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High speed induction motor update

Discussion in 'Electronic Design' started by colin, Aug 18, 2007.

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  1. colin

    colin Guest


    If anyone remembers I was messing about trying to modify a 2 pole 240vac
    induction motor to go at higher speed and lower voltage driven from a dspic
    based 3ph pwm setup.

    well I got some of that magnetic paper wich goes black in the presence of a
    magnetic field,
    I put a circle of it in the center of the stator but was very little to see,
    it apears the paper only goes black if the megnetic field goes
    perpendicularly through the paper,
    wich it doesnt with it aranged this way.

    but anyway I had a realy extra strong cup of coffee and traced how id
    conected the 4 windings to the 3phase
    (there were 16 slots wich made it awkward)
    and realised id got one phase backwards.

    changing that it now runs at a whoping 20krpm.
    it realy starts to sing,
    and its not realy balanced well enough as it only made for 1500rpm.

    it was only an experiment,
    I only put a few turns of wire loosly through the slots,
    I also need to bore the rotor out so it can sit in the middle of a 1M long
    12mm dia shaft.
    I think I might need to trim the cooling vanes on the rotor too.
    however I realy need a motor 2"x2" this one is a bit to big to fit in easily
    at 70x70,
    its from an old 8" floppy drive to drive the main spindle.
    ive been looking around at all sorts of junk,
    but nothing just the right size yet.

    my usual stockists only seem to have larger motors.

    Colin =^.^=
  2. You must be driving the motor with a much higher frequency than 50/60 Hz.
    Probably more like 800 Hz. I'm surprised that an induction motor was used
    in a floppy drive. It is more likely a synchronous motor. Even so, I wonder
    about the 20krpm. You might not get very high torque at such high speeds,
    and there may be heating problems due to magnetic losses. You may need
    additional shaft support bearings if it extends so far from the motor
    bearings. If the shaft end vibrates and goes off-center, at 20krpm it may
    become a lethal weapon.

    What is this gadget supposed to do?

  3. John Larkin

    John Larkin Guest

    I suspect there's a fairly simple basket of wire one could shape that
    both suspends and spins an aluminum sphere, driven by 2 or 3-phase
    varoable-frequency AC. In a partial vacuum, such a gadget could rip
    any rotor apart.

  4. John Larkin

    John Larkin Guest

    Modern chem lab centrifuges and turbomolecular vacuum pumps use
    magnetic suspension bearings and ac drive. They measure rotational
    speeds in kilohertz.

  5. colin

    colin Guest

    Ive wound it with 2 poles the same as it was before, so 20krpm =666hz.
    I havnt checked it properly thats just going from the drive frequency.
    if the motor slows down much the slip increases and it rapidly slows down
    so its not far off 20k.

    theres only 6 turns of wire on each segment to keep the inductance down so i
    can drive it from 24v.
    it struggled to get to 20k as my current limit kicked in before it reached
    ful voltage
    so i probably need a few more turns. but this was just an experiment.

    I dont need torque, well no more than necessary to overcome bearing and wind
    and im not realy sure how much that is.

    although im puting less than 50w into the motor most of that is copper/iron

    the bearings dont seem to get warm exept when I had to much preload.

    the floppy drive was an old one, very old, sa801 8".

    it does scare me a bit when it gets going, there are bearings at each end
    however and the motor in the middle,
    I also have 2 more bearings inbetween.

    Colin =^.^=
  6. colin

    colin Guest

    they have magnetically suspended high speed rotors in that gravity
    experiment satelite,
    but theyr driven by compressed gas.

    im sure someone here could derive the failure speed of an aluminium rotor of
    a given size.

    they dont use titanium in jet engines for nothing, well they got hot as well

    Colin =^.^=
  7. colin

    colin Guest

    The electronics and software will go to over 100krpm,
    its hard to get the moving parts, bearings etc,
    to run smoothly enough even at 10krpm.

    just getting the shaft straight is taking ages.
  8. JosephKK

    JosephKK Guest

    colin posted to
    There are inexpensive motors and controllers that will do those
    things readily available on the market. And they are less than 2
    inches (50 mm) diameter and can deliver up to 400 watts (over 1/2
  9. JosephKK

    JosephKK Guest

    colin posted to
    Hardly, dental drills and similar have done and do 100,000 rpm for
    decades routinely.
  10. colin

    colin Guest

    yes but they dont have a 12mm dia 1M long shaft.

    It probably wouldnt be so difficult if I had an engineering workshop instead
    of an electronice workshop lol.

    Colin =^.^=
  11. John Larkin

    John Larkin Guest

    Yikes! That will wobble itself to death at that speed. The fan shaft
    on the JSF has similar issues; it transmits 32,000 hp to the forward
    lift fan.

  12. colin

    colin Guest

    well with bearings at 400mm intervals if its 10um out of tru in the middle
    it does wobble a lot
    at over a few k rpm
    the catastrophic problem is at the point of resonance
    wich is ~ 5krpm, it absorbs a lot of power too.

    if I can get it to have no visible bend on my dial guage wich is probably
    about 1um,
    then it runs fairly smoothly,
    however to straighten the last 10um is proving difficult,
    I can bend it so its straight but an hour later it just goes back.

    another bearing inbetween so theres 200mm between bearings is better and il
    probably need it,
    but the extra friction is a problem and even with these it still needs to be
    straighter than it is.

    Colin =^.^=
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