The major goal of this thesis was to investigate the air-gap magnetic flux density of a permanent magnet synchronous motor. Several magnet geometries were studied in order to find out how to gain maximum air gap flux density. Two rotor geometries with embedded magnets have been investigated thoroughly. One of the rotors has 3 pieces of magnets per pole and resembled a U-shape whereas the other has five pieces of magnets per pole resembling a dove-tail. The magnet arrangements have been altered slightly in both geometries in such a way that the original geometry remains intact. However, before performing investigations on these two rotor geometries, variation of air-gap magnetic flux densities with the height of magnet have been investigated. Chapter 4 has been dedicated to all the investigations and computations performed.
In subchapters 4.1, 4.2, and 4.3, investigation results performed for the variation of air-gap magnetic flux density with different magnet heights have been presented. Three different cases of embedded magnets have been investigated to find out the variation of air-gap magnetic flux density with magnet heights at no load. The results show that mere increasing the height of magnets could not help to achieve higher air-gap magnetic flux density. After certain height of the magnet, the air-gap magnetic flux density saturated and thus increasing the height of magnet could not help in achieving the required torque output. This only makes the machine overweight and increases its construction cost. One of the three cases in which two pieces of magnets were arranged in a V-shape also revealed that the height together with the width of the magnet need to be optimized to achieve required air-gap magnetic flux density in the machine.
In subchapter 4.4, the investigations on the rotor with U-shaped arrangement of magnets have been presented. The magnets arranged in U-shape were oriented so as to produce a Halbach array resembling arrangement. The results of the investigations performed on this kind of magnet arrangements show that a maximum output torque of about 1054 Nm could be produced.
Moreover, optimization in the size of the radially arranged magnets, the machine was able to generate a torque of about 1068 Nm. This also shows that a proper optimization of the magnet shape and size could help to generate a higher torque for a given amount of supply current.
76 However, it is also necessary to be careful in sizing the magnets as increasing the size could increase the overall mass of the machine.
The investigations with embedded magnets arranged in dove-tail fashion have been presented in subchapter 4.5. Unlike U-shaped-arrangement of magnets, the magnets were oriented to provide as high radial flux towards the stator of the machine. The results show that the maximum achievable torque with this arrangement of magnets is about 840 Nm. Moreover, the arrangement of magnets in dove-tail fashion is difficult as it involves five magnet pieces to be embedded in the rotor to produce one pole.
The results of computations of the d-axis and q-axis inductances show that the saliency ratio (Lq/Ld) for the rotors with both the dove-tail-shaped arrangement of magnets and U-shaped arrangement of magnets is 1.6. The computed d-axis and q-axis inductances in both the cases is Ld = 0.6 and Lq = 0.9.
Despite the dove-tail arrangement of magnets with radially directed magnetic orientation produced almost as much air-.gap magnetic flux density as with the case of U-shaped arrangement of magnets oriented to produce Halbach array, the torque output of the machine with the latter arrangement showed to be higher. Comparing the results of the investigations, the U-shaped arrangement of magnets seem to be promising with a higher torque output capability.
The magnets in the dove-tail-shaped arrangement could be oriented to produce a Halbach array to investigate if the torque output of the dove-tail-arrangement of magnets is limited due to magnetic orientation when compared to the U-shaped arrangement of magnets.
77
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