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Van de Graaff Question

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DrZoidberg

Wed May 24 2006, 04:13AM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

Looks good on the picture. If you get anything less then 10" sparks out of it you have done something wrong.
If you want to spray the belt the lower roller needs to be metal and it also needs to be grounded and not insulated. The spray comb is located under the lower roller but with enough distance to any grounded metal parts.
To figure out the reason for the small sparks you should try to mesure the short circuit current. You can use a normal multimeter for that but connect a capacitor of e.g. 0.1 uF in parallel to the meter or it won't work. The current should be more than 1uA. If it's much lower there is a problem with the combs or the roller/belt material combination.
If the current is high enough the problem has to be leakage e.g. through dirt or a conductive column or the upper roller is not inside the sphere. Remember that not only the roller but also the comb and all other metal parts need to be as deep as possible inside the sphere.

teravolt

Wed May 24 2006, 07:59PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I bought one of those 200kv VG kits from Science First and they use an aluminuim roler in the top and a soft polyethiline roler on the moter shaft. you may ask them what there belt is made out of. you can use a HV source like a dubbler but it may be nessisary to add a lot of series resistance say 100M ohm to get a spray from your brush. you may not need a lot of voltage like 5-10kv for your source. I think your belt is the culprit. if the materials in the rolers are swaped in a standerd grouned VG it should chang the polerity of the output and still work. anybody can correct me on this but I think that it is about 70kv per inch for sphieroidal VG.

GimpyJoe

Wed May 24 2006, 08:36PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

At school I found a small demo VDG that used a nylon lower pulley and a steel upper pully. The belt was just a big rubber band. Make sure the tube that supports your sphere is very non-conductive and won't absorb moisture. On dry days the little VDG would get surface tracking along its grey PVC tube ten inches long! And also, humidity makes an enormous difference. On a relatively humid day the school VDG would get a weak 1/2" spark, but on really dry winter days it could do up to three inches!

Dr. Drone

Thu May 25 2006, 12:57AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

cbfull

Thu May 25 2006, 02:32AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

That is actually a very nice looking VDG. The top terminal looks as though it was made for a VDG, do you mind telling us where you got the parts for this machine?

I started out having the lower roller attached to the motor shaft in exactly the way you have. The problems I ran into with this setup caused me to turn the rollers 90 degrees and set up a drive pulley system. The tension in the charging belt was making the motor shaft run slightly off-center and it became very difficult to align to keep the belt centered. I also quickly realized that because I was using an induction motor, it could only be operated at one speed, no matter what. The pulley/belt drive arrangement allowed me to optimize belt speed, since I had no idea how it would perform. Not to mention, I can make the belt go faster if I decide to make a new belt out of a much thinner material that I recently found.

If I would have known in the beginning that I was going to end up driving the belt like this, I would have made the bottom enclosure big enough for "beehive" pulleys. Instead I just change the pulleys for whatever speed I want. The only drawbacks are that I have several of those stiff black "V" belts for the different combinations (with a beehive setup you only need one belt), and a belt drive tends to be noisy.

I think you will be very happy with your machine once you have worked out the bugs. I got mine to supply 35 microamps of charge when it was complete.

Tesladownunder

Thu May 25 2006, 06:38AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

The key to running belts centered as I understand is to have the middle of the roller slightly thicker than the outer edges. On the 8cm roller on my treadmill it looks like a "bump" of 2-3mm.

Peter

colin heath

Thu May 25 2006, 05:34PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Hi All,
Thanks for the many great tips

The rollers i machined with a crown so they suffer from no belt tracking problems although i made the top roller also adjustable for this reason. The sphere is one of those gazing ball type things for the garden (homebase in uk). I cut a hole in it then carefully turned the lip inside sphere to prevent leakage. Tried again today when sun came out and could here it striking at 4" but not see it. Although sunny it's been raining for a week until later on this morning so still not low humidity yet. So should i go for metal lower roller if I am going to sparay the belt? or will it work the other way?
Cheers
Colin

cbfull

Thu May 25 2006, 10:40PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

If you want to spray the belt at the lower roller (this is the way it's usually done) then yes, the roller will have to be metal. I have been told that aluminum apparently can cause some sort of particulate to pollute the inside of the machine, as well as the belt. I think this might be after years of frequent use.

Keep in mind that there is a limit to how much charge a belt can carry. I spoke to someone online that has built several VDGs, one of them began as a self-charging and he later switched to spraying. He said that the results were very much the same, so keep that in mind if adding a HV supply is going to be very trying.

Another suggestion is to seal up the unit so it is virtually airtight. Weather stripping and caulking can help to seal. This way, you can keep the inside very, very dry with dessicants. I did this with mine and it was pretty reliable even in high humidity.

ReddyK

Thu May 25 2006, 10:41PM

Registered Member #92 Joined: Thu Feb 09 2006, 03:11PM
Location: Indian Trail, NC
Posts: 39

Hi Colin,
Yes, you want to use a non-insulated metal roller if you are going to spray the belt. You also want to add a second comb on the inside of the belt positioned over the roller and run it to ground.

I would caution you to select a power supply with a current of 100ua or lower and watch that there is not a capacitor on the output of the supply.

If you choose not to spray the belt, additional combs can be added to the inside of the upper and lower rollers to double the current, as you would be sending down positive ions as well as negative ions up the belt.
Hope this helps,
Steve

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