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A question about DC motor (input power, output power)

  1. Feb 12, 2009 #1
    I have a question. I set up a voltmeter and and ammeter (properly connected in parallel and in series respectively) so that when I input power into a motor in the form of DC current, both the voltmeter and the ammeter read positive values.

    Then, I removed the voltage source (i.e. battery). Now what I don't understand is, if I manually rotate the motor in one direction, the motor (acting as a generator), I get a positive voltage reading and a negative current reading.
    If I rotate the motor (generator) in the other direction, I get a negative voltage reading and a positive current reading.

    I don't understand, how can current flow against the voltage established by the generator? Can someone please explain this phenomenon to me? Thanks.
  2. jcsd
  3. Feb 12, 2009 #2


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    Science Advisor

    Well think about the difference between a load that consumes power (a resistor or a motor for example) versus one that supplies power (a battery or generator for example).

    For the resistor the voltage drop is positive at the terminal that the current enters whereas with a (dischaging) battery the current leaves from the positive terminal. The motor behaves in the same way as the resistor in this regard (because it's absorbing power) whereas the generator behaves in much the same way as a battery. It's all about the direction of power flow.
  4. Feb 12, 2009 #3
    Thanks for the fast and comprehensive response.
  5. Aug 3, 2011 #4
    Most of the DC motors take AC as an input and then uses the DC by rectifying it.
    And the negative sign you're taking about is because many of the DC motors have transformer type construction inside it.
    And it causes 180 degree phase difference between the current and voltage that's why current shows opposite sign.
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