What influences the synchronous speed of a motor?

• A. The supply voltage and load current
• B. The supply frequency and the number of poles
• C. The type of winding used
• D. The rotor material composition

Answer: (B)

The synchronous speed of a motor is primarily influenced by the supply frequency and the number of poles in the motor’s design. Synchronous speed is calculated using the formula:

[ \text{Synchronous Speed (RPM)} = \frac{120 \times \text{Frequency (Hz)}}{\text{Number of Poles}}. ]

This indicates that the speed at which the motor operates synchronously with the rotating magnetic field is directly dependent on these two factors.

The supply frequency is determined by the power supply and stays constant for AC motors; therefore, any changes in frequency will directly affect the synchronous speed. Similarly, the number of poles in the motor design determines how many divisions there are in the rotating magnetic field. More poles result in a lower speed, while fewer poles result in a higher speed for the same frequency.

Other options, such as supply voltage and load current, do not directly influence synchronous speed; instead, they affect the torque, efficiency, and overall performance of the motor. The type of winding can influence aspects such as the impedance and specific characteristics of the motor but does not directly set the synchronous speed. Lastly, rotor material composition impacts the rotor's performance and efficiency rather than its synchronous speed.

Thus, the correct answer highlights