By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
This paper summarizes the construction and initial experiments of a corona type electrostatic motor modeled after similar motors built by V.E. Johnson in 1921. My version of this motor self-starts at one microampere (1mA) with 12 kilovolts (kV) Direct Current (DC). One variant achieved 4,200 revolutions per minute (rpm). Besides documenting the construction and describing the initial performance test results, the paper concludes with a few thoughts on further work. This project was not without its potential safety hazards. Precautions included insulation for all high potential wiring and maintaining a safe distance from the high voltage DC power supply. I do not recommend any experimenter duplicate this work unless they are very familiar with the dangers involved.
This corona type electrostatic motor has a horizontal polycarbonate (or acrylic) disc rotor, with its shaft supported on sapphire jewel v-bearings. The rotor is constrained by its shaft to rotate. The two corona point electrodes are tangential to the rotorâ€™s periphery and are oriented in the same rotational direction. The two corona points deposit opposite electric charges on the rotor surface. The respective sections of the rotor then have the same polarity as a given corona point and are repelled. As this electrostatic force applies torque to the rotor, it begins to rotate about its axis. The rotorâ€™s resulting momentum moves that particular charged rotor section (positive or negative) closer to the other electrode. The rotor is then attracted as it approaches the corona point of opposite polarity, is discharged and acquires the same polarity as that electrode1. This cycle is a continuous process â€“ it repeats itself and the rotor can reach high rotational speeds.
Referring to the diagram and photographs, the motor is simply a 0.375â€ thick polycarbonate (LexanÂ®) disc with a central axis (shaft) supported by a frame. The polycarbonate disc in one prototype was 1.5â€ in diameter, the other was 3â€. Each disc had an aluminum hub fixed in place with cyanoacrylate adhesive, e.g., super glue or similar. The shaft is a press-fit into the aluminum hub. The two tangential electrodes and input terminals are mounted on insulating acrylic posts. The rotorâ€™s shaft was a 0.125â€ or 0.1875â€ diameter steel rod with both ends machined to a 60o point. The motor uses sapphire v-bearings with a low coefficient of friction (CoF). The steel-on-sapphire static CoF is 0.15. Not needing a lubricant (attracts dust â€“ increases CoF) is another advantage for this combination.
Other material combinations are certainly acceptable if the pivot point geometry minimizes contact within the v-bearing. Wear and deformation are two other important aspects. The static and dynamic CoF for many material combinations are available from jewel bearing manufacturers, e.g., Bird Precision, Moser Company and Swiss Jewel Company websites. Another source is the CoF table found in the Engineers Handbook at http://engineershandbook.com/Tables/frictioncoefficients.htm, the cited reference or its other editions. CoF values are shown as static or dynamic (sliding) and with or without a lubricant. The range static of friction is the region up to the impending motion. Static friction is the force needed to overcome the resistance to movement of two surfaces in intimate contact. Dynamic or kinetic friction is associated with movement and is always less than the static values. The details on setting v-bearings into setscrews are straightforward, but the process does entail careful machine work and attention to detail. For example, one must burnish the steel pivot points to minimize surface asperities – the smoother the better. Secondly, hardening the steel minimizes its deformation under load.
Equipment and Procedures
The motor used a 12kV power supply, a vintage surplus item that started life in an oscilloscope many years ago. Modern equivalents can include semiconductor diode multipliers connected to a high voltage transformer. Alternatively, the 1B3GT vacuum diode tubes (new-old stock) are still available at reasonable prices. These diodes have a 30kV peak inverse voltage (PIV) rating. An analog 0 to 40mA ammeter was connected in series to measure the current. All wiring was designed for high potentials, e.g., silicone rubber insulated type with a 15kV rating. Secondly the ammeter was placed on an insulating block. The motor and power supply was mounted on stand-off insulators. All components thus acquired the 12kV potential and were not tied to an earth ground. The shock hazard in this case was touching anything except the on/off switch on the power supply. I ensured the circuit was disconnected and the power supply capacitors were discharged each time I made an adjustment. Another safety precaution was keeping one hand in my pocket during this process. This practice can prevent one from completing a potentially lethal circuit â€“ one hand to the other through the heart. The rotor speed was measured with an inexpensive photo-tachometer.
The threshold self-starting current was one to 2mA with an ammeter in series with the motor and 12kV power supply. Rotor rotation was barely discernible and almost totally silent at high shaft speeds. The rotor would also continue spinning from the residual charge in the voltage multiplier capacitors. The motor was smooth running from 2mA to 3.25mA, but there was little performance gain with a higher 5mA current. As one would expect, rotor speed increased with the motor current. The smaller motor achieved steady state operation at 5.25mA (12kV), with a rotor speed of 4,200 rpm. The larger of the two motors achieved a shaft speed of ~1700rpm at similar currents.
Conclusion and Further Work
These particular electrostatic motors self-started at currents down to 1mA and achieved excellent performance. Further work could include collecting more comprehensive rotor speed data to illustrate the relationship between it and input current. Another avenue of research is operating the motor from the earthâ€™s electrostatic potential. A potential gradient exists between a ground (earth) conductor and an insulated point a given height above ground. This potential varies with the local topography and climatic conditions, but it can reach about 100 to 120 volts/meter and higher values,. An antenna ~10m above ground could then acquire a ~1.2kV potential (all things being equal). Therefore, it is conceivable to construct a small electrostatic motor to operate from this potential, as Dr. Jefimenko did circa 1970. A challenging project with some practical difficulties to surmount.
Â Â Â O.G. Jefimenko, Electrostatic Motors: Their History, Types and Principles of Operation. Electret Scientific Company, Star City, WV, 1973. Note â€“ This is the 2011 revised edition.
Â Â Â R.C. Weast, Ed., Handbook of Chemistry & Physics (64th Ed.), CRC Press, Inc., Boca Raton, FL, 1984.
Â Â Â J.L. Meriam, Mechanics, Part I â€“ Statics (2nd Ed.), John Wiley & Sons, Inc., New York, 1959.
Â Â Â T.E. Raney, Jewel Bearing Assembly Fabrication (unpublished work), 15 October 2003.
Â Â Â C.L. Stong, The Amateur Scientist, Simon & Schuster, Inc., New York, 1960.
Â Â Â R.A. Ford, Homemade Lightning: Creative Experiments in Electricity (2nd Ed.), The McGraw-Hill Companies, Inc., New York, 1996.
Â Â Â S.L. Lerner, Physics for Scientists and Engineers, Jones & Bartlett Publishers, Sudbury, MA, 1996.
Â Â Â A.D. Moore, Electrostatics: Exploring, Controlling and Using Static Electricity (2nd Ed.), Laplacian Press, Morgan Hill, CA, 1997.
Â Â Â J.H. Moore, et al, Building Scientific Apparatus: A Practical Guide to Design and Construction (2nd). Addison-Wesley Publishing Company, Inc., Redwood City, CA, 1989.
 O.G. Jefimenko, Electrostatic Motors: Their History, Types and Principles of Operation. Electret Scientific Company, Star City, WV, 1973, pp. 74 â€“ 75 and 80 â€“ 82.
 R.C. Weast, Ed., Handbook of Chemistry & Physics (64th Ed.), CRC Press, Inc., Boca Raton, FL, 1984, p. F-17.
 J.L. Meriam, Mechanics, Part I â€“ Statics (2nd Ed.), John Wiley & Sons, Inc., New York, 1959, pg. 251.
 T.E. Raney, Jewel Bearing Assembly Fabrication (unpublished work), 15 October 2003.
 C.L. Stong, The Amateur Scientist, Simon and Schuster, Inc., New York, 1960, pg. 274.
 O.G. Jefimenko, Electrostatic Motors: Their History, Types and Principles of Operation. Electret Scientific Company, Star City, WV, 1973, pp. 126 â€“ 128 and 134.
 O.G. Jefimenko, pp. 25 â€“ 26.