AN INVESTIGATION INTO THE PERFORMANCE OF A FLUX CONCENTRATION MAGNETIC GEARBOX AND MAGNETICALLY GEARED MOTORS
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Abstract
In today’s world, renewable energy sources like wind and ocean energy have come more into prominence as a source of sustainable energy. The energy conversion devices used in these cases, utilize mechanical gearboxes to match the speed of the input shaft to that of the electromagnetic generators. The mechanical gearbox however, has certain disadvantages such as wear and tear, vibration and noise, need for lubrication and no overload protection. In order to mitigate these issues, direct drive generators were used wherein, the gearboxes were removed and the input shaft was directly connected to the synchronous generators. If the input speed on the generator shaft is low, a high torque would be required to generate the power. To generate this high torque, the synchronous generator should be of significantly larger in size. An alternative approach is to replace mechanical gearboxes with magnetic gears. The magnetic gears being contactless mechanisms reduce wear and tear, vibration and require less maintenance. They also have inherent overload protection which makes them highly reliable. However, when compared to its mechanical counterparts, magnetic gears have low torque densities. Although significant progress has been made to improve the torque densities, the torque densities achieved in magnetic gears are quite low compared to mechanical gears. This research is focused on studying and comparing different typologies of coaxial magnetic gears designed to enhance the torque densities. Three main typologies - the idealized design with the cage rotor between the inner and the outer rotor, an outer cage rotor design and a design with consequent poles on the outer rotor were studied. A parametric sweep analysis of the geometric parameters was conducted to maximize the volumetric torque densities in each case. This was done using finite element analysis. The highest torque density of about 336 Nm/L is obtained with the idealized design and was considered for further designing magnetically geared motors. A magnetic gear with flux concentration Halbach rotor was designed, simulated and experimentally tested. The design used ferromagnetic pole pieces acting as retaining sleeves to hold the radially directed magnets. The design produced a torque density of 185 Nm/L when simulated using 2D finite element analysis. However, owing to the complicated assembly process, a measured torque density of 112 Nm/L is obtained when tested experimentally. A Magnetically geared motor achieved by integrating a magnetic gear with an electrical machine was analyzed. At first an internal stator machine was studied with both integral slot and fractional slot winding designs. Halbach magnets were used to increase the torque densities of these machines. However, torque ripple generated was quite high making these machines less viable. In order to reduce the torque ripple, a two layer winding distribution approach with each phase winding shifted by one slot was considered. This approach however reduced the torque density of the machine. An outer stator magnetically geared motor with integral slot winding design has also been studied. Two different cage rotor designs – the rectangular modulator lamination supports with bridge connection and circular modulator lamination support were analyzed and simulated using 2D finite element analysis method. Both the designs used Halbach magnets on the inner and outer rotor. The machine designs were conceived to achieve a zero torque on the inner rotor. The design with rectangular cage rotor produced a torque density of 130 Nm/L with considerably low torque ripple values of about 1% of the steady state value whereas, the other design with circular cage rotor produced a torque density of 117 Nm/L and a torque ripple of about 2% of the steady state value.