A Slightly Different Version

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A Slightly Different Version

Postby mikek8632 » Fri Dec 27, 2013 7:53 pm

I assembled my coil in November. I had waited until issues like the low inductance secondaries had been widely discussed and various remedies devised. However, I finished testing before the now official position of a 5 turn primary had been announced.

My secondary coil measured 221 ohms. The wire used was single enamelled 34 AWG giving an estimated total turns of 1395 (using wire diameter). If you calculate the theoretical resistance from that total wire length, it works out to be 226 ohms. So that number of turns is close to reality. Therefore it’s no mystery that the self-resonant frequency is somewhat higher than the 1800 turns of 36 AWG stated in the manual. (1780 turns would fit the winding space, giving a resistance of 458 ohms).

Depending on the top load, my secondary is resonant at around 320kHz. If you wound the same space with 35 AWG that frequency would shift down to around 285kHz and 36 AWG would bring it down to 254 kHz. These are rough figures with all sorts of assumptions, but you get the idea.

My primary capacitor measured very close to the nominal value at 67.6 nF. The primary winding with 6 close-spaced turns measured 5.48 µH. The overall length of that winding is 20mm. The theoretical resonant frequency of that combination is 261kHz. I measured a steady 256kHz at low power levels using a scope. At higher power levels, the frequency started at a similar value but would progress down with each cycle and would be in the 230kHz range before the secondary resonance started to dominate.

If the 6 turns were spaced to give an overall length of 23mm, the inductance measured 5.0 µH which gives a theoretical frequency of 274kHz. Spacing still further to give 26.5mm overall measured 4.7µH and so 282kHz.

Although a 4.7µH winding was tried, it caused breakdown between the primary and the lower/mid part of the secondary, even at low power settings. This is with the primary being positioned so that its lower edge was at the same height as the standard primary.

All testing was done with the mains current monitored with a clamp-type AC ammeter. The length of the breakout point was varied to produce the minimum current at "¾" power. By that, I mean the power control knob in the 3 o’clock position. The frequency was always set to minimum to reduce current draw. A spark gap was set up using a grounded aluminium rod at around 45 cm from the breakout point. Arcs to ground are particularly demanding on the electronics, so the distance was adjusted to make them infrequent. This spark distance was monitored to make sure that tuning for minimum current did not sacrifice performance. The coil was placed on a sheet of aluminium foil (40cm x 70cm) to give a reproducible ground plane. This was connected to the mains ground via one of the holes in the end of the oneTesla box. The spark gap rod was also connected to the same point.

The supplied breakout point was replaced with an easily adjustable version using K&S telescoping brass tubes which were to hand. The one in the videos is a 17/32" and ½" pairing. The inner end is anchored to the bolt holding the toroid. It has been angled up to eliminate strikes to the ground plane which made it hard to determine maximum spark length. It also keeps any sparks well away from the primary and from power cords. The primary winding was held in place by two polycarbonate clamps and 3mm nylon bolts which allowed easy adjustment of winding spacing. The primary wires were routed directly into the Perspex box via two adjacent holes. Holes were also drilled for the earth connection to the secondary. The secondary wires were terminated by soldering to adhesive copper tape securely anchored by 4mm nylon bolts screwed into tapped holes. The ¼" metal mounting bolts at the top and bottom of the secondary were replaced with 6mm nylon bolts. This is all shown in the pictures: http://www.smugmug.com/gallery/35635619_bmv7Dd

There are two short videos on Youtube: http://youtu.be/GeL5A54yOnQ is a Christmas carol which is almost entirely 2 note chords and so is a good test of the polyphonic capabilities of the controller. The second video http://youtu.be/65_VPWHLoeY shows a run to determine spark length followed by a tune for all the Aussies out there. A snapshot from that video is shown below.
Ground Strike.jpg
Total length of spark is 55 cm
(47.29 KiB) Downloaded 217 times


With all the twists and turns, the total spark length is 55cm. The maximum I ever saw was a few cm longer but at somewhat higher mains current. The configuration in the videos and photos is with a 6 turn primary spaced to 23mm overall length. The breakout point end was 233mm from where it contacts the toroid. The mains voltage was 243V and the current drawn at ¾ power was between 1.6 and 1.8 amps. At all times during the tuning procedure I didn’t turn up the power past a 2 amp reading and so far I haven’t had to replace any IGBTs. With the IGBTs, I didn’t use the supplied thermal pads or paste. I substituted a phase-change type thermal pad. These eliminates the need for paste which most people use far too much of, and so actually increase thermal impedance. I also gave my secondary coil an extra 6 coats of polyurethane varnish.

I don’t feel I have found the optimum tuning as the current keeps reducing as I increase the length of the breakout point, and I have it at maximum. I could increase the primary frequency by reducing turns or the capacitor, but I am also tempted to make a 35 AWG secondary which may to be close to an optimum. While it probably won’t make much difference to the spark length it may reduce current consumption which should translate to a more robust unit.
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Re: A Slightly Different Version

Postby ican » Fri Dec 27, 2013 9:00 pm

I did a 36AWG 2000 turn winding on my new secondary and the resistance with the toroid and breakout point in place is 460ohms... I also used a wider primary form, to reduce coupling and flashover, and I have 6 turns on a 4" primary form. Check out my videos on youtube in the link in my signature to see some very cool coil-to-coil arcs - which incidentally, also make up my signature banner. =)
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Re: A Slightly Different Version

Postby DJKOR » Fri Dec 27, 2013 10:01 pm

Bugger. Just when I thought I was done with tuning my coil (for fear of breaking it again), I see some more positive progress that makes me kind of want to start tuning mine again.

Those are some good results and measurements you have there. I once managed to get my current draw down to 1.54A at 3/4 power and mid-level frequency on fixed frequency testing, but in it's current state I'm getting about 2.2A to 2.4A at 3/4 power (though I only run to 1/2 power at most as I haven't gotten much of a gain running at higher power).
Image
My build thread: viewtopic.php?f=15&t=998 | 246kHz Primary / 267kHz Secondary
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New Secondary Gives Longer Sparks

Postby mikek8632 » Tue Jan 21, 2014 10:36 pm

In my previous post, using the 34 AWG secondary supplied in the kit, I achieved a maximum spark length of 55 cm with a current consumption of around 1.6 – 1.8 amps at a mains voltage of 243V (3/4 power and minimum frequency). That used a spaced, six turn primary. The current consumption was reducing with increasing top loading, so felt I still hadn’t reached an optimum.

Rather than going to a 5 turn primary or using a smaller primary capacitor, I preferred to lower the operating frequency to get closer to the original oneTesla design which used a 36 AWG secondary. This should increase switching efficiency and reduce heating of the IGBTs.

Because a close spaced, 6 turn primary with a 68nF capacitor is resonant at about 260 kHz, the secondary should resonate around the 280-290 kHz mark to allow for the capacitative loading once a full length spark is established.

With just the toroid as top load, my 34 AWG secondary resonated at 330 kHz. (This was measured by connecting the primary to the high impedance output of a signal generator and maximising the secondary field with an oscilloscope probe about 20 cm from the toroid.) Because inductance is proportional to the square of the turns, and the resonant frequency proportional to the reciprocal of the square root of inductance, the resonant frequency is proportional to the outside diameter of the winding wire. This an approximation to reality, but predicts that a 35 AWG secondary would be resonant at around 285 kHz.

The problem with enamelled wire, particularly of fine gauge, is that its insulation thickness and hence outside diameter, can vary considerably (single build, double build, heavy-coated etc). This directly affects the resonant frequency for a set winding length. As well, the outside diameter is often not specified with any precision on websites and 35 AWG is not as readily available as other gauges. Less than 50g of wire was needed for the secondary, so it was economic to order a 50g spool of 0.14 mm from the UK. This is the metric equivalent to 35 AWG and had polyester insulation with an outside diameter of 0.154 mm. This was a smaller OD than I expected, and using the approximation of direct proportionation, would be resonant at around 275 kHz. However the completed coil resonated at 286 kHz which was near ideal. The coil winding setup I used is shown in the last 3 photos at http://www.smugmug.com/gallery/35635619_bmv7Dd The coil received 6 coats of polyurethane varnish.

After experimenting with different breakout points, it was apparent that smaller was better. The following figures used a 40mm length of 0.2mm tinned wire taped near the top of the toroid and oriented at about 45 degrees to direct the discharge up and away from the primary etc. The mains current being drawn was 1.25 – 1.30 amps at 245V (3/4 power, lowest frequency). The longest spark length measured from a video frame was 60 cm (23.5”) and that jumped a gap of 50 cm.

This is a nice reduction in current consumption with an increase in spark length compared to the 34 AWG secondary, so the exercise was worthwhile. It looks like the secondary resonance may be fractionally too low, but I suspect any further improvements in performance would be small. To check that, I have some small value, high current capacitors which could be paralleled to the 68 nF capacitor to take the primary resonance to slightly lower frequency.
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