Slowing down a ac motor

Discussion in 'The Projects Forum' started by martgreg, Apr 5, 2008.

  1. martgreg

    martgreg Thread Starter Member

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    Hi I am still working on my fan project ... but am trying a simpler approach and was wondering if it possible to get a regular fan from the store and slow down the rpm by adding a variable resistor "a potentiometer"

    Remember this only has to work for a while ..not production,

    Or will i end up killing my self and burning down my school.

    The motor will be 120 volt ac , 60 htz .35 amps or close


    any help will be greatly appreciated.. or any more info needed would be grand :)

    thanks again ..

    this is such a helpful forum..
  2. Audioguru

    Audioguru New Member

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    Look in Google for AC Motor Type and you will see some that some types sync with the mains frequency and other types that are "universal" can be slowed by reducing the duty-cycle of the power with pulse-width-modulation.
    Which type of motor are you talking about?

    A variable resistor in series with a motor must be huge and expensive since it will get very hot. The motor might be the wrong type for a huge variable resistor anyway.
  3. martgreg

    martgreg Thread Starter Member

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    umm ...

    I am basically pulling it from a brand new box fan.... is there any way of me looking at it .. or getting any info from it that would help me find out what type it is ?


    thanks for your help

    :)

    mg
  4. jpanhalt

    jpanhalt E-book Developer

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    Does the motor have carbon brushes? Very few small fan motors have brushes today. Also, a motor like AG is referring to will often be labeled, AC/DC; whereas, the motors that can't be controlled with a resistor we be AC only.

    John
  5. Audioguru

    Audioguru New Member

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    I think most fan motors are a cheap induction type which syncs to the mains frequency and need the frequency to be changed (very complicated circuit) to change the speed.

    The big ceiling fans switch how many poles are used to change their speed.

    I have a 30 years old drill that has sparking brushes. My fans don't have brushes.
  6. jpanhalt

    jpanhalt E-book Developer

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    But, if the fan is shaded pole, it can be contolled with phase. That is, a light dimmer, which I think is easier and cheaper than a big potentiometer. John
  7. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Depends on the type of motor. My first thought was a light dimmer (lovely solution to so many problems controlling AC), but those work by reducing RMS power by cutting off part of the sign wave, changing their shape.

    If it is a frequency dependent problem than this might work, but I've never built it. It was a solution I put up for another post.

    [​IMG]
  8. jpanhalt

    jpanhalt E-book Developer

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    Actually, for shaded pole motors (often used in fans), there are commercial speed controllers for about $20. Those controllers are basically just triac light dimmers.

    If the OP is willing to open the case of the fan, he can determine if it is shaded pole, either by posting a picture here of the outer laminations or by going to Wikipedia and looking at the picture there. In the alternative, one can often just look though the cooling holes and see the tell- tale single coils of the shaded pole as shown here (from Wikipedia). John
  9. martgreg

    martgreg Thread Starter Member

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    Hey thanks guys...

    Here are a couple of pictures from a motor that I might end up using.....


    This one has 6 wires from the motor....

    grey and white go to a little black box ( forgot whta this was.. someone told me)

    then 4 wires ( white , grey , red ,brown,) go to power cord and 0 1 2 3 speed switch.

    I took this apart and connected the white wire to the ribbed wire section of the power cord . Then attached one of the wires ( brown) to the otheer smoothe wire of the power cord... crossed my fingers and plugged it in...

    still alive and it spins.. but i dont know what speed ... lol...

    a speed control would be great...

    Attached Files:

  10. jpanhalt

    jpanhalt E-book Developer

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    Does not look like a shaded pole motor to me. It looks more like a conventional squirrel cage induction motor where the number of poles and synchronous speed is determined by the way the wires are attached. Apparently, from the labels on the switch, it is a 3-speed motor. With that type of motor, it will be difficult or impossible to get speeds that are in between the three choices without using a variable frequency drive. Even then, the range of useful speeds may be quite limited.

    See: Wikipedia for induction motors.. Here is another photo from that source. John
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