Which formula is used to calculate synchronous speed?

The formula used to calculate synchronous speed is based on the relationship between the frequency of the AC power supply and the number of poles in the motor. The correct formula, which uses 120 in the numerator, is derived from the need to convert frequency (measured in Hertz) into revolutions per minute (RPM).

In this context, the value 120 accounts for two factors: it converts the frequency in cycles per second into cycles per minute by multiplying by 60, and it also accounts for the fact that each complete cycle results in a rotational speed dependent on the number of magnetic poles. The formula then becomes:

Synchronous speed (RPM) = (120 x Frequency) / Number of Poles.

This derivation aligns with the fundamental principles of electromagnetism and motor operation, where the number of poles determines how many times the magnetic fields can complete full cycles in a given time frame dictated by the frequency of the electrical supply.

Other formulas presented in the choices either use incorrect values or fail to adequately convert the frequency into RPM correctly based on the principles that govern synchronous motors. So, the selection of this formula is essential for accurately calculating the synchronous speed of single-phase motors.