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Re: Oil-immersed RFCs
From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Friday, September 05, 1997 4:01 PM
To: Tesla List
Subject: Re: Oil-immersed RFCs
>
> From: Owen Lawrence[SMTP:owen-at-iosphere-dot-net]
> Sent: Tuesday, September 02, 1997 9:21 PM
> To: Tesla List
> Subject: Re: Oil-immersed RFCs
>
> > From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
> > Sent: Monday, September 01, 1997 5:02 PM
> > To: Tesla List
> > Subject: Re: Oil-immersed RFCs
> (snip)
> > You are correct. The reason for many sets of windings is MORE to
> > insure adequate breakdown voltage protection than to gain
> > inductance... although you obviously get BOTH. If you increase
the
> > width of each section you increase the number of turns and thus
the
> > inductance of each section, but the voltage rating of that
section
> > will then DECREASE.
>
> Here's a quote from the Radio Amateur's Handbook:
>
> "Many of the popular sizes of RF chokes consist of a number of
small,
> flat sections of wire, called pies, separated about an eighth of an
inch on
> a ceramic form. The wire itself is wound in a honey-comb pattern,
so that
> the turns are slightly separated. The purpose of both spacings is
to
> minimize the capacitance action between adjacent wires and groups
of wires.
> This must be taken into consideration because it has the unwanted
effect
> of turning the choke into a resonant circuit all by itself or of
nullifying
> its choking action by providing a low-impedance path of its own to
the very
> RF currents the choke is supposed to block off."
>
> No doubt this was meant to apply to lower voltages, but it still
suggests
> that there are other reasons chokes have the geometry that they do.
I'm
> just mentioning this (out of my own ignorance) because this is the
first
> time that I ever heard that having many sets of windings is for
increasing
> breakdown voltage protection. Comments?
>
> - Owen -
>
>
Owen,
The primary purpose of honey-combing the wire layers in an RF choke
is indeed to reduce the self-capacitance. It does this at the expense
of size, as there is more air between wires when it is honey-combed.
Honey-combing is NOT so good for high voltage work, however, as it
has the wire crossing over a great many other wires.
As to the use of pies, well, this has been a trick used in the making
of high voltage windings since the turn of the century. When used for
extreme high voltage production, the goal is to keep the WIDTH of
each layer small, so that the maximum voltage between ANY two
windings (especially BETWEEN layers) is kept way below the breakdown
level of the insulation. Usually multiple pies are used, with an
adequate separation between each pie so that there is no breakdown
between sections. When making a high voltage pie the layers are
generally kept narrow. I have seen 20KV transformers built using
several pies in which absolutely no additional insulation was used
between layers, other than the existing wire insulation.
When I answered the original post concerning the building of HV RF
chokes, I didn't mention honey-combing at all because it is NOT good
for HV work. I mentioned the use of pies because that IS good for HV
work. Your quote from the Radio Amateur's Handbook is mostly
concerned with the reduction of the self capacitance so that the coil
does not self-resonate at too low a frequency, or act like a coil in
parallel with a capacitor.
HV RF chokes for Tesla coil use have a particularly nasty
environment. It is normal for the choke to have to hold back many
tens of kilovolts of RF spikes. The only way you can really reduce
the self capacitance of these particular chokes is to space wind
them. In everything there is a trade-off. Honey-combing, by the way,
is a form of space winding. Really good for many types of RF chokes,
but horrendous for Tesla coil service!
Hope this helps.
Fr. Tom McGahee