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Motor resistance value isn't correct

  1. Sep 18, 2012 #1
    Motor resistance value isn't correct....

    I have an electric motor that registers a .12 A draw off of a 2.76V source. This obviously indicates to you and me a resistance of 16.23 Ohms.

    The problem is my Ohmmeter function on the MM. It reads out 1.2 between the motor leads, and this arithmetically can't be so.

    There was a split second when the ohmmeter spiked to about 16.3 Ohms, but only for a split second, and then fell back to 1.2 Ohms.

    Any advice to as why the meter is showing this impossible reading? ( OK, not impossible, but you get the idea.)
     
    Last edited: Sep 18, 2012
  2. jcsd
  3. Sep 19, 2012 #2

    uart

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    Re: Motor resistance value isn't correct....

    A motor is not a resistor and therefore it does not follow the V = RI.

    Google "back emf"
     
  4. Sep 19, 2012 #3

    sophiecentaur

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    Re: Motor resistance value isn't correct....

    The motor needs to be running for it to take only 0.12A at 2.76V. When it is stalled, it can pass much more current (and can burn out).
     
  5. Sep 19, 2012 #4
    Re: Motor resistance value isn't correct....

    Ohhh. I see. So the reading I am getting is commutator touching the coils of wire?

    I am a spring chicken when it comes to electronics. I am currently taking an AC/DC course at my technical school. So excuse my Newbie sense of direction.
     
  6. Sep 20, 2012 #5

    vk6kro

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    Re: Motor resistance value isn't correct....

    I have an electric motor that registers a .12 A draw off of a 2.76V source. This obviously indicates to you and me a resistance of 16.23 Ohms.

    I get 23 ohms for this.

    Anyway, the motor generates "back EMF" when it is turning and this opposes the supply voltage to reduce the supply current.

    If the figure of 1.2 ohms was correct, the motor would draw 2.3 amps when it was not turning. ( 2.76 volts / 1.2 ohms = 2.3 amps). This is why it is not a good idea to stall an electric motor.
     
  7. Sep 20, 2012 #6
    Re: Motor resistance value isn't correct....

    Hmm, I think the reason the reading spiked to 16.3 ohm for a brief period is your probes were not touching the terminals properly. It may be caused by a jerk while taking reading.
     
  8. Oct 2, 2012 #7
    Re: Motor resistance value isn't correct....

    I meant a current of .170 A
     
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