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So, I understand that relays, motors, and other electromechanical devices are almost always electromagnetic devices. Why is this? I know that electrostatic motors exist, but what makes them substantially worse than electromagnetic motors?

Note that I'm asking specifically about macroscopic devices. I'm aware that electrostatic electromechanical devices are common in MEMS.

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    \$\begingroup\$ Aren't some of the paper-pickup mechanisms in copiers electrostatic? Also, some ink-deposition methods? Electrospinning of membranes? \$\endgroup\$ May 10 '18 at 14:22
  • \$\begingroup\$ @ScottSeidman That's entirely possible! I was thinking more about machines analogous to motors, where a high power is required. \$\endgroup\$
    – Hearth
    May 10 '18 at 14:23
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    \$\begingroup\$ @ScottSeidman: Sure, there are plenty of applications for static electricity, but just try building a 700 watt electrostatic motor that can run your washing machine! \$\endgroup\$
    – Dave Tweed
    May 10 '18 at 14:26
  • \$\begingroup\$ Do you count piezoelectric motors and actuators? They are reasonably common. \$\endgroup\$ May 10 '18 at 16:40
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    \$\begingroup\$ Electrostatic audio speakers. \$\endgroup\$ May 10 '18 at 17:00
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Have you calculated the voltages you'd need in order to get the equivalent force or torque? It's much easier to build compact machines of equivalent power based on high currents rather than high voltages.

Making a machine compact basically boils down to being able to concentrate the field adequately. It's easy to concentrate magnetic fields by adding more turns to coils and using iron pole pieces to guide them.

There are analogous structures for electric fields, but a key problem is that air (and other dielectrics) break down under high fields, allowing the electrons to leave the conductors, which ruins everything. Even in a hard vacuum, spontaneous emission puts a limit on how strong a field you can develop.

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  • \$\begingroup\$ This does cover motors and speakers, but I'm curious about relays as well. It seems like an electrostatic relay would avoid the occasional problem of a relay messing up hall effect measurements, though that might be a rare enough problem that it doesn't really matter. \$\endgroup\$
    – Hearth
    May 10 '18 at 14:25
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    \$\begingroup\$ There is no substantial difference between speakers and relays or solenoids. Don't you think that E-field leakage would mess up nearby electronics, too? One alternative is piezoelectric machines. \$\endgroup\$
    – Dave Tweed
    May 10 '18 at 14:29
  • \$\begingroup\$ The difference I refer to is in the power level. Relays aren't intended to actuate anything more than a contact, whereas speakers convert electrical power to audio power, and solenoids to mechanical power. But yes, you're right. Leakage will always be a problem, but it seems to me that magnetic fields are potentially more problematic than static electric fields. That could very well be wrong, though! \$\endgroup\$
    – Hearth
    May 10 '18 at 14:32
  • \$\begingroup\$ Relays happily power higher current devices : car fog and spot lamps in 10’s of amps... \$\endgroup\$
    – Solar Mike
    May 10 '18 at 14:47
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    \$\begingroup\$ Electrostatic relays in the form of e.g. MEMS switches are an appealing technology which have been the focus of significant research and development attention for some time now. The small scale of MEMS devices means that useful functionality (switching a contact) can be achieved with reasonable field strengths. Commercial offerings are available and you are starting to see them in markets. \$\endgroup\$
    – user49628
    May 10 '18 at 16:22
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There is ongoing research into electrostatic machines. A prototype rotary actuator is described by: G. Reitz, B. Butrymowicz, J. Reed, B. Ge and D. C. Ludois, "A switched elastance electrostatic machine constructed from sustainable elements for rotational actuators," 2017 IEEE Energy Conversion Congress and Exposition (ECCE), Cincinnati, OH, 2017, pp. 2389-2395

The described machine operates at 9000 volts and has a liquid dielectric. It produces 2 N-m of torque at stall, but the internal drag torque increases with speed to equal the torque produced at about 1100 RPM. The machine was constructed in a NEMA 42 frame. C-Motive Technologies Inc., is said to be commercializing this technology.

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One macroscopic application of electrostatics that's commercially available is Electrostatic Loudspeakers (ESL). These are generally expensive speakers, limited to "audiophile" applications.

More information here: https://en.wikipedia.org/wiki/Electrostatic_loudspeaker

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Here is nice book detailing several kinds of electrostatic motors...

enter image description here

http://rexresearch.com/jefimenko/jefimenkoesmotors.pdf

I would not call them "worse", but they are different than magnet motors.

As others have noted above, you will need very high voltages and/or lots of surface area to produce even relatively low torque, but for low toque, high speed, high reliability, low power applications an electrostatic motor can be a good choice especially if you already have a high-voltage/low-current supply available.

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