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Synchronous Generator ( again )

  1. Jan 26, 2012 #1
    So I decided to take a brake from electronics, and get back to physics of synchronous machines, or alternators if you will.

    A great deal of help was mr. jim hardy, trying to explain the alternator in the intuitive way. And so he succeed. I got the basics right. Now I want to tackle some more physics about it.


    So we have a stator and a rotor.

    http://pokit.org/get/e919530b23e3274b923d0063cf13fb01.jpg [Broken]

    Blue-from rotor
    Red-from stator
    Black-resulting
    all flux or mmf

    I believe that this represents the rotor, but "unwrapped"?

    I want to talk about torque angle.

    The picture shows the case where we have a resistive load. This means current will produce a field exactly pi/2 ahead of the rotor field, am I correct?

    This means that the field of the rotor is always perpendicular to the field of the stator?

    Does this mean that the δ=pi/2? Or the torque angle?

    OR

    is the torque angle, the angle between, rotor field and the RESULTING field?

    I have tons of more questions, but lets settle this for now.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jan 26, 2012 #2

    jim hardy

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    i hope i understand the question -
    and i think it stems from the derivative relationship between flux and voltage.



    well, start your thinking from open circuit
    in which case your sketch shows machine at instant of voltage zero crossing.

    in absence of armature reaction (no armature current)
    there's no armature MMF
    so
    the stator field IS the rotor field;
    so torque angle is zero.

    when resistive current begins to flow
    there appears a MMF component due to armature reaction
    which is perpendicular to rotor field
    (remember voltage is derivative of flux and d[itex]\Phi[/itex]/dt is zero at the poles, max between poles);;
    so instant of max voltage is 90 deg from your sketch and so will be resistive load current peak,
    so armature reaction MMF will be 90 deg from rotor MMF
    so armature reaction adding in a perpendicular MMF rotates total field relative to rotor
    so we begin to develop a torque angle between rotor and total field.


    you can watch this with a stroboscope synched to terminal volts(total field) and shined on the shaft keway..

    now, were armature current 90deg out of phase with terminal volts
    the armature MMF would be aligned with rotor not perpendicular
    so there'd be no rotation of total field
    hence no torque angle
    which meshes up nicely with observations that reactive current neither transfers electric power nor produces torque..

    here's another writeup
    ecee.colorado.edu/~ecen4517/materials/SynchronousMach.pdf

     
    Last edited: Jan 26, 2012
  4. Jan 27, 2012 #3
    So the torque is always present?

    When do we say that the machine is in synchronism? When the torque angle is 0, I assume nothing happens then.

    But I was told that when mechanical torque equals electrical torque, we are in synchronism?

    Is this correct?



    And isn't this dangerous? If we have a very big torque angle, don't we have big torque?
    Lets say that the prime mover is spinning the machine clockwise. Mechanical torque would be then clockwise.

    But we have a electric torque, which is in opposite direction. Doesn't this tend to twist the shaft of the rotor?
     
  5. Jan 27, 2012 #4

    jim hardy

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    more later

    i'm embarassed, made a mental mistake unwrapping the machine to match your drawing will correct that.

    re your question ""Doesn't this tend to twist the shaft of the rotor?""

    quick answer -

    yes the torque twists the shaft that's why it is stout steel
    the torque is in proportion to electrical power generated
    in my English units it is:

    horsepower = 2 X pi X torque X rpm / 33,000

    and it's pretty apparrent that the 33000 units converter term comes from 60 sec/min (accounts for rpm ) X 550 ft-lbs/sec (one hp=550ftlbs/sec)

    now the generator rotor is pretty stout

    on our big turbine we measured the twist.

    going from zero to full power , about one million horsepower, it twisted 3.2 degrees , mostly in the steam turbine.
    whole thing probably 150 feet long

    as i said in another post, torsional oscillation is possible
    turbine shaft had a resonant point at 7 hz and the damping was very light
    so that is a frequency to be avoided in any electrical load connected to generator.

    if you google "subsynchronous resonance" you can find tales of broken turbine shafts.

    there's also a torsional resonance of the whole machine's inertia against the electric grid, 1 hz typical for steam turbines
    i dont know what windmills would be. Hopefully not close to any blade or tower natural frequency.

    old jim
     
  6. Jan 27, 2012 #5
    Yes I find those tales very entertaining. You can learn a lesson from there too. I will google those pictures. I am seriously considering, building my own synchronous machine. I want to see this over-excitation and under-excitation myself. I can only vaguely understand what it is. I want to understand how does a machine consume, or produce reactive power. That is still far-fetched to me. I want to know the physical explanation, rather than the phasor representation.

    Any thoughts here mr. jim?
     
  7. Jan 27, 2012 #6

    jim hardy

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    The torque is in proportion to the resistive current, hence to the electrical power going in or out as in previous post.
    no power out, torque equal zero (well there's windage but we are thinking of torque from magnetic effects).
    Electrical power out and it's a generator torque opposes rotation;;
    electrical power in and it's a motor torque aids rotation.
    that agrees with the fellow's formula torque = K sin (torque angle) - zero at zero, + for positive angle and - for negative angle , unlike cosine.

    it's in synch when the rotor is spinning at same speed as stator field.
    note for an isolated machine there's nothing for it to be in synch with so the term is meaniingless then
    when there's an external voltage like the grid which is far more powerful than our prime mover, we are locked into synch with the stator field by the magnetic torque.
    we are not big enough to change the grid speed much, can only pump energy into it or extract energy from it at fixed speed of the grid.
    so all the generators on a grid are spinning in unison with only small differences in their torque angle.

    IF a machine ever gets past the 90 degree torque angle, note torque drops off again (sin function) and it will accelerate. When sine goes negative it becomes a motor and tries to pull back into synch, toward zero torque angle.
    this is a very violent maneuver both electrically and mechanically and breaks things.


    when the torques are equal the rotor will be in step with stator field neither accelerating nor decelerating. So it's in synch.
    were torques unequal their difference wouldn't their difference accelerate machine , torque/moment of inertia?
    i can't say it's wrong but perhaps not complete.
    these torques are very real and well behaved betwen -90 and +90 degrees.

    what makes it click for me is this thoought experiment:

    stop everythiing in your mind and de-energize the machine..
    remove the rotor from the machine so you have a hollow cylinder with 3 phase windings surrounding it. You're looking down the axis of the cylinder.
    place a child's compass in the center of the cylinder.
    Apply some DC current through one phase of the stator. Will the compass align with that phase?
    Now swap your DC current to the next phase. The compass will follow.
    And so on.
    You could make the compass spin by switching currents in sequence phase A, B, C.

    Tesla figured out that by applying sinewaves to phases A, B, C he'd get a resultant magnetic field that rotated and didn't pulsate,
    which was very handy for a motor.
    If he put a strong electromagnet in the cylinder it would follow the rotating field
    just like the child's compass
    and if he put a squirrel cage rotor in there it would get currents induced in it that made it follow the rotating field albeit at slightly slower speed. Hence slip of induction motors.

    SO if you took an induction mtor and placed inside its rotor a strong permanent magnet,
    it would start and accelerate to near synchronous speed on the induction principle and when slip got low enough the permanent magnet would grab the rotating field and follow it driving slip to zero.

    might you remember the old Garrard Synchro-Lab turntables of late 1950's ? (oops probably not)

    look up "amortisseur windings on alternator"

    old jim
     
  8. Jan 27, 2012 #7
    I understood everything. I am beginning to form a very very clear picture. I came up with the analogy. Care to listen it out?


    Take 1 bicycle. Put it upside down. Spin the front wheel very hard. The wheel would continue spinning for a long time, due to low friction and in ideal world it would spin indefinitely Right?

    This would represent, in my analogy a generator, operating with no load. So you have a prime mover turning the shaft, turning the rotor but its doesn't have any load, so no torque.

    Now put your hand gently at the tire, and try to slow it down. Your hand would represent the load, and you would need POWER to continue spinning that wheel at the same speed it was spinning before.

    So my load would be my hand. I would need power that comes from my prime mover, to overcome that torque.

    This is something I came up with, to get me a little better navigation through all those powers and modes of alternator.

    Is my analogy valid? Or am I completely off?
     
  9. Jan 27, 2012 #8

    jim hardy

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    just saw your new post.

    want to think on it so i dont embarass myself again.

    keep after it. i wake up at night operating things in my head, have to because my math is so bad.

    When you get your two alternators try this

    connect their stator windings in parallel
    connect their fields in parallel and apply maybe 12 or 24 volts AC to the fields
    now they think they're rotating in synchronism although they're standing still.
    turn one and watch the other follow
    turn one and hold the other, feel them transmit torque


    if you do that experiment you've built something called a "Selsyn" , a widely used as position transmitter.


    that should help your brain accept it. Tactile information takes a different neuron route into our cerebrum than does spoken or reading.
     
    Last edited: Jan 27, 2012
  10. Jan 27, 2012 #9

    jim hardy

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    it sounds right to me !
     
  11. Jan 27, 2012 #10
    So this is actually possible? Guess I have a project to do after 4. semester.

    I think I just had an "aha" moment when you explained those rotor and stator currents.

    There is a lot going on in this synchronous machine :(

    But, nevertheless I am getting there.
     
  12. Jan 27, 2012 #11
    Thank you very much kind sir. You filled a lot my knowledge gaps today.
     
  13. Jan 28, 2012 #12

    jim hardy

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    in post #2
    this line
    ""in which case your sketch shows machine at instant of voltage zero crossing.""

    should have said
    "in which case your sketch shows machine at instant of voltage peak"

    my bad, as noted earlier...:redface:
    sorry about that.
    haste makes waste.
     
  14. Jan 28, 2012 #13
    Its ok, I've got what you meant to say. I had that sleepless night. Trying to simulate this machine in my head. Still have a little tweaks to do, but I think I am there. Funny thing is, I am not studying this for my test, or anything, I am studying this because I find this synch machine very interesting.
     
  15. Jan 28, 2012 #14

    jim hardy

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    when you can work the machine in your head, the equations are sure easier to remember.

    Trouble is our mind can believe things that aren't physically possible
    i have to keep on cross-checking my thought processes against each other, eliminating the ones that lead to impossible conclusions.

    i have seen your fluency with equations and admire it.
    you'll have success at whatever you choose to do.

    The ability to swap back and forth between the math and the mechanics of something is a real valuable skill in industry. It allows you to communicate successfuly with audiences of vastly different educational levels.
     
  16. Jan 28, 2012 #15
    Thank you. I've just grasped the idea of under and over excitation. I mean, not learning it by heart, but really understanding what those phasors mean.

    I will on my final exam, ask from my professor, to draw me ANY situation of mentioned 6 (phasors) and I will have to tell if it is generator, motor, no load, under excitation, over excitation. I am getting very comfortable with synchronous machine.

    Needless to say, without your guidance, I would be nowhere. Thank you.
     
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