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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I've finally built an HV transformer that hasn't failed within seconds! About 500 secondary turns and run from my multipurpose inverter (off-line half-bridge). I used overhead transparencies between layers, and submersed the whole mess under oil. Arcs will initiate at up to 1cm, despite estimates based on a turns ratio of 500:14 and half-bridge drive yielding 5kV output. I left the spacers in the core, as I've experienced this to give voltage boosts, which it seems to be doing here too. Any explanations? I don't think any resonance effects are going on as my inverter isn't struggling, and the drive frequency was simply picked at random. I've uploaded a youtube video.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Uzzors wrote ...
I've finally built an HV transformer that hasn't failed within seconds! About 500 secondary turns and run from my multipurpose inverter (off-line half-bridge). I used overhead transparencies between layers, and submersed the whole mess under oil. Arcs will initiate at up to 1cm, despite estimates based on a turns ratio of 500:14 and half-bridge drive yielding 5kV output. I left the spacers in the core, as I've experienced this to give voltage boosts, which it seems to be doing here too. Any explanations? I don't think any resonance effects are going on as my inverter isn't struggling, and the drive frequency was simply picked at random. I've uploaded a youtube video.
The peak output from a half-bridge driven HV transformer will be approx. 2x turns ratio*input (in most "experimenters" conditions) becuase the leakage resonates with secondary capacitance and produces ringing on the output.
As to the spacers, possibly they affected the leakage so the ringing frequency became a harmonic of the driving frequency, giving slight peak output boost as effect of series resonance.
Registered Member #1132
Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84
That is some nice results from that homemade transformer.Also nice that you manged to make it multilayer without arcovers.I guess the oil helps a lot.
I still haven't did any more work on it but i plan to keep going with the easier single layer, but make two identical secondarys to get twice the voltage without putting more strain on the insulation. But i will still keep this first experemental secundary as it has a little less turns, so it dosent make a relay high voltage but the current is high because of that. Thats why the arcs are so bright.Also this lower voltage will be easier to rectify into DC and then meaby try making a pour homemade HV cap (and make sure you don't touch it when full)
Registered Member #1132
Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84
Until the IGBTs get here i trued some MOSFETs in the bridge and had to run it at only 40V. It actually put out quite a lot.
It actually welded two iron screws together in a few seconds, and i cant seam to get them apart anymore without a vice. Normally when things weld up like that they pop a part again with a little twist.
The resonant frequency of the transformer can range anywhere from 10Khz to 200Khz and its very easy to find. The whole thing draws a very big current when its in resonance. Also the resonance of it changes with load (bigger the load bigger the freqency) My transformer seams to work best at 80Khz.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I used "Eurol Hykrol" hydraulic oil. Nothing special with it I would assume, just what I found at the gas station. Any oil will do as long as the actual transformer is done properly, and any air bubbles are removed. There are small air pockets in my transformer, due to the use of scotch tape instead of something the oil could soak into. If they happen to ionize and eat away to much insulation the oil should seep in and heal the damaged area however, so I'm not too concerned. The overhead transparencies I used are good for at least 10kV per sheet, so each layer should be adequately insulated from the others. The oil only serves to help remove air, and insulate the path between layers on each end of the secondary.
As for the drive frequency, I'm running it at 80-90kHz. I measured the parallel resonant frequency to be 108kHz, much lower than anticipated but not surprising. No efforts were put in reducing self-capacitance. The core spacers actually helped by increasing the resonant frequency, otherwise the drive frequency would need to be unreasonably low to stay under the parallel resonant frequency. Good thing I put them in from the start! What surprises me is that the inverter is enjoying the load so much, I've never had it run so cool while driving a flyback! Then again I've only run monitor flybacks, and that was possibly over their resonant frequency. (Dominating capacitive reactance in case this seems insignificant.)
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> What surprises me is that the inverter is enjoying the load so much,
If you are just using a voltage-fed square-wave inverter then I think you got lucky in two areas:
1. The leakage inductance limits the current drawn from the inverter to a tolerable value when the output is shorted or drawing an arc. If you'd wound both windings on the same limb it probably wouldn't tolerate a s/c so well.
2. The drive frequency and its harmonics must not fall on any of the transformers series resonant frequencies otherwise excessive current would be drawn at these frequencies.
I know about the 2nd situation because I tried to do something very similar to this and had a CT in place displaying the inverter load current. It just happened that the 3rd harmonic of the inverter excited the series resonant frequency of the particular transformer. Current flow at 3xFswitch was very high and the inverter failed within a few seconds of power-up, with the traditional time-honoured white flash, bang and shower of sparks!
Registered Member #1132
Joined: Mon Nov 19 2007, 06:15PM
Location: Slovenia
Posts: 84
I did try out a new transformer, and it worked pretty good for quite some time, but then it arced over between the two secondarys. Again the outage out of it is not very impressive , its better for high currents.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
rp181 wrote ...
Ive found that crayola marker tubes are a perfect fit over my ferrite core, it will allow for closer coupling and larger distance between coils.
I got my cores with nice bobbins already made (from a TV) When constructing my transformers I always make sure all materials used are good for at least 120C (or more if possible). This way I can run the transformer hot without worrying about meltdown The usual "cheap plastic" usually softens at around 80C, not really useful for transformers.
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Homemade HV transformer.
(from a TV)
The usual "cheap plastic" usually softens at around 80C, not really useful for transformers.