The slip frequency approach is used to model working regimes of the induction motors. In induction motors the stator winding generates the rotating magnetic field with speed w1. The rotor turns with some slipping speed w2 < w1. That can be represented as the following equivalent model.
The picture of torque vs. slipping frequency of AC motor.
The slipping frequency approach does not allow the calculation of the eddy currents in the stator.