Electrostatic motors reach the macro scale | Hacker News Reader

Electrostatic motors reach the macro scale

spectrum.ieee.org

128 pointsnamanyayg64 comments

There are other types of motors that rely neither on magnetic fields nor electrostatic fields, for example the ultrasonic motor which is commonly used as a focusing actuator in high end camera lenses. They use the piezoelectric effect to cause a semiconductor material to physically deflect and "push" the rotating part around in circles... or in a line in the case of a linear USM.

https://www.piezo-motor.net/

https://www.meddeviceonline.com/doc/what-are-canon-s-linear-...

Those are pretty neat. Their main advantage is being able to move quickly and with extreme precision (think down to nanometers). They're not particularly strong or efficient.

I'm sure this has already been invented, but I don't know the proper terminology:

Two or more motors with different characteristics that are somehow geared together, so that the X% errors in speed/position of a more-powerful motor get fixed by simultaneous activation of a finer motor (possibly in "reverse") and so on down the chain.

I suppose it hits diminishing returns whenever the main source of error comes from all the connections or gears.

I wager the electric motors actually have a very low angular velocity error on average. However given the way they work, during a revolution you likely have non linear rotation of the shaft on a basic motor due to the classical physics of the rotational load accelerating between its push/pull magnetic poles. Theres some interesting work in the motor driver space trying to smooth out the curves in the motion profiles of the trusty stepper motor (see TMC2209 driver promo content) and surely similar work for other electric motors. I bet one of our peers here could speak more authoritatively than my conjecture above.

This book describes a family of devices that work both as electrostatic generators and motors

https://www.amazon.com/Electrostatics-Exploring-Controlling-...

The cool thing is that this motor shouldn't take any power to maintain a static force against something. (In the same sense that a table takes 0 watts to hold something above the floor)

Only insulation resistance should be the loss at very low speeds.

static part of electrostatic!

I'm curious about their leakage current though

To compare different dielectric liquids you would want to see the dielectric spectroscopy plots:

https://en.wikipedia.org/wiki/Dielectric_spectroscopy

And the different types of viscosity vs temperature plots.

Hydraulics can do this. Pump in the oil, close a valve.

Mechanics: worm gears, ratchets. Static friction: e.g. tuning pegs in pianos (no gears).

This reminded me of the Propylene Carbonate on my Amazon wish list. A few years ago, I heard about HASEL actuators[1], which started out as zip lock bags with a dielectric fluid inside, aluminum foil, and high voltage power supply. The fluid turned out to be Propylene Carbonate.

I don't know chemistry... so I figured I'd wait until I was actually ready to try things with it (having the power supplies figured out) before I got it in the house. I'm not there yet, but I understand 1700 volt transistors are a thing now.

[1] https://www.artimusrobotics.com/

"C-Motive team succeeded in producing an organic liquid dielectric with low viscosity and a relative permittivity in the low 20s." Nice.

I'm curious if that's any use in variable capacitors; heck the picture of the cross section looks like a variable capacitor. (Except I guess the discs are non-continuous)

a variable capacitor will present some mechanical resistance and thus conversion of mechanical energy into heat, at low speed this is negligible (unless you need the system to behave reversibly), but if you are going to rotate your variable capacitor at speeds where frictional heating becomes an issue (perhaps the generation of heat limits your application, or the loss of mechanical energy limits your application) then yes these types of dielectrics will be of interest to you.

Otherwise, conventional dielectric can be more compact for the same maximum energy storage in the capacitor, since no simultaneous optimization on viscosity was performed in their selection, resulting in a wider range of dielectrics being viable.

How fast do you plan to turn your variable capacitors?

I wasn't interested in the rotation speed; more the high voltage - I know there are vacuum variable caps for high voltage work, but don't know what the current state of other dielectrics at a few kV is.

This will likely be a maintenance hell when the liquid ages and get particles in it.

Many organic liquids/solvents are cancer juice, I wonder if this is too.

The sustainability angle is a bit suspect, although of course it's smart of them to make that pitch. Many large motors don't use permanent magnets, and even for the ones that do, neodymium isn't all that rare and is mined in the US. Most of our copper comes from friendly sources too (US, Chile, Peru, Mexico, Australia). When we talk about sustainability for EVs, I think the main concern is batteries, not motors.

But the most interesting (and problematic!) part of their design is the use of a dielectric liquid to increase field strength. They don't give any specifics, but reliability and weight issues aside, I'd imagine that drag-related losses would get significant at high RPM. Maybe the point is to go slow?

How many RPM is "high?" I have a router that can go up to 40,000 RPM, reciprocating sander that goes about 2000 RPM, et. al. But if I want to actually do good high-quality sanding, lower RPM with higher momentum is better.

Also how big is a "large?" motor? Are we talking tens of horsepower, or single-digit horsepower? My drill press's motor is about 2 horsepower and my router is about 2.5 horsepower, for reference.

Unfortunately your drill press is now manually driven. When I found out it was 2 horsepower I “borrowed” it from your shop and turned it into a tiny go-kart

Look it's a 1950's Craftsman I got from a guy who wired the hot and neutral backwards when he replaced the original motor. They did things differently back then.

> Also how big is a "large?" motor?

The motors discussed in the article were described as "fractional horsepower"

that question was obviously not directed to the authors of the article, but to doe_eyes:

> Many large motors don't use permanent magnets, and even for the ones that do, neodymium isn't all that rare and is mined in the US.

What doe_eyes is referring to are motors or generators where the magnetic fields are generated by currents through windings. Under certain conditions it is more LCO efficient to generate magnetic fields that are stronger than even Neodymium magnets can supply.

Your dig about fractional horsepowers is thus misplaced.

Wasn't meant to be a dig.

The featured article is about electrostatic motors that provide less than one horsepower. Therefore any comparison to "large motors" is kind of off subject.

Most motors connected directly to AC power are likely to be induction motors, which require no special magnets. From fans, to fridges, to AC etc. Because they don't need drive electronics they can be very cheap compared to BLDC and the like, which would need power conversion from AC.

Turbine generators typically use Synchronous motors, which often use DC to generate the opposing magnetic field ("exciters"), though they can also use permanent magnets for the same effect.

They seem to be looking for low RPM, relatively higher torque applications. Maybe like a stepper motor? It’s claimed that you don’t need a gearbox.

“Ability to hold a position with virtually no energy losses”

https://www.c-motive.com/technology/

That could be useful for robotics. Gear trains in robots are a headache.

The ability to hold a position without much power is a big win. Steppers need almost full power when stationary. But that's probably because these electrostatic motors are run as servomotors, with only as much power applied as is needed at the moment.

Could it also lower the cost of simracing direct drive wheels?

While stepper motors produce torque based on the current flowing through them, electrostatic motors produce torque based on the voltage applied across them. As long as the motor is stationary, the voltage does not need to change, which means that there is no additional power needed to hold position even against an external force.

I am not correcting you since what you say is correct, but looking at other comments I thought it would help to expand a bit for the other readers.

In your static torque situation, the electrostatic motor draws 0 current at your constant voltage. And thus 0 power (instantaneous power is instantaneous voltage times instantaneous current).

Whereas a conventional electromotor would be called stalled in such a situation, in which case the coils of the electromagnets in the motor behave like inductors.

With the result that for a constant applied voltage the current increases linearly with time, until the parasitic winding resistance of the coil limits the current.

This means a stalled electromotor will consume energy without performing mechanical work, and all this energy will be dissipated as heat developed over the winding resistance of the coils.

If that heat cannot escape the electromotor fast enough, the insulation of the electromotor coil will be compromised, and you gradually loose loops of the coil as they short, further decreasing the total winding resistance (since the shortcut means a loop of single turn winding resistance less), which increases the current, and thus the power into the motor.

This thermal runaway eventually destroys your motor.

That is why you should immediately shut of electric motors as soon as you detect a stall condition (typically you will hear mains hum as the stalled motor is pounding whatever stalls the motor at twice the mains line frequency), and allow it to cool, while you resolve the cause of the stall condition.

So next time you use your bar blender in the kitchen, and you notice its struggling, or worse blocked, immediately stop the motor / back off, or let it cool. Don't just press the "boost" button for prolonged durations unless you like buying blenders over and over.

They probably mean sustainability due to frictional heat losses in drive trains / gear boxes.

The US is not a very friendly nation...

"Benjamin Franklin built and demonstrated a macroscopic electrostatic motor in 1747,” says Krein. “He actually used the motor as a rotisserie to grill a turkey on a riverbank in Philadelphia” "

I want to know more about Ben Franklin's electrostatic turkey roaster

Cool thanks!

>He actually used the motor as a rotisserie to grill a turkey on a riverbank in Philadelphia

Benjamins aeolipile?

I wonder how well the dielectric fluid holds up if the motor is spinning at several thousand RPMs for dozens of hours?

I suspect heat is a limiting factor here.

Also, if the vanes of the rotor are spinning _in_ the fluid, doesn't this also make it a torque converter? If so, then suddenly stopping the motor could be catastrophic depending on how much kinetic energy is in the system at the time.

From the context, I'd guess they want their motor to spin at single digit RPMs, not thousands of them.

That's the stuff magnetic motors are bad at.

Yeah, all you have to do to get a conventional motor to spin faster is to make it longer. When people show off their high speed motors they are actually showing off the easy part. It is much harder to get high torques, because you need to increase the number of poles and the diameter.

utter nonsense.

small RPM but high torque electromotors are made by making them longer, make a motor axially longer by a factor L, and the torque will multiply by L, while the volume also increases by L.

suppose we followed your advice and multiplied the diameter by the same factor L to get the same increase in torque, now the volume is multiplied by L squared!

And, for a simpler linear version that's used to drive air: electrostatic speakers [1]!

[1] http://jazzman-esl-page.blogspot.com/2016/12/wire-stator-esl...

I counted at least 30 electric motors in my (non-EV) car. 3 in each mirror. 2 in the HUD.

All of those will end up in a landfill/junk yard. Seems like a huge waste.

The materials will probably be recycled

Who will buy a 0.5 hp motor whose parts must be bathed in an exotic dielectric fluid, and that needs a 2000V supply, and will they buy another one?

The dielectric is internal, and the motor will presumably have a built-in driver to adapt from wall voltage to 2kv. Neither of those issues are at all an issue.

"No user-serviceable parts inside", and nobody qualified to work on it within a 1000 mile radius.

A big factor with this will be how inexpensive they are relative to something that's more maintenance friendly. If you can get five of them for the price of a similar traditional electric motor I imagine it might be worth planning to swap and never service yourself. Particularly the longer they tend to last.

I get it now. These will be lighter and cheaper than induction motors, due to containing a lot more plastic and a lot less metal and wire.

Nobody fixes fractional Watt induction motors. When a coil burns out (or whatever), it goes to the landfill (perhaps along with the whole appliance).

I couldn't get past the first two paragraphs without the article losing all credibility.

> And although there are many different kinds of electric motors, every single one of them, from the 200-kilowatt traction motor in your electric vehicle to the stepper motor in your quartz wristwatch, exploits the exact same physical phenomenon: electromagnetism.

Well, basically your whole experience of the world is just electromagnetism, nothing more. And electrostatics is part of electromagnetism theory.

> In some applications, these motors could offer an overall boost in efficiency ranging from 30 percent to close to 100 percent, according to experiment-based analysis.

What practical electric motor is even close to 30% efficient? This is laughably low.

Edit: it's BOOST over the current efficiency.

> What practical electric motor is even close to 30% efficient? This is laughably low.

I think that means 30% over existing performance.

Got it, missed that part.

Still, assuming efficiency can't get past 100%, 100% boost can be achieved only on something that already has only 50% efficiency.

I assume that they are looking at in laymans terms, and saying that if an electromagnetic motor is 80% efficient, an electrostatic motor could increase efficiency of the remaining 20% by 30% to 100% (86-100% total efficiency). Not that it makes any sense, but I think that was their intention.

i wish people would just always use percentage points and never deviate from that.

I wish people would just explicitly state they can't believe their eyes that 100% efficiency can be asymptotically reached, i.e. no known law of the universe prevents us from building electromotors that are 90%, 99%, 99.9% etc... absolute efficient.

Then we could just remind them that Carnot efficiency does not apply to electrical / mechanical energy conversion.

This is absurd, detecting that a wrong interpretation is inconsistent (because it would result in overunity violation of energy conservation) and instead of rejecting the misinterpretation of the parent, concocting an even more convoluted interpretation.

All because people refuse to believe 100% energy conversion between electromagnetic domain and mechanical domain is impossible?

Turbosets have been doing this for a long time already, the Carnot efficiency limit does not apply to non-thermal energy conversions...

THey are in fact talking about absolute efficiency.

Conventional electromotors are designed with high efficiency... at a certain range of RPM and torque. For lower RPM's permanent magnet electromotors suffer dramatic decreases in efficiency and torque, unless you use a gearbox, which also produce heat due to frictional loss.

These electrostatic motors can achieve quasi reversible performance (i.e. asymptotically close to 100% efficient, not a violation of thermodynamics, since neither electrical nor mechanical energy are thermal forms of energy).

Turbosets also reach nearly 100% conversion efficiency.

Electrostatic motors started their niche with miniature motors, since they were more compact and it becomes progressively harder to miniaturize winding coils. Where a simple electrode surface would be more space efficient.

Pay attention to Macroscale in the title, its these miniature-niche low RPM motors slowly capturing larger torque and higher RPM lebensraum from the gearboxed permanent magnet electromotors.

Yeah, that was some poor quality for IEEE. I'm pretty sure they confused "electromagnets" and "electromagnetism."

As far as the "boost in efficiency" sentence, I reread the paragraph around it several times and still have no clue WTF they are trying to say.

Spectrum is aimed at the general public and its articles are often university press release quality.

> I reread the paragraph around it several times and still have no clue WTF they are trying to say.

some advice: next time you don't understand some article, consider the possibility that it is not a case of:

> Yeah, that was some poor quality for IEEE. I'm pretty sure they confused "electromagnets" and "electromagnetism."

but that the problem might be your lack of understanding that you already detected. it's a press article, it can't teach you an undergraduate course in physics condensed to a few paragraphs of text...

I have taken several undergraduate courses in physics. If you have some clue what they are saying with:

> In some applications, these motors could offer an overall boost in efficiency ranging from 30 percent to close to 100 percent

please illuminate. Brushless DC motors across a wide variety of applications already exceed 50% efficiency. Perhaps there are applications in which they cannot reach 50% efficiency, so a 100% boost in efficiency would be possible.

The key is “in some applications.” Not in all applications. There are areas where brushless DC motors are inefficient, these are better in those areas - my guess is low speed, high torque use cases.

A) that's the sort of thing I wish the article had mentioned.

B) I know it's research, but low speed high torque situations usually involve a reduction gear-set. They make some surprisingly compact ones these days.

B) Yes, which involve losses and packaging changes. Exactly why a motor like this would be useful…

Also, piezoelectric motors are a thing - sure, they're super tiny and for specific purposes (very fine movement), but beyond the fact that they use electricity to generate the vibrations that they then use for movement, I don't think it would be considered "electromagnetic".

Actually, in that same vein would be a Nitinol or similar "shape-memory alloy" motor - run power through it to have it change shape, then remove power to let it relax.

So yeah, unless I'm misremembering or grossly misusing terms, "piezoelectric" and "thermoelectric" electric motors both exist...

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