Home made Wimshurst machine and Van De Graaf generator

[Nicolas]

Hi,

I am planning to build a Tesla Coil, but not a fully newbie about high voltage. 

A Wimshurst machine built when I was a teenager, 27 years ago (oh my god), I just took it from my parents home and restored it a little bit. Long time ago I remember reaching 10 cm sparks, but for now I do not reach more than 3 cm... I have to check what is wrong ! 60 cm PVC disks.



And much more recently... I just finished a Van De Graaf generator. Passive one. PA / PTFE pulleys. Rubber Theraband belt. 50cm topload. Outrunner DC brushless motor, direct drive on the bottom pulley. I still need to improve it, but I am now at 50 cm sparks :











System for easy connection with the topload (removable) :




Top comb :




25 cm strong sparks :


If I want to increase, I have many leakages. I must improve, I still have some metal screws on the top from where leakages come from. I will have to replace them with PA screws. We can see some strong more than 50cm long sparks, so should be reachable with improvements :



50 cm sparks with a smaller electrode :




Really an incredible machine. With just friction... No need to turn at high RPM, speed can be very low and electricity is generated almost instantaneously. 50 cm sparks almost every second. The most difficult in this machine is to have a stable belt rotation. Not easy, not perfect for now on my machine, sometimes big instabilities occur (depending on the speed). Belt can move so much sometimes that it jumps and can be damaged by the combs.


Nicolas

[davekni]

And much more recently... I just finished a Van De Graaf generator. Passive one. PA / PTFE pulleys. Rubber Theraband belt. 50cm topload.

Impressive machine!  Would be interesting to measure output voltage.  One measurement option is to see how heavy a thin conductive disk can be lifted off the top by electrostatic repulsion.  Requires care to accurately center disk on top.  Disk slides off once electrostatic repulsion reduces normal force to the point where friction no longer keeps the disk in place.  Thus actual electrostatic force required to dislodge disk is not quite as much as disk's gravitational force unless centering is perfect.

[johnf]

hopefully you made your pulleys so that they are tapered from the middle ie the middle is greater diameter than your outside edges.
doing this gives automatc belt centering much like railway wheels stay on the centre of the track.
commercial VDG's run at about 70km/Hr belt speed.
Otherwiswe nice build

[DashApple]

Very nice van d'graph generator impressive output ^^

I was looking to make a small one someday , With the rollers you have used nylon and PTFE ? What one sits at the bottom / vs the top ?

[Nicolas]

Would be interesting to measure output voltage.  One measurement option is to see how heavy a thin conductive disk can be lifted off the top by electrostatic repulsion.  Requires care to accurately center disk on top.  Disk slides off once electrostatic repulsion reduces normal force to the point where friction no longer keeps the disk in place.  Thus actual electrostatic force required to dislodge disk is not quite as much as disk's gravitational force unless centering is perfect.

Good idea ! How do you link the disk weight to output voltage ? I imagine you use Coulomb's law to get the charge, but then, to link voltage and charge I will need capacitor value (sphere and dish) ? How do you estimate it ? Thanks for your explanations, I would like to estimate this output voltage.

hopefully you made your pulleys so that they are tapered from the middle ie the middle is greater diameter than your outside edges.
doing this gives automatc belt centering much like railway wheels stay on the centre of the track.
commercial VDG's run at about 70km/Hr belt speed.
Otherwiswe nice build

Initially my two pulleys were tapered from the middle, by 3 mm :



But : this was horrible, I think probably too much tapered, and the belt came so much in the center that it folded and folded on itself !
Then I read that it was better to have only the drive pulley to be tapered in the middle.

So finally I remade two pulleys, the bottom drive pulley is tapered in the middle by 1 mm. The top one is flat.
But you are right,  perhaps I still should change and tapered the top pulley by 1 mm too ? What do you think ?

With my motor, my belt should run at around 60 km/h at full speed. So close to your value (70 km/h). But when run at full speed I have many leakages for now, I must improve by removing metal screws I think.

I was looking to make a small one someday , With the rollers you have used nylon and PTFE ? What one sits at the bottom / vs the top ?

Yes, I used PTFE roller at the top, and Nylon at the bottom. Not sure it will affect the power if you do the opposite. I just think with PTFE on the top you charge the top load with electrons (negatively).


One more precision if some people read this topic to make a VDG machine : on the first pictures the belt is red. BUT this was not a Theraband belt (just an other latex band brand). When I took the pictures, I damaged the belt. And then I changed the belt with Latex Theraband (Gold one, the strongest in the range, I measured it at 0.55mm thickness, same thickness as previous red band). The change has been impressive : the machine works really really much better with this new "gold" belt. So probably the previous red belt was a little bit conductive. This new one made a huge step in power and sparks length (probably more than 2 times longer). The pictures in the dark with sparks are taken with the "gold" Theraband belt.

[Mads Barnkob]

Very nice VDG!

I got motor, rollers, band and combs in a box for a VDG one day. The topload I have, is a huge half toroid bottom with mushroom top. Its about 70 cm in diameter.

How is your motor powered? Is the blue pack a battery, to keep it isolated from a external power supply?

[davekni]

Good idea ! How do you link the disk weight to output voltage ?

Electric field E on top of the sphere (presuming semi-infinite distance to other objects) is V/r, where V is the sphere voltage and r is the sphere radius.  Energy density (J/m^3) of that field is 1/2 * E^2 * e0, where e0 is air (vacuum) permittivity, 8.854pF/m.  Force is energy per unit distance, so J/m^3 is the same as N/m^2.  What matters for voltage measurement is force (weight) per unit area.  As long as the disk is thin and disk diameter somewhat smaller than top-load radius, weight/area is the only needed parameter.  In other words, if you can lift a 1mm thick aluminum disk, it doesn't matter if it is 10mm or 100mm diameter.  Either way, 1mm thick aluminum (2.7g/cc = 2700kg/m^3) is 2.7kg/m^2 for 1mm thickness.

With a bit of algebra, V = r * sqrt(2 / e0) * sqrt(force / area) = r * sqrt(2g / e0) * sqrt(mass / area) where g is gravitational acceleration on earth's surface, 9.8 N/kg.
For MKS units, that becomes:
     V = (1.49MV * r_in_meters) * sqrt(kg / m^2).
For your 0.25m radius top load:
     V = 372kV * sqrt(kg / m^2).
Thus, if you can lift 1mm thick aluminum (2.7kg/m^2), voltage is 372kV * sqrt(2.7) = 611kV.

Measurement weights can be constructed various ways.  Coins are easy if you can find ones with appropriate areal density.  Sharp fields around rims will have some effect.  I've used aluminum foil disks formed to top-load curvature, with various weighting sandwiched between pairs - flattened bits of clay or other putty, coins and double-sticky tape, etc.  Weigh and divide by area to get areal density.

I believe the above is all accurate.  If I've made a mistake, presumably USpring will add a correction or clarification

[Uspring]

@David:
Your derivation looks ok to me. I've got the same result with a different approach. One remark, though:
The force on the disk depends on V/r. That happens to be the field strength at the spheres surface. In the example given for 1mm aluminum, that turns out to be 611 kV / 25 cm = 24 kV/cm. That is pretty close to the breakdown field strength of 30 kV/cm in air. The surface field strength wont go above 30 kV/cm, which will limit the force on the disk, even if the VDG voltage is much higher. I don't know, whether the voltage can go much higher than 750 kV (=30 kV/cm * 25 cm), since then the corona will start drawing a significant current from the sphere. In any case, the max surface field strength given by the breakout will limit the lifting force on the disk.

[Nicolas]

How is your motor powered? Is the blue pack a battery, to keep it isolated from a external power supply?

My motor is powered by a li-ion battery (6S2P 21700 LG cells), the blue pack yes. Just because I have this kind of batteries in hand...
Once I saw a spark reaching, I think, the battery pack. So now I connected the battery negative polarity to the ground, and I did not see anymore spark. Not sure it is linked or not.

Measurement weights can be constructed various ways.  Coins are easy if you can find ones with appropriate areal density.  Sharp fields around rims will have some effect.  I've used aluminum foil disks formed to top-load curvature, with various weighting sandwiched between pairs - flattened bits of clay or other putty, coins and double-sticky tape, etc.  Weigh and divide by area to get areal density.

I believe the above is all accurate.  If I've made a mistake, presumably USpring will add a correction or clarification

Clear thanks ! I still have the sphere piece of steel I cut from the bottom... So I think I could use it to test... Not sure it is light enough, but it has the good curvature ! I will try to approximate the voltage to check. With 50cm sparks, should be above 1 million, right ?

First I will change all the metal screws by PA ones. Then I will try to bend a little bit my top roller. And then I will try to measure the voltage. I keep you informed.

Thank you all !

[davekni]

Clear thanks ! I still have the sphere piece of steel I cut from the bottom... So I think I could use it to test... Not sure it is light enough, but it has the good curvature ! I will try to approximate the voltage to check. With 50cm sparks, should be above 1 million, right ?

How thick is the steel?  May be fine. Somewhat smaller stainless steel balls I bought have thin walls.
As USpring pointed out, voltage is unlikely to be much above 750kV due to excessive corona discharge from top sphere.  Higher local field strength at smaller ground ball can start arcs of 50cm below 1MV.

And then I will try to measure the voltage. I keep you informed.

Great!  I'm looking forward to seeing your results.

[haversin]

With a bit of algebra, V = (1 / r) * sqrt(2 / e0) * sqrt(force / area) = (1 / r) * sqrt(2g / e0) *

Maybe this is a typo. The correct equation is
V = r * sqrt(2 / e0) * sqrt(force / area)
you got the correct answer of 611 Kv for the 1 mm aluminum plate.

I measured the thickness of some aluminum foil with a micrometer and found it to be .018 mm. Putting this value into the above equation with r = .25 gives a voltage 82 Kv to lift it off the VDG.

[davekni]

Thank you!  Yes, a typo that I carried through the post, though I did calculate with r rather than 1/r.  I've edited (corrected) my original post so that no one will get confused looking at this thread in the future.