As we know that, induced emf is directly proportional to the speed of rotor so induced current is also doubled when the speed of rotor in a generator is doubled.
Rotor speeds typically lie in the 120–210 m/s range, but mostly between 150 and 190 m/s, with a tendency to be higher with a smaller rotor diameter [6]. Currently the smallest rotor diameter used industrially is 28 mm with rotor speed up to 150,000 rpm, though some machines can reach 160,000 rpm.The rotor speed, rotational speed of the rotor, is usually measured in rotations per minute (rpm) or tip speed in feet per minute.
The synchronous speed of the rotor in RPM N = 120f/P where f is the frequency of the stator current and P is the number of poles. The operating speed No = N - Ns where Ns is the slip speed. The frequency of the rotor current fr = Ns x P/120, so fr = 0 if the slip is zero.