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Re: Wood
Hi Alex,
Most people are surprised to learn that wood is commonly used in large
distribution transformers and other high voltage devices filled with oil
even in these "space age" times. Despite today's fancy plastics, wood will
last indefinitely in such applications and has been well proven for over
100 years now.
Wood may have a problem absorbing moisture which makes it "too" resistive
and it can flashover. Soaking it with oil vastly reduces this and I would
think it would actually work better than many plastics since its surface is
rather rough. I have heard that it's very slight conductivity also reduces
static charges building up on the surfaces and thus renders it even better
at resisting flashover. Since it is slightly conductive, it is not good
for holding static charges but TCs usually have so much power going through
them that this is not noticed.
The real problems I can see are not knowing how much water content there is
and thus not being able to be sure it is ok. The oiled surface my also
pick up dirt that may hurt it's standoff. Also, I wonder if plywood would
fall apart after soaking in oil and the glues my do something unforeseen to
the voltage standoff. Oak is a very good choice but I would stay away from
pine due to the high sap content. Hard maple is very strong and can be
worked to high precision but it would saturate with oil slowly. Of course,
the kiln dryed types are what you want. Here in Colorado, it is very dry
and I use wood a lot for high voltage stuff. In a humid area, I would
worry much more. Oil soaking solves this problem if one can stand having
everything being all oily.
Wood soaked with oil will burn, so you have to design and use it with that
in mind. An advantage is that if you use the right kind of oil, the fumes
will not be very toxic unlike most plastics.
Of course, wood is cheap, very easy to get, and there are all kinds of
wonderful tools available for working with it...
Terry
At 01:18 PM 5/12/99 +0100, you wrote:
>Dear All
>
>I was looking through various posts about materials for SG boxes and
>Primary supports, and I noticed that many people have said that wood is
>a bad material for these purposes. However, I have used it very
>successfully in my coil in all HV areas and as yet it hasn't even got
>warm or displayed any kind of tracking. What I did was to soak every
>piece throroughly, several times, in transformer oil. I am using 8mm
>plywood for most of these bits, and I found it was quite absorbent.
>I allowed this to soak in for several days, at which point I liberally
>applied varnish. Due to some oil still oozing from the wood, it takes an
>age to dry (think in days here) but I found that once it had done so it
>set quite hard and the wood actually felt like plastic to the touch.
>I tested this with my NST output, even trying to induce tracking by
>using pins as electrodes and it didn't draw any current or spark. I
>think that the oil must have displaced the water (2% or so, I hear) that
>is common to most woods - the Xfrmr oil is very non-viscous and
>penetrating - I spilt some on the concrete and thought there would be a
>stain there forever, but in three or four days it had disappeared.
>
>Anyone else have any experience with using wood at HV successfully?
>
>Alex Crow
>