I'm just left wondering why old jim did not answer sooner.
I feel like I'm maybe making a nuisance of myself here by overposting, so i stay on EE and Nuke threads mostly. And i wait for others to have their say usually somebody else gives a better answer than i could have.
(I think this is due to transformers and some measurement inaccuracies of the AVR sensing voltage - correct me if I'm wrong)
i think you're exactly right on that one.
So, secondly i think you misunderstood my question.
Quite likely, i do that a lot. It's called 'Asperger's Syndrome".
Anyways, my first initial question in this thread is that when a generator is importing VAR, is that current doing any work that makes the prime mover's work on the shaft easier?
No. VARS transmit no work.
There's two different ways to show that.
1. . Recall VAR current is 90 deg out of phase.
Watts is Volts X Amps X cosine of angle between them, and since cos(90deg) = zero that product is also zero - no watts so no torque. That's a math solution.
2. . If you draw two sinewaves 90 deg out of phase, label one Volts and the other Amps,
now quadrant by quadrant multiply them to get 'instantaneous' watts. You'll see by symmetry that there's equal positive and negative products so the area under the resulting curve is zero. That's a graphic solution.
Now shift them back in phase and repeat - all area is now positive (because negative volts times negative amps = positive watts).
Observe if you shift them 180 out all area is negative, which means you swapped from generator to motor.
Now it's true that VARS exchange energy between source and load for fractions of a cycle but they average out to zero. That's the positive and negative areas you saw in the graphical solution.. In a single phase machine that would make a pulsating torque but it would average zero.
Also I think that even though the current is supposed to 90 degrees leading the voltage, it still must be somewhat lower (89? 89.9?)due to heat losses in the windings providing som real power (?)
I believe you could say that. But don't get hung up on that thought. Practically, there'll always be a resistive and a reactive component to load current so it'll have some angle between -90 and +90 deg.. For learning we break load current into ideal components at zero and 90 degrees and study them separately.
And, IF a generator is importing huge amounts of reactive current from another generator (all protection functions are disabled, machine will eventually burn or pole slip will occur etc), still with prime mover input -- will that current make it easier for the prime mover to turn the shaft?
Hmmmm there's a possibliliy we're not on same track here. Try this thought:
That imported reactive current does not transmit any torque to the shaft so it niether helps nor hinders the prime mover.
What it does do is this: it adds to the flux that's made by the rotating field of the underexcited machine. Now - if that underexcited machine has so little excitation that its field can no longer handle the torque of its prime mover, it will accelerate above synchronous speed and try to run as an induction generator. That can be mechanically pretty violent so we don't want it to happen.
But the reason the machine sped up is not because the other one helped it, instead because its own field got too weak to hold on to the torque from its engine.
Let me lift a phrase fom an old post last January
If as a kid you ever played with magnets, put one on top of a table and one underneath and moved them from below, you know magnets can transmit force.
Of course so can electromagnets. It shouldn't be much of astretch to realize they can transmit torque, too.
Rotor of a synchronous machine is an electromagnet that physically rotates. A stator with external voltage applied is also an electromagnet and its field rotates because of the precession of three phase currents around windings. In a motor the stator field drags the rotor along, in a ngenerator the rotor drags the stator field around.
We had a long discussion with a cool guy named Bassalisk about this last January, you might check these two threads. He had some good drawings and some very good thoughts.
https://www.physicsforums.com/showthread.php?t=538388
https://www.physicsforums.com/showthread.php?t=571174
another question:
when reactive imported current is opposing the excitation current (in the rotor, right?), does that mean that the excitation current is canceled out?
Well - it's flux we should be thinking of, and magneto-motive force MMF.
Flux is MMF divided by (properties of the iron magnetiic path ~ reluctance)
and MMF is amp-turns.
The amp-turns of the reactive armature current oppose the amp-turns of the field
so it's the MMFs that cancel each other out.
Both currents still flow but the end result is not unlike they canceled one another. That's called Armature Reaction and is a big part of those two links i gave.
Or does the AVR somehow turning down the excitation by some kind of detection of something?
The AVR measures reactive current and modifies the excitation to make the machines share vars gracefully. Often they measure phase B current and add that to phase AC voltage, note they're already 90 degrees out of phase so it's a dirt simple way to get var correction. It'll be called "Reactive Droop". Typical setting is 5% meaning full reactive amps would lower voltage by 5%. That way a machine that's hogging vars will be cut back by its AVR. Or a slacker will be kicked up a little.
If machines are tied together by transformers you'd subtract transformer impedance from that 5% setting. Note that with a high impedance transformer , > the droop you want, you might have to reverse the AVR droop setting to make it look like there's 5% impedance between machine and bus. Then it'll be callaed a "line loss compensator" or "reactive rise" or something the manufacturer found to his liking. Be careful with that adjustment - it chenges the reactive compensation from negative into positive feedback so the AVR gets real touchy, might need more damping.
Sigh it's endless, but really fun. I envy you that setup - have a good time !
I sense you want an intuitive feel for these machines. Until tha sinks in that i can't make myself believe the formulas, afterward they come easily.. I hope i have helped you
Wow - an engineer on a boat in China. Just like "Sand Pebbles" , one of my favorite movies.
old jim
