What determines the speed of an AC motor?

The speed of an AC motor is primarily determined by the frequency of the AC supply voltage. In an alternating current (AC) motor, the speed at which the motor operates is directly proportional to the frequency of the electrical supply to the motor windings. This relationship is described by the formula:

Speed (RPM) = (120 × Frequency) / Number of Poles

In this formula, the frequency of the electrical supply (measured in hertz) and the number of poles in the motor are critical factors. As the frequency increases, the speed of the motor also increases, which is why frequency is often the primary method to control the speed of AC motors.

Other factors like voltage, torque, and load also play roles in the overall performance and efficiency of the motor but do not directly dictate its operational speed in the same fundamental way that frequency does. For instance, while voltage affects the power and torque capabilities, it does not change the inherent relationship between frequency and rotational speed. Torque and load can influence how the motor performs under different conditions but are not the primary determinants of speed. Thus, understanding that the speed of an AC motor is fundamentally linked to the frequency of the alternating current is essential for effective motor control and application.