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Thread: 3-phase complex power calculations

  1. #81
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    Final answer I get is 12959 volts

    I convert Delta load to wye and solve it for phase voltage

    Vsource - VD across line reactance = Load voltage

    This gives v source per phase multiplied by SQRT 3 to line line to line voltage of 12959 volts.

  2. #82
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    Quote Originally Posted by roy167 View Post
    Final answer I get is 12959 volts

    I convert Delta load to wye and solve it for phase voltage

    Vsource - VD across line reactance = Load voltage

    This gives v source per phase multiplied by SQRT 3 to line line to line voltage of 12959 volts.
    Well then your method and execution would evidently be correct since you got the correct answer.
    BB+/BB=?

  3. #83
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    Quote Originally Posted by mivey View Post
    The something wrong is:
    ...and that is not to take away from David who first detailed Phil's error as shown below. But for some reason, after multiple members agreed with David's assessment, Phil could not seem to understand and suggested David should go back to school.

    David's circuit analysis skills may have laid dormant but he certainly did not forget how to use them and evidently needed no refresher.


    Quote Originally Posted by david luchini View Post
    Should be easy enough to verify. With a Z of 290+j106 or 308.8<20deg, IAB would be VAB÷Z or 12500<0deg ÷308.8<20deg = 40.4<-20deg...
    IBC = 40.4<-140deg, and
    ICA=40.4<100deg.

    IaA= IAB-ICA= 40.4<-20deg - 40.4<100deg = 70<-50deg.

    Since the question tells us that IaA = 70<-20deg, it is clear that a load impedance of 290+j106 is not correct.

    Try a load impedance of 304.6-j53.7...capacitive, not inductive. That would give an IAB= 40.4<10deg, IBC= 40.4<-110deg, and ICA=40.4<130deg.

    So, IaA=IAB-ICA= 40.4<10deg - 40.4<130deg = 70<-20deg.
    A capacitive load impedance of 304.6-j53.7 will give the proper line current.
    Quote Originally Posted by david luchini View Post
    Phil, here is your mistake. VAn will not equal VAB/Sqrt(3). VAn will equal (VAB<-30deg)/Sqrt(3).

    Using the wrong VAn (7216.9<0deg V) will calculate the delta impedance as 290+j106, just as you mentioned in post #23.

    Using the correct VAn (7216.9<-30deg V) will calculate the delta impedance as 304.6-j53.7, as I mentioned in post #24.

    Hope this helps.
    Quote Originally Posted by david luchini View Post
    What we noticed is that your method produced the wrong load impedance, the wrong load current, and the wrong source voltage.

    Your method is, quite frankly, irrelevant since it produces the wrong results.
    BB+/BB=?

  4. #84
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    Well guys if this was a PE exam question, it would be pencils down and the clock would have run out by now.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  5. #85
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    Quote Originally Posted by kingpb View Post
    Well guys if this was a PE exam question, it would be pencils down and the clock would have run out by now.
    There are problems just like this on the PE exam. You don't get to discuss with your fellow examinees though!
    BB+/BB=?

  6. #86
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    Quote Originally Posted by mivey View Post
    Well then your method and execution would evidently be correct since you got the correct answer.
    I can't be sure of that. NCEES is known to include wrong answers in the options.

  7. #87
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    Quote Originally Posted by roy167 View Post
    I can't be sure of that. NCEES is known to include wrong answers in the options.
    Yes they do. Common calculation and concept errors seem to be available answers. As a perfect example, an impedance problem would probably have both the correct answer and the answer Phil gave. Another answer might be one given by 12500@0d ÷ 70@-20d.
    BB+/BB=?

  8. #88
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    When I solve it using the method above, you don't need to go into phasor etc for this kind of problem. It is quick and dirty and most importantly correct.

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