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Comment by mrfusion | Hacker News Reader

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

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.

That’s a great explanation. I wonder if anyone has looked at using materials other than air between the parts?

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.

There would also be the viscosity losses of an oil-filled motor.

> 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.

Is there anywhere that explains how a traditional electrostatic motor works?

For many structures of motors with magnetic fields, there are equivalent structures of motors with electric fields, which you can obtain by replacing each pair of windings that creates a north-south pair of magnetic poles with the 2 plates of a capacitor, which create a positive-negative electric pole pair when the capacitor is charged.

For example, you could have a stepping motor where the rotor has 2 metal plates at 180 degrees on the cylinder, and you apply a fixed voltage on them, so that one is positive and the other is negative.

On the periphery of the stator, surrounding the rotor, you have 2 or more pairs of metal plates spaced at equal angles. The 2 plates of a pair, at 180 degrees on the stator, are connected together forming a capacitor and you have as many capacitors as pairs.

Each stator capacitor will correspond with a phase of the stator. When you apply voltage on one phase of the stator, the rotor will rotate to allign with the plates of that phase (the negative rotor pole to the positive stator pole and similarly for the opposite pole).

To advance 1 half step, you charge the capacitor of the next phase and the rotor will move half-way between the plates (being equally attracted by them). Then you discharge the previous phase capacitor and the rotor will move to be aligned with the plates of the next phase, achieving a full-step movement. Applying a sequence of stator capacitor charges and discharges will achieve any movement you want.

The same transformation described above for a stepping motor can be done for any other kind of electric motors. Even the permanent magnet motors and the hysteresis motors have correspondents in motors with electrets, but those are unlikely to have so good characteristics as the motors with magnets.

In a motor with magnetic fields, if you want to supply the rotor with a current, you need gliding contacts, which have a limited lifetime, and you also have a permanent power consumption. These 2 disadvantages make preferable the induction motors, permanent magnet motors, hysteresis motors or variable reluctance motors, all of which are worse for other characteristics.

For a motor with electric fields, the rotor needs a voltage, but the only power loss is due to leakage currents, so the rotor could incorporate a battery to provide the voltage, which could last many years.

Unfortunately, the limitations due to electrical discharges make this potential advantage irrelevant, except maybe for use in vacuum outside Earth.

Thanks. That makes sense.

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.

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...

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