The Van De Graaff Generator invented in 1929 by Robert J Van De Graaff uses a couple of rollers, combs, and a rubber (or nylon) belt to create a charge of a few thousand volts. This charge is drawn off into the center of a hollow metal sphere, this last process takes advantage of the Faraday Ice Pail effect to multiply this charge to tens or hundreds times the input voltage.
The charge production occurs in three main areas: the rollers/belt, the combs, and the output terminal.
There are two main types of Air Insulated VDG's, externally excited and self exciting. This instructable will focus on the latter as any self excited VDG can be easily converted to an externally excited unit by attaching a high voltage DC source to the lower comb and removing the comb from ground.
The rollers in this unit must be selected so that they are as far apart on the triboelectric series as is possible. Below is a simple chart for reference:
- Human Hands (if very dry) +++ Positive +++
- Rabbit Fur
- Human Hair
- Steel (neutral)
- Hard Rubber
- Nickel, Copper
- Brass, Silver
- Gold, Platinum
- Styrene (Styrofoam)
- Saran Wrap
- Polyethylene (scotch tape)
- Polypropylene Vinyl (PVC)
- Teflon --- Negative ---
For this unit I used a PVC(vinyl) roller for the top and a Nylon Roller for the bottom, this will give a unit that has a negative output voltage. To create a unit with a Positive output voltage simply reverse the roller arrangement.
Instead of the standard spherical output terminal I recycled a helium tank as the output terminal.
This is not going to be a specific article on how to build this particular VDG (Van De Graaff [saves typing]) but rather what you must do to build a VDG. I will however try to explain the processes involved for each of the components necessary so that you will have a concept of what it is supposed to do which will aid you in your own construction and troubleshooting.
Whether you decide to build a large 700KV model or a small table top unit the components are exactly the same only the size is different. Also when it comes to VDGs, size does matter, as the output voltage is determined by the size and shape of the output terminal, the size of the rollers, the speed which the charge is delivered to the upper roller, and the distance between the rollers.
So while this unit which is just about 4 feet tall and delivers approximately 500,000 Volts I have a table top version that is 22 inches and only delivers about 80,000-90,000 volts. So if you don't want to commit to a large project then you can build a small unit and then scale it up once you are familiar with construction techniques.
The last photos are of my first VDG project and you will see the components are the same as in the large one. This was created from an online article here:
Step 1: Parts List
* ~2 inch OD PVC Pipe (1 1/2 inch ID)
* 4 inch ID PVC pipe - 2-3 feet long
* 4 inch mounting flange
* 4 inch connectors
* Steel rod for axle
* 'Skateboard' Bearings, or other sealed bearing
* Electronics perfboard or circuit boards
* Stick Pins
* Lamp Dimmer Switch
* 120V AC Toggle switch (may be optional, see step 2)
* Fuse holder
* AC Electric sewing machine engine or other 1/8 to 1/2 HP AC engine
* Output Terminal (Stainless Steel Gazing Globe, Helium Tank or other metallized globes
* Nuts, Bolts, Screws, etc.
Step 2: Motor and Mounting
I didn't get too fancy with this step because the base that I used did not have a lot of room to mount the motor so it is attached to the underside of the base with shims to level it. The drawback to this is that there is limited adjustment and the motor vibration travels to the whole device. Perhaps when I rebuild the base I will mount this on a spring loaded platform.
Motor speed is controlled with a standard lamp dimmer switch. You should hook it up first to see if it starts at the highest voltage or the lowest. If it starts at the highest you will need to add an On/Off switch and set the dimmer to its lowest setting when starting to avoid straining the belt and bearings.
Step 3: Roller Construction
The lower roller is from a set of furniture rollers and has an OD of 1 5/8" this has a pre-drilled center hole and I had to enlarge the hole slightly and use some alcohol to get it to slip onto the motor shaft. This permanently mounted the roller to the shaft once the alcohol evaporated. Also this roller had a slight crown to it, this is desirable as it helps the belt track straight when operating.
The upper roller is a 2 3/4" long section of 2" diameter white PVC pipe. Two skateboard wheels were used as the center hubs for the upper roller. I bolted them together and mounted them in a drill press and sanded them until they could be securely pressed into the PVC pipe. The roller was placed in a chuck holder of a lathe and the ends were sanded to give the center a crown. Bearings were inserted into the wheels in the roller and an axle is passed through the center hole of the roller and with locking collars to keep the roller centered and the axle was mounted into bearings at each end. See the picture for more detail.
Step 4: The Base
Most people recommend acrylic or other plastic building materials to prevent moisture absorbance. It you choose to use wood or particle board a coat or two of urethane should coat it well enough to prevent moisture issues.
A hole was cut into the deck for the belt to pass through and a PVC closet mounting flange was mounted to the deck. The column is 4" diameter PVC pipe from a hardware/home center and was sold in precut 2' lengths. You can cut your own but you need to be able to make the end square so that the column does not lean to one side or the other as this will throw off the tracking and make the belt jog to the side at high speed.
There is not much wiring necessary. Put the dimmer switch and the fuse holder in series with the hot side of the AC power line.
Step 5: The Support Column
1. Separate the upper and lower rollers.
2. To support the output terminal.
The support column should be selected to allow adequate clearance for the belt . The height of the column depends in part on the size of the output terminal; the general rule is that the distance between rollers should be at least 1.5 times the diameter of the output terminal. So for a 12 inch diameter output sphere you should have at least 18 inches between the rollers. On my unit I used a pre-cut 2 foot length of PVC to ensure I was working with square ends, as I did not have a reliable way of making square cuts on the PVC. When you add in the distances for the upper connector and roller assembly and the distance as it passes through the base to the motor it adds almost another foot. So in this unit the distance is probably twice the ideal length between rollers. A shorter column would have been better all around to reduce charge losses, but I had to work with what I have. If you have access to a power miter saw or your local home supply store has a power PVC tube cutter and can cut square ends then you may be able to have them cut to your specific needs.
Secondly the entry hole in the output terminal will fit over the 4 inch PVC pipe and comes to rest atop the coupler that joins the support column to the roller assembly. The inner sleeve has a corona dampening collar to reduce corona discharges and the sleeve itself helps stabilize the output terminal on the support column.
Step 6: The Belt Construction and Adjustment
I have tried many different materials for the belt and some just don't work and others work but are tedious to fabricate.
The two most readily available materials to use are exercise rubber banding or common roller shade material. If you ask your local hardware store they may be willing to give or sell you the cut off pieces as most stores usually discard these anyway.
The rubber belt will give the best charging current of the two but suffers from stretching or lifting at higher speeds this causes problems maintaining proper tracking. The rubber belt material is also very difficult to cut in a straight line as it tend to stretch even with a very sharp blade so you must take your time and be extra careful. I find it easier to lay the material flat and use a straight edge to make a line then use a ruler and razor to cut along the line. It is always best to have one square (factory) edge to ensure your belt is straight
The roller shade material doesn't provide as much charging current but once 'broken in' can be operated at higher speeds without lifting or having tracking problems. The roller shade material is easier to cut as it does not stretch when cutting, it can be cut with a rotary craft knife or scissors.
In any case the belt should be narrower than the roller. For example if your rollers are 2 inches wide than you should have a belt no wider than 1 3/4 inches. Remember when using a rubber belt the width decrease by about 1/4 to 1/3 when under tension so cut to at or slightly narrower than the roller. Also you must splice the end together at an angle because if they are cut square across they they will lift over the roller and eventually damage the bearings.
In previous builds I had spliced the ends at a 45 degree angle. But I recently read how others are using a V and reversed V to splice them this makes the center of the belt pass over the roller at the splice and spread outward evenly across the roller.
I have found two products to be the best for securing the belts to themselves. For Rubber belts the best glue I have found is suprisingly super-glue. For the Vinyl roller shades a product called Vinyl Welder. I recommend allowing the adhesive 18-24 hours to cure before using the belt.
If you opt to install the motor on an adjustable platform you can make adjustments there or you can make a spring mounted upper roller assembly as I have done to adjust tracking. Or if you want to mount the bearings inside the hub you can simply cut slots along the sides of the PVC column (as in the last picture) and use a threaded axle and secure the axle with lock-nuts.
Step 7: Combs
You can fabricate them with stick pins and perfboard or as in the third picture by salvaging pieces from other equipment. This was the metal tear off strip from an adding machine.
When designing the mount you should provide a means to change the distance from the comb to the roller. It is also desirable to be able to move the comb around the the roller. I find the comb at or near the separation point to be most effective. Others have said the best point is directly above the top roller and directly below the bottom roller. Again experimentation will determine the best placement for your build.
Step 8: The Output Terminal
The main thing is that it should have a smooth finish to reduce corona leakage, no sharp turns or angles which would tend to draw off the charge.
The problem with metal spheres is that they are EXPENSIVE!!. Up to about 12" spheres can be had for less than $60.00, but any larger and the price shoots right up. Do a search for Stainless steel Gazing Globes and you will find that for a 20" diameter globe you can expect to pay $165.00 to $240.00 online; while a 24" sphere will run into the $1000.00 range.
For smaller units the 8" metallized plastic ornament balls can be used as shown in the third picture.
If you go looking close to or after the holidays you can buy these for 50%-75% of the original price.
Another option people have tried is joining two stainless steel bowls together and taping over the rim to reduce corona discharge. This can be an extremely inexpensive way to go as the bowls do not need to be high quality and if you make a mistake cutting the entry hole on a $2.00-$3.00 bowl its not as devastating as it is on a $30.00-$50.00 Ball
Once you have your output terminal the next step is to create the entry hole because the charge has to be applied to the inside of the sphere to obtain the higher voltages. I recommend an angle grinder or a cut off wheel for cutting stainless steel to make the entry hole. I would advise against a jigsaw as the vibration may crack the sphere.
The ideal shape would be to have the sides of the entry hole curve back up and into the sphere, but Stainless steel is not malleable and if bent to much will fold and buckle. Therefore the use of a corona ring is highly recommended. This is just a soft copper, ring that is soldered or press fit around the entry hole as I have done. The ring significantly reduces the corona leakage at the entry hole and allow the output terminal to charge to a higher potential.
Step 9: Pictures of the Van De Graaff Generator Operating