[Amps] Re: Step-start calculation

[Tomm Aldridge]

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 at 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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