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Calculating self-resonant frequency of HV Transformer?

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Ash Small

Fri Feb 08 2013, 08:44PM

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

Patrick wrote ...

well thats why i suggested S2 or E glass, since youd only need ~40% epoxy for ~60% glass volume, since the glass has better E and M field behavior than epoxy... but if your asking for v/mil, generally 500v/mil, is the breakdown, so for less than 0.1 inches id say 300v/mil operation for a good quality epoxy or PMMA.

So ~0.1" for 50kV? How does this compare to oil? or Polystyrene?

How suitable is this wire for the secondary? Do I need something with thicker insulation?

(It's the top one in the table, 35SWG)

EDIT: Some 'strange' links have appeared in this post, don't click on 'compare' or 'insulation'

EDIT: I think I had an adware virus on my laptop, but I think it's gone now.

Patrick

Fri Feb 08 2013, 09:17PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

So ~0.1" for 50kV? How does this compare to oil? or Polystyrene?

How suitable is this wire for the secondary? Do I need something with thicker insulation?

(It's the top one in the table, 35SWG)

NO!, i said 500V per mill was the breakdown, to prevail i suggested 0.1" / 30kV... (which i think is resoanable, though im sure there will be a torrent of comments to follow.) ive also had epoxy survive 750v / mil for 20 sec, about 3 times before major flamage occured.

Ash Small wrote ...

EDIT: Some 'strange' links have appeared in this post, don't click on 'compare' or 'insulation'.

crap, i hope one of us hasnt aquired an adware/malware program...!?

And as for steve's comments, ash must avoid 2 matters right? recirculating current and skin effect resistance heating, both related to the thickess of his foil, right?

EDIT: im trying to remember the Unitrode pdf's on this matter, both are related to skin depth...

Ash Small

Fri Feb 08 2013, 09:45PM

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

Patrick wrote ...

NO!, i said 500V per mill was the breakdown, to prevail i suggested 0.1" / 30kV... (which i think is resoanable, though im sure there will be a torrent of comments to follow.)

Yes, my mistake. I'll wait for 'other comments' but I need to do some research here myself, comparing epoxy, polystyrene and oil, for starters.

This thread has been really helpful though, I just need to sort out the primary and the insulation now.

I have about 960mm to work with altogether, if 40mm is taken up by the secondaries, that leaves ~560mm for primaries/insulation, if I use 'pie windings'. (actually, 50 turns of insulated 0.2mm wire is ~12mm (48mm for secondaries), so I might have to increase Bmax a bit (or add more layers to the secondaries, increasing inter-layer capacitance).)

Tony Matt

Sat Feb 09 2013, 01:25AM

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

Just to confirm the Steve said "2.5 amper per square milimiter".
This 2.5A/mm2 is parameter for design of class A insulation, continuously duty used in transformers, motors, solenoides, I have rewind equipment in the 60s using class A insulation. Materials -Paper, cotton, in oil or inpregned with vernish, termoplastic films (as polyethylene).
When the class B insulation becames available, the parameter 3A./mm2 becames to be used in the design for continuosly duty. The class B insulation is cheaper than class A, believe. Materials: Mylar or polyester film, celeron, micanite, thermostable films impregnated with alkide or polyester vernish.
And the class F design parameter could be 4-5A/mm2. continuously
All data above refer to natural ventilation.

Using forced coolling ONAF or evem OFAF more A/mm is allowed, reducing the $$ and size of equipment.
Some info for Mylar:

Class A = 90C, Class B = 130C, Class F= 155C, etc...

Ash Small

Sat Feb 09 2013, 02:09PM

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

Thanks for that information, Tony. I'll probably use ODAF (Oil Directed Air Forced) cooling, with Class A, if space permits, which might enable me to use polystyrene bobbins. I'm hoping to post some more drawings by the end of the weekend.

Ash Small

Sat Feb 09 2013, 07:53PM

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

Well, I've made a few changes, and produced a cross-sectional drawing of my proposed 50kV transformer.

The changes I've made are as follows:

Increased volts per turn from 25VPT to ~28VPT

Reduced the number of pancake secondaries from four to three, but increased the number of layers in each pancake from 10 to 12.

Each primary turn consists of 16 windings of 0.8mm wire @ 0.5mm^2 per wire, total CSA = 8mm.

Twelve turns of primary altogether. Peak rectified voltage = 240Vrms X 1.414 = 340Vpeak, 340V/12=28.3VPT

Total number of secondary turns = 50 per layer X 12 layers per pancake X 3 pancakes = 1800 turns, 1800 turns X 28.3V per turn = 50,940V

So theoretical output is just over 50kV.

Primary and secondary are arranged as 'pie windings'.

I'm working on 6mm insulation everywhere (except between layers of windings), some of which will be oil, some of which will be plastic. I'm still considering using polystyrene, but I'm considering other plastics including epoxy, etc.

Any comments will be welcomed, thanks.

Tony Matt

Sun Feb 10 2013, 12:51AM

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

You may do all insulation in paper, including the bobins forms, or all with 0.1mm mylar, including the bobins forms....
The primary has to be simplified ... go to Home Depot and check 0.3mm or thinner aluminum roof sheet, (or copper sheet)

I gess just 2 x 16 turns aluminun (ou copper) sheet 0.3x40mm wond over top of core simplify the primary,

The pancakes may be more, the voltage between the layers looks high.. 2800V
My experience is limitate to 1500V between layers, in your case 27turns per layer .

Ash Small

Sun Feb 10 2013, 01:17AM

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

Tony Matt wrote ...

You may do all insulation in paper, including the bobins forms, or all with 0.1mm mylar, including the bobins forms....
The primary has to be simplified ... go to Home Depot and check 0.3mm or thinner aluminum roof sheet, (or copper sheet)

I gess just 2 x 16 turns aluminun (ou copper) sheet 0.3x40mm wond over top of core simplify the primary,

I did consider that for a few days, but came to the conclusion that it would be much easier using wire. I'd have to join several pieces of sheet/foil together for each turn. If I then need two or more to carry the current it becomes complicated. (How will the overlaps affect skin depth, etc? How do I join them? solder?)

Tony Matt wrote ...

The pancakes may be more, the voltage between the layers looks high.. 2800V
My experience is limitate to 1500V between layers, in your case 27turns per layer .

My calculations give 1400V per layer, within your 1500V limit. (I checked, and still make it 1400V)

EDIT: your idea of using paper bobbins sounds promising.

EDIT: I misunderstood your primary idea, Others have suggested I should use pie windings. I will give your suggestion some thought, but will it not lead to more leakage inductance?

Tony Matt

Sun Feb 10 2013, 02:18AM

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

In a pancaque the 51th turn is wond on top of the 49th turn, the 52 turn is on top of the 48th turn ..........the 99th turn is on top of the first turn. ( the voltage between the 49 and 51 may be 56V only , between the 48th and 52th may be 96V and so and so )
if a layer of 50 turns is running at 28V per turn the total volt per layer will be 1400V ( between the first turn and the 50th turn)
Then the voltage between the first turn to the 51th turn will be 1428V,.... and the voltage between frst and the 100th will be 2800V ...Note that the 100th turn is laying on top of the first. So, the insulation between layers has to be sized to manage 2800V plus a safety factor.

Tony Matt

Sun Feb 10 2013, 02:41AM

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

Yes, the leakage dispersion inductance is a pain when you deal with hV transformer that requires big distance between the coils.
The the goal is to keep 100kV insulation between any pancake and primary, 100 kV between any pancake and core, 25 kV beween any two sequencial pancakes (if 4 pancakes are in the secondary).
As the secondary pancakes are in series and works as a uniique secondary coil the leakage inductance is not different in both arrangements.
But for construction, a unique primary simplifies the construction a lot.

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