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FW: 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



> 
> From: 	Felix[SMTP:73374.1547-at-CompuServe.COM]
> Sent: 	Sunday, August 31, 1997 2:15 PM
> To: 	tesla-at-pupman-dot-com
> Subject: 	Oil-immersed RFCs
> 
> Tom,
> In your RFCs, is the number of 'slices' of winding driven much more
by 
> breakdown voltage than by inductance requirements? 

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. 

Decreasing the winding WIDTH on the other hand, ensures that the
inter-layer voltage is LESS. The extreme example would be a set of
plexiglass plates set so close together that only a SINGLE turn would
fit per "layer". In effect you would be winding a flat spiral pancake
coil. The method that I outlined for making coil sections will work
admirably well as long as the insulation on the wire can withstand
the voltage that exists between wires that are touching one another. 

I've been looking at
>  solenoid formulas and wire tables and offer this line of thought:
>      Using no. 22 wire, which gave about 35 tpi on my Tesla
secondary, one of
> your slices of winding would have about 35 layers in its 1-in
radial depth.
> Postulating a two-slice RFC facing 60 kv spikes, we get roughly 1
kv per 
> layer of winding or, as you point out, 2 kv between the ends of
adjacent 
> layers---or easily 5 kv if the layers are not laid down absolutely
evenly
> and some wires slip down into a lower layer
>   Now 2 to 5 kv sounds to me like a lot for the few mils of enamel
on my no.
>  22 Phelps-Dodge Thermaleze, although I know volts/mil are higher
for 
> insulation in thin layers.

Yes, but in your example you limited yourself to TWO sections. Let's
say you used FIVE sections instead. This would give you a breakdown
per section of 60KV/5 which is 12KV. That will give you about 1.2KV
maximum between the windings from one LAYER to the next (and since
there are TWO sets of insulation involved, you only need 666 volt
rated insulation). Using ten sections would reduce that to 333 volt
insulation, etc.

>    This suggests that the choice of RFC wire gauge may be more
critical than
> it looks at first sight. I've never seen data on breakdown voltage
of
> enameled wires in the 22 to 28 ga region, far less at the high
frequencies
> of interest, but if for instance the insulation breakdown were
about the
> same over this range of wire gauges, then the thinner wire would be

> better due to fewer volts per layer.

You do not gain anything substantial by using wire that is thicker
than necessary. #28 can easily handle about 300 ma of current. But of
course, feel free to use whatever you happen to have as long as it
can handle both the projected maximum voltage and current.

> I also notice that Tesla secondaries with 500 to 1000 turns and
several feet
>  of arc evidently achieve turn-to-turn breakdowns of 0.5 kv or
more, and in 
> the literature I haven't seen any concern about needing heavier
secondary 
> wire at higher arc length. This suggests that for our RFCs,
something around
>  1 kv between any two adjacent wires may be OK. This would call for
dividing 
> the total inductance of each RFC into about two to five 'slices',
depending 
> on how precisely your electric drill technique can place and
separate the 
> layers. As a by-product it would also give the several millihenries
that some
>  coilers favor.
> How does all this sound to you?
> The fun thing about coiling is that the circuit diagram can be so
simple in
> appearance, yet conceal so many unexpected subtleties.
>     Regards, Felix
> 
> 

Felix,
Glad to see that you are using your head and thinking. You are asking
the right questions, and for the right reasons. By the way, when I
wind such sections, I am consistent in always having the inner wire
hole at my left and the upper wire hole at my right, so that when I
inter-connect sections there is no danger of arcing to the inner
wire. I also have the two holes at the same "angle" so that
interconnections can be made in a very straight forward manner. 

Allow adequate spacing between sections! Remember that the voltage
between sections can be many tens of kilovolts when few sections are
used.

If you want, you can make the plexiglass sections so that they will
fit inside a 3" diameter PVC pipe. This can serve as a protective
shield to prevent damage to the wires. Use bolts to get the
connections to the outside of the case.

Hope this helps.
Fr. Tom McGahee