Explain V-curves and inverted V-curves of synchronous motors.

V-curves show armature current versus field current at constant load. At under-excitation, motor draws lagging current (acts as inductor). As excitation increases, current decreases to minimum at unity power factor (normal excitation). Further increase causes leading current (motor supplies VARs like capacitor). Inverted V-curves show power factor versus field current - power factor peaks at normal excitation. Over-excited synchronous motors are used as synchronous condensers for power factor correction.