km1h@juno.com wrote:
>
> Would a few experts care to help this chap out....and me too? I could
> never find 2 people in a row to agree.
Hi Carl,
The turns are shorted for two engineering reasons.
1.) To prevent unwanted stray resonances
2.) To prevent auto-transformer effects from creating excessive
voltages.
Shorted turns generally do not cause a circulating current problem in
air wound inductors because flux leakage is so high. The shorted turns
generate opposing flux, and it forces the flux from the active coil
section to harmlessly leave the coil center.
A problem usually arises only if the shorted turns are very few in
number, the current path is very short (a low impedance), and the active
coil section is long and densely wound.
If the inductor is wound on a toroid of high magnetic conductivity
(little flux leakage) shorting turns is often a big mistake. The flux
can not escape the area of the short, and circulating currents and core
heating can become severe.
I generally sweeep the network and look for stray resonances, and
measure voltages along the inductor. I prefer to short turns only when
and where necessary.
Recently I measured the change in a #14 inductor with a shorted area,
and without. The Q changed from 142 to 138 when the turns were
shorted....hardly a big deal.
73 Tom
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