Issue with energy conversion in synch. machine

[FOIWATER]

Hello,
Assume a synchronous generator is running unloaded and has a fixed input power for any given short time frame (which is a practical consideration for electric power plants). When an electrical load is applied, the machine slows down. I understand this.. what I don't understand is the return of the rotor to synchronous speed. It seems like if you analyze both situations 1) when the synch machine was being rotated at synch speed and 2) when the rotor was being turned at synch speed + delivering electrical energy for the same input power that something doesn't add up?

Thanks

 

[meBigGuy]

Why does the generator slow down? Is it because whatever is causing it to rotate it slows down? when power is drawn, whatever is causing it to rotate must supply the mechanical power.

 

[anorlunda]

A simple model.

Think of the grid as an infinitely big AC voltage source at exactly synchronous frequency. Power from the generator to the grid is proportional to the angle difference between the generator and the grid. That angle is the integral of the generator-grid speed difference.

The generators rotor accelerates proportional to the difference in mechanical power in minus electric power out. Increase mechanical power and it will accelerate a bit, making the angle increase, and thus electric power increase. That continues until power in matches power out, and speed matched grid speed once again.

Speed gen = ∫ power in minus power out dt

Angle gen = ∫ Speed gen minus speed grid dt

Power out is proportional to angle gen.

Speed grid is fixed at 60 or 50 hertz.

Equilibrium comes when the rate of change of speed and rate of change of angle are both zero.

If the grid is not infinite it is a bit more complicated, but this is a good approximation.

 

[FOIWATER]

I was having trouble understanding why, when a synch. machine is loaded down, it picks it's speed back up following the decrease in speed. I understand why it initially slows down. I do not understand why, for a constant input power, the rotor will speed back up.

 

[anorlunda]

I understand why it initially slows down.

No you don't understand that either.

The answer is in post #2. Please try harder to understand that rather than re-asking the same question. Often, understanding of physics requires some math. A long string of sentences will not clarify as much as a single equation.

 

[FOIWATER]

"No you don't understand that either."

The rotating of the field winding causes voltage to be induced in a stationary armature, this voltage opposes the field voltage due to lenz' law. when a load is connected, a current flows which creates a magnetic field opposing the rotating field. For a fixed input power this creates a torque imbalance which causes the rotor to decelerate. I don't understand why upon decelerating, it makes an attempt to accelerate. where does that torque imbalance come from

If you say I don't understand it pin point which part of this is wrong. I appreciate positive feedback.

 

[anorlunda]

OK, I'll give you two hints, then you're on your own.

1) three times you mentioned speed, but you ignored what I said about angle. Angle is the key to understanding synchronous machines.

2) what are the conditions necessary for acceleration/deceleration to cease, and for the power and angle to be steady, either before or after your change in load?

 

[FOIWATER]

I'm fine with being "on my own" I will find the answer in a more positive environment

this isn't a case of some one slapping away a helping hand, I appreciate help as much as anyone can but I find intolerance to studious people irrational.

 

[meBigGuy]

I am really confused (happens a lot)

Is this a generator or a motor?

If you load a motor it slows down. You want to understand why it speeds back up and stays synchronous?

If you load a generator, the force to keep it rotating increases and the device driving it must supply more power.

I don't understand what you are asking. You ask about a generator, but the questions seem to apply to a motor.

BTW, I too dislike the "I'll force you to learn something because I know more than you and I think you are being lazy" attitude that sometimes prevails here. But you are actually not asking a coherent question.

 

[jim hardy]

I was having trouble understanding why, when a synch. machine is loaded down, it picks it's speed back up following the decrease in speed. I understand why it initially slows down. I do not understand why, for a constant input power, the rotor will speed back up.

Your scenario wasn't thoroughly defined in original post... i think that's at root of the misunderstandings here.

Is your machine in parallel with a lot of others, ie on the grid, or standing alone with its load?

For now, i'll assume the latter.

I do not understand why, for a constant input power, the rotor will speed back up.

Think about that question a moment: Any machine whose output power is more than its input power will slow down as the energy imbalance is made up by expending its rotating inertial energy Iω².

An electric power plant senses shaft speed, and upon deviation from desired speed very quickly(like as fast as the valves can move) adjusts the mechanical power supplied to generator.
In my plant a 3% speed deviation would have swung mechanical power across its full range of ~800 megawatts in under a quarter second.

So the answer to your question is - the machine will not speed back up unless something restores the power balance. That might be the prime mover, or other machines in parallel.

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you hinted at an effect known as 'armature reaction' - maybe later ...

But you're on the right track, for to help visualize what's happening you can think of the rotor and stator fields as rotating electromagnets one dragging the other along. Playing with a couple permanent magnets helps one "feel" the elasticity of what in a synchronous machine is called Power Angle" .hoping above helps...old jim

 

[FOIWATER]

Thanks Jim