Correct Answer - Option 1 :

\(f = \frac{{PN}}{{120}}\)
__Concept:__

The synchronous speed of a synchronous machine is given by

\({N_s} = \frac{{120f}}{P}\)

Where f is frequency and P is the number of poles.

A synchronous machine always rotates with a synchronous speed.

**Explanation:**

From the above relation, the frequency can be written as,

\(f = \frac{{PN}}{{120}}\)