What is the frequency of rotor emf?

When the motor is stationary,rotor conductors are being cut by  rotating flux at synchronous speed.therefore,the frequency of rotor current(or emf)is the same of supply frequency.but when the rotor starts revolving,the rate at which the rotor conductors are being cut by the rotating flux depends upon the relative speed between the rotor and the stator revolving magnetic field , called the slip speedthe frequency of the rotor current (or emf) induced by the relative motion between rotor conductors and the stator revolving magnetic field is given by the equation: