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,

Reply to
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

Reply to
John Larkin

I'm interested (intellectually; no application, yet) in motors that can turn

100,000 rpm or more.
Reply to
DaveC

I don't know if this represents any kind of upper limit, but the US Navy is working with American Supercondustors on a 36.5 megawatt motor for ship propulsion.

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Reply to
Paul Hovnanian P.E.

On Mon, 7 Jun 2004 10:59:51 -0700, Paul Hovnanian P.E. wrote (in article ):

Pretty cool designs.

Reply to
DaveC

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.

Reply to
Tim Wescott

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote (in article ):

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?

Thanks,

Reply to
DaveC

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

Reply to
Tam/WB2TT

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.
Reply to
Tim Wescott

Really, the electromagnetic part of such a motor is no biggie. The mechanical part, especially related to first critical speed of high speed rotating machinery, is the part that requires the most attention to detail. If the motor is ever operated at the speed where the natural frequency of the first bending moment matches the rotational frequency, the vibrations tend to build to enormous magnitude in just a few revolutions. Very stiff structures can tolerate a rapid acceleration through the first critical speed, and then operate safely above that speed. But, getting a machine to tolerate that speed, even for a moment, is quite tricky. The other problem is ball bearings, for the most part, can't handle

100,000 + RPM. At the least, they need continuous oil mist cooling to remove the heat. Conventional journal bearings would need a continuous flow of cool oil to survive. Air bearings are a good choice, and are used in a lot of high-speed drilling and machining spindles, like Westwind. I'm pretty sure these run above first critical speed, just to look at the structure of them. The induction motor rotor, bearings, etc. are all the same diameter, about 1/2 to 3/4" diameter, with a flange at one end to act as a thrust bearing.

Jon

Reply to
Jon Elson

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.

Reply to
John G

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.

Reply to
Tim Wescott
[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

Reply to
Tim Auton

----------------- Dental drills.

-Steve

Reply to
R. Steve Walz

Those are mostly air turbines nowadays. They can go very fast, and are nicely self-cooling. Some NMR experiments benefit from spinning the sample, which they do with air motors at speeds like 40 KHz: 2.4 million RPM.

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John

Reply to
John Larkin

On Mon, 7 Jun 2004 17:22:26 -0700, Tim Wescott wrote (in article ):

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.

Reply to
DaveC

On Mon, 7 Jun 2004 16:57:28 -0700, John G wrote (in article ):

In my original post, forget electric power source. I'm interested *only* in the possibility of the motor to turn fast enough to spin a turbine to drive an aircraft.

Again, we're not talking about propellers, but turbines.

Reply to
DaveC

On Mon, 7 Jun 2004 15:08:58 -0700, Jon Elson wrote (in article ):

But what about conventional (fuel) turbine engines. Surely they turn in the

100,000 rpm range, and use ball bearings.
Reply to
DaveC

But the turbine is the bit that gets spun by the hot gases. That in turn drives a compressor to provide the air for the combustion chamber. In jet engines that is it - the hot exhaust gas drives the plane. In a turbo fan a geared down output from the compressor shaft drives the big fan (effectively a propeller) that you see in the front of a modern jet engine.

Which bit, out of that lot, do you propose to replace with an electric motor? The whole thing is a bit circular (apart from the fan) and it is hard to see how you could break the loop to put your motor in.

It looks a little as if you think that it is the rotating turbine that provides the thrust that drives the plane. It isn't - quite the opposite, in fact.

d Pearce Consulting

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Reply to
Don Pearce

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.

Reply to
Mjolinor

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