Unbalanced magnetic pull in cage induction machines for fixed-speed renewable energy generators

The paper addresses unbalanced magnet pull (UMP) due to rotor eccentricity in cage induction generators used in wind turbines. The rationale for the work is to investigate ways to reduce bearing wear. The paper studies both static and dynamic rotor eccentricity with axial uniformity and also non-uniformity (e.g., due to a misplaced bearing). The damping action of the cage is discussed and a suitable analytical model used to calculate the UMP. Both sub- and super-synchronous UMP is investigated. It is found that the characteristic for the UMP varies depending on whether there is static or dynamic eccentricity. A use of additional damper windings is also investigated and it is found that these can help substantially reduce the UMP when the machine is lightly loaded with dynamic eccentricity. A previously investigated 10-pole machine is used in the simulations. The analytical model has previously been validated for this. The range of simulation covers both motoring and generating modes (±10% slip). This, together with the simulation of the UMP damping windings, represents a good contribution to the literature since they have not been previously addressed. © 2011 IEEE