AN INVESTIGATION INTO THE TORQUE CAPABILITIES OF HIGH GEAR RATIO MAGNETIC GEARBOXES
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Abstract
Mechanical gears can be as small as those in traditional mechanical watches or as large as those in mechanical marine turbines. They can be seen in almost all transportation tools, for example, bicycles, cars, trains and airplanes. Though they have been studied and refined for centuries, there are still some disadvantages. For instance, mechanical gears often create a large amount of noise and vibration. They require regular maintenance as the gears need to be lubricated. In addition, if a gear is overloaded it can catastrophically fail. Unlike conventional mechanical gears, magnetic gears can create speed change without physical contact. The force between magnetic gears is not created by geared teeth but instead, it is created by the magnetic poles. There will be a small air gap between different rotors which means no lubricant or maintenance is required. When one rotor rotates, the other one will also rotate driven by magnetic forces. Since the permanent magnets are used, the forces will not recede or disappear as long as the magnets are not overheated. If overloaded, a magnetic gearbox will simply slip poles. Therefore, in many applications, magnetic gears can be more reliable, efficient and safer. The goal of this research has been to investigate the torque capabilities of high gear ratio magnetic geared devices. The performance has been investigated based on the gear ratio and torque density. A new type of flux focusing cycloidal magnetic gear (MG) was investigated that could operate at a gear ratio of -25:1. The flux focusing topology was used because it increased the air gap flux density and therefore enabled a higher torque density. Using 2-D finite element analysis (FEA), the volume torque density was calculated to be 291 Nm/L with an outer diameter of 0.228 m. A -20:1 prototype cycloidal magnetic gear was designed. It had a calculated volumetric torque density of 260 Nm/L. The cycloidal magnetic gear was mechanically difficult to construct and therefore only the inner rotor of the cycloidal magnetic gear was constructed. In addition, the eccentric air gap will cause bearing failure. To achieve an even higher gear ratio, a nested multistage magnetic gear (MSMG) was designed with a 2-D FEA calculated torque density of 424 Nm/L. The desired gear ratio was 59:1 so that the performance could be compared with a Sumitomo mechanical gearbox which had the same gear ratio. In order to minimize the rotor torque ripple, the harmonic field interaction between the inner stage and the outer stage of the magnetic gear had to be mitigated. A unique flux concentration Halbach rotor structure was proposed. The rotor structure was shown to shield the outer rotor from the inner rotor harmonics. The nested multistage magnetic gear contains 4 rotors and complex mechanical structure. In order to provide sufficient mechanical support, the mechanical axial length had to be very large and this negated many of the benefits of using the nested coaxial rotor structure. A two-stage series connected 59:1 gear ratio multistage magnetic gear was also designed for wind turbines. The 6.45:1 first stage magnetic gearbox had a diameter of 0.633 m and the 3-D FEA calculated peak torque and torque density were 4.79 kNm and 159 Nm/L. The measured torque and torque density were 4.25 kNm and 141 Nm/L. While the 9.14:1 second stage magnetic gearbox had a diameter of 0.507 m and the 3-D FEA calculated peak torque and torque density were 1.04 kNm and 136 Nm/L, respectively. The series connected multistage magnetic gear had the advantage of being more modular as different gear ratios can be obtained by changing the pole pair combinations for one of the series connected magnetic gears.A two-stage series connected 59:1 gear ratio multistage magnetic gear was also designed for a hydropower application. Non-magnetic rods were used to reduce the losses and the mechanical deflection. The 2-D calculated torque density for the stage 1 magnetic gear was 371 Nm/L. And the 2-D calculated torque density was 344 Nm/L for the stage 2 magnetic gear. A stator was also designed that was inserted inside the stage 2 magnetic gear. In order to try to reduce the torque ripple, the stator had a fractional winding distribution with 1.25 slots/pole/phase. In order to understand the fundamental torque density capabilities of rotating magnetic devices, a 3-D analytical based model for an axial and radial magnetic coupling was developed. The models were derived using Maxwell’s magnetostatic equations and by using magnetic charge boundary conditions. The Laplacian equation was solved by using the separation of variables principle. The surface charge model was then used to obtain the field and torque expressions. The results from the analytical based model were compared with commercial FEA software. A good agreement was achieved for the radial magnetic coupling. However, it was shown that in order for the axial coupling to be modelled accurately a volume charge model needed to be considered. The analytical based model was significantly faster than the FEA models as at most two integrals needed to be numerically solved. The study of the magnetic couplings provided insight into the upper torque bound of magnetic rotary devices. The torque and torque density benefits of radial magnetic couplings relative to axial magnetic couplings were discussed.