V curves and inverted V curves are drawn between armature current verses. Field excitation and power factor field excitation, respectively.

Variation in DC excitation of a synchronous motor causes variation in both the speed of the motor and the power factor. This can be explained as follows:

1. Speed of Motor:
- The speed of a synchronous motor is directly proportional to the frequency of the AC supply and inversely proportional to the number of poles in the motor.
- The frequency of the AC supply remains constant, so any change in the speed of the motor can only be caused by a variation in the number of poles.
- The number of poles in a synchronous motor depends on the excitation current provided by the DC source.
- If the DC excitation current is increased, the number of poles also increases, resulting in a decrease in the speed of the motor.
- Similarly, if the DC excitation current is decreased, the number of poles decreases, leading to an increase in the speed of the motor.
- Therefore, variation in the DC excitation of a synchronous motor directly affects its speed.

2. Power Factor:
- Power factor is a measure of the efficiency of electrical power utilization in an AC circuit.
- In a synchronous motor, the power factor can be controlled by adjusting the excitation current.
- When the excitation current is increased, the power factor of the motor improves, i.e., it becomes closer to unity.
- On the other hand, if the excitation current is decreased, the power factor deteriorates, i.e., it deviates from unity.
- Thus, variation in the DC excitation of a synchronous motor causes variation in its power factor.

Conclusion:
Overall, the variation in DC excitation of a synchronous motor affects both its speed and power factor. By adjusting the excitation current, it is possible to control these parameters and optimize the performance of the motor according to the desired requirements.