# RPM Limit on Theoretical motor?

## Main Question or Discussion Point

Ok so I was trying to design a flywheel for a project of mine and an idea occurred to me. Why not use a gas instead of a solid wheel so you didn't haven't worry about it exploding? Could you spin a conductive gas using electromagnets situated around the gas chamber? Whats the maximum speed at which you could switch the electromagnets with today's technology to maximize the speed?

## Answers and Replies

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billy_joule
Science Advisor
I would guess the low density and fluid friction would mean it would have terrible energy density and efficiency

SteamKing
Staff Emeritus
Science Advisor
Homework Helper
What's a gas wheel?

Flywheels store energy by virtue of their rotational inertia. For a flywheel made from a solid circular cylinder, the mass moment of inertia I = mr2, and the amount of energy stored when the angular velocity of the flywheel is ω rad/sec is:

E = Iω2 = mω2r2

By making a 'wheel' out of gas, the mass m will be much smaller than if the wheel is made of metal, and the amount of energy which can be stored for a given angular velocity will be proportionately reduced.

I'm ok with low energy density i'm more interested in creating something that would spin the gas at very high rpm. Not so much for a useful purpose but as something interesting to do.

russ_watters
Mentor
How do you "spin a gas"?

Ok a more accurate description would be to induce a state in the gas in which the majority of the gas particles have an orbital trajectory about a single axis.

Whats the maximum speed at which you could switch the electromagnets with today's technology to maximize the speed?
This depends on size and construction of machine. Out of curiosity, I've done some short high speed test runs on small induction machines (Pn< 3 kW) up to 10 000 rpm. They all survived . Big machines (Pn>1 MW) would probably selfdestruct much more before that.

anorlunda
Mentor
Ok a more accurate description would be to induce a state in the gas in which the majority of the gas particles have an orbital trajectory about a single axis.
You mean like The Sun?

mfb
Mentor
I don't see how you would get the force right for neutral particles. You can collect a cloud of ions, of course, like a small cyclotron. They can reach relativistic speeds, but the density is extremely low (and you get synchrotron radiation) - not practical as energy storage.

You mean like The Sun?
Hadn't thought of that comparison but ya basically.

I don't see how you would get the force right for neutral particles. You can collect a cloud of ions, of course, like a small cyclotron. They can reach relativistic speeds, but the density is extremely low (and you get synchrotron radiation) - not practical as energy storage.
Ok cool. So the limit is constrained by how effectively you can bend the trajectory of the ions around the track.

mfb
Mentor
Cyclotrons are limited by relativistic effects and at some point the size becomes impractical. Even higher energies can be reached with other accelerator types. That has nothing to do with flywheels any more, however.

russ_watters
Mentor
Ok a more accurate description would be to induce a state in the gas in which the majority of the gas particles have an orbital trajectory about a single axis.
That's just a definition of the word "spin". I'm asking physically how you hope/plan to do it. I'm unable to envision the concept.

That's just a definition of the word "spin". I'm asking physically how you hope/plan to do it. I'm unable to envision the concept.
Think like a cd or a galaxy or the event horizon on a black hole. I want to use electromagnets to create a similar state in a body of gas. Since the gas if ionized will be attracted to the negative pole on an electromagnet I want to use electromagnets in a circle to rotate the body of gas around the center of a container at high speed.

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Cyclotrons are limited by relativistic effects and at some point the size becomes impractical. Even higher energies can be reached with other accelerator types. That has nothing to do with flywheels any more, however.
That's fine i,m not looking to accelerate anything to close to light speed. Id prefer to get a larger amount of gas ionized or not,moving at a couple hundred m/s

When I was a kid, I would drive past industries that generated a lot of dust, and I'd regularly see cone-shaped air cleaners. They used to capture my imagination.
http://en.wikipedia.org/wiki/Cyclonic_separation
These were constructed from galvanized steel and would be about 6-8 feet in diameter. It was a good use of spinning air as it captured a massive amount of pollution.

RonL
Gold Member
Ok so I was trying to design a flywheel for a project of mine and an idea occurred to me. Why not use a gas instead of a solid wheel so you didn't haven't worry about it exploding? Could you spin a conductive gas using electromagnets situated around the gas chamber? Whats the maximum speed at which you could switch the electromagnets with today's technology to maximize the speed?
Your thoughts remind me of how I found out about MHD generators,....they were around a few years before I imagined one in my mind.
I was thinking about alternators and how spin speed and heat would bring them to a state of shrapnel, the idea that came sounds similar to what you are describing.
Things you might consider......
The mechanics of a vortex tube cooler (spin speed of the air)
An alternator with all components ground to a fine powder
A sandstorm with silicon particles in suspension
Mix a quantity of iron and copper powder in the vortex flow, guided through a magnetic field.

Feeding particles in a layering pattern where they produce an electrical current, then draw from that, would be the goal to reach.
Spin speed and heat becomes almost irrelevant. Participial mass and speed determine voltage and current, recycle of the powders should be easy enough.
An alternator of sorts that has a very high wear factor, but super cheap to replace

Perhaps there is something in that dribble that coincides with your thoughts, or maybe will trigger in your mind a thought that makes your vision click. :)

mfb
Mentor
That's fine i,m not looking to accelerate anything to close to light speed. Id prefer to get a larger amount of gas ionized or not,moving at a couple hundred m/s
A couple of hundred meters per second is possible with conventional solid flywheels.
You can't get a large collection of ions. Every ion has at least one elementary charge, if you have too many they repel each other too much.