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shaded pole motor control

Discussion in 'Electronic Design' started by Ian Macmillan, Nov 26, 2008.

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  1. Does anyone have experience with speed control of a small shaded pole
    motor about 25W, with a ghastly PF of about 0.36 ?.

    This drives the water circulation pump in an experimental setup much too
    hard, and the churning
    measurably heats the water.

    I thought a variable frequency inverter might be nice, but wondered how
    sinusoidal it would need to be. I doubt that a simple phase shift let-it-
    slip-more controller, would be good enough. Works for fans up to a point.

    Rather than suck it and see, I thought I'd politely probe the brains of
    the experts first.

    All the best
    Ian Macmillan
     
  2. Try a standard TRIAC light dimmer (Check & Make sure that the motor gets AC
    though, a bulb won't mind but the motor will!)! Shaded-pole motors are often
    speed controlled by regulating the voltage down.
     
  3. MooseFET

    MooseFET Guest

    Shaded pole induction motors can only be reduced in speed slightly by
    lowing the voltage. On something where the load increases rapidly
    with speed, like a pump, you may have a little luck. The torque of
    the motor decreases when the speed falls below the "pull out speed".
    This makes it suddenly go down to a very slow speed as the voltage is
    decreased.

    Decreasing the frequency and voltage together is the way to go if you
    need a significant decrease.
     
  4. MooseFET

    MooseFET Guest

    Yes, I tried it many years back. It doesn't work.

    Can you cycle it on and off?

    How about running a pipe back around the pump with a valve in it to
    dump a lot of the water back around to the input.

    You can use a stepwise sine wave like shape to do it. If works a lot
    better if you place an extra inductance in series with the motor to
    keep the sharp edges from causing current spikes.

    My brother made a system that would power up an induction motor using
    a square wave drive. He was using a transformer to step up from a 12V
    battery to mains voltage. The motor got fairly hot but it did work
    for an hour without serious overheating. He was working with a 1/4
    horse motor.

    A company called "Flytronics"(SP) used to make a system for making a
    sine like power with a trick that you can borrow. What they did was
    used a transformer with many taps on the primary. The primary was
    driven by transistors with rectifiers in series with them. It worked
    very nicely but they charged an awful lot for them. I have my own
    version I have long wanted to try and will offer it here:

    Imagine 3 binary weighted transformers. I am thinking of center
    tapped "primaries". The actual transformers would be 12V, 24V and 48V
    step down transformers run backwards. The "secondaries" would be in
    series. The transformers would be way over rated for the intended
    power but low cost ones.

    The drive on the "primaries" would be transistors that are bypassed
    with rectifiers so that current can flow back onto the input power.
    The transformers are never allowed to see a high impedance.

    If you want more points and more complexity without adding more
    transformers, you can go with this idea:

    Imagine that the transformers are base 3 weighted and driven by full
    bridges. By turning on the two lower transistors of the bridge, you
    can short out the transformers. This gives you minus, zero, plus as
    the three points.
     
  5. JosephKK

    JosephKK Guest

    I suppose you could try the way the big boys do it and build a
    variable frequency drive.
     
  6. Mike Monett

    Mike Monett Guest

    1. An ordinary light dimmer can work over a limited range. Sometimes you
    have to swap the leads to kill an oscillation.

    2. Put a plain light bulb in series. Start with 25Watt.

    Mike Monett
     
  7. Ben Bradley

    Ben Bradley Guest

    I don't have actual experience doing it, but I recently read this
    article and found it interesting (all those emails I've sign up for
    sometimes have articles I actually want to read). The controller
    described may be overkill for your app, but it describes the
    characteristics of such motors very well, and that may help you figure
    out what you can reasonably do:

    http://www.embedded.com/design/210800291


    From that article I get the impression you can't control the speed
    of such a motor very well without feedback of its actual speed to
    measure the 'slip.' These things appear to be near-constant torque
    when slowed down below their peak torque. That works well on fans
    which have increasing load with speed, and may also work with your
    app, presuming the pump similarly gives an increasing load with speed.
     
  8. Shaded pole motors need variable voltage to control their speed, Buck
    the voltage as described above.

    Cheers
     
  9. Artemus

    Artemus Guest

    If you don't care about efficiency and don't need variable speed
    a power resistor in series with the motor is the simplest solution.
    Experiment with a variac to see what voltage you need on the
    motor and determine the ohms needed. Or just leave the variac
    in place.
    Art
     
  10. JosephKK

    JosephKK Guest

    Gosh that is only about 40 years old. Over 75 if you count tubes.
     
  11. Fred Abse

    Fred Abse Guest


    AKA Magnetic Amplifier, or Saturable Reactor.
     
  12. Not the same thing! In a saturable reactor, a DC current
    saturates the core of an inductor, reducing its inductance.
    In the arrangement described above, there is no DC magnetization,
    and the core does not saturate.

    Jeroen Belleman
     
  13. One way to control speed is to add series resistance. The technique
    varies the impedance seen looking into the primary of the transformer.

    Cheers
     
  14. Fred Abse

    Fred Abse Guest

    True.
     
  15. Fred Abse

    Fred Abse Guest

    Conceded. Stupid of me.
     
  16. Fred Abse

    Fred Abse Guest

    By definition? Seems a bit narrow.

    It's still controlling current using another current in a magnetic device.
     
  17. Fred Abse

    Fred Abse Guest

    There are lots of what are referred to as "magamps" used in switch mode
    power supplies as secondary regulators:

    http://www.ferroxcube.com/appl/info/square.pdf

    I did a Spice hysteretic model of one of their magamp toroids.

    Metglas make (made?) a range of suitable cores, too.
     
  18. JosephKK

    JosephKK Guest

    It is not so much as saying old is good, but that i find it disingenuous
    to represent it as a new circuit / discovery.
     
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