Real power of generator depends upon power input. when load on a generator increases, stator current increases. Due to counter emf the speed is reduced which is again attained by opening of the fuel valve. Due to increase in current terminal voltage drops which are corrected by increasing excitation current. Question is that the reactive load also require current due to which counter emf would increase then why only excitation current is increased to meet the VARS demand? Has reactive power no effect on power input?

 

I like to always refer to the reactive capability curve provided with a synchronous generator. This curve represents the limits of operation of a particular generator, and in my estimation, is useful in describing how the energy input to the generator can be "split" or divided.

As we all know, AC power systems with real and reactive components can be represented with a right triangle. One leg of the triangle represents watts; the other side of the right angle represents VArs; and the hypotenuse represent the total energy, VA.

If the excitation is sufficient to keep the generator terminal voltage equal to the voltage of the electrical system with which the generator is synchronized to, then there is no reactive component (VArs) at the generator. All of the power, VA, is going out as watts, and none as VArs.

If excitation is increased, which would try to increase the generator terminal voltage above the voltage of the electrical system with which the generator is synchronized, then lagging VArs will be present at the generator terminal. If the torque being input to the generator are held constant as excitation is varied, then the amount of watts will decrease as the amount of VArs increases in the lagging direction as excitation is increased.

The VA (hypotenuse) of the power triangle isn't changing; just the division of energy between real and reactive components. So, as the angle of the hypotenuse increases above 0 degrees, if the length of the hypotenuse (the total energy input to the generator) remains the same, the length of the adjacent side of the power triangle decreases, which means the real power output (watts) decreases.

Now, how all of that is affected, or not, by counter emf is unknown to me. Counter emf has never been monitored on any synchronous generator I have ever seen operating, even ones in the lab at university. It's always a theoretical parameter, which is the domain of generator designers and manufacturers.

I, as a technician, have to believe that any and all effects of emf and counter emf and back emf and air gap and conductor sizing and conductor spacing and twists-per-turn and all of those things which can't be changed during operation were all properly accounted for by the generator designers and manufacturers and anyone choosing an excitation system (AVR, Automatic Voltage Regulator).

I don't know if increasing excitation to change the VAr component of the power triangle actually changes the torque input to the generator or not. I know it doesn't change the energy flow-rate being admitted to the prime mover. But, I also know it does have an effect on the total power <b>output</b> of the generator.

One last thing about the reactive component of electrical power. When we put AC energy into an electric motor (usually an induction motor), the power factor of that motor tells us how efficient that motor is at converting the electrical power input to torque output. Since the power factor is a number less than one, that means that some of the energy is "lost" to reactive elements and not all of the energy input is being directly converted to torque output (horsepower, or, "real power"). Since there's no real difference between motors and generators, I think it's relatively the same with synchronous generators, except that the power factor can be "controlled" with the exciter (AVR).

But, how all that relates to counter emf, again, that's all "egghead" stuff.

And when you find a counter-emf meter, please write to let us know! Take a digital photograph, and we'll ask you to post it somewhere we can all view it!