1 MegaVolt, Dual Belt Van de Graaf (VDG) Generator

This bad-boy is a hair-raiser.   I contacted Fred Graff at Physics Playground about his Van de Graaff generators (I saw them on Ebay) for some high-voltage experiments.  I asked him if he had any higher-voltage devices than the 700kV Van de Graaf generators he has been selling, and he said he was thinking about building one that was around a mega-volt.   I told him that if he’d build it, I’d buy it, and promptly sent him a deposit.     Since he builds these in his spare time he let me know that it might be a couple of months, especially since the sphere is very large (it turned out to be 27 inches in diameter) and has a long lead-time.    So, at the appointed time, the VDG arrived in three large boxes; all in very good order.  Fred told me that one of his shipping secrets is to insure the boxes for at least a thousand dollars each, which causes them to cross some mystical super-special-handling threshold.  Here’s how the components arrived:

1 MV VDG components

After slapping on the casters, I bolted the tube-mount platform to the top of the base, which contains the motors, lower rollers (felt covered), and electron combs (seen through the center hole).   Fred uses wing-nuts for pretty much everything that comes apart to make assembly and dis-assembly very straight-forward.     I also asked him to make the support tube from 1/4″ acrylic instead of his usual 1/8″ stock to give the unit more strength.    That added about $150 to the price:

So, after bolting down the acrylic tube, I washed the belts & rollers, then installed the first belt.  The top rollers seem to be made of Vinyl. This VDG has two motors and two belts.  Each belt is 5.5″ wide.  To install the lower rollers, the motor belt “pulley” must be removed, then re-installed after insertion (I did the same for the combs to prevent them from snagging/tearing the latex belts).  The pulley has to then be adjusted in/out so the motor drive belt stays on while running (tweak, tweak).  The image on the right shows more detail about the sphere mounting hardware.   Even with only one belt installed, and no sphere, I was getting 1″ sparks off the aluminium straps:

Here’s the base with both belts & roller sets installed.   The latex belts smell strongly like chocolate candy!  The motor speed controller is the red box on the left.   Running the motors  faster doesn’t increase the voltage, just the charging rate.   The only thing I found missing was a ground-wire connecting the second brush/comb in the base.   Adding one made a big difference in the rate-of-charge (as expected).

After slapping on the smoked acrylic sides (Velcro attachments for easy access), and slipping on the top-sphere, here’s how it looks (standing 7 feet tall):

Fred has put quite a bit of thought into his units.   Notice the cut-out in the acrylic tube for belt access (installation, cleaning and maintenance).  If anything, the VDG is over-engineered (I can handle that).   This unit should be good for many years of use.   Fred includes the discharge wand — believe me, it’s needed!   After dropping on the sphere, with no cleaning, I was able to pull sparks over a foot long in the living room, and with a bit of tweaking I was able to get sparks over 27″ long before they started jumping into my ceiling.

The discharge wand can also be used as a sort of “voltage regulator”;  I taped it to a light stand, and placed it close to the sphere, so the electrical potential (voltage) can only increase until it jumps (sparks) the distance, thus limiting the sphere voltage to that level.   Closer equals lower voltage (it’s still high, so don’t be fooled!)

I really enjoyed working with Fred; he’s been very helpful, enthusiastic, and easy/fun to get to know.

Usual disclaimers:  Don’t use a VDG if you have a pacemaker or other biological implant.   These devices are not toys and can generate lethal voltages.   Operation of a VDG will create Ozone and other potentially dangerous gases, including, but not limited to NOx – the major (brown) component of air pollution.  Use only with adequate ventilation.

Note: A sphere of 27″ in diameter has a capacitance of roughly 38.15 picofarads, with a maximum sustainable electric field of just over a million volts.   Its energy storage is given as E = 0.5 * CV^2  = 19.1 Joules.   Energy discharges greater than about 5 Joules are considered very dangerous.   These are theoretical maximums, and are less in practice due to sphere imperfections, dirt/dust/humidity…

Another great link on VDG’s & other HV devices is Bill Beaty’s site.    His site also contains a lot of other interesting and useful information.