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Homework Help: Synchronous machine stator current

  1. Dec 31, 2015 #1
    1. The problem statement, all variables and given/known data

    Explain how the synchronous machine stator current can be varied if the load is constant. Justify your answer with phasor diagrams.

    2. The attempt at a solution

    If load is constant then load angle, the angle between induced e.m.f. and terminal voltage remains the same. Phase angle is the angle between stator current and terminal voltage, so if I change power factor that will alter stator current? But how can power factor be changed if the load is constant? Does this make any sense?

    Are these the phasor diagrams that are expected to see?

    Thank you for your comments.

    Attached Files:

  2. jcsd
  3. Dec 31, 2015 #2


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    This can be achieved by changing the excitation of the machine. But the alternator must be connected to the grid or infinite bus to make this happen. In this situation, frequency(rpm of the generator) and terminal voltage are constant as the machine is tied synchronously with the grid. Now if the excitation of the machine is increased, induced voltage(or generated emf) increases and supplies a current to other machines. This way, this extra current flows between the machine and other machines and not through the load. This current is superposed on the load current of the machine. One interesting thing is that the total current I of the machine increases and its power factor decreases such that IcosΦ remains constant. This way, the active power(kW) supplied by the machine is constant but the reactive power supplied(RkVA) increases.
    Last edited: Dec 31, 2015
  4. Dec 31, 2015 #3
    Thank you, it starts making sense now, so changing excitation will alter stator current. I think my phasor diagrams show all that, don't they?
  5. Dec 31, 2015 #4


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    In your phasor diagrams, terminal voltage V is not constant. I believe they are showing the effect of armature reaction on the terminal voltage of an isolated generator at different power factors. You can google "alternator on constant load and variable excitation" or refer any good machinery book to learn this in terms of phasor diagram.
    Last edited: Jan 1, 2016
  6. Jan 1, 2016 #5
    Right, that seems a good phrase to google with. Thank you.
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