Using the information from (5-9), the circuit phasors can be plotted for constant power as the field current is varied. This is shown for three values of field current in Figure 3. Note that the synchronous generator can operate with a leading power factor, unity power factor, or lagging power factor. Since the magnitude of af ˆ E is proportional to field current excitation, the leading power factor condition is sometimes referred to as under-excited operation and lagging power factor condition is referred to as over-excited operation for the synchronous generator. Note that the current a ˆ I and the voltage af ˆ E follow the lines of constant power given by (7-8). If the armature current magnitude is plotted versus field current for several power levels, the resultant plots are the generator V-curves shown in Figure 4. The points marked a, b, and c on the upper curve correspond to the operating conditions in Figure 3.