Abstract :
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
