Re: Rebuilding an OBIT

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

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