Van de Graaff Generator

March 21st, 2009        Mark Rehorst

When I was a kid I had a very small Van de Graaff generator (VDG) and it was a lot of fun.  I always wanted a larger one but never got around to making one until Alex told me about someone bringing one to school and demonstrating it in his class.  Then I was at an Ikea store and saw those bowls and the decision was made for me - it was time to build one!

Click small images to see larger, more detailed images.

Van de Graaff generator

I originally built the generator with a positively charged sphere, as shown in the photos and plans on this page.  I have since swapped the rollers so the sphere is negatively charged and find it gives longer, more impressive sparks.  Now the VDG throws off painful, blue, lightning-like 10" sparks to my fingers within a few seconds of plugging it in. It produces such sparks every second or so.  If you want that sort of performance I recommend that you build your generator with the teflon roller up in the sphere.

How VDGs work:

A VDG works on the principle of triboelectricity.  Triboelectricity is what you experience when you rub a balloon on your head and your hair stands up.  When two different materials come into contact and are then separated, some electrons jump off one material and are acquired by the other.  That puts a positive charge on the first material and a negative charge on the other.  A VDG merely automates this process.  

A VDG has two rollers and a belt with a motor to drive it all.  The rollers are typically made from plastic and the belt from rubber.  As the rollers turn the belt is moved along.  As it moves away from contact with a roller, the belt acquires one charge and the roller the opposite charge.  Brushes are used to concentrate the charge on the belt at one end and to pick it off at the other end, inside a metal sphere.  There are a lot of web sites that will tell you all about how VDGs work, in more detail than I want to get into.  Check any of these sites:

http://www.howstuffworks.com/vdg.htm
http://www.eskimo.com/~billb/emotor/vdg.html
http://en.wikipedia.org/wiki/Van_de_Graaff_generator
http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/vandeg.html

More info than you can digest in a lifetime:
http://www.coe.ufrj.br/~acmq/electrostatic.html

Lots of practical, DIY info and help for the asking here:
http://tech.groups.yahoo.com/group/VanDeGraaffGenerator/

Here's a brief biography of the inventor:
http://tvdg10.phy.bnl.gov/vandegraaff.html

Here is Van de Graaff's original patent on the machine:
Van de Graaff patent 1991236

Building a VDG:

There are really only two tricky parts when it comes to making a VDG.  You need a metal sphere for the top electrode and a rubber belt.  You can use a motor or a hand crank to turn rollers/belt.  The sphere came to me in the form of a pair of steel salad bowls from Ikea for about $6 each.  The rubber belt is made from a "resistance band" from a Sports Authority store.  I used 4" PVC pipe for the main structural tube and a flange to hold it to the wood base.

Clicking the image below will get you a .pdf file that has sort-of detailed construction drawings in case you're interested in trying to make a similar device.  Right click to save the file to your computer.

Construction Drawings

Making rollers:

The rollers are made of 1 1/2" PVC pipe with end caps.  I drilled holes in the end caps then put a threaded rod through the roller.  I used nuts to center the roller on the rod.  Skate wheel bearings allow the rollers to turn smoothly.  I soaked the bearings in degreaser and flushed them out before using them- a little grease in the wrong place will kill the output from a VDG.  The bearings are retained on the threaded rod with more nuts and then they are retained in the side walls of the pipe with sheet metal screws.  The generator turns smoothly and quietly.  OK, well, the word "quietly" is stretching the truth a bit, but it isn't nearly as loud or unpleasant as a vacuum cleaner.

roller under construction

Putting humps in the center of the rollers prevents the belt from slipping off.  That is done by wrapping each roller with duct or masking tape, letting the tape pile up thickest at the centers of the rollers.  Roller surface materials are chosen from a triboelectric series - Teflon for one and aluminum for the other.  Don't worry, the rollers don't have to be made of those materials, only a thin layer on the surfaces of the rollers.  I used plumber's Teflon tape for the bottom roller and aluminum tape for the top roller.  (NOTE- this makes the sphere acquire positive charge- swap the rollers for negative charge)   The belt rubber (latex) is between aluminum and Teflon on the triboelectric series so I get extra fast charging of the VDG.  You can use any materials you want to make your VDG.  Using the triboelectric series table will help you select those that provide the highest output current.

Teflon is negative
relative to the rubber belt in the triboelectric series so the teflon roller becomes negatively charged and the belt carries positive charge (by becoming electron deficient) up to the sphere.  Aluminum is positive relative to the belt so the belt carries negative charge (electrons) downward from the sphere.  If the roller locations were switched the generator polarity would be reversed.  The positively charged, upward moving part of the belt passes close to the negatively charged, downward moving part of the belt.  If your belt tension isn't high enough the two parts of the belt may touch each other due to the electrostatic attraction.  That would cause all sorts of problems for your generator.  This sort of problem can be overcome by increasing the belt tension, using bigger rollers to move the two side of the belt farther apart, or using more rollers to separate the sides of the belt further.  I think you could also fit a piece of plastic inside the pipe to isolate the two sides of the belt.

It's a bit of work to mess with all the skate bearings.  I think it would be easier to use a couple bicycle wheel hubs for the rollers.  Then the axle wouldn't have to rotate with the roller.  You could use a pair of small pulleys with an O-ring as a belt to couple the motor to the bottom roller.

    top roller, brush, belt   bottom roller, belt, and brush   top roller, brush, belt, bowl    

The sphere:

The size of the sphere (and its smoothness) ultimately determine the maximum voltage you can get from a VDG.  If you want super high voltage you need a really big sphere.  Ikea sells hemispherical steel salad bowls in different sizes for very low cost.  A pair of these bowls makes a nearly perfect sphere.  I used a fiber reinforced cut-off wheel on a Dremel tool to cut a hole in the bottom of one bowl so it would fit over the 4" PVC pipe.  The hole is a little over-sized so that a piece of slit urethane tubing would fit on the sharp edge and hold the bowl in position on the pipe.  It is important to keep the sphere as smooth as possible to maximize the charge that can accumulate on it and thus the voltage - covering the sharp edge reduces charge leakage into the air.

cutting the bowl    partially cut bowl

Once the generator was working properly it was time to install the top half of the sphere, an unmodified bowl.  I found that the seam where the two bowls meet was the place where the most corona discharge occured and that by covering the entire seam with duct tape the output voltage of the generator increased dramatically.

The belt:

I went against conventional wisdom and tried using a bicycle tire tube for the belt.  This time the conventional wisdom was right- I got no output from the generator at all.  That set me off on a web search to find out what others were using in their VDGs.  I found my answer in a Yahoo discussion group devoted to VDGs.  I ran out to the local Sports Authority store and bought a package of resistance bands and used it to make the belt for the generator in less than an hour.  When I turned on the power the generator immediately started throwing off sparks!

top roller, brush, belt

The resistance bands are 6" wide and about 48" long.  I cut one down to about 1 1/2" wide with a pair of scissors.  A little contact cement closed the ends to form a loop.  My rollers are about 38" apart so I had to stretch the belt to get it over both rollers.  Stretching the belt made it narrow, like you see in the pictures, so I put in a fresh one that I cut to 2 1/2" wide.  After stretching it was about 2" wide and covered the rollers almost completely.  The wider belt charges the generator faster.

The motor:

The motor isn't critical.  Some people use old electric drills or motors taken from electric fans.  It doesn't take a very powerful motor to turn the belt/rollers when you use ball bearings on the roller shafts.  You could probably get away with a battery operated motor.  I didn't have any junk motors laying around so I bought a surplus 117VAC motor from Herbach and Rademan for $8.  It runs at about 1800 rpm.  It is mounted to the base board of with a pair of 4" angle irons.  The motor is coupled to the bottom roller shaft with a small piece of urethane air tubing that has been slit length-wise - the same stuff I used to cover the sharp edge where the bowl was cut for the bottom half of the sphere.  The air tubing can flex a little so alignment of the shafts is not critical.  Nylon tie-wraps hold the tubing to the the motor and roller shafts.  

If I were doing it all over again I think I'd bolt the motor right to the pipe and eliminate the box.  The whole thing could be operated on a table top in a horizontal position, assuming the belt would be stable in that position.  I'm not sure the great length is really necessary either.  It might be OK to go half as long as I did.


Many people have emailed me about this project and many have duplicated it.  One thing I am often asked is how to wire the motor - apparently Herbach & Rademan doesn't always supply a wiring diagram.  I have prepared a diagram for you to download here:
Herbach & Rademann capacitor start motor wiring diagram


motor and bottom roller   base of the pipe   shaft coupling close-up

Brushes:

Brushes are not critical at all.  I drilled holes in the pipe at the same level as the rollers then threaded some wire through them.  At the top of the VDG the bare wire is aluminum taped to the metal bowl (the photo just shows it hanging and touching the bowl- I later added a piece of tape).  At the bottom roller the end of the brush wire connects to the frame of the motor which is grounded to the AC power grid (use a 3-wire line cord!).  The brushes are made from metal tape- I used aluminum at one end and copper at the other but the material doesn't matter as long as it is metal.  Just fold the tape over the wire next to the roller, then snip away at the tape with a pair of scissors to make a bunch of pointy corners.  Make sure that the brushes don't touch the belt.

top roller, brush, belt    bottom roller, belt, and brush

Finishing up:

Put it all together, make sure the belt and rollers turn freely.  Get all your valuable electronics out of the room.  Plug it in and see what happens.  If it starts charging you'll know it when the hairs on your arm stand up even though you're standing a foot or two away from the VDG.  Now you're ready to put the top bowl in place.  Set it over the other bowl and  wrap duct tape around the entire length of the seam where the bowls meet.

How does it perform?

I originally built the generator with a positively charged sphere (aluminum roller at the top of the column).  When I first turned it on the generator immediately threw 2" sparks about every 1/2 second.  One thing I discovered after looking at the VDG operating in the dark was that there was one point on the sphere that was discharging into the air continuously.  That point was along the seam where the two bowls meet.  I covered the seam with duct tape and suddenly I was getting 8" sparks!  I have since swapped the rollers so the sphere acquires a negative charge and I get 10" sparks that look a lot more like lightning.  Another thing I discovered was that putting a plastic bag from the grocery store over the sphere also increased the length of sparks.  I think the bag prevents some of the more "casual" corona discharge and allows the voltage on the sphere to build up even higher.  When you get zapped with plastic covering the sphere you really get zapped!

Update 4/4/12

Please note: performance of any VDG is critically dependent on the humidity of the air in which you try to operate it.  I tried to operate my VDG on a rare rainy day in Phoenix once and got no sparks from it at all.  If you like in a humid place or it is raining outside you may not get much or any output.  I have seen patents from the 1930s for VDGs with heaters built in to dry the air inside the machine to improve performance in humid environments.  If you live in a humid place try blowing warm air from a hair dryer into the machine to see if it works better.

Need higher voltages?  Look at this: http://www.pelletron.com/charging.htm
Here's the original Pelletron patent.
Maybe my next project...



You can email me here:



Home

This page was created using KompoZer.  Support Open Source Software!