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Synchronous Motor Phasor Diagram

  1. Sep 8, 2016 #1
    It's not really a problem but I was studying Sync Motors for assignment and didn't understand few things.

    1. The problem statement, all variables and given/known data

    http://s1320.photobucket.com/user/ajaytulsiani/media/-%20ia_zps8oujbzrh.jpg.html http://s1320.photobucket.com/user/ajaytulsiani/media/synch_zps3bry0aah.jpg.html
    -%20ia_zpsgbz2izmh.jpg synch_zps3bry0aah.jpg
    In the first picture...what is the point of -Ia. I mean when synchronos machine is working as alternator then current is Ia. Now the sync machine is working as motor so direction of Ia is reversed. That is shown in phasor. what is the point of adding a -Ia. And shouldn't power factor be between Ia and Ef instead of -Ia and Ef.

    In second picture if you look at third picture, isn't direction of IaXr wrong? Shouldn't it be towards left like the arrow be pointing left so the phasor looks like: Vt = Ef + IaXar?

    2. Relevant equations
    For first question...current flows out of armature in case of sync generator while it flows into armature in case of sync motor. Power factor is between load voltage and load current, which is Ia and not -Ia.

    For second question equation for motor is Vt = Ef + IaXar. But if you look at second picture, third phasor, then it appears that Ef = Vt + IaXr

    3. The attempt at a solution

    Well i just tried reading it again and again but didn't understand why IaXar phasor is otherway around?
    Any help would be appreciated...

  2. jcsd
  3. Sep 9, 2016 #2
    There was a reply here with a link. Is there any reason why the link was removed?
  4. Sep 9, 2016 #3


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    Staff: Mentor

    Judging by the mentor's memo, the link was advertising, and was unrelated to synchronous motors.
  5. Sep 9, 2016 #4


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    Staff: Mentor

    Let me see if I understand what you are asking: Are you questioning the need to consider -Ia for a motor and +Ia for a generator?

    Let's assume the machine is largely resistive, just for clarity. Then however you draw the phasors or label them, the voltage phasors need to be only a few degrees different from the current phasor to comply with being a resistance. Otherwise, if they are almost 180° apart, I guess you'd be illustrating a motor having negative resistance (which, strictly speaking, is what a synchronous generator really is when you have it feeding into the grid).
  6. Sep 10, 2016 #5
    Yeah i didn't understand why they've used -Ia for motor and +Ia for generator.
    If you look at second picture, the third phasor.
    the motor equation is Vt = Ef + IaXar.
    Isn't it shown wrong in the picture, the third phasor? Shouldn't IaXar be pointing towards Ef?
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