Basically induction motor working on induction principle like transformer where iron core is must, to have flux linking produced by primary(stator) rotating field(Ns), which would produced secondary voltage E2(S=1)in bars at stand still, and SE2 when rotor rotated with slip S= (Ns-N)/Ns. Hence rotor cannot rotate with out iron core, made up of iron stampings. Otherwise, the negligible induced EMF in secondary bars, as the little current would not produced enough torque to overcome friction and cooling fan load…..