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Re: Primary Heating



Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Tesla list wrote:
> 
> Original poster: "Peter Lawrence by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> 
> Are you suggesting that the current flow in a thick primary conductor is not
> uniform across the cross section of the conductor, and that this
non-uniformity
> would be different for inner turns verses outer turns? Wouldn't this have
to be
> something other than skin effect since inner and outer are both operating at
> the same frequency? Are you suggesting that the inner facing surface of a
> spiral has different magnetic coupling hence voltage and current than the
> outer facing surface of the same portion of that spiral conductor, since
it is
> geometrically closer to the center.
> 
> -Peter Lawrence.
> 
> >Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <paul-at-abelian.demon.co.uk>
> >
> >Hi All,
> >
> >David Rieben wrote:
> >> I definitely have a good, solid low resistance ground connection
> >> fron the inner primary turn
> >
> >Fair enough - had to be eliminated.
> >
> >Bert wrote:
> >> Perhaps "current bunching" on the innermost turns (due to proximity
> >> effect?) so that these turns develop higher Joule heating?
> >
> >Well that sounds like the next best explanation.  I suppose I'm going
> >to have to write that program to plot the magnetic field. As a quick
> >look we can use acmi to show the distribution of self-induction
> >across the primary.  If this is divided by the area of each turn,
> >we end up with a set of numbers proportional to the B field strength
> >in the region of each turn [*]. Eg for a 12 turn primary, 5" inner,
> >12" outer, see
> > http://www.abelian.demon.co.uk/tmp/flat-pri-B.gif
> >
> >Is that enough of a concentration in the center?  If the inner field
> >were say 5 times the outer field, does that mean the proximity loss
> >is 5 times higher in the inner turns?  It would only make a difference
> >to the heating of the inner turns if the proximity loss was
> >significant (equal or greater than) compared with the straight line
> >AC resistance (ordinary skin-effect) loss.  I've no idea how to work
> >that out.
> >
><SNIP>

Peter,

Proximity effect does result in non-uniform "current bunching" across a
conductor. Proximity effect and skin effect are similar in the sense
that both phenomena reflect changes in current distribution within a
conductor due to time varying magnetic fields. Skin effect shows up in
an isolated conductor through the influence of the conductor's own
changing magnetic field. Proximity effect occurs when multiple current
carrying conductors are in close proximity to one another such that the
current flow in (and magnetic flux created by) nearby conductors further
influence the distribution of current flow in a conductor. This is the
case for windings in an inductor, resonator or transformer. 

While skin effect tends to force current flow (symmetrically) towards
the outermost portion of a conductor, proximity effect causes additional
(asymmetrical) current bunching to occur. When large numbers of
conductors are in close proximity with current flow in the same
direction, proximity effects can have a significantly larger impact on
the AC resistance of a conductor (Rac) than skin effect alone. 
Some fairly readable descriptions, with graphical representations
showing how current distribution is altered through proximity effects
can be seen at: 
http://www.worldenergysavings-dot-com/facilitylosses.asp

Here's where it gets interesting - it can be shown that the power losses
from proximity effects in a non-magnetic conductor immersed in a
transverse AC magnetic field scale as the square of the local magnetic
field. Now look at Paul's B field plots from ACMI from an earlier
posting:
http://www.abelian.demon.co.uk/tmp/flat-pri-B.gif

If power losses scale as the square of the B field, then a B field
that's 5X as great implies losses that are 25X as great... and this may
indeed be the root cause of preferential inner turn heating. 
BTW, the shape of the resulting current distribution in a spirally wound
coil also suggests that a primary wound with a flat conductor may in
fact have a significantly lower Rac than a similar coil wound with
round/tubular conductor.

Other References: 
Carter, G. W., "The Electromagnetic Field in its Engineering Aspects",
2nd ed., Longmans, Green, 1967, pages 248 - 260pp for skin and proximity
effects.
http://perc.nuigalway.ie/publications/pesc1997.pdf

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Coins Shrunk Electromagnetically!
http://www.teslamania-dot-com