Generator connected to an infinite bus bar

[greg997]

Hi. I am supposed to explain using phasor diagram how change in excitation current affects power factor and active power of the generator. I have a few different values for excitation current, real power and power factor.
Basically, increasing excitation current causes decrease in power factor and real power.
But what I don't understand is why the real power decrease as well. Hmm, I do understand because P= VIpf.
But in textbooks it is always only two scenarios are described. Constant excitation + change in load and constant load + change in excitation. And for changing excitation, the power output is considered constant. So only power factor changes. It is confusing.

So the question is. When alternator is connected to infinite bus bar, when changing the excitation current, does the real power change or not?

How can I use phasor diagram to show that?

 

[Valkarie]

The answer to your question is yes, when changing the excitation current, the real power does change. This can be shown in a phasor diagram by plotting the voltage phasor and the current phasor at different excitation currents. The voltage phasor is usually drawn as a fixed vector with a given magnitude and phase angle while the current phasor is drawn as a variable vector whose magnitude and phase angle changes depending on the excitation current. As the excitation current increases, the magnitude and phase angle of the current phasor will increase as well. This will cause the real power to increase but the power factor will decrease. To show this in a phasor diagram, you need to plot the two phasors for each excitation current and then draw a line connecting the two points. The slope of the line indicates the power factor and the length of the line indicates the real power. You can then see how the power factor and real power changes as the excitation current changes.