When we introduce an iron star inside the stator of a 4-pole 3-phase AC motor and then generate the rotating magnetic field, the star being made of a ferromagnetic material is magnetized and starts to rotate at synchronous speed (1800 rpm). On the otherside, when we swap the star with an iron disk it doesn't rotate at synchronous speed. Instead it rotates at a lower speed (aprox. 1200 rpm) and we start to see slip.
My question is: Why does this happens? The iron disk being made of a ferromagnetic material shouldn't start to become magnetized and to rotate at synchronous speed? Are there two magnetic fields in the iron disk: the one created by the magnetized material and the one created by induced currents on it?