Parallel R-L circuits, modeled motor

  • #1
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Homework Statement



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.

Homework Equations




Not sure yet....


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!

Brad
 

Answers and Replies

  • #2
Astronuc
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  • #3
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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??
 
  • #4
Astronuc
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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

http://www.allaboutcircuits.com/vol_2/chpt_3/2.html
 

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