Hi Steve,
Actually, I can’t take the credit for originating the idea of the shaft collars. I mentioned the blistering problem to this illustrious list of coilers and one of its other resident members came up with that suggestion. I cannot recall who it was at this moment, but I’m sure a review of the archives under the subject matter “G-10 blistering” could pull it up.
I am running considerably larger electrodes than you (3/8” diameter flying electrodes and 1/2” diameter stationary electrodes), but then again, I’m running with a considerable higher power ceiling than your 4.8 kVA, too. I am also using the tungsten rod pieces only and did not drill out brass bolts length wise to place the flying electrodes in. I can run over 15 kVA with my system, so I wouldn’t have considered going any smaller than what I have with my spark gap electrodes. I have not experienced any unreasonable electrode erosion with my setup, either. The stationary electrodes do tend to get a bit of erosion on one side over long term operation, but nothing unmanageable.
David
Sent from my iPhone
> On Jul 3, 2018, at 10:22 AM, Steve White <steve.white1@xxxxxxxxx> wrote:
>
> Hi David,
>
> You are correct about the machinability of the aluminum nitride sheet. It is a bear as would be expected because it is a ceramic. Fortunately it comes in just the right size (1.5" x 1.5"). I only had to drill a 3/16" hole in each one which is the size of my stationary electrodes. I used a rotary die grinder held in a custom holder. Running the die grinder at 25,000 RPM with a carbide cutter gets the hole drilled reasonably easy. You have to be really careful because the ceramic is extremely brittle. I glued the heat shields to the auxiliary heat sinks using exhaust manifold crack filler. My stationary electrodes are mounted on large aluminum posts approximately 1.5" x 1.5" x 7" so I have a good thermal mass. The problem was that the face of my mounting post is about 2" from the firing point. This leaves a long 2" of 3/16" diameter tungsten to conduct the heat from the firing point to the mounting post. So I augmented this with the small auxiliary heat sinks with the attached heat shields to get the heat conduction distance of the tungsten rod down to 1/2". The auxiliary heat sink mounts between the firing point and the mounting post. The heat sink is in contact with the mounting post using heat sink grease for good thermal conductivity. This seems to have solved the problem of excessive stationary electrode erosion.
>
> I was very concerned about having the flying electrode just sitting in a hole in the G10 rotor held in place by only a set screw as I have seen on some RSGs. So I did something different. My flying electrodes are mounted on the G10 rotor using brass bolts, washers and nuts. I drilled a 1/8" hole through the center of each bolt to hold the 1/8" tungsten electrode. I drilled and tapped 2 of the 6 bolt head faces and put set screws in them. The set screws hold the tungsten rod in place in the bolt. I then use a brass nut to tighten down and hold the entire flying electrode assembly in place. I believe the brass bolt holders give me much better cooling and prevent the blistering that you have seen on your G10. My other thinking was that the larger diameter brass bolts would be conducting most of the current instead of the tungsten. I have seen no noticeable erosion on my flying electrodes at 4.8 KVA even though they are only 1/8". I have been thinking about increasing the air gap on my ballast to allow me to run at 6 KVA, so I wonder if my 1/8" flying electrodes will still hold up. We will see.
>
> I like your aluminum shaft collar set up idea. If I had thought of that at the time, I may have used that method instead of the drilled-out brass bolts. I am sure there is much less rotating mass to balance with the aluminum shaft collars than with the brass bolts.
>
> Steve
>
> ----- Original Message -----
> From: "David Rieben" <drieben@xxxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Tuesday, July 3, 2018 6:27:28 AM
> Subject: Re: [TCML] Re Please review my coil plan
>
> Hi Steve,
>
> You bring up some good points about trying to utilize multiple pairs of stationary electrodes in a RSG. I have also tried the two pair of two (4) stationary electrodes and have run into the same issue of very critical spacing to get reliable firing. I had tried this approach of two pair of two stationary electrodes set 180 degrees apart with the thought of better quenching, but found that there was no noticeable improvement of the output. If anything, it seems that the output may have been somewhat diminished, maybe due to the increased gap losses from twice the spark gap bridges.? I have read somewhere that it is possible to “over quench” with multiple spark gap commutation, where the gap losses overcome the acquired benefits of the increase quenching.
>
> While I personally did not notice an appreciable increase in stationary electrode erosion, I DID notice increased heating issues of my (8) flying electrodes, since they were now each firing on a 200% duty cycle from their previous (2) stationary electrode setup. Even though they were spinning at around 3000 rpm, the 3/8” x 1.75” long tungsten slugs were getting hot enough that they were actually beginning to “blister” the G-10 material of the disc they were mounted in. I ended up ditching the (4) stationary electrode setup, returning to the single pair stationary electrode setup, as well as purchasing (8) 3/8” thick x 3/8” diameter (estimated 1” outside diameter) adjustable aluminum shaft collars (McMaster-Carr) to place on each rotary electrode on each side of the disc for increased thermal dissipation. They also proved an excellent lock down mechanism of the flying electrodes to prevent their slipping inside their respective holes in the disc, which can potentially become a real issue with electrode crash. I have had no further overheating problems with the rotary electrodes since taking these measures.
>
> I have not considered the ceramic heat shield of the stationary electrode support blocks. I have just assumed with my setup that the thermal mass of the brass block supports, in addition to the natural air circulation caused by the nearby flying electrodes would suffice for addressing this thermal load. Of course, as I mentioned above, the natural air currents were not able to address the thermal load of the flying electrodes when set up with the 4 stationary electrode setup! I would have to assume that these aluminum nitride ceramic sheets are NOT easily machinable, as in drilling a hole through them for the passage of the stationary electrode to pass through?
>
> David
>
> Sent from my iPhone
>
>> On Jul 2, 2018, at 11:28 PM, Steve White <steve.white1@xxxxxxxxx> wrote:
>>
>> Jan,
>>
>> There are a couple of potential problems with using double stationary electrodes (4 gaps to fire simultaneously instead of 2) if I am understanding you correctly.
>>
>> The main problem with 4 gaps is that you have to get the gap spacing twice as close as with 2 gaps for the same firing voltage. I made my SRSG with the ability to use either 2 or 4 stationary gaps. When I tried it with 4 gaps, I found it very difficult to get consistent firing even though I am using a 14.4 KV pole pig. I eventually went to 2 gaps and get good consistent firing. The usual reason for using 4 stationary gaps instead of 2 is to decrease the quenching time however, given the diameter of the large tungsten electrodes which would be used for a high-power coil, the impact of this is negligible. I would just stay with 2 stationary gaps. You can try it with your stationary electrodes and see if the erosion is acceptable. If not, you can make the auxiliary heat sinks with ceramic heat shields for the stationary electrodes as I described earlier. I don't have any problem with flying electrode erosion with a rotor speed of 3600 RPM due to the excellent air flow. If your r
>> otor speed is high enough, I don't think you will have a problem with flying electrode erosion. Since you are contemplating a ARSG you will have to experiment with the speed.
>>
>> Another potential problem with having 4 stationary gaps is uneven electrode erosion. If the stationary electrodes wear unevenly, then this make it even harder maintain a very small gap spacing. This uneven wear also becomes a problem as you increase the number of flying electrodes if they wear unevenly. If the wear is uneven, the electrodes can collide if you try to adjust some electrodes to compensate.
>>
>> One thing to remember is that the more flying electrodes that you have, the harder it will be to achieve precise electrode hole placement and to balance the rotor. As you probably know, the rotor with the flying electrodes needs to be made to a very high level of precision and balanced very precisely or else it will shake your coil apart or worse. I see from your transformer picture that you have a lathe. That will help a lot. With a lot of careful thought, I was able to make my rotor with just a bandsaw, a drill press, and a rotary die grinder. At the time, I didn't have a lathe or a milling machine, which I now have.
>>
>> The conclusion, as I see it, is to use as few stationary and flying electrodes as possible. This minimizes the uneven erosion problem and makes your gap maintenance easier.
>>
>> The G10 (FR4) rotor material that you are contemplating is perfect.
>>
>> Steve
>>
>> ----- Original Message -----
>> From: "Jan Ohlsson" <jan@xxxxxxxx>
>> To: tesla@xxxxxxxxxx
>> Sent: Monday, July 2, 2018 8:04:10 AM
>> Subject: [TCML] Re Please review my coil plan
>>
>> Hi Kurt,
>> Your big coil is really impressive and it was not my intention to be
>> disrespectful to a pioneer like mr Burnett. A calculation method that makes
>> it possible to establish a good balance between BPS, ballast inductor and
>> capacitor is of course of great value, regardless of if the charging system
>> really is resonant in the true sense or not.
>>
>> And thanks David,
>> You and others have certainly convinced me of the importance of varnish...
>> Yes, the bomb shelter is a great place, but I will be dependent on a
>> counterpoise type ground, and I have gotten the impression that this concept
>> is not that much tested for tesla use.
>>
>> I was planning on an epoxy/fiberglass disk for the rotary, slightly larger
>> in diameter (40 cm), in the hope that this would cool the electrodes better.
>> But from ypur experiences with your high powered coil, I realize that
>> electrode erosion might be a bigger problem then I expected. I will try to
>> achieve as much cooling as possible for the stationary electrodes, and will
>> try to locate larger diameter rods. Perhaps double sets of stationary
>> electrodes would also share the load and keep up better?
>>
>> All assuming that I first succeed with the transformer and ballast, of
>> course...
>>
>> Kind regards,
>> Jan
>>
>>
>>
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