Re: Rebuilding an OBIT

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stefan Richter by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Stefan.Mac.Richter-at-t-online.de>

Question:
Is there an usable  formula for switch mode transformers????
Kind regards, Stefan Richter

Tesla list schrieb:

> Original poster: "Ed Phillips by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>
>
> Tesla list wrote:
> >
> > Original poster: "Jochen Kronjaeger by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <Kronjaeg-at-Stud-Mailer.Uni-Marburg.DE>
> >
> > >
> > > TRANSFORMERS DO NOT SATURATE DUE TO TOO MUCH CURRENT BEING DRAWN !
> > >
> >
> > Then how come that you need bigger cores for higer power? Why not just
> > thicker wire? Sorry - have to disagree.
>
>         You can disagree all you want, but you're wrong; look at any book or
> handbook on transformer design.  Higher power transformers use bigger
> cores because of the space taken by the windings, which need to use
> larger wire because of the increased current.
>
>         The number of turns required on any winding on an iron (steel)
core is
> determined by the core area, frequency, and maximum allowable flux
> density in the core.  In english units and for typical transformer steel
> operating at 60 Hz, the following approximation is useful:
>
> #TURNS = RMS VOLTAGE ACROSS WINDING x 6.5 / CORE AREA IN SQUARE INCHES
>
> Less turns or core area will result in higher flux density in the core,
> with increased "iron" losses and heating. This relation is true whether
> the transformer is shorted or not, or whether it is drawing load current
> or not.
>
>         I see lots of discussion here which indicates guys are violating this
> law by such practices as attempting to run a 120 V transformer on 240 V,
> using an un-gapped variac as a current limiting reactor, etc.  Some over
> voltage is possible, as transformer designers typically design the
> windings and core area such that the operating flux density is perhaps
> 70 to 80 percent of saturation, but as the voltage is increased above
> the design value the magnetizing current will increase and, as
> saturation is reached, go to unacceptable values.
>
> >
> >
> > Regards, Jochen
>
> Ed