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after how many turn on a HV transfomer you start insulation

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Andre

Fri Oct 12 2012, 06:21PM

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

Pinky's Brain wrote ...

Just to be sure, when you say 30 Hz do you mean one short square pulse every 1/30th of a second ... or do you mean a near square wave at 30 Hz? (Pretty sure you can't get close to that with the size of transformer you want.)

a near square wave at 30hz, why you said it would not be posible?

also I'm still confused, about the layers, let say I'm doing 100 layers and is an inch wide, so each layer would have 400V, how about if I do 200 layers, half inch wide, each layer would have 200V then, is there a formula to all this?

Sulaiman

Fri Oct 12 2012, 07:00PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

When I wind hv hf transformers I;

1) Insulate the lead-in wire (heat-shrink or ptfe tubing, or whatever I have)
2) leave a gap at the ends of the windings so that the inter-layer insulation is wider than the windings
3) wind the first layer, wrap with insulation and run the wire back down to the start end of the winding, insulating above and below the lengthwise wire.
4) wind the next layer (in exactly the same direction as the first), insulate, run the wire back to the 'start' end and continue.
5) insulate the lead-out wire then put a couple of layers of insulation to insulate the outer layer from the core.

Running the wire back along the winding does make the overall winding a little thicker, it helps to wind N+(1/4) or N-(1/4) turns per layer to spread out the extra thickness.

This helps with insulation (1/2 the voltage stress) and it definitely lowers the self-capacitance of the windings, which raises the self-resonant frequency.

P.S. it's not practical to make a very low leakage inductance hv transformer
(multiple interleaved primary and secondary layers)
just wind the primary (or primaries) first (as the wire is usually thick/stiff)
insulate the primary from the secondary with a full-width fairly thick insulator
(paper/card/plastic sheet etc.)
to get a nice level surface to wind the secondary on and continue as above.

Find the natural self-resonant frequency of the secondary and design to operate at a frequency somewhat lower to allow for external capacitance.
IF using squarewave drive operate at 1/3 the resonant frequency OR much much less.

Good Luck !

P.P.S. if you manage to wind a reliable 40 kV transformer I will be surprised/jealous :<)

Pinky's Brain

Fri Oct 12 2012, 07:33PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Andre wrote ...

a near square wave at 30hz, why you said it would not be posible?

It will saturate unloaded with 400 turns on the primary (can't put much more on the primary unless you want to switch more than 1 kV on it).

Andre

Fri Oct 12 2012, 07:39PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
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Thanks Sulaiman, is very helpfull information, how do I find out the natural self-resonant frequency of the secondary?
the reason I wanted a very low leakage inductance hv transformer is because I read is one of the keys of a close to square wave pulse transformer.

Sulaiman

Fri Oct 12 2012, 08:05PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

First ... in my opinion, a reliable 40 kHz squarewave +/-40 kV transformer output is not possible,
I doubt that there is one on this planet !

I'll give a GBP100 prize to the first person that delivers one to me !
(must be able to drive my 100 MOhm//3pF probe to qualify)

(i.e. 'squarewave' meaning F+3F/3 +5F/5 +7F/7 +9F/9 at least)

Explain what the transformer is for and maybe someone here can come up with a viable option.

Andre

Fri Oct 12 2012, 11:18PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
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first part was to get to 40KV
second part was to get the output to pulse at 30hz

Ben Solon

Sat Oct 13 2012, 12:34AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

Those are the specs you've been telling us about but not the objective. What are you USING the transformer for?

30hz will not work because then the core will saturate. The peak flux density in a core is given by (V*t)/(N*Ae). The acceptable limits of flux in ferrite is usually ~.3T. You havn't given your core cross sectional area, primary voltage or number of primary turns yet, so let's hear them. We'll walk you through just why this setup is approaching or already past impossible.

The easiest way would be if you gave us your USE of the transformer, then we could give you an altrnative method of aproaching it.

Pinky's Brain

Sat Oct 13 2012, 01:01AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Lets say 400 turns primary so you'd get ~40 kV out with 1 kV in and a 16000 turn secondary ... you'd need a core area of 1000/(60*400*0.3) = 0.14 m2 :)

Ben Solon

Sat Oct 13 2012, 01:27AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

Pinky's Brain wrote ...

Lets say 400 turns primary so you'd get ~40 kV out with 1 kV in and a 16000 turn secondary ... you'd need a core area of 1000/(60*400*0.3) = 0.14 m2 :)

and for those of you out there not so mathematically inclined: the core(if circular in cross section) must have a radius of ~8.31102 inches! 211cm in all important metric, but the imperial value inspires more fear don't you think

more: 16000 turns without stacking factor on that core is 21.222 kiliometers. try finding 13.1867 miles of continuous wire with thin enough gauge that it wont fill your core window. then wind it!

Sulaiman

Sat Oct 13 2012, 04:33AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Andre, are you trying for 30 Hz bursts of 40 kHz for sonar etc.?

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