How is the speed of a synchronous motor controlled?

The speed of a synchronous motor is directly influenced by the frequency of the power supply. In synchronous motors, the rotational speed is determined by the formula: speed (in RPM) = (120 × frequency) / number of poles. This relationship means that when the frequency of the electrical supply changes, the synchronous speed of the motor will also change proportionally.

Frequency variations adjust the number of cycles per second at which the magnetic field rotates, which in turn dictates how fast the motor synchronizes with this rotating field. Since synchronous motors operate at a constant speed that is synchronized with the supply frequency, any alterations in frequency will yield changes in speed. Thus, altering the frequency of the power supply is the primary method to control the speed of a synchronous motor effectively.

Other options, while related to motor operation, do not influence speed control in a synchronous motor in the same direct manner that frequency adjustments do. For example, variations in load can impact the performance and efficiency of the motor but do not alter its synchronous speed. Adjustments to rotor size or armature winding typically pertain to other characteristics of the motor rather than speed control.