Condition monitoring of wound-rotor induction machines
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Condition monitoring enables diagnosis of the inception of fault mechanisms in electrical machines, thus averting failure and the need of expensive repairs. Therefore, it is valuable to develop efficient methods of condition monitoring. The idea would be relatively low cost and/or non-invasive system, which is still sufficiently powerful in terms of monitoring by online detection of developing faults. In this research, an overview of existing condition monitoring techniques is given, and issues related to induction machine faults are discussed. Therefore, this research develops a relatively simple yet powerful model for studying the behaviour of a wound rotor induction machine (WRIM) or doubly fed induction generator (DFIG) in healthy and faulty conditions based on the impedance matrix. The first part of the work presented in this dissertation builds the fundamental impedance matrix that can predict the behaviour of the WRIM or DFIG in a healthy condition. A theoretical model is necessary so that any stator or rotor winding configuration in the machine can be incorporated. The effect of rotor skew is considered in this model. Then, the Motor-CAD package is employed to predict the electromagnetic behaviour of the induction machine during steady-state and transient-state operation. Motor-CAD has been used for examining the induction machine parameters. The second part of the work develops the impedance matrix to detect unbalanced rotor-phase impedances. This can simulate rotor faults in the machine. The method leads to the calculation of stator current components when there are unbalanced rotor-phase impedances and it is verified experimentally using a four-pole wound rotor. The method is verified by inversion of the voltage matrix equation and solving for the currents in the wound motor. Experimental results (torque and current characteristic) are compared with computer predictions for the test machine. The third part of this thesis develops the fundamental impedance matrices for both rotor eccentricity detection and unbalanced magnetic pull (UMP) calculation. It puts forward a concept for detecting and measuring eccentricity faults in the WRIM. A simple and new approach using pole-specific search coils is introduced, and a theory is developed to illustrate that rotor eccentricity leads to the generation of air-gap flux waves with pole-pairs of p𝑚 ± 1, where p𝑚 is the number of pole-pairs of the machine. Once again, this technique is used here to detect rotor eccentricity in a four-pole wound rotor machine and is verified experimentally using a rig for measuring UMP. The investigation uncovers several aspects of the damping effects of pole-specific search windings which can also be used to suppress UMP.
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