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What happens when alternator load is increased

  1. Sep 9, 2016 #1
    So I'm studying synchronous machines and what happens when load is increased.

    In case of alternator(synchronous generator) if the load is increased, like more lamp is added in parallel,
    then the armature current would increase. But where does this extra current come from?

    The rotor cannot go faster because:
    there is locking between rotor magnetic field and armature rotating magnetic field.
    If the rotor accelerates, then the armature rotating magnetic field also accelerates, that means the frequency of output current changes. that can't happen as bus is assumed to be at constant voltage and constant frequency?

    What exactly happens and where does this extra load current come from?

    In case of sync motor when the shaft is loaded, the resultant air gap flux is behind leads the field flux. So when the shaft is loaded(load torque increased), the speed reduces and the air gap flux is a bit more in phase with field flux. Since the rotor has to move at constant speed, the air gap flux has to go back to initial position(leading field flux), so it draws more armature current.

    Armature flux leads the air gap flux. Air gap flux is phasor addition of armature flux and field flux.

    If armature flux is increased, air gap flux goes back to initial position.

    What actually happens when alternator load is increased? And if the rotor does speed up how does it know it has to speed up?
  2. jcsd
  3. Sep 9, 2016 #2
    You add more alternators. You bring up more power stations.
  4. Sep 9, 2016 #3

    jim hardy

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    Last edited: Sep 9, 2016
  5. Sep 9, 2016 #4
    If load current increases, then 2 things are needed to maintain constant voltage.
    1) Fuel consumption must increase.
    2) Field current must increase.

    To elaborate:
    1) If the alternator is regulated for constant voltage, which I am assuming, & the load current increases, it is also true that the power has increased. An alternator transduced mechanical power to electric power. Since electric power has increased, the mechanical power inputted must increase. So more fuel must be burned to keep steady voltage.
    2) The armature (usually the stator) has a large self inductance. The increased current will result in a larger voltage drop across said inductance, which reduces the terminal voltage. To compensate, the filed current must be raised so that the generated open circuit voltage increases, such that after the inductive drop is accounted for, the terminal voltage is at the target value.

    Did I help? BR.

  6. Sep 9, 2016 #5


    Staff: Mentor

  7. Sep 9, 2016 #6
    Thanks for replying.

    Okay so I looked at the phasors.
    As per them, when load current increases the resultant flux(air gap flux) tends to slip back.
    So the field mmf rises to pull the resultant flux(air gap flux) back towards it, back to original place.

    But how does the field current know it has to rise? The field flux(by rotor only) of sync motors are independently controlled. So how does it vary as per the load? I mean it can't be instantaneous, right?

    How does field current know it has to change and by how much does it change? How does it calculate?

    So if i remove the load, the armature current will go zero, then the rotor will accelerate, right? Then how will it come back to sync speed since synchronous motor has to run at sync speed.

    As per the phasor diagrams in the link: https://www.physicsforums.com/threads/synchronous-generator-excitation.829603/#post-5215184
    the rotor is first pushed ahead and this causes the armature current to flow to bring the resultant flux back to the original place.

    But what if the load current is increased without increasing mechanical power manually? I mean how does the rotor know it has to accelerate. And where does the extra input power come from to supply to additional load?

    One explanation could be that increased load current could send load current more lagging to voltage. Like power factor goes low and low, so resultant phasor addition of armature flux and field flux is again back where it was.

    Like at first alternator is not loaded. When it is loaded lightly, it's like leading armature current will flow, so the leading current is like 90 degrees leading the terminal voltage, so the resultant flux is back where it was. now when machine is loaded, the armature current rises but it starts lagging. Like it moves clockwise. As machine is more and more loaded, the armature current is in phase with terminal voltage and then starts lagging with more armature current.

    Not sure if this really happens. Any insight would be helpful?
  8. Sep 11, 2016 #7


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    There is some mechanism present that senses the voltage drop and applies more current. This is called the voltage regulator.

  9. Sep 11, 2016 #8
    Ah...okay. like some feedback element.
  10. Sep 11, 2016 #9

    jim hardy

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    You must decide what you will vary and what you will hold constant in your thought experiment. I stated those conditions on my drawings
    Here's keeping both field current and terminal volts constant , just applying torque. That's not unrealistic for a machine tied solidly to a stout bus(infinite bus is a useful concept)


    Observe "Condition 2"
    fixed terminal volts determines radius of circle
    note armature amps and armature mmf have same magnitude and direction.
    To restore unity power factor at that load you (or rbelli's voltage regulator) must raise field amps.

    You'll have to think one step at a time.
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