Operation at Constant Load with Variable Excitation

Operation at Constant Load with Variable Excitation

Now let us consider a generator operating at constant load.

We have,

If the field current of the alternator is varied to change the excitation e.m.f. E, the power angle δ chrnges m such a way as to maintain E sin 8 constant. With change in E, the tip of phasor  moves on a line parallel to  at a distance E sin P_oX_s / V from it.

Also tip of current phasor traces a line perpendicular to V at a distance of I cos ϕ = P_o/V from the origin to make I cos constant. This is shown in the Fig. 3.7.1.

The current phasor I is always at an angle of 90° to phasor IXS as shown in the Fig. 3.7.1.

The excitation required to obtain unity power factor is known as normal excitation while excitation above and below normal excitation is respectively known as over and under excitation. Thus a generator supplies a lagging power factor when over excited while a leading power factor when under excited.

As the excitation is varied, the current and load angle is minimum for normal excitation. Both current and load angle increases as the excitaion is changed above or below the normal excitation. For under excitation power factor becomes more and more leading while for over excitation power factor becomes more and more lagging. 

Review Question

1. Explain the effect of variation of excitation of a synchronous generator connected to infinite busbar on the armature current, power factor and load angle.