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Re: Oil-immersed RFCs?
From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Saturday, August 30, 1997 3:37 PM
To: Tesla List
Subject: Re: Oil-immersed RFCs?
>
>
> From: Felix[SMTP:73374.1547-at-CompuServe.COM]
> Sent: Friday, August 29, 1997 6:25 PM
> To: tesla-at-pupman-dot-com
> Subject: Oil-immersed RFCs?
>
> Suppose you want a pair of 5 millihenry chokes to protect neon
transformers,
> The consensus seems to be that you wind them single-layer to
prevent
> layer-to-layer arcing. But the solenoid equations translate this
into
> things the size of a 10-gallon bucket, or alternatively a baseball
bat,
> roughly speaking. Do chokes have to be that cumbersome? Why not
wind
> them multi-layer and immerse in oil? I have seen corona occur
within
> oil so I assume that you'd have to have enough windings to keep
> layer-to-layer voltage below some value, based on the 50 to 60 kv
> spikes that you're presumably trying to choke, and the corona-ing
> propensity of the high frequency components of those spikes.
> Can anyone tell me of criteria for spacing layers of windings to
> make the multi-layer, oil-immersed choke work?
>
>
Felix,
Multi-layer chokes are indeed possible, and they certainly help to
keep the physical size down. You have to be careful about a few
things, however. First, keep the number of turns PER LAYER small so
that the INTER-LAYER maximum voltage is kept to a reasonable value.
Keep in mind that the voltage that will appear BETWEEN two layers
will be equal to the SUM of the voltages across each layer. For
example, if you had 100 volts between each turn, and 10 turns on each
layer, then you would have 2,000 volts (max) between two layers.
The wire size should be at LEAST as large as the wire used on the
windings of the transformer secondary. I usually use something like a
#28 for neons up to about 60 ma. Others prefer a thicker wire. In
reality it only has to be able to handle the current from the
transformer secondary, NOT any of the TC primary currents.
To prevent breakdown between layers you should place insulation
between each layer. This insulation should extend well beyond the
ends of the coil winding layers so that arcing cannot sneak around
the edges. Layers should be consistent in size, and smooth. When a
layer is finished, then you insert a piece of insulating material
such as transformer paper snug under the wire and then continue
winding but start the wire moving in the opposite linear direction.
This will ensure that the winding does not go near the outer edge of
the insulating material. The insulating material should go completely
around plus an extra amount to ensure that sparks cannot sneak
"under" the insulation. Don't be a cheapskate here! It is better to
have too much insulation rather than not enough!! If the insulation
is particularly thin, then you can always use more than one wrap per
layer.
Multiple coils can be created on a single form, but make sure you
allow adequate spacing between each of them. If each coil section had
a 10KV rating, then the maximum voltage that might exist BETWEEN two
sections is 20KV. You can create as many sections as you want.
It is very important when building multi-coil chokes to make sure
that the wire going to the BOTTOM of any given coil section is kept
carefully insulated from the top and sides of the same coil, as the
voltage that will sometimes appear BETWEEN these two can be many TENS
of kilovolts at times!!! Create as many sections as needed to achieve
the desired voltage rating.
Don't like putting all that insulation between each layer? Then try
the lazy-man's version of a high voltage winding. Imagine two thin
plexiglass disks about three inches in diameter with a 1/4" thick
one inch diameter plexiglass disk wedged in between them as a
separator. Glue the assembly and drill a 1/16" hole about 1/2" from
the center of one of the 3" disks. Take the end of a reel of wire and
thread it through this hole from the inside out. Tape the wire down
from the outside of the plexiglass so that it can't come loose during
the winding process. Drill another small hole near the rim. Later
this will be used to secure the outer end of the wire. Put a bolt
through the center holes of the plastic disks and secure the bolt
tightly with lock washers and nuts. Clamp the bolt into the chuck of
a variable speed drill. Applying speed slowly at first, begin winding
the wire between the two large plastic disks until it is almost full.
As the winding proceeds, wiggle the wire back and forth slightly so
that it distributes itself fairly evenly. When done, cut the wire and
thread it through the rim hole. You can make as many of these
"sections" as you want. Mark the winding direction on the plastic for
later reference in phasing the sections all the same.
You can connect the top of one section to the bottom of the next
section to increase inductance and voltage rating. I like to leave at
least 1/4" separation between sections.
I usually immerse my coils in something like varnish for several
minutes and then hang them up to dry.
I have never oil immersed such coils. Keep things as simple as
possible!!
Actual value of the coil? I don't calculate them at all. I build one
section and then measure the value, and then build as many sections
as needed to attain the desired value.
Although some people are recommending 5-10 millihenry chokes, my
experience is that you can use smaller values, as the effective
"frequency" of what you want to filter out is usually quite high.
Higher than the coil's operating frequency in most cases.
Hope this helps.
Fr. Tom McGahee