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4hv.org :: Forums :: Tesla Coils
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enamelled wire voltage rating

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Stuart
Wed Oct 08 2008, 01:40PM Print
Stuart Registered Member #1747 Joined: Wed Oct 08 2008, 12:57PM
Location:
Posts: 6
Hi, This is my 1st post here, so Hi to everybody:)

I have recenlty stopped work on my gas turbine to concentrate on my tesla coil (i must have a subconcious love of building things that are dangerous)

I have a MOT which i am planning to re-wind for a 15kV supply. The secondary has already been removed.

I have wound high voltage transformers (up to 6kV) before but ive had plenty winding area and space to fit additional insulation between the windings, here it will be tighter.

I have 0.767v/turn open curcuit from the transformer (both measured with a few turns and calculded from the 300 primary windings at 230v) so for 15kV i need about 19600 turns.

My open window area is 43mm x 20mm, take that down to 42 x 19 to allow for some turns of Polyimide tape (RS components 468-421 70microns thick, 4.8kV insulation voltage) as insulation and winding holding.

Using wire with a maximum overall diameter of 0.165mm (0.132mm dia copper, or 35-36AWG or 38-40SWG) i can wind 250 turns allong the 42mm dimension. for 19600 turns, ill need 78 layers, these being a total thickness of 13mm. I have 19mm which leaves me with 5mm for insulating tape layers and any possible inaccurate winding layering. Theoretically at 0.07mm thick, i can fit 70 layers but i think thats optimistic. If i taped every 3rd layer, that would give 26 layers of tape.

Every 3rd layer would be 575v so i shouldnt have any issues(?)

My wire supplier (Comax) has gave me data sheets which shows the insulation on that wire is 0.021mm thick and has a breakdown of 110v/micron giving me a maximum of 2300v between any two wires. (see data attatched)

How often should i place additional layers of insulation which will be the 70micron tape able to withstand 4.8kV? Is every 3rd or so layer excesive?

Will i even need additional layers of insulation? I think so just due to breakdown of air around the wires.

Depending on the answers, i may go for more windings to get as high a voltage as possible (up to a max of 20kV perhaps(?)) by balancing windings with insulation.

Thanks,
Stuart
1223473241 1747 FT0 Winding Wire Dimensions
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Dr. Dark Current
Wed Oct 08 2008, 04:58PM
Dr. Dark Current Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi, sorry but I think 15kV is absolutely unrealistic for a MOT. You will have big problems with corona and even if it holds for first few seconds, it WILL arc over sooner or later.
15kV is going to jump more than 2cm in air and even more by surface tracking, so you would need to make sure that there is more than ~3cm of insulation tracking surface between winding and core.

The enameled wire will indeed stand a lot of voltage, but that's only assuming there is NO scratch or nick in the enamel. It's true that a non damaged enameled wire will stand more than a kilovolt.

If you can wind the layers on top of each other, with turns side-by-side and can make sure the wire does not slip near the lower layers, you would theoretically need no insulation between the windings. The best way to secure the layers in place is to varnish them before winding the next layer, this way the turns are held in place and won't slip.


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Sulaiman
Wed Oct 08 2008, 05:16PM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I would consider a couple of options;

1) Use a center-tap winding... e.g. 7.5 - 0 - 7.5 kV
This will reduce the probability of flashover to earth/frame/core/primary etc.
(i.e. just like a Neon Sign Transformer)

2) Put the transformer under oil !
I find it difficult to insulate a dry high voltage transformer.
It's also easier to repair.

3) Consider a lower voltage.
5 - 0 - 5 kV for example would be significantly easier due to corona problems.
For the same power throughput the spark gap would need to be a little better due to lower primary impedance
but there would be very little overall performance change.
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Stuart
Wed Oct 08 2008, 09:48PM
Stuart Registered Member #1747 Joined: Wed Oct 08 2008, 12:57PM
Location:
Posts: 6
Hi,

Thanks for your replies,

Dr K, I had considered this but thought that by wrapping the windings effectively creating 15 of 1kV winding sets would deal with this issue. If at all times i kept all connections potted or wrapped so as to minimise the chance of it happening.

Sulaiman, would you advise winding it as two separate coils, side by side where high's are on the outside and the grounded sides are closest to the core.

I see what you are saying about 5kV - 0 - 5Kv. My thought was, whilst i had the oppertunity to go for a higher voltage and get a higher energy (v squared) into the caps then i should try and acheive it.

Thanks,
Stuart
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