Regenerative power frequency back to line power

What about the frequency of the small amount of the power which back to the network its depend on the speed of the rotor and number of poles f=60/N.P?  In another word assume I have AC generator give AC power source what is the voltage and frequency of this power? How to calculate it?

The frequency of the line power is fixed by the utility source. The speed of the induction motor rotor is immaterial to this. The frequency output of an actual generator is dependent on the speed of the generator armature, but an induction motor rotor is not a wound armature, it's a chunk of aluminum (or copper) and steel. It has no field of its own, outside that induced in it by the rotating magnetic field provided by utility power. The power an induction motor provides when running above synchronous speed is power AT LINE FREQUENCY, just as when it is operating in motor mode it draws power AT LINE FREQUENCY.

To do what you're suggesting, the motor would need to be a synchronous design with either a wound rotor excited by a separate power supply or a permanent magnet rotor, and even then, it couldn't affect the line frequency. Once a generator has been synchronized and connected to the distribution network, attempting to drive it ABOVE the line frequency is essentially impossible. You can increase the torque on the shaft as much as you want, but past a certain point the generator will only trip - it will not go out of phase with the line frequency unless it's THE major contributor to that network.