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Question on alternators

  1. Jan 19, 2012 #1


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    Hi all:

    My question is regarding why electric alternators, dynamic retarding machines, or any rotating source of voltage becomes loaded down when a load is provided across it. I of course understand it in terms of energy, that the load is consuming the voltage that is produced.

    But i'm looking for a purely electrical explanation..

    I work on a diesel electric fleet of komatsu mining equipment..

    When an engine becomes weak, the controller backs off field excitation for the alternator, so it decreases it's output and becomes easier for the engine to rotate, They do this so the engine never stalls out, even when it becomes weak.

    So I was wondering HOW reducing excitation, and ultimately the output of the alternator, makes the alternator easier to rotate. Why is there a physical resistance for it to turn as it is increasingly loaded?

    I know this is simple and I should know it, but I do not.

    I want to reference counter emf in a generator, and say that the more voltage is generates, the more counter emf it generates as well, and somehow reference that to physical work required.... any help appreciated

    PS: I know already that one could say, "well it of course requires more input if you expect a greater output to feed more loads..." or "compare it to a mechanical load, the more load the harder to turn..."

    But I'm really looking for electrical explanations here, and ones that properly illustrate what's happening with reference to induced voltages, and actual loading

    let me try and talk myself through it:
    the engine rotates the alternator.
    the static exciter excites the field.
    output is produced in the alternator, due to the fact the conductor is cut by moving lines of flux of the rotating field (rotating field alternator)
    the output is a function of the induced voltage, which is a function of the field excitation from the exciter, and the rotational SPEED of engine.
    as the generator is loaded, current flows to the loads, voltage is dropped across them due to their resistance.

    OK I'm stuck here... like... if the generator wasn't capable of producing enough voltage to power the loads, than you would assume it would just simply...not power them sufficiently? but I don't understand why it becomes harder to turn....... :(
    Last edited: Jan 19, 2012
  2. jcsd
  3. Jan 20, 2012 #2


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    For anyone interested I solved this...

    Assume an alternator with rotating inner field, inducing voltage into an outside stationary armature. As the alternator is turned, and the field provided with a current (through brushes and slip rings) it produced a magnetic field. The field turns with the prime mover, as the static field in the field expands, it also appears to the armature as though it is collapsing. It crosses the armature while it turns and induces a voltage in it. The current that flows however, will be in the opposite direction as the current who's field induced the voltage that caused the armature current to flow. (lenz' law) As such, if we connect a load to the stationary armature, a current will flow, and a field will be establishes around the current in the armature winding. This field will oppose the field in the rotating field. The more loads connected in parallel to the generator output... the lower the apparent resistance... for a fixed induced voltage, more current will flow, this current will strengthen the magnetic field that acts against the original field. Therefore making it harder to turn.

    Works same way with dynamic braking when a load is applied across armature of generator, this is why dynamic retarding grids are connected in parallel... so as to limit the apparent resistance, increase the armature current, and strengthen the magnetic field that oppose the prime mover field.

    Good times!
  4. Jan 21, 2012 #3
    I would like to understand it simply as, more the load on Alternator more would be its current. Like you said, these currents will make electro-magnets on the stator iron, so, the stronger the stator electro-magnets, harder will it for the prime-mover to rotate the rotor (magnets) past them.
    (Actually, the stator electro-magnets are also rotating in nature (and rotates at the same speed as the rotor), but its axis is slightly off from the magnetic axis of the rotor, and that provides the force)
    Last edited: Jan 21, 2012
  5. Jan 21, 2012 #4

    jim hardy

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    have you as a kid ever played with magnets, placing one below and one atop a table, then dragged the top magnet around by moving the bottom one?

    if so, you must believe magnets can transmit a force from one to the other.

    if so, it should not be much of a leap to accept they can also transmit torque.
    and from there to electromagets transmitting torque.

    your field is a rotating electromagnet connected to crankhaft
    and armature is an electromagnet too but it can't rotate
    ... look at it it's bolted in place ...
    instead, the AC currents that flow in the stationary armature wires make a magnetic field that acts exactly like a second electromagnet, but one that DOES rotate, being dragged along by the field.
    That drag draws torque from the engine... in proportion to the power that's handed over to the load.
    When they reduce field they reduce power and hence torque.

    any help?

    old jim
    Last edited: Jan 22, 2012
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