Frequency of rotor current- When the rotor is stationary, rotor conductor is being cut by the rotating flux at NS. So the frequency of rotor current is same as the slip. But when conductor is started rotating, the frequency of rotor current depends on the relative speed between NS and N.
We know that,
For Rotor – f’=P/120(NS-N)
At running – f’=S120f/P×P/120
f’=Sf where f’=Frequency of rotor current
f=frequency of supply