Variable air gap axial flux permanent magnet machine with mechanical field weakening

Field-weakening of electrical machines enables rotational speeds to exceed the rated values. This is particularly simple in some AC machines such as induction machines where the field strength magnitudes are determined hence controlled by a component of current in the stator windings. In surface-mount permanent magnet machines, however, electrical modification of the field strength is problematic, and the torque-speed curve quickly drops to zero above the rated speed. A method of weakening the field is still necessary. This thesis describes two mechanical field weakening methods for axial flux permanent magnet machines, a type of machine that is popular for in-hub traction applications such as solar-powered electric vehicles, where the permanent magnets are surface mounted on magnet rings. After a review of the literature and the introduction of the two novel concepts for mechanical field weakening techniques, a simple design process is adapted to generate a first-pass design. This design process guides the design of the axial flux machine including estimates of the performance of the machine when utilising either of the mechanical field weakening techniques. The mechanical field weakening techniques are shown to improve the speed range of the machine. The two concepts for mechanical field weakening are (a) to vary the air gap between the two magnet rings (that constitute the rotor) and the stator (with the windings) and (b) to twist the two magnet rings around the main rotational axis, thereby offsetting the magnet poles and reducing the air gap flux density magnitude (and position). The two techniques are experimentally assessed on an off-the-shelf permanent magnet machine that has a single stator and double rotor structure. This unique experimental setup is necessary because it allows the air gap between both rotor magnet rings and the stator to be changed and, importantly, maintained at equal. In the case of twisting the magnet rings, the field weakening is achieved by spatially-displacing magnet rings so that the relative positions of the opposing magnet poles can be positioned sub optimally, reducing the air gap flux density. Increasing the air gap and/or the twist angle improves the overspeed capability of the machine, allowing machine operation up to 3.5 and 5.5 times of the rated speed, respectively. The machine is then controlled using a specially developed vector control drive system. Vector control requires accurate estimates of the position of the dq reference frame (the air gap flux density and position). The two techniques affect the rotor-developed magnetic field; both flux density magnitude and its phase (position). The research then investigates a method of tracking the change in the d-axis position of the air gap MMF as the twist angle is changed. The proposed method confirms that the d-axis rotational shift is αt/2 where αt is the twist angle. These results are then confirmed through experimental tests using the off-the-shelf machine, which has been adapted to include the mechanical field weakening techniques and its test rig. The proposed mechanical field weakening techniques and their impact on the magnetic conditions in the machine (both the flux density magnitude and position in the air gap) are validated by analysis, modelling, and importantly experimentally. The thesis proposes and assesses two mechanical means of field weakening by varying the air gap and the twist angle between magnet rings. The thesis also proposes a flowchart for the initial design of an axial flux permanent magnet machine with field weakening capability as well as revealing the change in air gap flux density (magnitude and phase). The significant force and torque required to develop the mechanical field weakening techniques is briefly assessed and is found to need further consideration when the two techniques are applied in real time and online, which is a suggested area for future wor