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Small Van de Graaff Generator
Article Number: 4 | Rating: Unrated | Last Updated: Thu, Jun 14, 2012 at 5:46 AM
Small Van de Graff Generator
The performance of a Van de Graaff generator depends heavily on the local conditions—humidity and temperature—and on the cleanliness of the generator and its surroundings. The electric field of the generator attracts dust particles—much like a television set—and these quickly create leakage paths to ground by corona discharge and conduction, greatly reducing the performance of the generator. Keeping the generator scrupulously clean will greatly enhance its performance.
The most common problem conditions are:
· Weak sparks
· No charge developed (especially if the generator was working perfectly yesterday/last week/last year)
Weak sparks - The machine runs OK, but only produces corona discharges or small, weak sparks.
This is usually due to accumulation of dust on the belt, insulating column and dome, or, in humid conditions, to adsorption of moisture in the same areas. These create leakage paths to ground for the charges, most of which leak away before reaching the dome or are dissipated as corona discharges from dust particles on the dome acting as tiny "needles.”
- Sometimes, running the generator for a longer period (about 10 minutes) will "burn off” the dust and cure the problem. If not:
- Clean the outside of the dome and the outside of the column with a clean, dry cloth.
- In humid conditions, or if humid conditions have existed since the last use, remove the dome and blow warm air down the column with a hairdryer. This will often blow off the dust and evaporate the adsorbed moisture. Warm up the dome in the same way.
- Inspect the belt and the inside of the column for accumulated dirt. If necessary, clean the belt as described below. Clean the inside of the column with a dry piece of soft cloth attached to a stiff wire. Be careful to avoid scratching the column.
No charge developed - The machine runs okay, but there are no signs of electrostatic activity. This can be just an extreme form of the "weak sparks” problem, which can be cured by the same measures. If these do not succeed, further problem issues may be a dirty or damp lower pulley which prevents any charge separation taking place, misadjusted combs, or a worn belt.
- Lower pulley problems can often be solved by removing the belt as described below and blowing warm air down the column with a hairdryer. If this does not work, the bottom cover should be removed and the pulley cleaned from below. DISCONNECT THE GENERATOR FROM THE POWER OUTLET BEFORE REMOVING THE LOWER COVER! Also clean the belt to prevent recontamination of the clean pulley.
- The combs are adjusted at the factory, and usually will not need attention. Visually inspect the machine to see that the combs are about 1/16” away from the belt. If they are misadjusted, follow the adjustment procedure below.
- After some years of use, the belt may become stretched and/or brittle with age. A loose belt or a degraded surface leads to inefficient charge transport, and eventually to no transport at all. In this case, the belt must be replaced. A new belt can be obtained from your distributor.
NOTE: If you acquire a spare belt at the time you purchase the machine, be sure to store it in a dry place away from sunlight or chemical vapors, or it may be as degraded as the old belt when you need to use it.
Maintenance & Storage:
If the Van de Graaff generator will not be used for some time, storing the machine as follows will help prevent problems on the next use.
· Store the machine in a dry area where it will not be exposed to moisture or chemical vapors which could attack the rubber belt material and/or degrade the surface of the insulating column.
· Cover the machine with a cloth or plastic bag to reduce the accumulation of dust and the adsorption of moisture on the column.
· Store the machine away from direct sunlight, which will cause the rubber of the belt to become brittle.
For safety, maintenance work on the Van de Graaff generator should only be performed when the machine is unplugged from the power outlet and fully discharged.
Basic maintenance procedures include belt cleaning/replacement and comb adjustment.
1. Remove the dome from the Van de Graaff generator.
2. Next, remove two screws from the upper comb and pivot the comb to expose the upper portion of the belt.
3. Pull up on the belt and remove the upper pulley sideways.
4. The belt can now be slid off the lower pulley and removed from the top of the unit through the column.
1. Draw a 24” piece of thick string through the belt. DO NOT PIERCE THE BELT, just loop the string through the belt.
2. Carefully lower the belt into the tube, using the string, so as not to drop the belt.
3. Re-attach the belt to the lower pulley.
4. Pull the string to expose the other end of the belt.
5. Slide the upper pulley into the loop of the belt and reposition the pulley axle in the notches on the top of the column. Make sure that the belt is not twisted inside the tube.
6. Rotate the upper comb back into position and reattach the two screws.
7. Check that the upper comb is about 1/16” away from the belt and tighten the screws.
8. Rotate the upper part of the belt by hand to center the belt on the pulleys and ensure that the belt does not catch on the combs.
1. Follow the above instructions for belt removal.
2. To clean the belt, wash it with a solution of a few drops of dishwashing detergent in water. Rinse with clean water and allow the belt to air dry completely before reassembling the unit.
3. Replace belt following the above instructions.
If the belt is clean, humidity is low, and Van de Graaff operation is substandard, the comb/
belt spacing may need adjustment. Factory settings for the distance between the belt and the comb are 1/16”. To adjust the comb settings:
1. Remove the dome.
2. Loosen the four comb retaining screws and adjust the upper comb to desired spacing.
3. Tighten the screws. DO NOT OVER-TIGHTEN THE SCREWS—there is a risk of stripping the threads in the plastic insulating column. Replace the dome.
4. Remove the base cover.
5. Loosen the lower comb retaining screws and set the appropriate spacing.
6. Tighten the screws and replace the base cover.
If fuse replacement is necessary, the fuse is a 5 x 20 mm cartridge fuse - 250 V, 1 A, time delay (slow blow).
Dome diameter: 21 cm
Base diameter: 20.5 cm
Column length: 21.5 cm
Column diameter: 4.9 cm
Belt dimensions: flat length: 33.5 cm width: 1.8 cm thickness: 1.75 mm
Power requirement: 110VAC/60Hz, 26 W
Electrostatic performance (depends on conditions)
Voltage: = 200 kV, Max. current: 3.0 mA.
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