Analysis of electromagnetic behavior of Permanent Magnetized (PM) electrical machines in fault modes
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- Institutt for elkraftteknikk 
Over the years, the use of PM machines has been increasing in the offshore wind industry and marine industry. The industries thrive on efficient function of the PM machines. These machines are prone to electro-mechanical faults due to environmental conditions and maintenance. Out of all these faults, stator internal faults are concerning as they can lead to insulation failures which may take around 30 seconds to expand and lead to a fire on ships, or on wind turbines. These type of faults develop gradually, which gives the opportunity to control the fault currents before they reach dangerous levels. Rolls Royce Marine AS is also working to tackle this problem for their hybrid propulsion shaft generator. DNVGL requires the generator to be made electrically dead during such event and the long-term propulsion should not be affected. During such conditions, WTs are either turned off or field weakening is used to develop a fault tolerant control (FTC) by the help of power electronics for the WT. FTC helps the machine not to be turned off completely, but less power is generated during fault conditions. An alternative efficient field weakening method using a Dual Rotor PMSM was suggested for both the applications. The DR-PMSM has two rotors instead of one, with identical surface mounted magnets on both rotors. One of these rotors has the capacity to rotate with respect to the other, in order to reduce the flux or completely short the flux path by misalignment of rotors. The machine stator is exactly like the conventional PMSM. The machine is capable of reducing the induced emf to zero by field weakening. In this thesis a transient 3D finite element model is presented to test the credibility of the machine. A 2D FEM of a conventional PMSM was also built to check the validity of the machine. It is seen that torque is a function of the active length of the machine, and if a gap is introduced between the rotors then the total length of the machine must be increased. Also, axial flux component which induces eddy currents in the stator teeth was studied. By modeling anisotropy in the stator iron, certain hot spots could be seen in the middle part of the stator. The forces that were in the shifting mechanism were studied and it was concluded that machine cogging can be reduced to reduce the effect of these forces. A machine prototype was also built which confirms the field weakening capability of the machine. The DR-PMSM works like a conventional PMSM but with flux weakening capabilities and can be implemented on marine and wind turbine applications for these type of fault conditions.