wrote:
All,
Transformers frequently have a gap in the core just to prevent the
flux walking behavior described in the article. The reason tape wound
toroid cored transformers behave so badly in this regard is the
absense of the gap or cut. A very small gap (.005") will shear the
BH loop over enough that the "double flux" condition will not be added
to by any appreciable remnant magnetization. EI lamination designs
and cut core designs do not suffer from this effect. They do,
however, suffer from the inrush phenomena described in the mentioned
article and more clearly in this reference:
http://www.qte.com/Main_Pages/Technical_Papers_Page/Inrush_Page/qte_tech_papers_inrush.htm
The only cures are to control turn on at 90 degrees (impress Vo COS wt
instead of Vo SIN wt), add enough line reactance (R or L or some
combo) to control the inrush due to this effect during magnetization
and then switch it out, or build the system robust enough to handle
the transformer saturation current for a few dozen cycles. For big
power supplies I would advise using the second method as it will also
perform the "step start" of the capacitive input filter on teh
secondary side.
Regards
Tomm KD7QAE (new ham (2002), old PS engineer)
exray@bellsouth.net wrote:
Jeff'
The magnitude of the current depends a great deal on the transformer
type....for instance a fair size torroid, say 1 or 2 KVA will
absolutely require some form of current limiting or you WILL trip the
breaker at turn-on!!!
As a matter of fact most x-ray machines of any size 'remember' the
polarity of the applied voltage so they always start with the
opposite just to prevent this kind of occurrence.
George W4IW
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