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AC Generators: Voltage or Current Source?

  1. Apr 12, 2005 #1
    hey i'm wondering whether an ac generator would be classified as a voltage source or current source, or neither (if thats possible)? i'm asking you guys since i have nooooo idea how i would go about determining this. Thanks!
  2. jcsd
  3. Apr 12, 2005 #2


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    Staff: Mentor

    Is this a homework problem? Think about how a generator actually operates: they run at constant rpm, providing constant voltage (voltage is determined by the physical construction of the generator - magnets and windings, etc.) at a constant frequency (determined by the rpm). Current varies as load increases -- but how?
  4. Apr 13, 2005 #3
    no this isn't a hwk question. this is to satisfy some of my own curiousity. and how? i dunno..thats why i'm askin! lol
  5. Apr 14, 2005 #4

    my understanding is that AC power comes in its own voltage/current package. That's why electric companies use transformers, not resisters, to bring down voltage before it gets to your house. assuming we have perfect conductors and blah blah, there is an increase in current proportional to the decrease in voltage. Power (voltage times current) remains constant. So i would consider an AC generator a _power_ source, rather than one of voltage or current. Tesla could probably put it better.
  6. Apr 15, 2005 #5


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    An ideal AC generator would be a time varying voltage source. A real AC generator can adequately be modelled as a time varying voltage source in series with a very small resistor.
  7. Aug 28, 2008 #6
    It can be either a CCS or a CVS. The speed is related to voltage, and the torque is related to current. Forcing constant torque results in constant current. As the load resistance varies, the voltage varies with it. Forcing constant speed results in constant voltage. As the load resistance changes, current varies inversely.

    In real world power transmission, constant voltage is the method employed. Insulators lose much less power than conductors. With constant voltage, to turn off a load, say a lamp, we open a switch and block a voltage source resulting in near zero current. The loss is V^2*G, where G is the conductance of the insulation. If constant current was used, to turn a lamp off, we would close a switch across the lamp shorting it resulting in near zero voltage across it. The loss in this off state is I^2*R, where R is the conductor resistance.

    The long and short of it is that V^2*G is much lower than I^2*R. So for now, power is distributed in the constant voltage mode. This is achieved by holding a constant speed, rpm, on the generator. A side benefit to constant voltage operation is constant frequency. The steady 50 or 60 Hz frequency can be employed to make synchronous motors run at a fixed predictable speed. If constant current ac generators were employed, the frequency would vary.

    Does this help?
  8. Aug 28, 2008 #7
    Beat me to it. But yes because a grid will operate at a fixed frequency, the method of generation needs to be a voltage source. Like cabraham said, if it was a current controlled type of generation then the frequency would vary.

    As far as an ordinary synchronous machine being a VS or CS, it can go either way.

    Voltage = kv * speed
    Current = kt * torque (sort of)
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