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Re: [Amps] Re: Step-start calculation

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Subject: Re: [Amps] Re: Step-start calculation
From: Will Matney <>
Date: Sun, 21 Nov 2004 23:17:00 -0500
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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.


Will Matney

Tomm Aldridge wrote:


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:

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.


Tomm KD7QAE (new ham (2002), old PS engineer) wrote:

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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