Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 213-227).

Electric machines are indispensable part in modern industry and they are widely used in many applications such as power generation, vehicles, elevator, robot, and so on. Owing to the diversity of the application occasions, different requirements have been proposed. In the applications, such as wind power generation, low-speed, high-torque and high-efficiency electric machines are required to match with the low-speed load. Currently, this problem is handled by either adopting a speed-boost mechanical gear, or using low-speed machine design. The former one causes mechanical wear and tear, audible noise and low efficiency, whereas the latter one increases the generator size and weight as well as raw material cost. In recent years, another option, namely integrating a coaxial magnetic gear into a permanent-magnet (PM) machine has been proposed. This option allows for directly mounting the outer, low-speed shaft with the load while enables the electric machine coupled with the high-speed inner gear to operate at high speeds. However, this magnetic-geared electric machine desires a complex structure, involving two rotating bodies and three air-gaps, which increases manufacture difficulty and cost. The research work of this thesis attempts to solve the speed-matching problem by developing a new electric machine. The incorporation of vernier concept with PM machine gives birth to the permanent-magnet verier (PMV) machine. Different from traditional synchronous machine, the rotor of the PMV machine rotates at a definite fraction of the synchronous speed, as if it were geared down from the high rotating field set up by the stator. The PMV machine, therefore, can be regarded as a combination of a gear with fixed gear ratio and an electric machine. This kind of machine is attractive in applications which require low speed and high torque, and mechanical gearing is undesirable. The main objective of this thesis is to present the design, analysis and control of the proposed PMV machine. After the introduction on mechanical gears, magnetic gears, and low-speed machines, the design details of the proposed outer-rotor PMV machine are provided. Moreover, the working principle, stable torque generation mechanism and structural features are presented. Due to the introduced vernier effect, it is important to perform the finite element analysis (FEA) for the proposed PMV machine. The electric circuit equation and the motion equation are coupled with the Maxwell’s equation to calculate the key parameters of the proposed machine. The analysis results of the magnetic field distributions, air-gap flux density distributions, flux linkages, winding inductances, back electromotive forces (EMFs), cogging torque and static torque are presented in detail. The performances of PMV machine are compared with other machines to show its advantages and disadvantages. The control strategy of the PMV machine as a brushless DC machine is also presented. The conventional 120-degree conducting, dual-closed-loop control strategy is used for speed and torque control and the experimental setup are given. The results are obtained and compared with the simulation results, thus verify the validity of the design. Finally, the potential applications for the proposed PMV machine are suggested. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2005. / Also available in print.

Multi-phase motors have several advantages over the traditional three-phase motors. In this study, the additional degrees of freedom available in five-phase permanent magnet motors have been used for three purposes: 1) enhancing the torque producing capability of the motor, 2) improving the reliability of the system, and 3) better adjusting of the torque and flux linkages of the five-phase direct torque controlled system. 1) Due to the fact that space and time harmonics of the same orders will contribute positively to output torque, a five-phase permanent magnet motor with quasi-rectangular back-EMF waveform is supplied with combined fundamental and third harmonic of currents. For modeling and analysis of the motor a 0 3 3 1 1 q d q d frame of reference is defined where 1 1q d rotates at the synchronous speed and 3 3q d rotates at the three times synchronous speed. Based on the mathematical model in the 0 3 3 1 1 q d q d frame of reference, it is shown that this system while having a higher torque density with respect to a conventional permanent magnet synchronous machine, is also compatible with vector control algorithm. 2) A resilient current control of the five-phase permanent motor with both sinusoidal and trapezoidal back-EMF waveforms under asymmetrical fault condition is proposed. In this scheme, the stator MMF is kept unchanged during healthy and faulty condition. Therefore, the five-phase permanent magnet motor operates continuously and steadily without additional hardware and just by modifying the control algorithm in case of loss of up to two phases. The feature is of major importance in some specific applications where high reliability is required. 3) High torque and flux ripple are the major drawbacks of a three-phase direct torque controlled system. The number of space voltage vectors directly influences the performance of DTC system. A five-phase drive, while benefiting from other advantages of high order phase drives, has inherently 32 space voltage vectors which permits better flexibility in selecting the switching states and finer adjustment of flux and torque. A sensorless direct torque control of five-phase permanent magnet motor is implemented. Speed information is obtained based on the position of stator flux linkages and load angle. Experiments have been conducted on a 5kW five-phase surface mount permanent magnet motor and a 3kW five-phase interior permanent magnet motor by using TMS320C32 DSP. The results obtained are consistent with theoretical studies and simulation analysis, which further demonstrate the feasibility and practical significance of the five-phase permanent magnet motor drives.

With the growing interests and high requirements in low-speed linear drives, the linear machines possessing high force density, high power density and high efficiency feature become in great demands for the linear direct-drive applications. There are many available linear machine topologies, but their performances for exhibiting the high-force density capability dissatisfy the industrial requirements. In order to solve this problem, the new machine topologies emphasizing on high force density are explored and studied. The objective of this thesis is to present the design, analysis, and application of permanent magnet (PM) linear machines which can offer a higher force density at the same magnetic loading and electric loading than the conventional machines. Although in recent years there are many emerging advanced PM rotational machines for direct-drive rotational drives, the development of advanced PM linear machines for direct-drive linear drives is sparse. In spite of the motion type of electric machines, the inherent operating principle is the same. By studying and borrowing concepts of the high torque density rotational electric machines, the linear machine morphologies of the promising candidates are designed and analyzed. The problems and side effects resulting from the linearization are discussed and suppressed. Two main approaches for machine design and analysis are developed and applied, namely the analytical calculation and the finite element method (FEM). By analytically solving the magnetic field problem, the relationships between the field quantities and the machine geometry are unveiled. With the use of analytical calculation, the machine design and dimension optimization are conveniently achieved. With the use of FEM, the machine design objective and its electromagnetic performance are verified and evaluated. Finally, the proposed low-speed PM linear machine is applied for direct-drive wave power generation. By mathematically modeling the wave power, generation system and the generator, the conditions for maximum power harvesting are determined. By using the vector control, the generator output power is maximized which is verified by the simulation results. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2005.

Thesis (MScEng) -- Stellenbosch University, 2003. / ENGLISH ABSTRACT: Axial Flux Permanent Magnet (AFPM) machines have become attractive because of significant improvements in permanent magnets over the past decade, improvements in power electronic devices, and the ever increasing need for more efficient machines in electric vehicle systems. In comparison with the cylindrical radial flux motor, the AFPM machine is better in a number of aspects: short frame; compact construction; high efficiency; brush less construction; good starting torque and high-power density. The common modes of failure and typical operating conditions of AFPM machines are discussed further. The focus of this research project is a prototype AFPM machine developed by the Electrical Engineering Department of The University of Stellenbosch. The machine considered has a power rating of 300 kW and an operating efficiency of 95 % at a speed of 2300 rpm. This specific machine is used as an example to illustrate the thermal characteristics of geometrically similar AFPM machines. The thermal characterization was achieved with the use of two numerical computer models. Firstly a fluid model was specially developed and experimentally verified. The objective of the fluid model was to calculate the mass flow rate of air through any geometrically similar AFPM machine. The fluid model was further used to investigate the effects of different magnet thickness and axial gaps between the stator and the rotor plates on the mass flow rate of air through the machine. The fluid model was verified with experimental testing that was done on a half-scale Perspex model. During the experimental testing the magnet thickness was varied between 2.5 mm, 5.0 mm, and 7.5 mm along with axial gaps of 6.5 mm, 7.5 mm, 8.5 mm, and 9.5 mm. The fluid model showed a correlation to within 10 % of the experimental mass flow rates. The results of these tests showed that the magnet thickness and axial gap between the stator and the rotor plates had no significant effect on the mass flow rate of air. The fluid model was based on one-dimensional, steady-state, and incompressible flow. The second numerical computer model was a thermal model. This model was used to calculate the transient temperature response of the AFPM machine. The model was based on a twodimensional transient finite difference solution technique. Experimental temperatures taken from the prototype AFPM machine were used to verify the thermal model. Correlations between the experimental and theoretical temperatures were within 5.8 % of each other. The thermal model was used to investigate the effect of geometrical changes on the temperatures in the AFPM machine. It was found that these geometrical changes had no significant effect on the temperatures in the AFPM machine. It was also established that increasing the air mass flow rate over about I kg/s had no further effect on lowering the temperatures. The stator was also identified as being the most critical component as it reached its maximum temperature limit before any other component. Heat pipes were considered as an alternative thermal management technique. The location of the heat pipe was limited to the stator. Further simulations were done to investigate the effect of the heat pipe properties on the amount of heat removed from the stator. Recommendations were made concerning the thermal management of the current and possible future prototype AFPM machines. It was recommended that a further more detailed investigation into the use of heat pipes be considered. This recommendation is substantiated by the fact that in this research project only one type of heat pipe was considered and its location was limited to within the stator. / AFRIKAANSE OPSOMMING: AFPM masjiene het meer aantreklik geword weens betekenisvolle verbeteringe in permanente magnete gedurende die laaste dekade, verbeteringe in elektroniese toestelle en die vraag na meer effektiewe masjiene in elekriese voertuigstelsels. Die AFPM masjien is beter as die Silindriese Radiale Fluksie Motor wat die volgende aspekte betref: die kort raamwerk; kompakte konstruksie; hoe effektiwiteit; borsellose konstruksie; goeie aanvangsdraaimoment; en hoe-krag digtheid. Die algemene vorms van faling en ook die tipiese werkstoestande van die AFPM word verder bespreek. Hierdie navorsingsprojek fokus op die prototipe AFPM masjien wat ontwikkel is deur die Elektriese Ingenieurs Departement van die Universiteit van Stellenbosch. Die masjien onder bespreking wek 300 kW per uur op en is 95% effektief teen 'n spoed van 2300 rpm. Hierdie masjien word gebruik om die termiese kenmerke van geometries-gelyksoortige masjiene te illustreer. Die termiese eienskappe is bepaal deur die gebruik van twee numeriese rekenaarmodelle. Eerstens is 'n vloeistofmodel spesiaal ontwerp en eksperimenteel geverifieer. Die doel van die vloeistofmodel was om die massa vloeitempo van lug deur enige geometries-gelyksoortige AFPM masjien te bereken. Die vloeistofmodel is verder gebruik om die uitwerking van verskillende magneetdiktes en aksiale gapings tussen die stator en die rotorplate op die massa vloeitempo van lug deur die masjien te ondersoek. Die vloeistofmodel is geverifieer deur eksperimentele toetsing wat gedoen is op 'n halfskaal Perspex model. Tydens die toetsing het magneetdiktes gewissel tussen 2.5 mm, 5.0 mm en 7.5 mm en die aksiale gapings tussen 6.5 mm, 7.5 mm en 9.5 mm. Die vloeistof model het 'n korrelasie van binne 10 % van die eksperimentele massa vloeistempo getoon. Die resultate van hierdie toetse het getoon dat die magneetdiktes en die aksiale gapings tussen die stator en die rotorplate geen noemenswaardige uiterking op die massa vloeitempo van lug gehad het nie. Die vloeistofmodel is gebaseer op een-dimensionele, gestadigde, onsamedrukbare vloei. Die tweede numeriese model was 'n termiese model. Hierdie model is gebruik om die transiente temperatuur respons van die AFPM masjien te bereken. Die model is gebaseer op 'n tweedimensionele, transiente eindige-verskil oplossingstegniek. Eksperimentele temperature gemeet op die prototipe AFPM masjien is gebruik om die termiese model te verifeer. Die eksperimentele en teoretiese temperature het binne 5.8% met mekaar gekorrelleer. Die termiese model is gebruik om die uitwerking van geometriese veranderinge op die temperatuur in die AFPM masjien te ondersoek. Daar is gevind dat hierdie geometriese veranderinge geen noemenswaardige uitwerking op die temperature van die AFPM masjien gehad het nie. Daar is ook vasgestel dat 'n vermeerdering in die lug massa vloeitempo yerby I kg/s geen verdere uitwerking het op die verlaging van die temperatuur gehaad het nie. Die stator is ge-identifiseer as die mees kritiese komponent aangesien dit sy maksimum temperatuur limiet bereik het voor enige ander komponent, Hittepype is oorweeg as 'n alternatiewe termiese bestuurstegniek. Die plasing van die pype is tot die stator beperk. Verdere simulasies is uitgevoer om die uitwerking van die hittepyp eienskappe op die hoeveelheid hitte wat verwyder word van die stator te ondersoek. Aanbevelings is gemaak m.b.t die termiese bestuur van die huidige en moontlike toekomstige prototipes van AFPM masjiene. Daar is aanbeveel dat daar in meer besonderhede ondersoek ingestel word na die gebruik van hittepype. Die rede hiervoor is dat daar in hierdie studie net gebruik gemaak is van een tipe hittepyp en dat die plasing daarvan beperk is tot binne die stator.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy