Design of a permanent magnet air-turbo generator and oscillation studies in miniature synchronous motors
The work described in this part of the thesis is involved mainly with uprating a 55W air-turbine driven permanent magnet generator to 250W. It is shown that the electrical performance of the generator can be predicted from a simple equivalent circuit comprising an, induced emf source with a series inductance and resistance. When matching the generator and turbine characteristics iron losses are included as an additional torque requirement. Analysis has identified that the most important parameters which determine the rating-of the generator are the stator flux, linkage, stator inductance, and number of pole pairs. Investigations have therefore centred around the calculation of these quantities. Previous design methods for calculating the parameters have been dependent on experimental data for particular magnet geometries. They are not sufficiently general to permit design calculations for magnets having radically different shapes or properties. Therefore the finite element method is used to predict the magnetic field distribution, from which the stator winding flux linkages and inductance, and the saliency torque, are predicted, an important parameter when matching the turbine and generator at starting. The finite element method offers significant advantages over analytical methods because it can account accurately for leakage flux, it can handle complex configurations of magnetic circuit and the directional properties of the magnet, and it allows different parts of the magnet to operate at different flux density levels. The performance of the generator is predicted with reasonable accuracy. Alternative rotors for the 250W generator have been designed and tested. Results have shown that the existing generator can be adapted to the 250W design simply by using a rare-earth magnet rotor, and selecting a suitable stator winding turns factor for matching the generator and bulb. Two methods of matching the load characteristics of the turbine, generator and bulb, to produce an acceptable system have been developed. A simple electronic protection circuit has been designed to prevent the 250W generator from overspeeding in the event of bulb failure. Additionally it can limit over-voltages, caused by variations in the pressure of the air supply, which would otherwise decrease the life of the bulb.