4 ms·

How light could we make a motor that worked on electric fields instead of magnetic fields?

Turing_Machine 5y ago
A lot of MEMS motors use electric fields, so... pretty danged light.
jbay808 5y ago
Shinsei corporation made a (macroscopic) electrostatic motor that operates on that principle. I guess they did it just out of curiosity because I don't think it has many advantages. It required high voltages and all the active surfaces had to be highly polished to prevent electric arcing. If I recall correctly it had to run in vacuum as well, for the same reason. But it was a pretty cool project.
- userbinator 5y ago
  Electrostatic motors have been around in some form ever since the earliest experiments with (static) electricity, so they predate even electromagnetic motors. The high voltages required and their relatively low torque has kept them from gaining much use. https://hackaday.com/2017/10/03/ben-franklins-weak-motor-and-other-forgotten-locomotion/ https://hackaday.com/2017/10/03/ben-franklins-weak-motor-and...
adrian_b 5y ago
For large motors that work in air, electric fields are not competitive with magnetic fields. The usable magnetic fields are limited by the saturation of the soft ferromagnetic parts or by the demagnetization of the permanent magnets at around 1 to 2 tesla, while the electric fields are limited by the electrical breakdown of the air at around 3E6 V/m. 1 tesla is equivalent with 3E8 V/m (multiplied by the speed of light), so the usable magnetic fields are 100 to 200 times greater than the usable electric fields. So any large motor must be made with magnetic fields, otherwise it would need to be huge for the same torque. On the other hand the motors with magnetic fields have large power losses in conductors (copper losses) and ferromagnetic materials (iron losses). Because of this, while large motors can have an efficiency above 99%, the smaller an electrical motor with magnetic fields is, the lower its efficiency is. So below a certain very small size, the efficiency of a motor with magnetic fields becomes so low that the motors with electric fields become preferable because of their higher efficiency.
- mrfusion 5y ago
  That’s a great explanation. I wonder if anyone has looked at using materials other than air between the parts?
  - adrian_b 5y ago
    Another poster mentioned a motor with electric fields made to work in vacuum, which may use much higher electric fields. A motor could be filled with oil, but the problem of preventing oil leaks through the bearings would be insurmountable. Keeping vacuum inside the motor would be even harder. Enclosing completely the motor in a case and using a magnetic coupling through the wall might be a solution to avoid holes in the case, but it would greatly increase the cost and it would be complex to ensure lubrication for the through-the-wall coupling. No such solution seems acceptable, so besides microscopic motors the only other possible application for motors with electric forces would be in some motors intended to be used in satellites, on the Moon, or in other such places where the motors could operate in vacuum.
    - snovv_crash 5y ago
      There would also be the viscosity losses of an oil-filled motor.
    - usrusr 5y ago
      > A motor could be filled with oil, but the problem of preventing oil leaks through the bearings would be insurmountable Shouldn't be too hard for stationary applications if you stick to a vertical axis and allow gravity to help keeping oil and air separate. I guess neighbor post's viscosity counterargument will do fine in its own though.
- mrfusion 5y ago
  Is there anywhere that explains how a traditional electrostatic motor works?
  - adrian_b 5y ago

A lot of MEMS motors use electric fields, so... pretty danged light.

Shinsei corporation made a (macroscopic) electrostatic motor that operates on that principle. I guess they did it just out of curiosity because I don't think it has many advantages. It required high voltages and all the active surfaces had to be highly polished to prevent electric arcing. If I recall correctly it had to run in vacuum as well, for the same reason. But it was a pretty cool project.

userbinator 5y ago
Electrostatic motors have been around in some form ever since the earliest experiments with (static) electricity, so they predate even electromagnetic motors. The high voltages required and their relatively low torque has kept them from gaining much use. https://hackaday.com/2017/10/03/ben-franklins-weak-motor-and-other-forgotten-locomotion/ https://hackaday.com/2017/10/03/ben-franklins-weak-motor-and...

adrian_b 5y ago

For large motors that work in air, electric fields are not competitive with magnetic fields. The usable magnetic fields are limited by the saturation of the soft ferromagnetic parts or by the demagnetization of the permanent magnets at around 1 to 2 tesla, while the electric fields are limited by the electrical breakdown of the air at around 3E6 V/m. 1 tesla is equivalent with 3E8 V/m (multiplied by the speed of light), so the usable magnetic fields are 100 to 200 times greater than the usable electric fields. So any large motor must be made with magnetic fields, otherwise it would need to be huge for the same torque. On the other hand the motors with magnetic fields have large power losses in conductors (copper losses) and ferromagnetic materials (iron losses). Because of this, while large motors can have an efficiency above 99%, the smaller an electrical motor with magnetic fields is, the lower its efficiency is. So below a certain very small size, the efficiency of a motor with magnetic fields becomes so low that the motors with electric fields become preferable because of their higher efficiency.

mrfusion 5y ago
That’s a great explanation. I wonder if anyone has looked at using materials other than air between the parts?
- adrian_b 5y ago
  Another poster mentioned a motor with electric fields made to work in vacuum, which may use much higher electric fields. A motor could be filled with oil, but the problem of preventing oil leaks through the bearings would be insurmountable. Keeping vacuum inside the motor would be even harder. Enclosing completely the motor in a case and using a magnetic coupling through the wall might be a solution to avoid holes in the case, but it would greatly increase the cost and it would be complex to ensure lubrication for the through-the-wall coupling. No such solution seems acceptable, so besides microscopic motors the only other possible application for motors with electric forces would be in some motors intended to be used in satellites, on the Moon, or in other such places where the motors could operate in vacuum.
  - snovv_crash 5y ago
    There would also be the viscosity losses of an oil-filled motor.
  - usrusr 5y ago
    > A motor could be filled with oil, but the problem of preventing oil leaks through the bearings would be insurmountable Shouldn't be too hard for stationary applications if you stick to a vertical axis and allow gravity to help keeping oil and air separate. I guess neighbor post's viscosity counterargument will do fine in its own though.
mrfusion 5y ago
Is there anywhere that explains how a traditional electrostatic motor works?
- adrian_b 5y ago