Which factor primarily influences the speed of an electric motor?

The speed of an electric motor is primarily influenced by the frequency of the current supply, particularly in alternating current (AC) motors. In general, for AC motors, the synchronous speed, which is the speed at which the motor would operate depending on the frequency of the electrical supply, can be calculated using the formula:

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

This formula shows how directly the frequency affects the motor's speed; as the frequency increases, so does the synchronous speed of the motor, assuming the number of poles remains constant.

While the voltage applied to the motor can affect its torque and can indirectly influence speed due to load effects, it doesn't directly determine the motor's maximum speed in the same way that frequency does. The type of load connected can affect how the speed is regulated under load conditions, but again, it does not change the inherent speed capabilities defined by the motor's construction and the frequency of the supply. Lastly, the type of motor construction can provide variations in maximum speed due to design factors, but the fundamental dependency on speed generation comes down to the frequency of the electrical supply.