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Dr. Brownout

Wed Mar 21 2012, 12:55AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

This is a pretty big VDG, and almost done with it. I need to add a contactor because of the motor inrush. But other than that it works quite well (Edit: I should add this is not a normal VDG this is a powered VDG, the belt is actually sprayed with a rectified 9KV NST, whie it is running you can see the corona being emitted from the bottom screen. From almost 4 feet away you can feel the charge from this. Develops a massive strike to a grounded discharge sphere.

Antonio

Sat Mar 24 2012, 06:13PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

What is the material of the support tube?

Thomas W

Sat Mar 24 2012, 06:57PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

looks like fiberglass

Dr. Brownout

Sun Mar 25 2012, 04:38AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

Yes the big housing is fiberglass. The belt is 13.5" wide and 88" in length. The brushes are made from brass screen, there are four of them, two on the bottom and two on the top.

The transformer is 9KV NST rectified which is what is spraying the belt. The discharge terminal is 7" X 34".

This performs very well, even during those high humid days it performs good. My longest discharge is through a 4' acrylic tube to grounded sphere on the other end of the tube. It took it a minute or two to charge up enough to shoot down the tube but it did it.

I still need to add a contactor to turn the motor on, because the inrush of the belt motor is killing my switch, I have the contactor I just have not installed it yet.

I also have a 27" aluminum dome I am going to fabricate to fit on the VDG, I think the charge should be even better with the dome rather than the toroid I am using, then I can free up the toroid and use it on a coil

Dr. Brownout

Sun Mar 25 2012, 05:01AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

Here is a closer picture showing the top of the VDG, everything on top is mounted with a 1/2" phenolic. I will take some other photos and post them of the brushes, I have two on the bottom, one of the brushes sets at 90 deg to the roller and the second sits at around 280 deg of the roller. I played around wth the brush placement found it seems max discharge is when one of the bottom brushes is offset from BDC of the roller by about 10 to 20 degrees, I am not sure why but it does.

Each of the rollers are chrome and have a slight crown to each of them.

Antonio

Sun Mar 25 2012, 12:20PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Interesting to see a toroid used as terminal. Did you close the upper side of the hole? Because with the hole open the upper brush assembly is exposed and may leak charges to the air. A sphere, or just a half-sphere over the toroid, would provide better shielding and higher breakdown voltage. You don't need a powerful motor in a VDG. The required torque is insignificant.

Dr. Brownout

Sun Mar 25 2012, 01:33PM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

Antonio,
Thanks for the input, the dome I am going to use is a half sphere over a toroid, the belt motor is the only one I had so that is why the motor is so big. If you look closely the brush right now is way below the toroid, it is actually mounted to the phenolic mounting plate so there is a big gap inbetween the discharge terminal and the brush so I am sure there is plenty of leakage right now. That will be corrected when I replace the existing toroid with the dome.

The belt actually turns fairly easy the pillow block bearings keep the rollers turning very easy. There is a pillow block on top and bottom. So I could use a much smaller motor.

Even with all that leakage this thing charges up pretty darn good.

Newton Brawn

Sat Mar 31 2012, 02:09AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi Brownout!

Id like to see a dwg showing the construction ...

regards

Newton

Dr. Brownout

Sun Apr 01 2012, 04:48PM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

Hi Newton,
I can put some drawings together if you like. Its basically a rectified NST spraying the belt at the bottom, a couple of pillow blocks with bearings for the rollers mounted on phenolic. Not a whole lot to it. Playing with the brushes was biggest challenge as I found that not only the distance from the belt makes a difference but location with reference to the roller made a difference. I am not sure what kind of performance I would get if I used just one brush at the bottom. Right now I have two brushes or combs at the bottom, one is about 90 degress from TDC of the roller and the other is at about 280 degrees from TDC. It seems to work the best in that arrangement although I am not sure why just yet.

--Brian

Finn Hammer

Sun Apr 01 2012, 09:24PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Dr. Brown

You should have 1 comb at the bottom, and 2 at the top. This will enable you to double the charging current of the machine.
Study these 2 pictures from a VDG I have just finished, and you will get the picture:

The charge collector is situated just before the belt reaches the top roller, and it extracts the charge from the belt and delivers it to the top electrode via a 120Mohm resistor. It is also connected to the roller via a ball bearing ball and bronze contact. The charging current, in this case 40ÂµA creates a voltage drop along the 120Mohm resistor of 4800V, so the top electrode is 4800V lower than the roller. The top spray electrode is connected to the terminal, and it is the difference in potential between this spray electrode and the roller that forces this comb to spray charge to the belt on the down run, effectively doubling the charging current to 80ÂµA.
This is the classic VDG top terminal layout. It is a good idea to limit the current to the spray terminals with a high value resistor, since it reduces the risk of arcs to the belt, which destroy it. Always verify the machine operating in pitch darkness. Put out the light and allow a couple of minutes to let your eyes adjust. Seen in the dark, a faint corona glow on the tip of the combs is enough to spray the belt, and you will see excess charge leaving the belt to the roller just where it leaves the roller.
The spray terminals can sit half an inch from the belt without any bad effects, but it is my impression that to get a stable output, the charge collector should be no more than 1/16 inch from the belt.
Hope this helps.

Cheers, Finn Hammer

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