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,

 

[John Larkin]

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

 

[Doug Goncz]

The fastest permanet magnet DC commutating motors I know of are the ironless
rotor designs, or "pancake" motors.

Kollmorgen's my favorite brand.

There are pics at motionvillage.com, which now handles more than just
Kollmorgen.

I am told by Kollmorgen's tech support that "flux shift" limits the speed of
motors. The ironless rotor has near zero inductance, and so can reach high
speeds. A bonus is very low moment of inertia, so they're usually used in
motion control applications where acceleration and even the third derivate of
position, which can be called control or jerk, is very high. They aren't
particularly good as traction motors except for one thing:

They have extremely high overload ability. This is usually integrated into the
motion controller, but I have experimented a bit with obstacle surmounting use
of such a motor for a military application. The potential is there for electric
bicycle stunts like hopping picnic tables. You can't get that out of a
conventional motor, at like 110% of stall rating, the magnets depolarize and
remain forever lower in performance.

You see, with the ironless rotor, the magnets are just slugs inside the casing,
so available simple geometric shapes, typically cylinders, can be used, where
designing a custom shaped high coercivity (is that right right term?) magnet
adds $$$ to your motor design, which is one reason hard disk drives cost a bit.
Those little wedge shaped magnets in them are low volume, designed to fit the
drive, manufactured in smaller quantities.




Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

 

[Tim Wescott]

The fastest permanet magnet DC commutating motors I know of are the ironless
rotor designs, or "pancake" motors.

Kollmorgen's my favorite brand.

There are pics at motionvillage.com, which now handles more than just
Kollmorgen.

I am told by Kollmorgen's tech support that "flux shift" limits the speed of
motors. The ironless rotor has near zero inductance, and so can reach high
speeds. A bonus is very low moment of inertia, so they're usually used in
motion control applications where acceleration and even the third derivate of
position, which can be called control or jerk, is very high. They aren't
particularly good as traction motors except for one thing:

They have extremely high overload ability. This is usually integrated into the
motion controller, but I have experimented a bit with obstacle surmounting use
of such a motor for a military application. The potential is there for electric
bicycle stunts like hopping picnic tables. You can't get that out of a
conventional motor, at like 110% of stall rating, the magnets depolarize and
remain forever lower in performance.

You see, with the ironless rotor, the magnets are just slugs inside the casing,
so available simple geometric shapes, typically cylinders, can be used, where
designing a custom shaped high coercivity (is that right right term?) magnet
adds $$$ to your motor design, which is one reason hard disk drives cost a bit.
Those little wedge shaped magnets in them are low volume, designed to fit the
drive, manufactured in smaller quantities.




Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ )

Read about my physics project at NVCC:
http://groups.google.com/groups?q=dgoncz&scoring=d plus
"bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
in the search box

I've got a friend who used to work at a place that made it's own custom
brushless motors, and their fundamental speed limit was always the point
where the rotor exploded or otherwise failed.

Take anything you hear from a vendor with a grain of salt --
Kollmorgen's argument sounds like the Chevy dealers who told customers
that Fords would wear out quicker because the pistons in a V-8 put more
strain on the downhill side of the cylinder (Chevy didn't have a V-8
until 1955).

While there are a number of significant advantages to coreless motors,
demagnetization is a factor of the magnet, not the coreless technology.
Since brushless motors also use perfectly cylindrical magnets they can
also easily use the rare earths. For that matter there are rare earth
brushed motors out there as well.

If you really want to abuse a motor I'd recommend a brushless with rare
earth magnets. Brushless because then you can't burn out the brushes
(your controller is up to you), rare earth because it's very difficult
to demagnetize those.

 

[DaveC]

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

 

[Paul Hovnanian P.E.]

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/

 

[DaveC]

http://www.amsuper.com/

Pretty cool designs.

 

[Tim Wescott]

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.

 

[DaveC]

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,

 

[John Larkin]

The fastest permanet magnet DC commutating motors I know of are the ironless
rotor designs, or "pancake" motors.

Kollmorgen's my favorite brand.

There are pics at motionvillage.com, which now handles more than just
Kollmorgen.

I am told by Kollmorgen's tech support that "flux shift" limits the speed of
motors. The ironless rotor has near zero inductance, and so can reach high
speeds. A bonus is very low moment of inertia, so they're usually used in
motion control applications where acceleration and even the third derivate of
position, which can be called control or jerk, is very high. They aren't
particularly good as traction motors except for one thing:

They have extremely high overload ability. This is usually integrated into the
motion controller, but I have experimented a bit with obstacle surmounting use
of such a motor for a military application. The potential is there for electric
bicycle stunts like hopping picnic tables. You can't get that out of a
conventional motor, at like 110% of stall rating, the magnets depolarize and
remain forever lower in performance.

You see, with the ironless rotor, the magnets are just slugs inside the casing,
so available simple geometric shapes, typically cylinders, can be used, where
designing a custom shaped high coercivity (is that right right term?) magnet
adds $$$ to your motor design, which is one reason hard disk drives cost a bit.
Those little wedge shaped magnets in them are low volume, designed to fit the
drive, manufactured in smaller quantities.

So how fast can they go? I recall Excelon PCB drilling machines
running at 60 grand twenty years ago, with brush-type motors.

John

 

[Tam/WB2TT]

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
[email protected]
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]

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.

 

[Jon Elson]

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]

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.

 

[Tim Wescott]

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.

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 Auton]

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

 

[JeffM]

Chevy didn't have a V-8 until 1955.

Tim Wescott

Not entirely correct.
http://64.233.167.104/search?q=cach...chevy's+first+v-8"+1917+lasted-only-two-years

 

[R. Steve Walz]

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

 

[John Larkin]

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]

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.