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3-speed fan motor

Discussion in 'Electrical Engineering' started by [email protected], May 27, 2007.

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  1. Guest

    I notice that a lot of fans, including all I have recently bought, have
    3 speeds controlled by a switch. So I was wondering how they manage to
    control the speed (yeah, I know ... via the position of the switch :)
    Latest one I bought has parts more visible (I don't want to take them
    apart). There are 4 wires to the switch. So it would seem likely that
    one wire is common and the other 3 are selected by position, and none
    are selected for off. There's a 3uf capacitor and what looks like a
    shaded-pole winding in the motor itself.

    The motor type that seems to fit is "permanent split-capacitor", also
    known as "capacitor start and run".

    If the starting winding is permanently connected, is there any design
    change over that of a switched winding that is needed because it will
    be permanently powered? For example would the number of winding turns
    need to be greater? Or maybe different capacitor? Or at at different
    angle in the stator?

    I read that the switch selects taps in the main winding for speed. But
    is this just a selection that affects slip, or does it change the way
    the winding is slectrically configured? Could more taps allow more steps
    in motor speed? Is there a low end limit on the speed that can be set
    this way?

    I have noticed that it always is the case that the first position next
    to the off position is the highest speed position. Is this done because
    that is the best for starting the fan from a not-running state?

    I tested a couple of fans I have (the circulate air in the computer room
    by getting them spinning in the wrong direct by driving the backware with
    another face facing into it. Then I flip the switch and notice that the
    fan has no trouble getting started by quickly slowing down the rotation
    that's going in the wrong direction, to a stop, then going in the right
    direction. This works fine even which quickly switched to the slow speed.

    My father has a ceiling fan with a pull chain that increments the speed
    in steps with 3 speeds possible, and also has a wall switch with 3 speeds.
    Turns out there there are more than just 3 speeds possible based on the
    combinations of both switches. How does this happen? I have not looked
    at the wiring, but I assume there are as many wires as the wall switch
    needs to have going between the fan and the wall switch. Could the pull
    chain switch be selecting different taps on the other end of the winding?

    The ceiling fan has a reverse switch. I'm guessing that simply reverses
    the permanent start winding.
     
  2. JohnR66

    JohnR66 Guest

    Yes, these seem to be PSC. With the cap removed, they will not start. If
    already runing and the cap removed, the will continue runing, but current
    draw will increase. They are less efficient in this mode.
    I guess it is a matter of economics. Sizing the cap for each speed position
    might maximize efficiency, extra capacitors and more complex switch.
    Lower speed switches in more windings which means lower current thus less
    torque.
    I always thought this as well, but not positive.
    I noticed that some PSC motors will run backwards if power is applied while
    the shaft has been rotated at nearly full running speed backwards.
    Not sure.
    Not sure, but it would seem logical that this is what is being done.
    John
     
  3. Guest

    |
    |> I read that the switch selects taps in the main winding for speed. But
    |> is this just a selection that affects slip, or does it change the way
    |> the winding is slectrically configured? Could more taps allow more steps
    |> in motor speed? Is there a low end limit on the speed that can be set
    |> this way?
    |
    | Lower speed switches in more windings which means lower current thus less
    | torque.

    So that lower torque plus the drag on the blades results in the lower
    speed. If operated in a vacuum, the speeds might be a lot closer with
    only the mechanical drag?

    I notice the full speed on most of these fans seems to be about 1 rps
    different than the power frequency, or some multiple thereof. The
    observation is that there is a slight change in the timbre of the sound
    emitted by the motor that cycles around 1 per second, but can vary a
    lot, including with variation in air flow around the fan. That's at
    full speed setting. At lower speeds the sound is more like a flutter
    and does not seem to change as much.

    Some day I might take one of these fans apart and substitute a sync
    motor and see what happens :)


    |> I tested a couple of fans I have (the circulate air in the computer room
    |> by getting them spinning in the wrong direct by driving the backware with
    |> another face facing into it. Then I flip the switch and notice that the
    |> fan has no trouble getting started by quickly slowing down the rotation
    |> that's going in the wrong direction, to a stop, then going in the right
    |> direction. This works fine even which quickly switched to the slow speed.
    |
    | I noticed that some PSC motors will run backwards if power is applied while
    | the shaft has been rotated at nearly full running speed backwards.

    I used to have an electric clock that would go backwards about half the
    times it would be plugged in. I would guess the shade winding has been
    broken or the cap disconnected/loose.

    It was a fun clock to have.
     
  4. J. B. Wood

    J. B. Wood Guest

    Hello, and years ago many of the better quality box-type fans were
    intended to be used on the floor or in a window. For window fan use they
    had an "in/out" switch that would reverse the direction of blade
    rotation. Most of these fans had a special type shaded-pole motor with
    two sets of externally-connected shaded windings. Other than that the
    motor construction was conventional. With the shaded pole windings
    open-circuited you could cause the motor to rotate in either direction by
    manually spinning the fan blades in that direction. Although the motor
    would continue to run in this mode it produced minimal torque compared to
    having one of the shaded-pole windings in the circuit. Sincerely,

    John Wood (Code 5550) e-mail:
    Naval Research Laboratory
    4555 Overlook Avenue, SW
    Washington, DC 20375-5337
     
  5. Guest

    |
    |> |> I tested a couple of fans I have (the circulate air in the computer room
    |> |> by getting them spinning in the wrong direct by driving the backware with
    |> |> another face facing into it. Then I flip the switch and notice that the
    |> |> fan has no trouble getting started by quickly slowing down the rotation
    |> |> that's going in the wrong direction, to a stop, then going in the right
    |> |> direction. This works fine even which quickly switched to the slow speed.
    |> |
    |> | I noticed that some PSC motors will run backwards if power is applied while
    |> | the shaft has been rotated at nearly full running speed backwards.
    |>
    |> I used to have an electric clock that would go backwards about half the
    |> times it would be plugged in. I would guess the shade winding has been
    |> broken or the cap disconnected/loose.
    |>
    |> It was a fun clock to have.
    |
    | Hello, and years ago many of the better quality box-type fans were
    | intended to be used on the floor or in a window. For window fan use they
    | had an "in/out" switch that would reverse the direction of blade
    | rotation. Most of these fans had a special type shaded-pole motor with
    | two sets of externally-connected shaded windings. Other than that the
    | motor construction was conventional. With the shaded pole windings
    | open-circuited you could cause the motor to rotate in either direction by
    | manually spinning the fan blades in that direction. Although the motor
    | would continue to run in this mode it produced minimal torque compared to
    | having one of the shaded-pole windings in the circuit. Sincerely,

    I'm also curious why it seems to common that the first speed right after
    the off position is the highest speed. Could it be that this speed was
    necessary to counter the fact that the blade may be rotating in the wrong
    direction due to airflow through the window or such? I have tried all
    the fans I have ever owned by putting them in the lowest speed position
    while unplugged, the plugging them in to start. They always start OK,
    unless the fan is generally dead (after many years of service a couple
    of them have pretty much frozen up with all kinds of dust and gunk in the
    motor, which I should have cleaned out, but didn't). So it seems the
    first position being high isn't to start the fan from a stop, but maybe
    it needs it to start from reverse rotation.

    Also, it seems to me if a motor of this type is going to have 4 wires to
    the winding, it could achieve 4 speeds instead of just 3 speeds, with the
    taps being in different positions. Two of the wires would be on one end
    of the winding, and the other two on the other end. I'll label them as
    A, B, C, D. It could look like:

    A---B-----C------D

    Connect to B and C for high speed.
    Connect to A and C for medium high speed.
    Connect to B and D for medium low speed.
    Connect to A and D for low speed.

    It would be no more cost on the motor than redesigning for different tap
    positions on the winding. The switch would cost a bit more to have the
    extra position. But maybe it's not worth it in the market.

    Long long ago my grandfather had a fan that had a smooth speed adjustment.
    This was around early 1960's. I have no idea how they did that. I do not
    recall there being a zillion wires from the knob to the motor.
     
  6. Bud--

    Bud-- Guest

    Could be to give a reasonable speed acceleration. And to give a high
    enough torque to start against all kinds of dust and gunk. (I had a
    gunked up fan that would start on high, but not the other speeds.)
     
  7. Guest

    | wrote:
    |>
    |> Also, it seems to me if a motor of this type is going to have 4 wires to
    |> the winding, it could achieve 4 speeds instead of just 3 speeds, with the
    |> taps being in different positions. Two of the wires would be on one end
    |> of the winding, and the other two on the other end. I'll label them as
    |> A, B, C, D. It could look like:
    |>
    |> A---B-----C------D
    |>
    |> Connect to B and C for high speed.
    |> Connect to A and C for medium high speed.
    |> Connect to B and D for medium low speed.
    |> Connect to A and D for low speed.
    |
    |
    | The taps are not designed to work that way.

    Then how are they designed to work?


    |> It would be no more cost on the motor than redesigning for different tap
    |> positions on the winding. The switch would cost a bit more to have the
    |> extra position. But maybe it's not worth it in the market.
    |>
    |> Long long ago my grandfather had a fan that had a smooth speed adjustment.
    |> This was around early 1960's. I have no idea how they did that. I do not
    |> recall there being a zillion wires from the knob to the motor.
    |
    |
    | Its VERY simple. It used a variable inductor in series with the
    | motor, rather than one with fixed taps.

    It was a rather small one, then.
     
  8. Guest

    | Could be to give a reasonable speed acceleration. And to give a high
    | enough torque to start against all kinds of dust and gunk. (I had a
    | gunked up fan that would start on high, but not the other speeds.)

    Today I found a 3-speed fan that has LO as the first speed and HI as the
    last speed. There went the trend I had seen for decades. I bought it
    ($13) to see if it would work (Chinese made in Wal-Mart). It actually
    did work. No idea how long that will last, though.
     
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