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Homework Help: How much current does the motor draw to produce the required torque?

  1. Aug 6, 2012 #1
    1. The problem statement, all variables and given/known data
    A small electric car overcomes a 269N friction force when traveling 28.6km/hr. The electric motor is powered by ten 12.0V batteries connected in series and is coupled directly to the wheels whose diameters are 50.6cm. The 295 armature coils are rectangular, 10.0cm by 14.8cm, and rotate in a 0.590T magnetic field. How much current does the motor draw to produce the required torque?

    What is the back emf?

    How much power is dissipated in the coils?

    2. Relevant equations
    Not too sure...

    3. The attempt at a solution
    This is kind of weird as my homework goes from Waves and EM to this question :S regardless...

    I'm not really sure where to start on this, I was guessing that the armature coil would be producing a current I? So:

    B=╬╝In, where B=0.590T and n = 295/(0.148m) but I'm guessing that's not it at all...

    Any help?
  2. jcsd
  3. Aug 6, 2012 #2
    Okay I seem to got the first part

    Torque on Wheel = Torque on coils in motor

    TorqueWheel = ForceFriction*RadiusWheel

    TorqueCoil = B*I*(AreaCoil)

    Where I can solve for I or current. :)

    But now i'm stuck on calculating the back emf...
    I guess I know...

    V(battery) - IR = emf or -L(dI/dt) = emf but I don't have either R or L... Any help?
  4. Aug 7, 2012 #3


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    Is it easier than you think? You know the force and velocity so you know the power required. You know the battery voltage so presumably you can calculate the current from P=IV.

    Is the rest of the info unnecessary? I don't think there is enough info to calculate the motor efficiency?
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