Axial magnetic forces in induction machines at twice line frequency

Axial magnetic forces on the rotor and stator of an induction machine are due to variations in the magnetic energy stored in the air gap along the machine axis. A virtual displacement of the rotor along the axial direction, in which the magnetic permeance of the air gap changes, leads to a change in the energy stored in the magnetic field and also a force in the axial direction between the rotor and the stator. In actual practice, induction machines with imperfectly balanced phase currents and containing a dissymmetry, or unsymmetrical configuration, in their magnetic circuit across the diameter of the machine, are not uncommon. In such a machine the rotating pole magnetic field can produce an axial magnetic force at twice line frequency in addition to the normal steady-state axial force. In certain applications, the vibrations produced by such a machine may be intolerable and modifications may be required. The author analyzes two examples of twice line frequency axial magnetic forces that may be encountered in induction machines