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Counter emf generated in a motor at full speed

  1. Dec 1, 2015 #1
    1. The problem statement, all variables and given/known data
    An electric motor draws 20.0 A of current when first switched on. The motors windings have a resistance of 5.0 ohm. Once the motor is up to full speed it draws only 4.4 A of current. Calculate the counter emf generated in the motor at full speed

    2. Relevant equations
    I = V/R
    V=IR

    3. The attempt at a solution
    R = 5.0 ohm
    A1 = 20.0 A
    A2= 4.4 A n

    1. (20A)(5.0ohm) = 100 V

    2. (4.4A)(5.0ohm) = 22V

    3. 100V - 22V = 78V

    Im told im missing some "change", however I think this is correct, it is change in voltage?
     
  2. jcsd
  3. Dec 2, 2015 #2

    andrevdh

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    Homework Helper

    Looks fine to me.
     
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