Leakage on vandergraaf generator

[zargnut]

Hi,

I build a vandergraaf generator with my kid. It works well. Charge does build up and shocks me. If I don't provide a discharge path, the charge builds up. Shortly after I hear it arc'ing in the wall transformer which dies within a minute or so.

Here are the specs and a link to pictures

The base is aluminum. The motor is grounded to the base. The powersupply uses a DC ~12v transformer (not grounded) The bottom roller is nylon. The belt is neoprene. The pipe is PVC. The top roller is PVC. The axles on both rollers are aluminum in nylon and teflon bushings respectively. The brushes are tinned copper. The discharge ball is stainless.

https://picasaweb.google.com/116582478791072010058/VanDerGraaffGenerator?authuser=0&feat=directlink

I provided a ground line to the base (ran a wire to a grounded copper water pipe) , but that does not fix the problem. Does anyone see an obvious problem. I would think that grounding the base would solve the issue...

thanks
dan

[IanB]

It's hard to see the details from your pictures, but remember that electricity needs a circuit. Grounding to a water pipe won't complete the circuit if the water pipe is not part of the circuit.

Consider how the VDG generator works: it moves charge from the base electrode to the collection electrode attached to the sphere at the top. When the collection sphere wants to discharge, it needs to discharge back to the base electrode. Make sure you have enabled this circuit. In an ideal world the motor and motor drive power supply will be completely decoupled and isolated from the high voltage side. I think you should connect the base electrode to ground, but make sure the motor and motor transformer are well isolated and have no electrical connection to the high voltage bits at all.

[IanB]

I provided a ground line to the base (ran a wire to a grounded copper water pipe)

Have you made sure that when the collection sphere discharges it has a path to discharge back to the base? Make sure the sphere can't discharge to other vulnerable bits of the circuit in preference to the grounded base electrode.

[zargnut]

When I am touching it, it discharges from me to ground.  I never made a discharge wand. I forgot to mention that charge also accumulates on the aluminum base and you get shocked whenever you touch it to turn it off 

I thought that it was pulling electrons from ground and storing them on the ball like a capacitor. I assumed the return path was any ground, including me when I am standing on the floor.

Isolating the DC motor would be really hard at this point...

I got an AC sewing machine motor to incorporate that I could ground properly with a 3-prong plug, but that means a major rebuild... I was hoping for an easier - but equally robust solution. If I do use the AC motor, can I use a metal chassis and just ground the motor housing and the base?
 

[zargnut]

How is the discharge ball discharging back to the base? I thought the whole point of the PVC pipe and belt is that they are non-conducting and the distance precludes discharge. Can it jump back to the base?

If I build a smaller "accepting" electrode mounted about 1/2 way up to the discharge ball and grounded to the base, will that provide a preferential path the will prevent these issues... but still allow us to grab the globe for a shock/hair trick?

[Psi]

I provided a ground line to the base (ran a wire to a grounded copper water pipe) , but that does not fix the problem.

I forgot to mention that charge also accumulates on the aluminum base and you get shocked whenever you touch it to turn it off 

You are missing a proper earthing connection.

The metal base parts and the electrical DC ground need to be connected to earth (the actual dirt outside).
Normally you'd do this by connecting it all with mains earth, but if your house has a broken earth then the entire generator will float and start to charge up. This will include the metal base plate, all the electronics and the secondary winding of the mains power pack.
At some point the primary/secondary insulation in the powerpack will fail and it will arc over.

Personally i've never liked earthing high voltage to the household earth wiring.
For my tesla coil i have a metal rod i drive into the ground to act as coil earth.
(i only run my tesla coil outside, so i have easy access to the dirt for this).
When running something inside the house mains earth is really all you have access too.

[IanB]

I forgot to mention that charge also accumulates on the aluminum base and you get shocked whenever you touch it to turn it off 

How could that happen if the aluminium base is grounded? Are you sure it is properly connected to ground? Are you also sure it is connected to the lower charging brush?

[IanB]

How is the discharge ball discharging back to the base? I thought the whole point of the PVC pipe and belt is that they are non-conducting and the distance precludes discharge. Can it jump back to the base?

You should make a separate discharge electrode (something the size of a ping pong ball is good), and put this on the end of a metal rod, connected with a wire to the base. Mount the discharge electrode a short distance from the main collection sphere so that sparks jump across to it.

[IanB]

Personally i've never liked earthing high voltage to the household earth wiring.

But a VDG only generates microamps. It should easily be within the safe operating range of the household earth.