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Homework Help: Resistance of DC Motor Experiment

  1. Mar 17, 2012 #1
    Hey all, I have a small question that's been bugging me and I can't find an answer anywhere. How do you test the resistance of a DC motor?

    My main "practical" idea is to test it as you'd test any other piece of metal: connect a power supply, an ammeter and a dc motor in series, and connect a voltmeter to measure the potential difference across the motor (in parallel), then take all the readings and calculate using the R=V/I formula.

    However, I thought maybe it has something to do with back emf/induced current? In this case, would I connect a galvanometer instead of an ammeter (same setup as above), start up the motor, record the reading, then hold down the motor to see the supply current, and then measure the induced current? But then, would I use the induced current, the supply current or the net current in the R=V/I calculations?

    My third thought is very vague, and I have almost no backing for it, but would there be any way to measure the back emf? Because isn't that kind of the resistance of a motor?

    Please help, we were just given the aim of this experiment in class and told to figure out the rest if we wanted a head start for a practical exam coming up.
  2. jcsd
  3. Mar 17, 2012 #2


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    Homework Helper

    Presumably you are after the "effective resistance" of the motor.

    If you connected the motor and a fixed resistor in series with a given power supply [battery?] you could measure the current through the circuit.

    If you then replaced the the motor with a rheostat [variable resistor], you could adjust that rheostat until the same current was flowing.

    Then use what ever is necessary to measure the resistance the rheostat was set to to achieve that - should/must be the effective resistance of the motor.
  4. Mar 18, 2012 #3
    If, by resistance, you mean the resistance of the wires of the motor you may be able to measre this with an ohm meter.
    If you connect a battery and measure a current it must be the current when the motor is prevented from turning (stalled) you must take care to use low voltage so that you do not burn out the motor coil.
    When the motor is running the current will be much less than the stalled current because of the back emf you have identified.
    Once the motor is running the volts and amps tell you the power being supplied and not much about 'resistance'
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