Van De Graaff Generators & High Voltage Stuff
How does it work? In order to understand the operation of the VDG generator, we need to look at the process of induction charging. It is sometimes called frictional charging, but no friction is actually required for the process to function. It should also be noted that the rollers and the belt are all made of different materials, with as wide a spacing on the Triboelectric series as possible. The following description relates to the MK3 VDG generator shown earlier in this section, although the general process holds true for most VDGs .The lower cylindrical roller is made from PTFE and is directly driven from the drive motor via a gearing system. The charge transfer belt is made from a rubber coated webbing, which runs in contact with both the upper and lower rollers. At the start of the process, the lower roller and belt are in contact and the atoms of the two different materials form covalent bonds. A covalent bond could be described as a chemical bond, which is formed by two or more atoms sharing pairs of electrons. As the lower roller and the belt rotate, the two surfaces separate. The belt and roller taking their equal and opposite charges with them. The PTFE lower roller acquiring a negative charge in the process.
In close proximity to (but not touching) the lower roller and belt is the lower charge comb, sometimes misleadingly referred to as a brush. The comb comprises a number of sharp needle tips, which are electrically connected to earth. When the needle tips are in proximity to the roller and its negative charge, any air molecules, which are nearby, are subjected to very high voltage gradients and the electrons and positive nuclei are torn apart forming a plasma. This plasma can often be seen as a blue corona around the tips of the comb. The positively charged nuclei are attracted towards the negatively charged lower roller, but cannot reach it because the belt is positioned in front of the roller. These positively charged nuclei coat the belt surface and are carried by the belt towards the upper roller. Meanwhile, the free electrons are attracted to the comb and flow back to earth.
As the positively charged nuclei are carried up the belt towards the upper roller, they approach the upper comb, which is connected, to the top terminal or sphere. The upper roller is made of aluminium, which is toward the positive end of the Triboelectric Series. As the upper roller rotates, it acquires a strong positive charge. As the positively charged nuclei being carried on the belt come into the proximity of the upper comb, air molecules once again are subjected to intense voltage gradients, which separate the electrons from the nuclei. The positively charged nuclei along with those carried on the belt, are repelled from the upper roller and travel to the needle points at the comb.From there they are conducted to the inside of the top terminal or sphere. The sphere being a hollow object, acts as a Faraday pail and all the accumulated charge collects on the outer surface. As far as the charges on the upper comb are concerned, the inside of the sphere is effectively ground regardless of how much charge exists on the outer surface. Thus the sphere is charged to a positive voltage. Meanwhile the free electrons from the air molecules are attracted towards the positive charge on the upper roller. In the same way as the lower roller these electrons cannot get to the roller surface and instead coat the belt surface with a net negative charge. This negative charge eventually travels to earth as the belt surface reaches the lower comb again.

Last update 10/02/03