50KV 500ma HFHV Transformer Design / Want to Build 50ma Version / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

50KV 500ma HFHV Transformer Design / Want to Build 50ma Version

previous 1 2

Move Thread

LAN_403

Dr. Dark Current

Fri Jul 10 2009, 04:11PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I used my paint skills (

) to illustrate how I think the wire crossing could be done.

Steve Conner

Fri Jul 10 2009, 04:36PM

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

Dr. Kilovolt's sketch, and the method described by RickR, are similar to how I've seen it done, with specially shaped slots between bobbins. All you need to do is bring the wire in at an angle, so it doesn't sit against all of the layers as it makes its way down to the bottom of the slot, which applies the full pie voltage across two layers of insulation as Dr. Slack points out.

Finn Hammer

Sun Jul 12 2009, 07:26AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Dr. Kilovolt wrote ...

I used my paint skills (

) to illustrate how I think the wire crossing could be done.

With a lathe at hand, I devised this method to acheive the same end:

Cheers, Finn hammer

LutzH

Mon Jul 13 2009, 02:14AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hey, a sincere thank you to all!!!!

You have pretty much answered all the remaining questions I had, your help is greatly appreciated, thank you again :)

Reference the Diodes and Rectification:

I will not be needing this because I want the 50KV 20-30KHz HF output to feed a hungry FWVM. I also solved my problem today in how to make the equipotential rings for each stage of the 16 stage VM, to prevent major corona losses. Most companies which sell S.Steel tubing will wind a coil for you cheaply. I can just cut rings from this coil, and join the ends to make the hoops which I need.

I still need to calculate the diameter of the hoops for 75KV / stage but this should not be to complicated, the top will be the same as a Van DeGraff, a hemisphere, with a short cylinder section with a re-entrant edge. Because of the very high voltage I am not going to use a single water resistor for protection. Instead I will use resistors between each HV capacitor in series with each stack, and the same resistors in series between each stage in the rectifier column.

One interesting point when using 2 different value capacitors in a FWVM: In my small scale test VM it works best to have all the higher value caps on the bottom of the stack, but only in the two outside columns, not in the center one. The surprise was that there was no improvement by having them in the center column also. I suppose this makes sense because only the outside columns transfer the charge, and the center one basically filters the ripple. Last I will double up on the diodes in the first, and last stage for higher surge current capability. The testing in air will be at the 400KV level. I will post some pics on this one when it is done. Then if all goes well I will start on the final steps, and put it all in a SF6 gas mixture. It seems many commercial VM designs are far from ideal using the same value of capacitors in the entire unit.

Reference the transformer design it seems indeed that all of these tricks have been done at one time, or another by different members:) Sometimes the hard part in the future is in putting it all together, and finding the information.

I will do some research into the cost having the bobbins made, and I will post the results. If is is reasonable I suppose the target voltage does not matter, since each individual member could choose their own wire size, and type, and thus customize the end result. If it is to expensive then I we can still try the great construction suggestions contributed in the replies.

Thanks again.... Lutz :)

jpsmith123

Thu Jul 30 2009, 08:37PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I may have missed it, but I didn't see anything in the paper about capacitive coupling between the windings and the core (and associated consequences).

When I did a fullwave frequency domain simulation of a ferrite core (albeit using numbers for ferrite "conductivity" that I'm not 100% sure of), with a polyethylene bobbin a few mm thick, being driven at frequencies of 20 to 30 khz, with single layer windings being represented by conducting cylindrical tubes concentric to the core, I see a power loss in the core comparable to or exceeding that of the familiar "magnetic" losses that would be expected.

Spice simulations using the estimated-from-geometry capacitive coupling seem to confirm the results of the EM solver. IOW with, say, 20 kv peak @ 30 khz coupled through an estimated 20 pf to an estimated 50 kohm resistor (assuming core conductivity of 0.003 S/M), there's about 3.7 kv peak across the resistor representing a power loss of about 137 watts rms.

Of course at higher drive frequencies the problem is worse, and I wonder if this capacitive coupling may be responsible for the failures of the single layer windings that Steve Ward (IIRC) and others reported.

In any case it seems to me that single layer windings may have the disadvantage of the highest capacitive coupling to the core (for a given standoff distance), and multilayer windings the least, with a multiple pie type winding somewhere in between.

Regardless, it seems you'd want to use the thickest possible bobbin, made from a material with the lowest possible dielectric constant.

Steve Conner

Thu Jul 30 2009, 08:51PM

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

LutzH wrote ...

I can just cut rings from this coil, and join the ends to make the hoops which I need.

Don't join the ends! (Why not?)

The capacitive coupling problem could be solved by using a Faraday shield to divert the current straight to ground. If you put the primary coil under the secondary, there's your shield.

jpsmith123

Thu Jul 30 2009, 09:33PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I thought about putting the primary winding on first, but unfortunately that takes up more precious radial space (also, to be effective, it would probably need to consist of multiple wires in parallel, another PITA, in order to span the same area as the secondary, for a single layer winding at least).

As of right now I'm leaning toward grounded copper foil shielding between the core and the secondary (with the secondary being a multilayer winding with decreasing winding-width per layer).

It seems there are a million trade-offs between ease of construction, capacitive loading, losses, field enhancement, etc...it's starting to drive me crazy trying to figure it all out beforehand.

HV Enthusiast

Thu Jul 30 2009, 09:38PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Don't forget to put a shield between the primary and secondary. This goes a long way in reducing parasitic capacitance between primary and secondary. Also, i agree with Geordy. You shouldn't join the universal windings together, but rather keep them separate and then rectify them separately. This is how its typically done in commercial hv supplies.

Also, you should approach the drive scheme using a resonant type power supply, not a conventional half-bridge / full-bridge.

Also, when using a dielectric liquid for insulation / thermal, be sure to evaluate them according to dielectric constant. For example, if you used Fluorinert (which i think has a dielectric constant of around 2.8), your parasitics will increase by approx. factor of 2.8. I think most oils are around this range as well.

klugesmith

Tue Aug 04 2009, 04:31PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Finn Hammer wrote ...

With a lathe at hand, I devised this method to acheive the same end:

I was reminded of this thread while thinning my motor collection last weekend. This bobbin is clearly designed for quick starting (of the coil). A thick slice has been removed from picture for 400 pixel limit.

-Rich

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob