Midterm – Sustainable Energy – Wimshurst Machine update

Electrostatic discharge photo by Hugh Mitton

The goal of this project is to replicate the classic electrostatic generator known as a Wimshurst machine. Included in this project is the goal of studying DIY capacitors using different size Leyden Jars (see previous Wimshurst post for more info).

This project has stretched my limits in terms of craftsmanship. I used some machines for the first time and frequently got help from my classmates, especially the shop staff. Thank you so much, Justin and Crys! However, it has much farther to go

Current results:

A rough build

The wheels were custom-cut by Advanced Media Services, with a 15″ diameter. Clear plexiglass lets you see where the foil pieces are in relation to each other while spinning. This machine generates very low amperage but high voltage – with more foil pieces, it is capable of hundreds of kV (at ~50 mA).

Wimshurst Spin Test

The major issue so far is the fit of the pulleys. They are 1/4″ bores but they do not sit correctly on the 1/4″ non-metallic shaft. Significant wobble prevents the wheels from spinning smoothly and prevents the charge combs from staying close enough to collect.

The wheels and pulleys:

Wheels and pulleys

I started with 3/16″ bore wheels, but could not find pulleys with small enough bores that were a reasonable price. Also, 3/16″ acrylic rods are not stiff enough to support the wheels.
I found clothesline pulleys in different sizes at Home Depot, and spent a long time drilling/sawing/grinding away the shaft on them to release the pulleys. Unfortunately they actually have slightly different bores. But one set has to be glued to the shaft, so those had a higher tolerance for offset. Even accounting for that, and after gluing the smaller pulleys to the wheels, there was still significant wobble. I will probably get new pulleys from McMaster-Carr and replace the central shaft with fiberglass or another more durable non-metallic material.

Cutting the foil strips took a surprisingly long time, for several reasons. One, I initially bought tape with extremely thick acrylic bonding, which bunched up beneath the foil and made the pieces useless (in addition to being nearly impossible to cut smoothly). Two, even after getting better foil tape, the pieces have to be cut as smoothly as possible to avoid corona discharge effects at any sharp corners (i.e. a “bleed” of energy). The sticky backing gummed up even non-stick scissors, necessitating a round of careful cleaning every four strips.

Aluminum foil tape

More strips results in better yield, so I will most likely cut many more.

A big part of this project was the chance to experiment with different capacitors, so I spent a long time gathering, cutting, and prepping different tubes and foils. Here are some shots of the Leyden jars in progress:

A saltwater Leyden jar made of a soft drink cup

This jar showed very little conductivity when connected to a small battery.

Rolled plastic sheets with aluminum foil

This jar was able to store all of the charge from a 1.5V battery and release it when a circuit was completed. It is probably capable of much more.

Primary Leyden jars (partially assembled)

These are the same size called for in the original Make magazine plans, and should be able to hold around 150kV if made well. I had some trouble getting a good reading from the multimeter, and will have to keep working on them.

These jars use ultra-thin tube protectors for a dialectric, a steel bar to conduct down from the charge combs, and copper wire to connect the steel bar to the inner foil wrap. I cut and drilled larger plastic rods to provide insulation and support for the steel. Cutting the tube protectors was nerve-wracking, as it is very easy to fracture plastic tubes. Even simply drilling out the plastic supports surprised me, as it was difficult to clamp them in a vise to go in the drill press – their smooth surfaces caused them to slip around in the vise once the drill contacted them.

I have several more tubes to try, all with much thicker dialectric pieces. They may store much lower voltage – bad for sparking, but safer to leave around.

To be continued!

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