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Re: Primary-Secondary spacing

> Original Poster: "Christopher Boden"
<chrisboden-at-hotmail-dot-com>
>
> We are building the primary mounts this week and
I am wondering the best
> spaceing for the Primary in relation to the
secondary.
>
> The secondary is wound on a 6" form and is
moveable in the Z axis so that we
> can adjust coupling.
>
> The primary forms create a .75" gap to the first
turn (each side) if the
> primary form is almost touching the sec, is this
enough?
>
> The primary supports are made from 1/2? UHMW and
will be bolted to a 2'dia X
> 1/2" thick UHMW plate with nylon bolts.
>
> Also:
>
> I have procured some large diameter nylon
threaded rod and was wondering if
> this would be a good idea.
>
> Drill a hole through the base of the secondary
and glue a nylon nut inside.
> Thread the nylon rod through the hole and have a
small stepper motor on the
> bottom of the shaft under the table. Coupleing
could be adjusted in realtime
> while the system is running and with the
operator across the room.
>
> Is this a dumb idea?
> Why?
>
> The only problem I can see is the whole phrase
"Drill a hole in the bottom
> of the secondary" at which point I can hear 200
quiet moans from across the
> modem connection :)
> It's that one guy out there that is
saying...hmmmmm that has me wondering.
>
> Have fun guys!
>
> Christopher A. Boden Esq.

Chris,

Actually there's probably many people on the list
thinking that's a really
good idea.  I seem to remember it being done by
one or two people on
the list.  Unfortunately, I can't remember who
they were.

I would recommend using a maximum of 10 kV per
inch between the primary
winding and the secondary winding.  These values
will vary somewhat based
on temperature and especially humidity - but they
should give a fairly
reasonable spacing for most situations.  Also,
make sure that there aren't
any sharp points on either the secondary (at the
bottom of the winding) or
the primary (where the primary goes down into the
coil base) - they will
lead to trouble!

Here's a little table of  the minimum spacing
requirements:

                     Minimum
Xfmr kV        Spacing
7.5 kV              3/4"
9.0 kV               1"
12.0 kV           1-1/4"
15.0 kV           1-1/2"

Your idea of the nylon rod is a really good one.
You shouldn't
have any problems with flash over down through
secondary
using the nylon rod.  If you have any concerns
about that
(such as if you're running 3x + secondary length
sparks),
then put some plexiglass disks slightly above the
lowest turn
and slightly below the top turn.  Silicon them in
place, and
you shouldn't have any problems.  How are you
going to
ground the secondary winding?

David L. McKinnon
D&M's High Voltage