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Parallel R-L circuits, modeled motor

  1. Sep 17, 2007 #1
    1. The problem statement, all variables and given/known data

    A 0.5hp motor is observed to draw 1000W and 10 A when connected across a 120V, 60 Hz source.

    The motor can be modeled as a parallel R-L circuit. Sketch the phasor diagram and find the values for R and L.

    ---There are more questions to this, but I might be able to do them, so want to start here. I can't understand how to get the answers in the back of the book(both in Ohms).

    The answers are 14.4 and 21.7 Ohms.

    2. Relevant equations

    Not sure yet....

    3. The attempt at a solution

    My phasor diagram has voltage(E=120V) leading Current(I=10A) by 90 degrees. So, in other words, the voltage arrow points in the + x direction and the I points somewhere down at an angle between 0 and -90 degrees(on a conventional x-y axis).

    I can't even get started....so any help to get me started would be great!

  2. jcsd
  3. Sep 17, 2007 #2


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    Staff: Mentor

  4. Sep 17, 2007 #3
    yeah, I have the equations for them.

    Total Power VA = IE

    Actual Power = P = IEcos(theta)

    Reactive Power = Pr = IEsin(theta)

    I don't see how these help though, I tried to use them, and solved for theta.....and got 33.6 degrees....but how does that help, and is that even correct??
  5. Sep 18, 2007 #4


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    Staff: Mentor

    Well how about 1000 W? This is the Real Power!

    So P = V2/R => R = V2/P = (120 V)2 / (1000 W)

    Then Apparent or Total Power = (120 V)(10A) = 1200 VA = S

    and S2 = P2 + Q2, where Q is Reactive Power,

    and Q = V2/X, where X is the Reactance = [itex]\omega[/itex]L for a pure inductive load, and [itex]\omega[/itex] = 2[itex]\pi[/itex]f

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