Which of the following point regarding normal, over and under excitation on a synchronous motor is true?

Conclusion of Synchronous Motor under different Excitation:

• As load on the motor increases, I_a increases regardless of excitation.
• For under-and over-excited motors, the power factor tends to approach unity with increase in load.
• Both with under-and over-excitation, change in power factor is greater than in I_a with increase in load.
• With normal excitation, when the load is increased change in I_a is greater than in power factor which tends to become increasingly lagging.

Additional Information

The synchronous motor is capable of operating at all types of power factors i.e. either UPF, leading, or lagging power factor.

• Lagging power factor: If field excitation is such that Eb < V the motor is said to be under excited and it has a lagging power factor.
   
• Leading power factor: If field excitation is such that Eb > V the motor is said to be over-excited and it draws leading current. So that the power factor improves.
   
• Unity power factor: If field excitation is such that Eb = V the motor is said to be normally excited i.e at 100% excitation.

Where Eb is back emf and V is terminal voltage.

V -curve for synchronous motor is shown below

As seen from the V-curve of the synchronous motor, when an overexcited synchronous motor is connected to an infinite bus bar and If the field current (If) is decreased, then armature current (Ia) will first decrease up to normal excitation (UPF) condition and then increased.