Draper (officially, The Charles Stark Draper Laboratory, a US-based not-for-profit R&D organization based in Cambridge, Mass.) is developing a new family of electric motors and generators based on electrostatic, rather than electromagnetic principles. Underpinning the initiative is a recent patent on aspects of the technology.
Almost all electric motors today use magnetism to generate torque. For centuries, engineers have known that forces from electric fields can also be harnessed to build motors, but these so-called electrostatic motors were too weak and problematic to compete with their electromagnetic counterparts. Even with a resurgence in interest over the last few decades, electrostatic motors suffer from various problems, such as high weight, low maximum rotation rate, very large volumes of dielectric fluid and power supplies requiring very high voltages.
However, by leveraging state-of-the-art materials, novel designs and decades of fabrication expertise, Draper is developing powerful new electrostatic motors that Draper says break the torque barrier suffered by their predecessors.
Draper’s approach to electric motors has numerous advantages over conventional electric motors, including significantly lower weight, higher efficiency, higher specific power and lower cost of materials. Draper’s new approach could benefit a vast range of technologies now dependent on electric motors, from electric vehicles to mobile defense applications, such as drones, and greatly extend their range and mobility.
Our e-motors use thin electrodes and electrets which reduce weight by 80% or more as compared to conventional motors. This translates to a range extension of up to 40% for drones and up to 25% for electric vehicles based on our simulations.—Sabrina Mansur, automotive business development manager at Draper
Electrets (“electricity” + “magnet”) are dielectric materials with a quasi-permanent electric charge or dipole polarization. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. Electrets have been used in more recent attempts at developing viable electrostatic motors (e.g., Genda et al.).
As described in the patent application, the Draper electrostatic motor includes a cylindrical rotor and a stator. Electrodes are placed on an inside cylindrical surface of the stator. Electrets and/or electrically conductive electrodes are placed on the cylindrical rotor and a dielectric fluid fills space between the rotor and the stator to prevent discharge of the electrets. A mask is used to charge portions of an electret cylinder or other curved surface.
The company says its electric motors are designed without the use of rare earth materials, an important consideration in light of rising prices for these materials and reports of China’s potential ban on the export of rare earth materials.
Electric vehicles, drones, robots and other products require ever-increasing performance from electric motors, but are limited by the weight and cost of materials needed to make a conventional motor—steel, copper coils and rare earth magnets. Draper’s recently patented approach to electric motors, which is available to license, replaces those materials with thin, light and widely available materials.
T. Genda, S. Tanaka and M. Esashi (2003) “High power electrostatic motor and generator using electrets,” TRANSDUCERS '03. 12th International Conference on Solid-State Sensors, Actuators and Microsystems. Digest of Technical Papers (Cat. No.03TH8664), Boston, MA, USA, pp. 492-495 vol.1, doi: 10.1109/SENSOR.2003.1215361