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Archived Back EMF

  1. Mar 12, 2008 #1
    1. The problem statement, all variables and given/known data

    An automobile starter motor draws a current of 1.9 A from a 5.5 V battery when operating at normal speed. A broken pulley locks the armature in position, and the current increases to 8.6 A.

    What was the back emf of the motor when operating normally? Answer in units of V.

    2. Relevant equations

    V=IR is the only one I can think of

    3. The attempt at a solution

    I calculated the resistance. But I don't know why it is at all relevant that the amperage should increase to 8.6 A, if the question asks for the back emf of the motor at normal operating speeds.
  2. jcsd
  3. Feb 22, 2016 #2
    Since the rotor is locked, there is no back emf in the armature circuit.
    Hence, Vbattery=IaRarmature
    When the motor is running normally,
    Back emf in normal running condition is 4.285 V.
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