Van de Graaff Generator Update

The coming of winter and dry Canadian air brings snow and other awful things, but there's always a bright side:  it's perfect Van de Graaff generator (VDG) weather!

I wanted to play with my VDG a couple weeks ago and discovered that it was producing absolutely no sparks, even though the air in my house was so dry my skin was cracking.  I set about troubleshooting...

First I tried wiping the tube down with alcohol to make sure it hadn't accumulated some sort of conductive dust (?).  Nope, that wasn't it.  

Then I tried swapping the fancy brushes I had pulled from a laser printer for the old standard aluminum tape with the edges clipped to make many sharp points.  Nope.  I put the fancy brushes back.

Then I checked the ground wire to see if the cats had chewed through it.  They had definitely been working on it, but hadn't chewed all the way through it yet.

Finally I took the belt off and examined the pulleys.  The top pulley, covered with Teflon tape, felt sort of rubbery, as if the thin layer of Teflon had worn through.  The bottom roller, covered with aluminum tape, likewise felt sort of rubbery.  Hmmm.  Close examination of the belt found its surface getting sort of crumbly and sticky.  Bingo!  The failing belt had left residue on the Teflon and aluminum rollers so there wasn't any triboelectric effect working any more.

It makes sense, if you think about it- as the belt goes onto a roller it stretches to conform to the roller surface, and as it comes off the roller, it relaxes again.  The  belt is like a pencil eraser scrubbing on a piece of paper, leaving crumbs behind.

I replaced the Teflon and aluminum tapes on the top and bottom rollers, and stitched together a new belt from some fresh therapy-band material, put it all back together, and POW!  The machine was throwing long, painful sparks into my hand again.

Bottom roller covered with fresh aluminum tape, fresh belt, and fancy brush.

Top roller, fresh Teflon tape, fresh belt, and fancy brush.

If you have a VDG and it mysteriously stops working, it may be time to replace the belt and clean or replace the coatings on the rollers.

One more thing- the rims of the two Ikea serving bowls used to make the spherical terminal at the top of the generator have no lips so it's almost impossible to stand one on the other, edge to edge, and have the top one stay in place.  It's a two person job to hold the top bowl in place to tape the two together, but once taped, you can't access the inside of the sphere.  That means you can't show disbelievers the inner workings of the generator.  Who would believe that all it takes is a rubber band, a small motor, and a couple plastic rollers to generate >500 kV if they didn't see it with their own eyes?

I got some help from Jake (the knight who does battle with a giant Tesla Coil at the Milwaukee MakerFaire) at the Milwaukee Makerspace and we TIG welded three small stainless steel screws to the inside edge of the top bowl.  After welding I cut the heads off the screws with bolt cutters and ground what was left down to nubs, just a few mm long.  The screws rest just inside the rim of the bottom bowl and keep the top bowl in alignment, without tape.

top bowl with three screws welded to inside rim.

One of the screws welded to the rim of the top bowl. 

Now when I run the generator, after a few seconds of operation I can draw 600+ mm long sparks to my arm.  If I stay away from it and just let the VDG run, it fires sparks into the air with a menacing "SNAP" every few seconds!

Look ma!  No tape holding the bowls together!

Here are a few fresh pictures of the generator doing what it does best:

Van de Graaff Generator Redux

Last September I did a blog post about a Van de Graaff generator that I built using 3D printed parts.  After talking to people, several emails, and forums posts, it has become apparent that a lot of people mistake VDGs for Tesla coils.  It's an easy mistake to make- they tend to look similar.  Both usually have a large spherical or donut shaped metal terminal on the top of a long column.  The primary difference between them is that the VDG produces high DC voltage while a Tesla coil produces high AC voltage.

The VDG is considerably simpler, cheaper, easier to make, and scale up or down as it is simply a motor (or hand crank) turning a pair of rollers with a rubber belt connecting them.  Here's a great video that explains in detail how a VDG works:

Performance of a VDG depends primarily on the materials used to cover/make the pulleys and the size of the top terminal.  You can think of the VDG as a capacitor with a current source charging it.  The top sphere has some finite capacitance relative to the earth.  The moving belt carries charge (therefore current) to/from the sphere to charge that capacitance.  It is the size of the top terminal that ultimately determines the maximum voltage that can be achieved because air only behaves as an insulator until the electric field in it becomes so high (about 30kV per cm) that it breaks down and becomes a conductor.  Once the air conducts (a spark), the charge escapes from the top terminal and has to be replenished by the belt.  The electric field (and voltage) is determined by the charge stored and the radius of the sphere.

This page has some basic calculations that apply to Van De Graaff generators.

Maximum voltage:  30kV x radius (cm)

Capacitance: 111.2 pf/m x radius (m)

Energy stored: 1/2*C*V^2

Upgrades:

A lot of my 3D printed designs get improved over successive generations.  This one is no exception.  I made a few mods and upgrades and it is now better than ever.

Top Terminal

My original design, with 11" diameter top terminal, allowed the voltage up to about 419 kV.  The capacitance would be about 15.5 pF and the energy stored about 1.36 Joules.  I recently acquired a pair of 14" salad bowls to replace the 11" bowls.  They will allow up to about 533 kV, 19.8 pF, and 2.81 Joules.  The increase in energy storage means an increase in pain from the sparks, among other things.

I originally cut the hole in the bowl to fit on schedule 40 PVC pipe.  I also changed the pipe - see below- which meant the approximately 115 mm diameter hole in the bowl was over-sized.  I designed and printed a new holder to mount the bowl on the new, slightly smaller pipe.

Brushes

I also recently took apart and old laser printer and found something useful.  In those printers, just before the paper exits, there is a grounded conductive brush that it passes over to discharge any electric charge that might make the paper sheets stick together.

I pulled that brush out and used it to replace the metal tape brushes that I had originally used in the VDG.  I don't have any measurements to back it up, but it seems like the generator charging current has increased because it seems like it generates sparks faster that it did with the metal tape brushes.

Pipe

Someone at the Makerspace wanted to build a VDG after seeing mine, and he wanted to use lighter weight pipe, so he bought a piece of 4" HDPE corrugated drain pipe.  It's much lighter than PVC, but all my parts were designed around PVC pipe, so I had to modify the designs and reprint a few of them.  The drain pipe seems to work just fine, so there's no need to spend the extra $ on schedule 40 PVC.  One $8 piece of drain pipe is more than enough to make 2 large VDGs.

HDPE is less hygroscopic than PVC, so the surface resistance should be higher even when the air is humid, so the machine may perform better in all weather conditions.

Teflon Tape for Top Roller

My original design used teflon plumber's tape to cover the top roller.  After using the generator a while I found that the tape was fraying.  Since it wasn't self adhesive, I had to use a couple narrow strips of duct tape at the edges of the roller to keep the teflon tape in place.  I did a search and found an alternative- self adhesive fiberglass/teflon tape from a vacuum food sealer.  One $5 spool of tape is enough to cover about 20 of the top rollers.  It seems to work as well as the plumber's tape, but hopefully won't fray or come off the roller like the plumber's tape (if it does, I have plenty to replace it!).

Top Terminal, Pulley, and Brush Holder

The printed base of the VDG was modified to accommodate the slightly thinner-walled HDPE drain pipe.  The printed top part of the machine was completely redesigned...

Cross-sectional view of the new top part of the VDG.  It fits snugly over the HDPE drain pipe.  The bottom bowl with the 115 mm diameter hole sits on the purple shelf and the orange lock ring screws down to hold it in place.  The roller is blue and the HDD bearings are green. 

This is what it looks like when assembled.  The brown tape on the roller is the new, self-adhesive Teflon tape.  The belt is folded to about 1/2 width because the pipe is a little too long (or the belt is a little too short) and belt tension combined with the crowning of the pulley causes it to fold.  I'll be shortening the pipe to prevent that.

 In this design there is nothing but friction (tight fit on the pipe) and the belt keeping the top part of the generator in place.  I may modify the design a bit to make it more secure, but I hesitate to use screws to secure it because I don't want to create any conductive sharp points that will allow charge to leak off the terminal.  Maybe zip ties...

I drew these sparks to my hand.  The fluorescent yellow filament I used to print the sphere support glows from UV light in the sparks.  I reprinted in black filament so it wouldn't fluorescence.

More of the same...

This blue brush-type discharge happens when I get my arm near the generator instead of my hand.