The speed of a synchronous motor depends on:
Supply frequency
The speed of a synchronous motor is determined by the frequency of the power supply it is connected to. This relationship is derived from the formula for the synchronous speed (in revolutions per minute) of a synchronous motor, which is given by:
N_s = \frac{120 \times f}{P}
- N_s is the synchronous speed in RPM (revolutions per minute).
- f is the frequency of the supply in Hertz (Hz).
- P is the number of poles of the motor.
Let's analyze the options given in the question to see why "Supply frequency" is the correct answer:
- Supply frequency: As shown in the formula, the synchronous speed is directly proportional to the supply frequency. Hence, a change in the frequency will directly affect the motor speed, confirming that the correct answer is supply frequency.
- Field resistance: Field resistance does not affect the speed of a synchronous motor. It primarily affects the field current and, consequently, the excitation but not the speed.
- Load only: The load applied to the motor affects the torque and power delivered by the motor but does not affect the synchronous speed. The speed of a synchronous motor is independent of the load.
- Armature current: Similar to the field resistance, the armature current affects the torque and efficiency but not the speed of the motor.
Therefore, the speed of a synchronous motor depends on the supply frequency.
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