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The power of a fan

  1. Apr 20, 2007 #1
    Hi guys

    I suddenly think of a question. In my home, there's a standing fan which is old so I cannot see the nominal power on the label. With a ohm meter, can I identify the power of the fan? The meter can measure resistance R, potential U and current I of an AC power supply.

    (This thread was posted in general engineering already, I hope to have more answers in this box)
     
  2. jcsd
  3. Apr 20, 2007 #2
    usually power is Voltage x Current...but i don't know if there's an addition to that for AC current
     
  4. Apr 20, 2007 #3
    You can't measure the power in AC just multiplying voltage by current. The reason is that the sinusoids of voltage and current are not in phase. That is, they do not reach their maximum at the same time. With a two channel oscilloscope you could visualize the tension and add a small resistor in series to visualize simultaneously the current (beware the earth/ground of the mains and the oscilloscope). You could see two sinusoids shifted and measure the phase difference [tex]\theta[/tex]. Then the power of the device can be calculated:
    [tex]P=V\cdot I\cdot \cos \theta [/tex]
    Here V and I are the RMS of voltage and current values as measured with a multimeter.

    For resistances the phase difference or "phase angle" is zero. But for motors and other inductive devices it is far from zero and you can't ignore it.

    Anyhow you van be sure that the real power is not greater than [tex]P=V\cdot I[/tex]
     
  5. Apr 20, 2007 #4
    No you cannot find the power of the fan with an ohm meter.
    The ohm meter will give you the resistance of some coil of the motor.
    The power absorbed is nearly unrelated to this resistance.
    This is easy to understand:
    Imagine that you remove this coil from the motor, you will still measure the same resistance, but this will not be related in any way to the power of the motor.
    The power depends, among other things, on the rotation speed, as well as the utilisation of the motor. If the friction of the fan is higher, the power absorbed might increase. It might also decrease if this friction blocks the motor, and then only will the power be related to the (static) resistance.

    In summary, this resistance just indicates you the power that will be absorbed when the motor is blocked.
     
    Last edited: Apr 20, 2007
  6. Apr 20, 2007 #5
    Yes. And this power is just the product of this resistance times the current when the motor is blocked. This current can be measured with an AC ammeter (but hurry-up please).

    And this current is not the mains voltage divided by the resistance. For a fan it is far lower.
    It is limited by the impedance due to the inductance of the motor.
     
  7. Apr 20, 2007 #6
    If the Ohm meter works by DC probing, the inductance of the motor will play no role.
    If the Ohm meter works with "high" frequencies, the impedance will be dominated by the inductance.
    In none of these cases will this measurement be useful to determined the absorbed power.
    The absorbed power depends on the rotation speed and on the load of the motor.
     
  8. Apr 20, 2007 #7
    lalbatross: You should read again, more attentively, the previous posts. Your post do not makes sense.

    On ohmmeter works always in DC. Some devices work in AC but they are usually called Impedance meters, Capacimeters, etc.
     
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