Wire Insulation Thickness

From:  Harri Suomalainen [SMTP:haba-at-cc.hut.fi]
Sent:  Wednesday, May 27, 1998 3:08 PM
To:  tesla-at-pupman-dot-com
Subject:  Re: Wire Insulation Thickness

>A coil with 1000 turns of enamel coated copper wire will have the follow
>volts per turn based on the different length discharge sparks.
>150" discharge spark = 2.769 MV = 2769 volts per turn.

I hate to be a spoil but there are couple of errors on this one. Arc
lenght is *not* a direct function of voltage. (I'll leave the arc lenght
vs.  voltage thread to the others. :)

The other problem is assuming voltage will vary linearly at the coil.
It does not. Having assumed a sinuoidal voltage profile is much
better approximation, I think.

>I don't know the exact voltage rating for enamel insulation but I bet its
>not high enough to hold up to 2769 volts or 1846 volts or even 923 volts.

OK, insulation is .0044 (#20) or .0031 (#24). For thin layers I'd be
certain that almost every brand of insulation is rated at least 500V/mil.
This will give 1550 or 2200V for the thickness of insulation.

However, depending on the perticular brand of insulation it may be
capable of much better. There are quite a lot of materials rated for
1-2kV/mil for those thin layers.

Then there's the question of insulation thickness. If you manage to
wind those wires with no space between them I'll be supprised.
A rational assumption would be perhaps 4mil space from wire to
wire. That means copper-to-copper distance is roughly doubled
to what plain wires would indicate.

The space has to be filled with something. If not, the air will surely
break down there becouse it will experience a much larger E-field
than the insulation. So, you coat it with some good insulator to have
wire embedded in coating.

Then you find out that you have actually bigger distances than you
first thought. All with good insulators. No air bubbles in the coating
either. The situation has changed for much better.

>I was thinking it might be a good idea to wind a plastic fishing line
>between the wires on a large coil to increase the insulation rating.  The
>extra spacing should help arcing between turns.

When the voltages get very large this may actually be needed.
Other space winding techniques should work as well. It is very
essential it gets a proper coating between the wires in any case.
Fishing line will most probably be a worse insulator than good
brand of coating so you may be better off if you have coating
between some not-so-good plastic between the turns.

Breaking may also happen if the coil arcs to surrounding air. (If not
preperly shielded or coated.) Those tiny spots might (will?) then arc to
each other once the way through air and broken insulation between
the wire and surface is there.

Harri Suomalainen     mailto:haba-at-cc.hut.fi

We have phone numbers, why'd we need IP-numbers? - a person in a bus