# Efficiently turning electric to kinetic energy

What are the most efficient electric mechanisms for creating kinetic energy?

Purely from a coulomb-volt to foot-pound perspective.

• Voltage is not an physical quantity of energy, so you can never convert voltage to foot pound or any other unit of energy. Mar 7, 2021 at 16:20
• Easier if you go to N m because the conversion is 1 Mar 9, 2021 at 1:06

I don't think anything beats an electromagnetic motor. Here's a link to the first result when I googled "Highest efficiency motor". It's 99% efficient. Note that you almost always need a set of gears to convert rotational force into the force you actually need, and a quick search seems to indicate a maximum efficiency of 98% per gear junction. Note that for some specific types of motion, different methods will be most efficient. For a high frequency vibration a piezo or quartz crystal might be more efficient. For bulk power transfer though, motor wins.

Just a side note, volts to footpound is an apples to aardvarks comparison. You need to compare input power to output power to analyze efficiency. You can produce the same power level motor in a variety of voltages.

• This is why actuator selection is so complicated, because there are usually so many other metrics to evaluate besides "bulk electrical to mechanical energy conversion". For instance (particularly in robotics) force/volume and force/electrical power sometimes matter more than mechanical power/electrical power. Plus, when you add gears you get all sorts of difficult to model dynamics in your system. Mar 7, 2021 at 18:34
• I'm not sure; an electrostatic motor would have high voltages and correspondingly low currents and so suffer less from $I^2R$ losses. These are usually small MEMS things but I'll bet they can have higher efficiency than motors with electromagnets. Superconductor-based devices might count but you'd have to argue away the power spent managing the cryogenic system.
– uhoh
Mar 8, 2021 at 3:42
• @uhoh Possibly in theory or for some special use case, but the theoretical articles I found quoted "high efficiency" without a theoretical efficiency figure and the highest efficiency practical units I could find seem to be about 95%, so the technology might be promising, but doesn't seem to have been realized. I would imagine to create their 44MW 99% efficient motor, ABB had to take voltage into account with their optimisation. The other concern I see with really high voltages that EM motors couldn't run at is that leakage discharges would probably become a big problem.
– K H
Mar 8, 2021 at 4:00
• +1 just for "apples to aardvarks". Mar 8, 2021 at 11:24

Might be accelerating a charged mass through an electric field. Efficient, but not a lot of energy in most practical cases.

• That's exactly what happens in a CRT screen (or, for that matter in a thermionic diode). The mass is very small, but it's in a vacuum, so that makes it pretty efficient. Mar 7, 2021 at 14:38
• This is probably the more correct answer. See comment.
– uhoh
Mar 8, 2021 at 3:44
• If you look at CERN though, they turn 200MW of power into 6.5TeV protons, and not a whole lot of them. I guess it depends on where you draw the systems boundary. Mar 8, 2021 at 14:10
• @MSalters Efficiency of the LHC in turning electricity off the grid into proton beam energy is ~0.02%. 87MW used to create 20kW of beam. There's a conversion to RF in there. Mar 8, 2021 at 15:49
• @SpehroPefhany Don't they also have to power active cooling systems? Mar 8, 2021 at 22:52

Powering a magnetically levitated block is more efficient, as there are no frictional losses.

But anyways, maintaining the cool temperature (for cooper pairs to sustain in a superconductor) is not so efficient, as we can't achieve a 100% efficiency in a thermodynamic system. (even from a theoretical perspective)

• Not sure where the kinetic energy is coming from in this scenario. If memory serves, there is a slight kinetic component to superconducting current, but it's usually dwarfed by the energy in the magnetic field. Mar 7, 2021 at 16:38
• @SpehroPefhany: It seems that this is more a modification of other machines, than a full scenario of its own. You could linearly accelerate a slug (railgun). Adding magnetic levitation to that machine lowers the frictional losses, now you have a maglev train. (Vacuum lowers losses also, now you have a Tesla Hyperloop). But you could instead rotationally accelerate (motor). And you can do that with magnetic levitation of the rotor, see for example MagMotor Mar 8, 2021 at 1:39
• @BenVoigt Just a nitpick, but IIRC if you separate the projectile from the rails you're effectively building a coil gun rather than a rail gun.
– K H
Mar 9, 2021 at 1:55