Under constant mechanical load, changing the excitation of a synchronous motor affects which of the following?

I. Power factor

II. Armature current magnitude

III. Speed of the motor IV. Real power output 

Effect of the change of excitation of a synchronous motor 

When changing the excitation of a synchronous motor under constant mechanical load, only power factor and armature current magnitude are affected; the speed of the motor and real power output remain constant. 

• Power factor: Excitation directly influences the power factor. Increased excitation leads to a more leading power factor, while decreased excitation results in a more lagging power factor. 
• Armature current magnitude: As power factor changes due to excitation adjustments, the armature current magnitude also changes. Higher excitation generally leads to a lower armature current when the motor is over-excited, and vice versa. 
• Speed: The speed of a synchronous motor is determined by the frequency of the power supply and the number of poles. It is independent of the excitation level. 
• Real power output: Real power output is related to the load and the motor efficiency. Since the load is constant, real power output remains constant regardless of excitation changes.