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General Question

  1. Jun 25, 2009 #1
    Hi I have a pretty simple question

    I was just looking at my FAN and was wondering what is the difference between putting it on 1 (low) and on 3 (high).


    Obviously it runs faster on 3 and therefore I assume it requires more energy.


    If V = I*R

    V stays the same right? 120 V? through the outlet?

    If that is true, I was thinking R increases since it needs more to go faster and is at a higher impedance.


    But that would mean CURRENT goes down to maintain the same voltage?

    That doesn't make sense. I would think current increases but the impedance doesn't go down....

    can someone explain?? thanks
     
  2. jcsd
  3. Jun 25, 2009 #2

    negitron

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    Rather than just hand you the answer I'd like to see if you can work it out yourself, so I'll add the following piece of information:

    When you're switching speeds on a fan, you're physically switching motor windings in or out of the circuit. And, yes, the voltage remains (more or less) constant. Does that help you?
     
  4. Jun 25, 2009 #3
    The resistance is higher (and current lower) when the fan is on the slow speed. On a side note, the fan is considerably more efficient at the high speed. So in theory you could use less power to move a given amount of air by leaving the fan on high and switching it on and off, than by simply leaving the fan on low. Although, chances are, your time is worth more than that.
     
  5. Jun 25, 2009 #4

    vk6kro

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    Fan speed controllers work by placing either an inductor or a capacitor in series with the fan.

    So, the supply voltage stays the same but the voltage getting to the fan motor is decreased, so the fan runs slower. This is mainly because the load on a fan motor increases with fan speed (because of increased air resistance) and the fan will only run as fast as it has power to combat the extra air resistance.

    Adding these components in series with the fan motor drops the current through the fan motor but because these components have reactance and not resistance, they do not waste much power.
     
  6. Jun 26, 2009 #5
    Multi speeds on ac induction motors are usually achieved by adding booster windings to the field core. These windings are wound on the same field core poles as the main winding and are switched in series with the main winding when lower speeds are required. For example if you run your fan on hi speed then only the main winding is connected. On med the main and med windings are connected in series. On low the main, med and low windings are all connected in series. There are other methods, but this one is the most common.
     
  7. Jun 26, 2009 #6


    If all it is doing is switching motor windings..then that will result in a stronger or weaker magnetic field? assuming the fan is using a DC electric motor?


    so more windings meaning faster motion because stronger field?


    but before I go on:

    at high speed is it lower or higher impedance.....compared to low speed....


    fleem said "The resistance is higher (and current lower) when the fan is on the slow speed."


    this means current is low on low speed and current is high on high speed?
     
  8. Jun 26, 2009 #7

    vk6kro

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    In desk fans and ceiling fans, the method of control (in my country at least) is to put reactance in series with the fan. I have repaired and replaced quite a few of these and that is always how they work. The fans are usually shaded pole single phase AC induction motors.
    Large industrial fans use more efficient methods.

    If your fan is using a DC motor, then reactance control will not work.

    DC motors can be controlled by reducing or increasing the supply voltage to the whole motor or just the field winding.

    If all it is doing is switching motor windings..then that will result in a stronger or weaker magnetic field? assuming the fan is using a DC electric motor?
    More turns in the winding will result in a stronger magnetic field.


    so more windings meaning faster motion because stronger field?
    If the motor is loaded (like a fan is) then a stronger field will give more power so the fan will rotate faster.

    at high speed is it lower or higher impedance.....compared to low speed....
    If it draws more current from the supply it is lower impedance because the supply voltage stays the same.


    fleem said "The resistance is higher (and current lower) when the fan is on the slow speed."
    this means current is low on low speed and current is high on high speed?


    Yes.
     
  9. Jun 26, 2009 #8

    Thank you!

    with respect to bold:

    if current is low on low speed that means resistance (or reactance) is higher compared with high speed (higher current)



    right?
     
  10. Jun 26, 2009 #9

    negitron

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    Right.
     
  11. Jun 26, 2009 #10
    thank you.
     
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