If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to

The correct answer is: A. increase in both armature current and power factor angle.

When the excitation of a synchronous motor is decreased, the back emf decreases. This causes the armature current to increase, in order to maintain the same torque. The power factor angle also decreases, because the motor is now operating at a leading power factor.

Here is a more detailed explanation of each option:

• Option A: Increase in both armature current and power factor angle. This is the correct answer, as explained above.
• Option B: Increase in back emf but decrease in armature current. This is not correct, as the back emf decreases when the excitation is decreased.
• Option C: Increase in both armature current and power factor which is lagging. This is not correct, as the power factor angle decreases when the excitation is decreased.
• Option D: Increase in torque angle but decrease in back emf. This is not correct, as the torque angle increases when the excitation is decreased.