The synchronous speed (Ns) of an induction motor is calculated using the following formula:
Ns = (120 * f) / P
Where:
Ns is the synchronous speed in revolutions per minute (RPM).
f is the frequency of the power supply in Hertz (Hz).
P is the number of poles in the motor.
The formula represents the ideal rotational speed of the motor's magnetic field in the absence of slip. In practice, induction motors always operate at speeds slightly lower than their synchronous speed due to slip, which is the difference between the synchronous speed and the actual rotor speed.