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Spark gap electrodes?

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dmg

Sat Apr 17 2010, 12:42AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

hello,
I need some advice on what metal should I use for a spark gap's electrodes (rotary gap), since the gap will be handling high power, electrodes tend to erode quickly. So I wondered what metal would one use for the electrodes, both "flying" and static, since tungsten is a good metal, however expensive, while on the other hand, steel is cheaper, but doesn't last as long as tungsten, but is easly replaced and can be bought for pretty cheap, and is also more readily available.

So does anyone have any advice on what metal should be used?

tobias

Sat Apr 17 2010, 01:06AM

Registered Member #1956 Joined: Wed Feb 04 2009, 01:22PM
Location: Jersey City
Posts: 172

I would choose tungsten. You can buy it in a small bar of 3,18 mm diameter (1/8") and ~70 mm long on a hobby or professional welding store. Its used as plasma arc starter on welding/cutting.
I used that material for plasma speaker electrode and it worked pretty well!

Arcstarter

Sat Apr 17 2010, 01:25AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

If shipping is not insane, maybe i could ship over a few small bars of tungsten in an envelope or something. Also i have some tungsten tipped electrodes with heatsink mounts, originally used in a spark gap style arc starter.

dmg

Sat Apr 17 2010, 02:35AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

Very generous offer arcstarter, but however I will have to decline that, sorry.
I do have both style of rods, but I just wondered what will be more relaible in the long run, since tungsten requires little to no replacing, but rods are expensive, and steel requires alot of replacing but is cheap.

also, say I deside to use tungsten, anyone know a good method to secure them? I had an idea of taking a threaded bolt and puting it in the center of the shaft (I am using helicopter RSG) and the just tighten that up.
so any more ideas are welcome

Herr Zapp

Sat Apr 17 2010, 02:41AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

gatedbreakdown -

Please define "high power", or specify what type ,of power supply you are using.

For 120ma or higher current power supplies, 3/16" or 1/4" dia pure tungsten rod works well as electrode material, and can be purchased at reasonable cost on eBay. It is a bear to cut accurately into short sections, so if you choose this route you may want the supplier to cut it into the required length.

I've also seen tungsten/copper EDM electrode material (powder metal composite) used, although in theory it should have a higher erosion rate than pure tungsten.

Brass or pure copper can also be used, although these materials will definitely erode faster than tungsten, but are inexpensive and can be machined easily.

Whatever stationary electrode material you use, mount them in large copper, aluminum or brass electrode holders as heat sinks, since the stationary electrodes will run considerably hotter tha the flying electrodes.

Regards,
Herr Zapp

Regards,
Herr zapp

dmg

Sat Apr 17 2010, 02:51AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

Herr Zapp,
I am going to use a supply that outputs around 10KV at 700-800 mA (perhaps lower? we will see), that will be rectified and fed into a DC resonant charging circuit.

so I think I will use 1/4 inch tungsten, or If im in a good mood Ill go get some 1/2 inch
I will probably have to see when I actually run the gap, and figure it out that way, but I am sure these thicknesses are good enough
originally I was planning on using fiberglass rods to support the stationary electrodes, but I think I will use brass

Thank you for your help everyone...

(EDIT: the 8-9kW supply might not be final, it most likely will be limited to 6kW due to my service limitations, I haven't made up my mind yet about my supply's design)

Herr Zapp

Sat Apr 17 2010, 06:27PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

gbd -

Before you get carried away with large diameter tungsten rod, review the following:

1. Even in a "helicopter gap", make sure that your drive motor (salient-pole, I assume) is powerful enough to accelerate and achieve synchronous speed with the large mass of rotating tungsten.

2. Check the electrode "dwell-time" based on the diameter of the rotating and stationary electrodes, the motor speed, and the radius of the gap location.

Regards,
Herr Zapp

dmg

Sat Apr 17 2010, 07:02PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627

I will take all those into consideration Herr Zapp, but one question,

what did you mean by;

"1. Even in a "helicopter gap", make sure that your drive motor (salient-pole, I assume) is powerful enough to accelerate and achieve synchronous speed with the large mass of rotating tungsten."

does that mean that the speed of the gap is constant, and doesn't fall or rise? or where you refering to the fact that the motor needs to have enough torque to turn such a load?

also, this gap is asynchronous, and therefore varaible speed.

radiotech

Sun Apr 18 2010, 05:58AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Why dont you buy one of these? Only $14.00 They used Zink. This gap is rated at 1000 Watts.

Also, do you think the spark energy will increase or decrease the torque loading of your rotary gap?

dmg

Sun Apr 18 2010, 06:19AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627

radiotech wrote ...

Also, do you think the spark energy will increase or decrease the torque loading of your rotary gap?

well the gap as Ive designed it, I dont think it will make any significant effects on torque, as long as the electrodes dont colide or something, this might have an impact if I were to use a gap with magnetized electrodes, or ring shaped mangets around the electrodes to provide perhaps magnetic quenching, however I dont see how that will effect the torque of the gap, since not all electrode materials are magnetic.

so I do not really understand what you said radiotech, can you explain further?

also, what year is that image from? finding something like that for 12 dollars seems a bit far fetched now days.
and one killowatt is unforchunatly too low of a rating.

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