Thursday, October 4, 2018

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.  



1 comment:

  1. This was a good read. I did nearly exactly the same when I built mine in 2012. I recently rebuilt it and was searching for formulas to calculate the voltages. I am using the same Ikea bowls you are actually but 535kv doesn't appear to be right. My Vdg is arcing over 15 inches to objects and will spew Carona over 25 inches to a lamp which then Sparks myself at it's cord. I am at see level also so the 75kv per inch would still apply.

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