The rate at which the flux cuts the rotor conductors is directly proportional to the difference between the speed of rotating field (N s) and that of the rotor (N). If the rotor speed were to become equal to the speed of the rotating field, there would be no generated e.m.f. (and hence current) in the rotor conductors. Consequently, there would be no motor action. Thus it is the slip of the rotor (Ns – N r.p.m.) which causes e.m.f.s to be generated and currents to flow in the rotor conductors. This is precisely the principle of operation of 3-phase induction motor.