Exploratorium Exhibit: Corona motor

High Voltage, Low Power Motor

Overview

In the early days of the Exploratorium, a Corona Motor was featured as one of the many exhibits. However, the exhibit was decommissioned due to problems with the power supply and the high voltages associated with the motor. With advances in electrical technologies and knowledge of the previous exhibits failures, it was decided that the museum would try again, and I was given the project.

A Corona Motor is a high voltage, low power motor that runs because of static electricity. A glass sleeve is surrounded by sixteen alternately charged copper electrodes. As the high voltage current runs through the electrode, it arcs to the insulating glass sleeve and gets attracted to the next electrode, pulling on the glass sleeve and causing it to rotate. The motor is powered by two high voltage power supplies linked in parallel, supplying 20,000 volts to the motor. Because of these high voltages and arcing from the electrodes to the glass sleeve, corona and ozone are both generated, causing the motor to have a subtle purple glow.

Motor Construction

The motor itself is actually a fairly simple thing to make. The most challenging aspect of making the motor itself was deciding how everything should be held together. The motor needed to be secure, but relatively easy to perform maintenance on, and we needed a way to alternately charge the electrodes. Our final design has the electrodes sandwiched between two cnc’d phenolic plates that hold the electrode at the correct angle and spacing around the motor. In each plate, half of the slots for the electrodes have holes drilled in them for small springs that connect the electrodes to a copper ring on the outside that is charged by the power supplies. One side of the motor is positively charged and the other negatively, ensuring that each electrode is charged appropriately.

We had the motor up and running about a month after we started thinking about and designing it. Then the challenge was to present it in a way that was appealing and safe for the public. We decided to house the motor in a plexiglass tube with 1/4" walls to ensure that there would be no arcing from the motor to a visitors hand. The plexiglass housing is mounted on two plexiglass tubes that extend into the table, allowing cable management and airflow for the motor.

Making the legs for the housing was rather difficult, as the larger tube is 12" diameter and the legs had to match the curvature for a seamless look. In order to accomplish this, we clamped the smaller tubes to the Bridgeport mill and used a 6" boring bar to shape the ends. This was the limit of our boring bar and our mill, and the entire mill shook as we did the operation.

I also built the table that the exhibit would be mounted and displayed on. It is made out of 1" steel square tube and TIG welded together. It features a closed cabinet for power supply housing and ventilation and prevents visitors from gaining access to the power supplies and other wiring. The completed exhibit also features a push button so the motor only runs when a visitor is interacting with the exhibit.

While we mostly had designs for this project, there were some challenging things that had to be designed for and thought through in the build process. Designing how the electrodes would be attached and charged was a challenge, but the solution we came to is elegant and allows for easy maintenance and replacement. The project moved very quickly, and was completed in 3 months. Additional refinements were made, but the exhibit was floor ready by the time I left for school in September.