Correct Answer - Option 2 : Number of poles and rotational speed

The rotor speed of the alternator is given by

\(N = {{120f} \over P}\) ........(1)

Where,

N = Speed of rotor in rpm

f = Frequency of generated emf

P = Number of Poles

From equation (1)

\(f = {{NP} \over 120}\)

\(f \propto {NP} \)

**So the output frequency of an alternator depends on both the number of poles and rotational speed.**