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Electrostatic Motor Plans


                            - William J. Beaty,        1988 Museum of Science

Here's a simple electrostatic motor that's based on 2-liter soda bottles and aluminum foil. It's construction does not require access to a machine shop. It draws a fraction of a microamp during operation, and can run at unexpectedly high speeds (1000 RPM!) It runs on a minimum of 5000 volts DC, which can be had from several different low-current electrostatic energy sources.

Any of the following can power this motor:

One of these motors is featured in the Electricity exhibit at the Museum of Science in Boston, powered by a hand-cranked Van de Graaff machine.


  • three 2-liter pop bottles, at least one with a METAL cap
  • roll of aluminum foil
  • rubber cement
  • silicone caulk
  • 13" metal rod, 1/8" dia. (could use coathanger)
  • Two 8" pieces of solid copper wire, or coathanger
  • wood plank (or metal, or plastic) for the base
  • duct tape
  • hookup wire for attaching the power supply




            |||     /     \     |~|
  stator  /     \  (       )  /     \   stator
  bottle (       ) |       | (       )  bottle
   (+)   |       | |       | |       |   (-)
         | |||---------    |-|       |
         |       | |       | |       |
         |       | (_______) |       |
    |____________________________________| base 
                  SIDE VIEW

                           ______  support
   stator    ___       ___/     _|\|_ 
   bottle  /     \   /     \   /     \
          |  (|)  | |  (|)  | |  (|)  |
           \_   _/   \     /   \ ___ /  stator
       brush |\|______/~~~rotor         bottle
       support            bottle
                           TOP VIEW
[Drawing of motor]
Highly recommended: ELECTROSTATICS by A. D. Moore (lots of projects), also others


Metal Rod

Cut the rod so it's about 1 in. longer than the middle bottle. Sharpen the rod using a file. Drill a hole in the center of the plank using a drill bit slightly smaller than the sharpened rod. Carefully force the rod into the plank, unsharpened end first. (Note: coathanger wire will work as the center rod, sort of. But it's very wobbly. 1/8" welding rod works much better.)(Note: instead of filing a sharp point, try attaching a piece of a sharpened pencil to the top of the rod. The sharp graphite point makes a good bearing.)

"Rotor" Bottle

Find the exact center of the bottom of the middle bottle, and drill a hole there that's slightly larger than the rod diameter. When slid onto the sharpened rod, the bottle should spin very freely. If the hole is too big, the bottle will rattle around and make the brushes drag on its surface

If you can find a bottle with a metal cap, make a dimple in the center of the cap. The dimple is there so the point on the rod will have something to ride in to stay centered. Take care not to poke through the metal bottlecap with the sharp rod! If you can't find a metal cap, glue a hard object such as a small glass test tube into the bottle cap. If you use the pencil point mentioned above, a plastic bottle cap might work (I haven't tried this.) You'll still have to make a dimple in the plastic somehow.

Precisely cut three broad strips of aluminum foil so they are just wide enough to give a 1/2" spaces when attached to the center bottle. You want the middle bottle to have three regions of foil, with half-inch gaps between the regions. Trim the corners of the foil so they are round, and test-fit them on the bottle and trim as needed.

Glue the foil to the center bottle as shown in the drawing. (It doesn't matter if the shiney side of the foil faces out or in.) I used rubber cement to glue the foil strips. I coated the whole bottle with cement, coated one side of each pre-cut foil strip, allowed the glue to dry a couple of minutes, then CAREFULLY layed on the strips and burnished them down with a spoon as I went. The end result should look like an aluminum coating on the bottle, with three broad foil sections separated by 1/2in gaps running vertically. No part of each foil section should touch any other foil section. Bubbles in the foil don't hurt anything, and can be punctured with a pin and flattened with a spoon. Instead of glue and foil, you might instead try using a roll of adhesive aluminum foil tape available at some hardware stores.

Two "Stator" Bottles

Glue large sheets of foil around the entire center areas of both of the two 'stator' bottles, leaving a 2 in. foil-free space at the bottoms. The bottom must remain clear of foil, and no foil on these bottles should come close to touching the wooden base or close to any duct tape you might use to connect the stator bottles to the base.

Commutator "brush" wires

The commutators (or "brushes") are pieces of heavy wire or coathanger 8in long, each attached to a stator bottle, and each extending sideways so their ends are very near (but not touching) the rotor bottle surface. After attaching them to the bottles, bend the tips so they point towards the surface of the rotor bottle. See the diagram and photos.

I attached them to the stator bottles by bending the wire ends into an S-shape and embedding the S-shaped part in silicone caulk on the foil bottles. After the glue sets, the remaining short ends of each S-shape should be bent so they make solid contact with the bottle's foil. Don't let the silicone insulate the wire from the foil, because the stator foil and the commutator wire must both be electrically connected to one of the power supply terminals.

Attaching the Stator Bottles

Attach the two stator bottles to the plank so they are spaced about 1/2" from the rotor bottle. I used nuts and bolts through the bottoms, which allowed me to rotate the bottles a bit for easy adjustment of the spacing between the commutator wire tips and the center bottle. (Yes, it was really hard to position the bolts inside the bottles!)

If you use tape to attach the stator bottles, make sure it DOES NOT reach up to contact the foil. Duct tape, masking tape and wood are slightly conductive, and when the humidity is high, they can provide an unwanted leakage path to ground, preventing motor operation.

The Bearing

The metal-cap-with-dimple bearing is pretty crude. I improved it by obtaining a 1/4" diameter test tube, cutting the bottom 1/2" off it (by nicking with a file and snapping by hand with gloves.) This I glued into the exact center of the bottlecap. The sharpened rod spun nicely against the glass. Avoid dropping the center bottle suddenly down onto the metal rod, or the sharp point will shatter the glass bearing.

Mark Kinsler has a better suggestion: use a bottle with a plastic cap, and screw a short sheet-rock screw through it so the point of the screw extends downwards into the bottle. Now use a wooden rod instead of a metal one, and screw a small phillips-head screw into the end of the rod. Place the bottle on the wooden rod so that the point of the sheet-rock screw rides in the head of the screw at the top of the wooden rod. He mentions that the slightest air current will make this bottle turn.


If it doesn't work: BOTTLE MOTOR DEBUGGING

It takes more than 5,000 volts to operate this motor. This source of voltage is available from "static electric" sources. Try the generators linked at the top of this webpage, especially the Van de Graaff generator. If you don't have access to a Van de Graaff, try the "TV SCREEN" electrostatic generator.

[Animation: pos and neg blocks on the sides, spinning 3-lobed rotor turns red or blue as it touches communators]
Animation made by Don Bangert

Any power supply will have two output terminals (although one terminal might be invisibly connected to ground.) The positive lead must connect to one stator-bottle's foil and commutator wire, while the negative lead goes to the other. If you use a Van de Graaff machine, use tape and bare wire to connect the foil on one stator bottle to the upper sphere, and connect the foil on the other bottle to the generator's metal base.

If your source of high voltage has just one output wire, then its missing wire is actually connected to ground internally. In this case connect the output wire to the foil on one bottle, and connect the foil on other bottle to electrical ground. Electrical ground can be found in many places, for example, use a metal faucet, metal sink, or the metal screw on the cover plate of a light switch or electric outlet. In a pinch you can use a metal tabletop for "ground", or use a few feet of aluminum foil layed on the floor, or even touch the other bottle with your finger to provide a crude ground-path out through your feet.

If it doesn't work: BOTTLE MOTOR DEBUGGING


When a high voltage is applied between the two stator bottle foils, one stator bottle aquires a negative charge imbalance, while the other one becomes positive. Also, a tiny spark jumps from the tip of each commutator brush to one of the foil sectors on the rotor bottle. The sector under the positive brush becomes positive, the one under the negative brush becomes negative. The rotor's foil sectors are then repelled from the alike-charged stator bottle and attracted to the unlike charged stator bottle. This sideway electrostatic force causes the center bottle to rotate, which brings new foil sectors under the brushes. Tiny sparks then jump to the new sectors and charge them, which makes them attract/repel from the stator bottles, etc. The foil on the rotor bottle that's under the commutator is always charged the same as the commutator, so it's always being repelled/attracted sideways.

The force is continuous, therefor the speed of the rotor bottle will keep rising higher and higher. In practice the rotor speed will not increase forever, but will stabilize because of air turbulence, bearing resistance and bearing chatter, etc. If the entire motor could be put inside a vacuum chamber, it would REALLY run fast. But then the sparks couldn't jump from the commutator wires, and you'd have to arrange some kind of sliding contact brushes instead. (Sparks cannot exist in vacuum, since a spark is made of air which has turned into plasma.)

Here's another way to visualize the motor: if you turn it sideways, you'll see that the motor is sort of like a waterwheel. On one side the excess negative charge is pouring into the side of the rotor bottle and "falling" towards the positive bottle. On the other side the positive excess is "falling" upwards and dragging the rotor bottle surface up. The moving charge gets temporarily stuck in the foil surfaces of the rotor, and it drags the rotor along with it as it moves from one stator bottle to the other. The motor rotates because it blocks the flow of moving electric charge, just as a waterwheel rotates because it blocks the flow of moving water.

Yet another way to imagine it: Normal coil-and-magnet motors often are built like this: three electromagnet coils in the rotor, two permanent magnets as the stator, a commutator, and they operate by magnetic attraction/repulsion. In the pop-bottle motor all the coils are replaced with capacitor plates. The rotor has three capacitor plates instead of three electromagnetic coils. The stator permanent magnets are replaced by (+) and (-) charged stator plates. All the electric currents have become voltages, and all the magnetic fields are replaced by electric fields. In electronics, the swapping of coils with capacitors and voltage with current is called the "electromagnetic dual." Capacitors are the "dual" of inductors. The popbottle motor is the "electromagnetic dual" of the common coil/magnet motor. Small slot-car PM (permanent magnet) motors represent one side of the electric/magnetic duality found in light, radio, electric energy, and all EM phenomena. The popbottle electrostatic motor is the other half of the duality. Light waves, radio waves, and electrical energy are where the voltage/current and electric/magnetic duality blends into a single thing called... Electromagnetism.




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