3 ms·

I’ve always wondered why no one is researching motors that use electric fields instead of magnetic?

blattimwind 7y ago
There are electrostatic motors, it's just that they aren't particularly interesting (low power density, very high drive voltages, poor efficiency). They do have one interesting property, which is that at zero speed they can develop nominal torque without using any power (unlike say a synchronous servo).
- mrfusion 7y ago
  How much of that is just a matter of more R&D though? I don’t see why any of that is inherent to electrostatic?
  - blattimwind 7y ago
    It's mostly Coulomb's law; electrostatic force is proportional to the involved charges divided by their separation. You don't have that problem in electromagnetic motors because strong magnetic fields don't break down the rotor/stator air gap. You might turn the air gap into a dielectric oil gap, but now you have fluid friction losses which again limit how narrow that gap can be. You'll note that this is a recurring pattern with electromagnetic vs. electrostatic implementations of roughly the same idea.
    - mrfusion 7y ago
      Good explanation. So it makes me wonder if we could improve on the capacitor if we could find its magnetic counterpart. Does that even make sense?
      - hwillis 7y ago
        like... an inductor? Inductors and capacitors are opposite in behavior, but a series capacitor can be replaced with a parallel inductor and vice versa. In that sense they are counterparts. It is not possible to make an inductor out of purely passive capacitive components, or vice versa. In fact the Gyrator[1] is a transistor circuit that exists specifically to act like an large inductor using capacitors, which are cheaper to build at large values. The function of a Gyrator inherently requires active power input; it isn't possible to passively convert the phase lag of a magnetic circuit into the phase lead of a capacitive circuit. [1]: https://en.wikipedia.org/wiki/Gyrator https://en.wikipedia.org/wiki/Gyrator
Synaesthesia 7y ago
You can’t generate strong static electric fields without it discharging, unlike magnetic fields made by currents.
hwillis 7y ago
Relative permeability is the characteristic of electrical steels that lets them carry magnetic fields. M19 steel has a permeablity ~8500x higher than vacuum[1]. Permittivity is the characteristic that allows dielectrics to transmit electrical fields. The vast majority of dielectrics are <100x higher than vacuum permittivity, and they're very unsuitable to make a motor out of. Materials with high permittivity have fairly low breakdown voltages. Also, humans have very low breakdown voltages; high voltage motors are often dangerous to be around. Fundamentally it's because materials lend themselves much better to magnetics. [1]: https://pdfs.semanticscholar.org/9303/19ba00b3c7329308df24798c4ec177d2946f.pdf https://pdfs.semanticscholar.org/9303/19ba00b3c7329308df2479...

There are electrostatic motors, it's just that they aren't particularly interesting (low power density, very high drive voltages, poor efficiency). They do have one interesting property, which is that at zero speed they can develop nominal torque without using any power (unlike say a synchronous servo).

mrfusion 7y ago
How much of that is just a matter of more R&D though? I don’t see why any of that is inherent to electrostatic?
- blattimwind 7y ago
  It's mostly Coulomb's law; electrostatic force is proportional to the involved charges divided by their separation. You don't have that problem in electromagnetic motors because strong magnetic fields don't break down the rotor/stator air gap. You might turn the air gap into a dielectric oil gap, but now you have fluid friction losses which again limit how narrow that gap can be. You'll note that this is a recurring pattern with electromagnetic vs. electrostatic implementations of roughly the same idea.
  - mrfusion 7y ago
    Good explanation. So it makes me wonder if we could improve on the capacitor if we could find its magnetic counterpart. Does that even make sense?
    - hwillis 7y ago
      like... an inductor? Inductors and capacitors are opposite in behavior, but a series capacitor can be replaced with a parallel inductor and vice versa. In that sense they are counterparts. It is not possible to make an inductor out of purely passive capacitive components, or vice versa. In fact the Gyrator[1] is a transistor circuit that exists specifically to act like an large inductor using capacitors, which are cheaper to build at large values. The function of a Gyrator inherently requires active power input; it isn't possible to passively convert the phase lag of a magnetic circuit into the phase lead of a capacitive circuit. [1]: https://en.wikipedia.org/wiki/Gyrator https://en.wikipedia.org/wiki/Gyrator

Synaesthesia 7y ago

You can’t generate strong static electric fields without it discharging, unlike magnetic fields made by currents.

hwillis 7y ago

Relative permeability is the characteristic of electrical steels that lets them carry magnetic fields. M19 steel has a permeablity ~8500x higher than vacuum[1]. Permittivity is the characteristic that allows dielectrics to transmit electrical fields. The vast majority of dielectrics are <100x higher than vacuum permittivity, and they're very unsuitable to make a motor out of. Materials with high permittivity have fairly low breakdown voltages. Also, humans have very low breakdown voltages; high voltage motors are often dangerous to be around. Fundamentally it's because materials lend themselves much better to magnetics. [1]: https://pdfs.semanticscholar.org/9303/19ba00b3c7329308df24798c4ec177d2946f.pdf https://pdfs.semanticscholar.org/9303/19ba00b3c7329308df2479...