Reduction of unbalanced magnetic pull in doubly-fed induction machine

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This thesis reports on an investigation into the unbalanced magnetic pull (UMP) in a wound rotor induction machine due to static rotor eccentricity. Wound rotor induction machines are commonly used in wind turbines as doubly-fed induction generators (DFIGs). It is important to maintain them in good working order and operational. There can be substantial bearing wear in these machines due to their almost continuous operation, and it is important to maintain low bearing wear of these machines in order to maintain good operation. These machines can have high maintenance costs since they are located in remote locations and in the nacelle of the wind turbine. When the rotor becomes eccentric (non-centred) bearing wear is increased and the maintenance requirement increases. It has been illustrated that the UMP is higher per unit in a wound rotor induction machine compared to the cage induction machine equivalent. In this study, a new measurement rig is developed in order to measure UMP. The methods for calculating UMP and splitting UMP and torque forces are addressed in this thesis. Furthermore, a new method for minimizing UMP is also introduced by using damper windings. Different tests and calculations are presented in this thesis. The understanding of the radial forces involved in producing UMP in a wound rotor induction machine is addressed. There is little literature on UMP in a wound rotor induction machine.
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