How it works
At constant real power load P on the shaft, varying field current If changes the excitation EMF Ef = kφω. With normal (unity PF) excitation, armature current Ia is minimum and purely in phase with the terminal voltage Vt. Under-excitation (lagging PF): the motor absorbs reactive power (like an inductive load), and Ia increases with a lagging angle. Over-excitation (leading PF): the motor supplies reactive power to the bus (like a capacitor), and Ia increases again with a leading angle. A synchronous condenser is an unloaded synchronous motor operated solely to provide variable reactive power by adjusting field current. The minimum of each V-curve corresponds to unity power factor operation.
Key points to remember
Each V-curve corresponds to a fixed mechanical load level — higher load means a higher minimum armature current and the entire V-curve shifts upward. The locus of minimum-current points (tips of V-curves) connects to form the unity power factor line. An over-excited synchronous motor is equivalent to a capacitor bank for power factor improvement and is used in industrial plants for this purpose. The relationship between Ef, Vt, and Ia in the phasor diagram: Vt = Ef + jXs·Ia (motor convention), where Xs is synchronous reactance. The angle δ between Vt and Ef is the torque angle; P = (Vt·Ef/Xs)·sin δ.
Exam tip
The examiner always asks you to sketch at least three V-curves for different load levels on the same axes, draw the unity PF locus, and shade the leading/lagging PF regions — label the over-excited and under-excited sides clearly and state which side absorbs or supplies reactive power.