Damper windings in induction machines for reduction of unbalanced magnetic pull and bearing wear

In large induction machines (such as cage large induction motor pumps in the petro-chemical industry and cage or wound-rotor induction generators in wind turbines) reliability and longevity is 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 p_m-pole pair machine there will be p_m±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 side-band flux waves and hence attenuate the UMP. Examples are put forward in terms of a 10 pole cage-rotor machine with static or dynamic rotor eccentricity and then extended to use a 4 pole machine 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 DFIG operation.