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Andre

Fri Jan 25 2013, 04:06PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
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Posts: 109

if a magnet wire is rated for 4.85 kV does it mean I can wind a secondary and go up to 4KV per layer?

this is the wire

Steve Conner

Fri Jan 25 2013, 04:36PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The voltage rating is probably for 60Hz or DC, it won't take into account high frequency corona.

Andre

Fri Jan 25 2013, 04:46PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
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Steve Conner wrote ...

The voltage rating is probably for 60Hz or DC, it won't take into account high frequency corona.

so for this type of wire how much voltage per layer can it take before it breaks down, and how did you came up with that answer?

StormInABottle

Fri Jan 25 2013, 09:00PM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

High frequency bites through most of the insulation s you can make So don't trust the rating. And don't be cheap and put some insulation between the layers.

Andre

Fri Jan 25 2013, 09:36PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
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Posts: 109

you guys are funny, you keep replying and saying what would not work, :)
but for 100points same question again.
how much voltage that wire can handle per layer?

I'm asking cause I could not find any site that talks about the wire insulation and voltage per layer when we are working with HV.

Newton Brawn

Fri Jan 25 2013, 10:09PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Andre'

Do not be stubbbbber.
Get two piece of such magnetic wire, twist then, and apply a square wave 1MHz e see what hapens. Raise the voltage up to the point that corona start to appears.
Record that voltage as Vc.
Then designe your trafo, such way that no more than 1/4 of that voltage Vc is between the adjacent turns. The insulation between layers may be 0.1mm thick Mylar, polyester or even craft paper vernished.
The max voltage beteween two adjacents layers are to be not more than 1.2kV.
These are just the begining of insulation design, for a reliable trafo, that could work continuosly for some years ....

Andre

Fri Jan 25 2013, 10:32PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
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Posts: 109

Thank you Newton Brawn, that seems like a nice test to do :)

do you know if anyone ever came up with the formula for it, running this type of magnet wire at that freq when it would break down? (I will do your suggested test, I'm just curious about it)

I'm trying to get away from using a lot of insulation because I'm using a small core, by increasing the insulation the secondary becomes less efficient, so I guess for a 40KV transformer I need to do over 40 layers.

Ash Small

Fri Jan 25 2013, 11:21PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Andre wrote ...

I'm trying to get away from using a lot of insulation because I'm using a small core, by increasing the insulation the secondary becomes less efficient, so I guess for a 40KV transformer I need to do over 40 layers.

You'll be extremely lucky to get 40kV from a 'small' core.

Bigger cores allow more volts per turn. I'm doubling up on the biggest cores I could find (using 2 pairs side by side) with the secondary wound around both of them, to try and get 50kV.I'm hoping for over 20 volts per turn before heating becomes an issue, and I'm hoping to get away with ~2000 turns on the secondary.

I'll also have pancake secondaries in series wound on elliptical bobbins to aid cooling (I hope to be posting some details soon, the bobbins haven't arrived yet due to the snow).

Make it easy on yourself and get the biggest cores you can find.

I've seen a few transformers on here with four sets of 'big' cores arranged in an 'X' to lower the number of secondary turns. If mine doesn't work, I'll try that myself.

Tony Matt

Sat Jan 26 2013, 12:01AM

Registered Member #3700 Joined: Sat Feb 19 2011, 12:59PM
Location:
Posts: 107

Andre'
If you really want a formula, get a copy of "Calculation of Corona Starting Voltage in Air-Solid Dielectric Systems" by H, C. Hallec , published by AIEE Transactions , April 1956.
Actually REPRINTRED by IEEE.
( reviewing your questions I am not sure that you will undestand the paper...)
Or
Follow the graphics:

Steve Conner

Sat Jan 26 2013, 07:10PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A classic newbie mistake is to try and make a HF transformer of way too high voltage, instead of using a lower voltage transformer with a CW multiplier. (Or building a Tesla coil instead if you really need a HF AC output.)

Even if you got the insulation right to avoid corona, self-capacitance would probably bite you in the backside. You have to drive the transformer at or below its self-resonant frequency, so you have to get that as high as possible, because the drive frequency limits the volts per turn you can get from the core.

I think a practical maximum from a TV flyback-sized core is about 15kV.

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