Question 40

The Correct Answer is C. ### Why Option C ("Frequency") is correct: The speed of a synchronous electric motor is strictly tied to the frequency of the AC power supply and the number of magnetic poles in the motor. The synchronous speed ($N_s$) is calculated by the formula: $$N_s = \frac{120 \times f}{P}$$ Where: * $N_s$ is the synchronous speed (in revolutions per minute, RPM). * $f$ is the frequency of the AC power supply (in Hertz, Hz). * $P$ is the total number of poles in the motor. Since the number of poles ($P$) is a fixed physical characteristic of the motor, the only way to increase the synchronous speed ($N_s$) is to increase the supply **frequency ($f$)**. ### Why the other options are incorrect: * **A) Inductance:** Inductance (a measure of a coil's opposition to a change in current) is a property of the motor winding and generally does not directly determine the synchronous speed. While high inductance can affect the motor's transient performance and power factor, it does not change the fundamental relationship between frequency and speed. * **B) Voltage:** Voltage primarily determines the amount of torque (rotational force) the motor can produce and the required power output. Increasing the voltage (within limits) increases the motor's available torque and power, but it does **not** change the synchronous speed, which is locked solely to the frequency and pole count. * **D) Armature current:** Armature current (the current flowing through the main stator windings) is a consequence of the applied voltage and the load placed on the motor. Higher load requires higher current, and higher voltage results in higher current (up to saturation). Like voltage, current affects the motor's torque and power delivery, but it has no influence on the theoretical synchronous speed defined by the frequency.