Design limits of electric motors?

[DaveC]

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,
--
DaveC
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[John Larkin]

On Mon, 7 Jun 2004 08:21:34 -0700, DaveC <me@privacy.net> wrote:

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,

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

 

[DaveC]

I'm interested (intellectually; no application, yet) in motors that can turn
100,000 rpm or more.
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DaveC
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[Paul Hovnanian P.E.]

DaveC wrote:

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?

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.

http://www.amsuper.com/

--
Paul Hovnanian mailtoaul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
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means he uses to frighten you. -- Eric Hoffer

 

[DaveC]

On Mon, 7 Jun 2004 10:59:51 -0700, Paul Hovnanian P.E. wrote
(in article <40C4AD17.57627DB1@Hovnanian.com&gt:

http://www.amsuper.com/

Pretty cool designs.
--
DaveC
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[Tim Wescott]

DaveC wrote:

I'm interested (intellectually; no application, yet) in motors that can turn
100,000 rpm or more.

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[DaveC]

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:

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.

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,
--
DaveC
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[Tam/WB2TT]

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:

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.

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,
--
DaveC
me@privacy.net
This is an invalid return address
Please reply in the news group

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

 

[Tim Wescott]

DaveC wrote:

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:


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.


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,

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Jon Elson]

DaveC wrote:

I'm interested (intellectually; no application, yet) in motors that can turn
100,000 rpm or more.


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

 

[John G]

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:

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.

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?

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.
--
John G

Wot's Your Real Problem?

 

[Tim Wescott]

John G wrote:

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:


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.

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?

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.

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Tim Auton]

John G" <Greentest@ozemail.com.au> wrote:

"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...
[snip]
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?

The quick answer is NO
No usefull purpose can be achieved by driving a turbine.

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
--
Love is a travelator.

 

[R. Steve Walz]

DaveC wrote:

I'm interested (intellectually; no application, yet) in motors that can turn
100,000 rpm or more.
--
DaveC
-----------------

Dental drills.

-Steve
--
-Steve Walz rstevew@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public

 

[John Larkin]

On Tue, 08 Jun 2004 01:52:21 GMT, "R. Steve Walz" <rstevew@armory.com>
wrote:

DaveC wrote:

I'm interested (intellectually; no application, yet) in motors that can turn
100,000 rpm or more.
--
DaveC
-----------------
Dental drills.

-Steve

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.

http://nmr.magnet.fsu.edu/rf/830-X-MAS2.htm

http://www.varianinc.com/cgi-bin/nav?products/nmr/probes/solids/index&cid=KNHPHNJFK



John

 

[DaveC]

On Mon, 7 Jun 2004 17:22:26 -0700, Tim Wescott wrote
(in article <10ca1lobqqlio8d@corp.supernews.com&gt:

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.

Correct.

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.

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.
--
DaveC
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[DaveC]

On Mon, 7 Jun 2004 16:57:28 -0700, John G wrote
(in article <Mh7xc.5$tx5.501@nnrp1.ozemail.com.au&gt:

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?

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.

Further most engines from old piston to modern turbines spin too fast
for propellors and have to be geared down to drive an aeroplane.

Again, we're not talking about propellers, but turbines.
--
DaveC
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[DaveC]

On Mon, 7 Jun 2004 15:08:58 -0700, Jon Elson wrote
(in article <40C4E77A.1040405@artsci.wustl.edu&gt:

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

But what about conventional (fuel) turbine engines. Surely they turn in the
100,000 rpm range, and use ball bearings.
--
DaveC
me@privacy.net
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[Don Pearce]

On Mon, 7 Jun 2004 22:16:45 -0700, DaveC <me@privacy.net> wrote:

On Mon, 7 Jun 2004 16:57:28 -0700, John G wrote
(in article <Mh7xc.5$tx5.501@nnrp1.ozemail.com.au&gt:

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?

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.

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
http://www.pearce.uk.com

 

[Mjolinor]

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10ca1lobqqlio8d@corp.supernews.com...

John G wrote:
"DaveC" <me@privacy.net> wrote in message
news:0001HW.BCEA26AB00030638F03055B0@news.individual.net...

On Mon, 7 Jun 2004 11:24:48 -0700, Tim Wescott wrote
(in article <10c9cn7hrsm07cb@corp.supernews.com&gt:


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.

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?

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.

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.

--

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.