Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A motor question, back emf

  1. Apr 14, 2005 #1
    A motor is designed to operate on 117 V and draws a current of 12.2 A when it first starts up. At its normal operating speed, the motor draws a current of 2.30 A. Obtain (a) the resistance of the armature coil, (b) the back emf developed at normal speed, and (c) the current drawn by the motor at one-third normal speed.

    Please let me know if im on the right track :)
    a) V=IR 117V = 12.2A * R
    R = 9.59ohms
    b) I = V-emf/R
    2.3A = 120V-emf/9.59ohms
    emf = 97.9V

    But i have no idea how to do 3... would i just divide emf / 3 and find current? (97.9/3)
     
  2. jcsd
  3. Apr 14, 2005 #2
    There is an inconsistency between a) and b). In a) you say V = IR, in b) you say V-emf = IR. Revisit your solution.
     
  4. Apr 14, 2005 #3
    emf = 0 in a)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook