[Prev][Next][Index][Thread]

Re: Reto Dale: Primary Coil Cross Section


-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Tuesday, February 01, 2000 6:50 AM
Subject: Re: Reto Dale: Primary Coil Cross Section


>Original Poster: "Megavolt Nick" <tesla-at-fieldfamily.prontoserve.co.uk>
>
>
>Hi Dale,
>

>Returning to your original question:
>> At what physical separation (air or some dielectric [insulator])
>> below a Pri does a ground plane
>> not act as a shorted turn ?
>It is not a binary thing, there is no distance at which it ceases entirely
>to have an effect.  It is an inverse square thing - it will absorb a
quarter
>as much energy at 2N distance from the primary than it did at N distance
>from the primary.
>
>In what Jim lux writes I think he is refering to the ground plane as the
>ground literally, which is not terrribly conductive, therefore the current
>flow within it will not be too great.

Nope.. what I was talking about is the shorted turn effect.  The flat copper
ground plane acts like a whole bunch of shorted turns in parallel...   And,
as well to the actual ground, which fortunately, isn't a very good conductor
(or is far enough away from the primary) so there isn't appreciable loss.

A shorted turn at infinity won't cause any loss, because no current is
induced (i.e. M =0).  As it gets closer, more current is induced (M>0), and
hence you'll get some loss from IR (unless the shorted turn is a
superconductor...)  Therefore, you can use the coupled coil formulas.  And,
for any given section on the coil coupled to any other section, the coupling
goes as 1/r.  This is because it is a "line source" not a "point source".
Charge is 1/r^2 because it is a point source.

Inverse square only really applies in the "far field" where the field is
primarily propagating electromagnetic radiation.  Closer in, where the field
is "storing" energy, aka the "near field" the inverse square law doesn't
hold. Ball park figures for where the change from near to far is
2*pi*wavelength, which, at 100 kHz, is about 19 km away.  (I acknowledge
that these are gross oversimplifications, BTW, before I get flamed...)



 As anyone who has made the mistake
>of using a strike rail which was continuous will testify as shorted turn a
>few inches above the primary of a medium power tc will have a very large
>current flowing within it.

Exactly, it is close, it has high mutual coupling, large currents are
induced, resulting in large IR losses.

>