Transient unbalanced magnetic pull in change-pole induction motors

In an ideal three-phase cage motor, which is both electrically and magnetically symmetrical, the radial forces acting on the rotor are in equilibrium, so that their resultant is zero. An out-of-balance radial force, usually referred to as an unbalance magnetic pull (UMP) arises when either or both forms of symmetry are disturbed. It is well-known that a transient UMP may be produced during pole switching in a machine which is fitted with either a pole-change winding or with two separate windings of different pole number. The force developed may be large enough to produce a pull over; that is the rotor and stator can come into physical contact. The authors explain and examine this phenomenon