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What will each wattmeter indicate?

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    What will each wattmeter indicate?

    Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three single-phase wattmeters are used to measure the power?

    The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!

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    Quote Originally Posted by Stephen30002 View Post
    Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three single-phase wattmeters are used to measure the power?

    The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!
    how did you come up with 311W? i'm having trouble with this question, not sure where to start.

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  5. baldscreen is offline
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    I get same as you...

    Quote Originally Posted by Stephen30002 View Post
    Three coils with a resistance of 4 ohms and impedance of 13 ohms each are connected in wye to a 120/208 Volt panel. What will each wattmeter indicate if three single-phase wattmeters are used to measure the power?

    The correct answer is 341W according to my answer sheet but I seem to keep getting 311W and I don't know what Im doing wrong. Any help would be great!
    I get the same answer as you and verified it through calculator on this site: http://www.rapidtables.com/calc/elec...calculator.htm

    Impedance Z = SQRT(4*4 + 13*13) = 8.823 Ohms
    Angle Theta = ATAN (Impedance/Resistance) = ATAN (13/4) = 72.9 degrees
    Total Power VA = V*V/Z = (120 * 120) / 8.823 = 1058.82 VA
    Real Power Watts = Total Power * Cos Theta = 1058.82 * Cos(72.9) = 311 watts

    Beats me how someone got 345 watts.

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  7. Allenxu is offline Junior Member Pro Subscriber
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    Quote Originally Posted by baldscreen View Post
    I get the same answer as you and verified it through calculator on this site: http://www.rapidtables.com/calc/elec...calculator.htm

    Impedance Z = SQRT(4*4 + 13*13) = 8.823 Ohms
    Angle Theta = ATAN (Impedance/Resistance) = ATAN (13/4) = 72.9 degrees
    Total Power VA = V*V/Z = (120 * 120) / 8.823 = 1058.82 VA
    Real Power Watts = Total Power * Cos Theta = 1058.82 * Cos(72.9) = 311 watts

    Beats me how someone got 345 watts.
    Impedance should be 13 ohms instead of 8.8

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  9. baldscreen is offline
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    I stand corrected...more or less.

    Quote Originally Posted by Allenxu View Post
    Impedance should be 13 ohms instead of 8.8
    You are correct on the impedance. I was thinking of it as reactance ohms.

    So, I'll try it again:

    COS(Theta) = (Resistance/Impedance) = 4/13
    Click image for larger version. 

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    Total Power VA = V*V/Z = (120 * 120) / 13 = 1107.69 VA
    Real Power Watts = Total Power * Cos Theta = 1107.69*4/13 = 340.8 watts

    Closer, but not quite the 345 watts given as the answer.

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  11. Lonpadams's Avatar
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    either luck or ???

    PF = R/Z = 4/13 = 0.3077

    E = I*R , 120 = I*13 , I = 120/13 = 9.23A

    P = I*E , P = 9.23*120 = 1107.7VA

    0.3077(PF) = W/1107.7VA , W = 1107.7*0.3077 = 340.84

    340.84(W)/9.23A = 36.93V

    36.93V/4OHMS = 9.23A

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