Re: transformer theory q

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

> > > Also using less turns in the primary will increase the number of
> >>secondary turns necessary to get one volt from the secondary.


	No way!!!!!  The real limit on the minimum number of primary turns is
core saturation, which is a function of turns/volt, frequency, core
area, and saturation flux density of the magnetic material.  By the way,
core flux density depends only on the applied voltage, not the current
in the windings as some have suggested here.  There have been a number
of good references on transformer design  given  here, and I suggest
consulting them before attempting any sort of transformer (or ballast
inductor) design or construction .

 
> > You sure about that?
> 
>         Its a subtle point.
>         The straight turns ratio, as others have noted, is a good
>         teaching tool.  However it oversimplifies some aspects of
>         real world transformer design.  These aspects, as covered
>         by others, become significant when 'power' is involved.

	All "power transformers" are designed to minimize the leakage
reactance, so the turns ratio gives the right answer.  The results are
exact for an unloaded transformer, and very close for any one operated
within its ratings.  I happen to have been looking at one of my very old
college EE texts (ca 1941) yesterday and it seems to me to cover all
significant aspects of power transformer design.  For a typical
transformer of those days the "design regulation" was about 5%.  That
meant that, at rated load, the voltage drop in the internal impedance
(wire resistance plus leakage reactance) was 5% of the applied voltage. 
Most of the drop is in the leakage reactance, and doesn't represent a
power loss.  The winding resistance must be kept very low to keep the
transformer from having excessive heating. 

	In looking at a modern text on pulse transformer design (about the
ultimate in power transformers) the only thing I see different about the
equivalent circuits given is attention to the distributed winding
capacitance, which has no effect at all at typical power frequencies.

>         (Historical note:
>         One of the things Tesla provided/sold to Westinghouse was
>         the 'know how' to make transformer design more real world...)


	Citations?  Tesla was not "into" power transformer design per se, and
there was already a lot of practical and theoretical design background
when he came on the scene.  The real innovator in transformer design was
Charles Steinmetz of what later became GE.

> > Seems Bassackwards from "the way we was larned in school".
> 
>         Certain simplifications take place in some training.

	Examples?  (See above).

Ed