Damper Windings in Induction Machines for Reduction of Unbalanced Magnetic Pull and Bearing Wear

In large induction machines (such as large cage induction motor pumps in the petrochemical industry and cage- or wound-rotor induction generators in wind turbines), reliability and longevity are advantageous. This is particularly relevant to wind turbine generators which can be inaccessible. There will be some degree of tolerance and wear that will lead to low-level rotor eccentricity. For a pm-pole-pair machine, there will be pm ±1 pole-pair flux waves set up by the eccentricity which will generate unbalanced magnetic pull (UMP), as well as additional higher space harmonics. This paper addresses the use of stator damper windings to reduce the sideband flux waves and hence attenuate the UMP. Examples are put forward in terms of a ten-pole cage-rotor machine with static or dynamic rotor eccentricity and then extended to use a four-pole wound-rotor machine. A tested analytical model is developed to include these damper windings and the wound rotor; they are shown to reduce the UMP, particularly in a wound-rotor machine. The simulations here are in terms of a wound-rotor machine, but this can be extended for doubly fed induction generator operation.