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Design limits of electric motors?

Discussion in 'Electronic Design' started by DaveC, Jun 7, 2004.

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  1. DaveC

    DaveC Guest

    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?

  2. John Larkin

    John Larkin Guest

    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.

  3. Doug Goncz

    Doug Goncz Guest

    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, which now handles more than just

    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.


    Doug Goncz ( )

    Read about my physics project at NVCC: plus
    "bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc.
    in the search box
  4. Tim Wescott

    Tim Wescott Guest

    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.
  5. DaveC

    DaveC Guest

    I'm interested (intellectually; no application, yet) in motors that can turn
    100,000 rpm or more.
  6. 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.
  7. DaveC

    DaveC Guest

  8. Tim Wescott

    Tim Wescott Guest

    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.
  9. DaveC

    DaveC Guest

    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?

  10. John Larkin

    John Larkin Guest

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

  11. Tam/WB2TT

    Tam/WB2TT Guest

    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?

  12. Tim Wescott

    Tim Wescott Guest

    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.
  13. Jon Elson

    Jon Elson Guest

    Really, the electromagnetic part of such a motor is no biggie. The
    part, especially related to first critical speed of high speed rotating
    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
    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
    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.

  14. John G

    John G Guest

    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.
  15. Tim Wescott

    Tim Wescott Guest

    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.
  16. Tim Auton

    Tim Auton Guest

    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.

  17. JeffM

    JeffM Guest

  19. John Larkin

    John Larkin Guest

  20. DaveC

    DaveC Guest

    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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