Design limits of electric motors?

After watching the PBS special on the building of the ocean liner Queen Mary II, I have no question of the size that electric motors that can be built.
Three (or is it four) huge motors in rotating pods push this behemoth ship at record speeds across the Atlantic.
But how fast can an electric motor potentially turn (though not necessarily the ones that drive the QMII)? Examples on-line?
Thanks,
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DaveC
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Ultracentrifuges and high-vacuum turbopumps use high-frequency AC-driven induction motors, with rotational speeds measured in KHz... million RPM territory as I recall. The limit is the destruction stress on the rotor.
Big ships are slow... 100 RPM and thereabouts. I wonder if the QEII motors are geared? The main reduction gear on a big steam turbine plant costs about a million dollars.
John
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I'm interested (intellectually; no application, yet) in motors that can turn 100,000 rpm or more.
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DaveC wrote:

I read a small article in a defense magazine about a high-RPM motor that used a variable reluctance design. This avoids the magnets and presumably lets you build a stronger rotor. They weren't thinking 100,000 rpm, though.
100,000 rpm is around 10,000 radians/sec, so a 2cm diameter rotor will experience 1,000,000 meters/sec^2 of acceleration at it's outer rim, or about 100,000 gravities. It would take a strong, lightweight material to stand up to that kind of acceleration -- and "lightweight" and "magnetic" don't usually go together.
You could get around the magnetic problem with a pneumatic motor -- the rim of your 2cm rotor is only traveling at 100 m/s, which is only 225 miles per hour, after all.
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Tim Wescott
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On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote

The thing that prompted my original question was seeing that QMII documentary. I began wondering if air flight could ever use electric motors to drive turbines that would provide equivalent thrust of jet turbine engines (let's put aside the question of a source of electric power; for now, let's say it's infinite).
I realize that low-speed electric motors could drive propellers, but is there any hope of an electric motor being able to drive a high-speed turbine?
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Dave, There are pilotless drone aircraft that use electric motors, though they drive low speed propellers. The cheapest high speed electric motor I can think of is a vacuum cleaner motor, about 10000 rpm. Gear it up?
Tam
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says...

25000RPM isn't unusual for an unloaded router. ...some a little higher.
--
Keith

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DaveC wrote:

OK, but you're talking more like 10-15 thousand RPM, not 100. Since the forces on the rotating components goes up as the square of the speed this makes a big difference.
10-15K is actually something of a sweet spot for small motors (up to 500W or so). Designing a larger motor would get you back into mechanical difficulties, but they could probably be overcome. So driving a jet (particularly a fan, which I think goes slower than my reference) should be easy from that standpoint.
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the
225
motors
engines
let's
is there

turbine?
The quick answer is NO No usefull purpose can be achieved by driving a turbine. The turbine and its heat are the source of the power. Where does you electric motor get its electricity from?
Further most engines from old piston to modern turbines spin too fast for propellors and have to be geared down to drive an aeroplane.
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John G

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[snip]
You might be able to compress the air and send it out the back at supersonic velocities. You can't do that with regular props, once you hit the speed of sound you generate shockwaves, not useful air movement. However, with a jet turbine type arrangement you could progressively compress and accelerate the air (as the density goes up so does the speed of sound) and thereby chuck it out the back faster than the speed of sound in the surrounding air.
I've only just thought of that, it's 1.20am and I've drunk some wine, so I may laugh at myself tomorrow morning.
Tim
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Tim Auton wrote:

Yes.
No. SoS is related to the average molecular mass and the square root of the temperature in degrees Kelvin - nothing else. This is because temperature is based on average molecular kinetic energy of collision, which is m*v^2 - and SoS is directly related to v.
Air expanding with a pressure ratio of about 1:0.58 reaches sonic velocity (this ratio varies slightly with the gamma of the gas, which is related to the moment of angular inertia of the molecules). Above this pressure ratio it's possible to use a divergent nozzle to further accelerate the flow to supersonic.
However, a supersonic exhaust is a waste of energy unless you're travelling supersonic - you can use the energy better by exhausting more gas at a lower speed (higher momentum transfer, less wasted kinetic energy). Hence airliners use high-bypass fans.
That's why the shuttle solid rocket booster burns a kind of rubber to produce a high molecular mass, lower velocity exhaust, during the early parts of the flight, and relies on LH2/LO2 fuel for a lower MM, higher velocity exhaust when it's travelling faster.
Basic rocket science :-).
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You're quite right of course. I shouldn't do my inventing while tired and drunk :)
Tim
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John G wrote:

I believe that the OP wasn't going to spin the turbine to spin a prop, he was more interested in spinning the turbine to drive the aircraft.
In theory a high-bypass fanjet motor could produce pretty much the same thrust if you spun the fan with an electric motor as with it's built-in turbine engine, and you'd get the same kinds of high-speed efficiency gains that you do from using a fanjet.
The real rub would be that "infinite source of electrical power" -- so far the only thing that really beats hydrocarbon fuels for power density is atomics, and while the US was crazy enough to seriously investigate atomic-powered craft in the 50's that would stay up for days they weren't crazy enough to continue the experiment once they developed intercontinental missiles. Even there they were going to use hot air from the reactor to drive the turbines; the weren't going to generate electricity then use motors.
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On Mon, 7 Jun 2004 17:22:26 -0700, Tim Wescott wrote

Correct.
But the stresses on high-speed electrics seem to be a limitation above a few thousand rpm. Yes, I think turning the existing turbine could get the aircraft running properly, but my questions focus on what design of motors can turn that fast.
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I can't understand this conversation, surely an aerofoil shaped bladed rotor would achieve the same whether it was inside a tube or outside it. If you dont have significantly higher pressure gas on one side of the blade then you will reach a speed where the effect is to create vacumn on the "high pressure side" rather than pressure increase at the back side (similar to cavitation on a water propellor), this will still create small ammounts of thrust I suppose but it would pretty quickly reach a maximum that you couldn't get past.
Even if I visualise the pressures in a thing with 10 or more rotors with different pitch (shaped) blades I can't see how it would work at all. I end up back at one "screw" pulling or pushing it's way through the air with all the limitations that standard propellors have. Enlighten me please.
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rotor
end
all
To me the only difference between a jet engine and a rocket engine is that the jet is the most complicated one way valve immaginable so that the explosion is directed out the back but one of the two components for combustion can still get in the front. Maybe I am looking at it wrong as I know less about aviation than I do about electronics :). So with my view in mind how is an electric motor going to replace some/all of the jet engine or is my understanding wrong.
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in
or
Yeah, you're right. A fan has inherent limitations; a jet adds energy to the fluid stream - so, since we've got an infinite supply of energy, drive the compressor part of a jet motor with the Electric Motor and replace the combustion chamber with a refractory heater that's arbitrarily hot, to expand the gas. Then lose the turbine, since we've got the infinite battery.
Cheers! Rich
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Mjolinor wrote:

--snip--
A turbofan engine (which is used by most airliners these days) gets most (about 80% IIRC) of it's thrust from the fan and about 20% from the turbine engine buried inside of it.
Ducted fans are used because a the tips of a large, fast propeller exceed the speed of sound and ruin the propeller efficiency. A relatively small-diameter prop with lots of blades ends up throwing a lot of air radially off its travel path. The answer is to put a multi-bladed propeller into a tube, and turn it with a turbine engine - that's a turbo fan.
So if you take that turbofan engine and replace it's turbine section with a big-ass electric motor you'll get almost as much thrust as before, but on electric power instead of kerosene.
Note that none of this applies to low-bypass engines, like the ones used in older jet fighters and the concord: Those engines get _all_ of their thrust directly from the hot, fast exhaust. It's great for supersonic flight because the exhaust is going so very fast, but for slower travel it's not good for fuel efficiency because a lot of air moving slowly produces more thrust than a little bit of air moving fast.
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What kind of "_-bypass" would you call the SR-71 engines? They have a bypass system that when it's going way fast, bypasses almost the whole engine - the only air they allow through the "compressor" is subsonic - the rest of the intake, which has already been compressed by the shock wave from the spike, goes through ducts, directly to the afterburner.
Which is kinda the opposite of what you're looking for.
But I'd say, since your first given was infinite electric power, would it be possible, given today's materials, to build an electric ramjet? I'd think you'd just put a white-hot element in place of the flame in the combustion chamber, or maybe a big-ass arc. :-)
Cheers! Rich
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Rich Grise wrote:

"Oddball". I was speaking, of course, of turbofans.

They had atomic turbojets in the 50's (no kidding, and very scary if you ever consider that airplanes do crash sometimes). Fortunately they never flew them. _Any_ heat source can be used as long as it transfers heat to the air quick enough, but I'm not sure if it'd work with a ramjet because of the speed of the air.
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