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speed control of shaded pole induction AC motor

Discussion in 'Electronic Design' started by [email protected], Apr 29, 2008.

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  1. Guest

    i have a small AC motor, its spec sheet says its "shaded pole
    induction" type. its input power is 35W. i'd like to be able to
    continuously control its speed with something small, say the size of a
    pack of cigarettes or smaller.. is there some way to do this?
     
  2. It is possible to control it over a limited range by:

    a) Coupling it to a speed-dependent load such as an eddy-current disc.

    b) Producing the same effect by passing a small amount of DC through the
    windings along with the AC.

    c) Feeding it with a variable frequency and variable voltage supply.

    d) Using an off-the-shelf thyristor dimmer or a series rheostat.


    The range of speed control with a) is very small, just a few percent,
    but this was used in the past for accurate speed adjustment of
    gramophone turntable motors.

    The range should be a bit greater with b) but most shaded pole moters
    run their laminations fairly near to saturation; so a large DC component
    would lead to saturation, excessive current and overheating.

    The best method is usually c), but that will still be limited to a range
    of less than +100% or -50% of the rated speed and it won't fit into a
    cigarette packet. The voltage must be varied at the same time as the
    frequency, so as to keep the magnetic field at about the correct value.
    I have occasionally driven small shaded pole and semi-synchronopus
    motors from a signal generator and 100v line amplifiers when I needed
    variable speed from a fixed-speed tape, belt or wire player. It worked
    very well but I had to keep a careful check on the motor to make sure it
    wasn't overheating.
    http://www.poppyrecords.co.uk/other/Dictabelts/dictabelts.htm

    Some motors have, to my great surprise, proved to be controllable to
    some extent by method d) when the load was very speed-dependent (fans or
    stirrers). It is a bit like the bumble bee which theoretically cannot
    fly - but carries on flying because no-one has explained that to it. I
    wouldn't recommend it, but if you try it and find that it works for your
    motor (without damaging or overheating it), well and good.
     
  3. You can probably make a controller for a 35 watt motor in a very small box.
    If you have 90% efficiency, you need to dissipate only about 4 watts of
    heat.

    It is possible to rewind a shaded pole single phase motor to be three
    phase. I did this with a 1/40 HP fan motor, and I used #14 AWG wire so that
    it ran on about 8 VAC, which could be produced by a 12 VDC battery. There
    are some monolithic three phase driver modules that can be controlled by a
    PIC, and handle several HP, and without the shading coils, such a motor can
    be driven at a wide range of RPM, with potentially better efficiency. But I
    did not actually measure the torque, HP, and efficiency of my modified
    motor, and I suspect it was not as good as it would have been if it had
    been designed originally for 3 phase.

    Paul
     
  4. whit3rd

    whit3rd Guest

    The rotor material determines if it burns up, or just gets less
    efficient, when it slows down. 35W sounds like a relatively
    high power, so I'm guessing it will burn up.
    Heating thus the disk, AND the rotor as the field slips.
    That will work, but remember it's 35W; too small for a full
    motor control VFD, too big for function generators.

    True variable speed on this kind of motor only happens if the
    designer put the right rotor material for type (a) speed control.
    I've seen stirrer motors that worked that way, but it's not
    the usual case.
     
  5. I have seen it used successfully for fine speed control of a very low
    power gramophone motor (Garrard 301/401), but the slip frequency was
    only a few %.
    I use a 100W P.A. amplifier (or two) to get the power for my particular
    appications. ...but a pair of Quad 50Es and a BBC TS/10 signal
    generator won't fit in a cigarette packet.

    Like a dog walking on its hind legs - the marvel is not that it works
    well, but that it can be made to work at all.

    If you want to design a controllable variable-speed motor, don't start
    with a shaded pole type unless there is some over-riding reason why you
    must. If there appears to be a good reason, try to design-out the
    reason.
     
  6. Guest

    Tim

    Shaded pole motors are frequency dependent. They are also low torque
    devices. Based on your input wattage of 35, this is a small motor.
    The big question is, what is your load? Fan, gears, etc?

    These motor can be controlled by changing the frequency or (depending
    on the load) by adjusting the voltage. Under a no load condition,
    changing the voltage will have very little effect. If the motor is
    loaded, the increased or decreased voltage will result in an increase
    or decrease of the strength of the magnetic field and will result in
    the motor speed to increase or decrease. This will usually result in
    an increase in coil temperatures.

    John
     
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