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High power spark gap 3kva

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Vigh holtage

Tue Dec 12 2017, 12:14PM

Registered Member #61739 Joined: Wed Aug 23 2017, 04:43PM
Location:
Posts: 44

Ok so i need to make a spark gap capable of quenching 3kva at at least 200 milliamps it has to quench 3kva without over ionizing or overheating and must be reliable for long runs.

Ok so i have 3 1500watt vacuume motors.... (4) 1/2hp induction motors 3450rpm, and unlimited acsess to 1/8 , 1/4, and 3/8 tungsten carbide bits that have 2in of good shank (i am a machinist and these bits get tosses in a bucket after the job so i cut off the shank portion on the diamond wheel and keep it ) i also have various h vac blower motors from the salvage yard

I own a lathe and mill/drill combo, so i can whip up a lot right at home.

Right now i use a 4 electrode 3 serries gap with an h vac blower mounted in a box and it works great up to a 15,000vac 120ma nst bank but quad mots over heat it badly and it wont quench even with a constant high power blast of air and over heats the tungsten ...

I am down for all suggestions ! Help me out here

Should i use one of my vacuume motors to make a sucker gap out of pvc ???

Ohh and pics of what i have to work with will be posted later, at work now

Blackcurrant

Tue Dec 12 2017, 03:37PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

Try using a horizontal rotary spark gap, propelller type.
the rotating part can act as a fan and help cool it at the same time, add fan blades for extra cooling?
Could you reconfigure your setup to use higher voltage so there would be less current for the same power in? MOT and a voltage multiplier
Easy option would simply be to use more metal to dissipate heat better
PVC melts too easy you can get PTFE tube offcuts on ebay

look at some old navy spark gap transmitters see what they used

Sulaiman

Wed Dec 13 2017, 05:58PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

One thing to consider if your rotary spark gap spins in a horizontal plane is safety,
IF the gap breaks AND the guard does not stop shrapnel then there is no hiding place,
a vertical rotary mainly has one line of fire,
as a machinist I guess that you will have that covered,
the comment is more for anyone else who may want to try a horizontal rotary anything.

Vigh holtage

Thu Dec 14 2017, 02:24PM

Registered Member #61739 Joined: Wed Aug 23 2017, 04:43PM
Location:
Posts: 44

I tried a 4 electrode propeller type with 1 propeller 10in dia, made from a vacuuume motor (asyncronous)

It power arced like a spinning fourth of july fire work and pulsed like a base drum

Couldnt get it to fire reliably at any rpm

Blackcurrant

Thu Dec 14 2017, 05:55PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

Very good point Sulaiman.
My first effort at a rotary gap was like the rest a vertical one but being small would ark over, so with mounting it horizontal it fixed that problem, however while thrashing it it melted and turned into a rotary fuse. Then I use tungsten electrodes and the thing was happy ever after. Other thing of note was two brown jets of gas coming off probably nitrogen dioxide.
I see all the large radio RSG seem to be wheels with the connecting electrodes at the top. All built into very protective housings!

Some thoughts
If it's an AC driven tesla coil then a Spark gap will let the voltage get to max before firing but if you are using less voltage now ie your 4MOT then you would need smaller spark gap which would be more difficult to quench, hence moving to a rotary gap. But this wouldn't give you any bigger sparks as the cap wouldn't charge up as much (max voltage to start with)? I bet the 4MOT charges you're cap up supper fast then just keeps the spark gap going even after firing.
I see from your youtube video that you can pull huge arks with that 4MOT supply. Some gaps use magnets to help break the arc or you could try two rotating parts or put something that would limit the charge rate to the cap so the gap has more chance to extinguish. Basically that huge distance of an ark you can pull needs to be chopped, force air through narrow slit like an air blade?

There has to be older threads with more ideas in. If I were you I would look into a DC powered coil (choke input filter type is a good idea) then you can use any speed of RSG to best effect.

Vigh holtage

Fri Dec 15 2017, 02:30PM

Registered Member #61739 Joined: Wed Aug 23 2017, 04:43PM
Location:
Posts: 44

Ok here is a video of the problem i am having and a (quick wipped up ) not permanate asynchronous spark gap i made for proto typeing the coil out before final touches and finish woodwork and paint. I like to make shure it works and is near good tune before it all goes together nicy nice like , nothing worse than putting all togetjer then having to rip it apart to change something after paint and finish work !

You can see the surging here, what about going to one of the induction motors shown in the video and modding it to be synchronous with a hugr 12in disck. ?

I also have pleanty of tungsten

I hope i explain thing correctly in the vid i have trouble finding the right words .

Vigh holtage

Fri Dec 15 2017, 03:16PM

Registered Member #61739 Joined: Wed Aug 23 2017, 04:43PM
Location:
Posts: 44

The problem with running a charging choke is it doubles the voltage and after rectifacation that puts it up in the 24,000 volt range and i really dont want to have to build a choke to handle that kind of voltage insulating layer after layer of 22g wire ,

However if i have to my local scrap yard has a giant bin of transformers of all sizes from huge street light balasts to old pole transformer cores soo i can do it just not looking in that direction yet. Would like to keep it under or around 15-16kv

It is 8000vac now
Rectified it would be 11500ish
And and after a choke 22kv !!!
With a doubler AND a choke 44,000volts lol too much for me ...
Can a parralel cap bank be used instead of a choke to make it fire correctly

profdc9

Fri Dec 15 2017, 11:05PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

I just built an angle grinder rotary gap, and I was having problems getting sustained firing.

With the voltage you have, 8 kVAC, you have to get the contacts close. I have a similar voltage to you because I am using 2 MOTs with a voltage doubler. I am using stainless for my contacts currently, and the two contacts were relatively flat, which I think is a problem. It helps to have sharper electrodes, but the ends of these wear faster. I now use the end of a screw arcing to the face of a bolt, both 1/4-20 size stainless, and I had to get them about 1-2 mm apart. I put the screw into a coupling nut so I could fine control the position of the end of the screw and get it very close, and then used a jam nut to keep the electrode in place. The coupling nut /jam nut combination seems to hold the end of the screw firmly at the correct distance.

I think that what might be happening is that after the arcing starts, the electrodes get hot, and this tends to stop the arcing. I think it has to be somewhat closer that the maximum distance to arc for the voltage you're using to ensure that the arc occurs at each approach of the contacts.

Dan

Vigh holtage

Fri Dec 15 2017, 11:40PM

Registered Member #61739 Joined: Wed Aug 23 2017, 04:43PM
Location:
Posts: 44

Thank you Dan all you guys help is very much appreciated but now that you mention it I'm just going to go ahead and build the real synchronous spark gap and stop messing around with the Prototype apparently it doesn't work with this much current as far as I can adjust it anyways I would like some advice on which motor I should use here are some pictures of the donor Motors

1/3hp 3450rpm 2 flats needed
3 - 1/2 hp 3450rpm 2 flats needed
And a dual output 1/4hp 1750rpm 4 flats needed

1513381253 61739 FT180967 20171215 182351

1513381253 61739 FT180967 20171215 182439

1513381253 61739 FT180967 20171215 182454

1513381253 61739 FT180967 20171215 182715

1513381253 61739 FT180967 20171215 182847

profdc9

Fri Dec 15 2017, 11:52PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

Using Paschen's law I made the plot attached to the message.

At 760 Torr, the gap size is about 1.5 mm for 8kVDC, which is about what I observed, and about 2.3 mm at 11 kVDC. It is probably somewhat greater if you use a pointy electrode so the field at the cathode surface is higher, but still it means you have to be pretty close.

Dan

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