Design optimisation and performance evaluation of flux switching machines for geared medium-speed wind generator drives

Akuru, Udochukwu Bola (2017-12)

Thesis (PhD)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: As wind turbines become large, the cost of energy increases because of the employed drivetrain (geared or direct–drive). Consequently, non–conventional geared medium–speed (MS) generators are gaining relevance, potentially due to better compromise to both the generator and gearbox costs. The study proposes a novel approach for the multi–objective design optimisation (MDO) of two variants of geared MS flux switching machines (FSMs) in their simple radial–flux structures––the 12–stator slots/10–rotor poles (12/10) and 12/14 machines, with major emphasis on rare–earth–free designs for industrial–scale applications. Based on finite element analyses (FEA), whereby torque density, torque ripple and power factor are prioritised, multiple design options are provided in different Pareto maps for the designer to make informed selections. From an initial optimal design comparison of the 12/10 machines based on rare–earth permanent magnets (PMs) in different wind generator drivetrains at 10 kW, the MS design yielded the best solution in terms of average torque densities per generator costs. Consequently, the focus on MS drivetrains was intensified to further evaluate the 12/10 and 12/14 rare–earth PM–FSMs as their power level shifts from 10 kW to 3 MW. As an indication of potentially adopting rare–earth–free designs, an improvement in terms of increased torque densities and reduced torque ripple values is obtained for the rare–earth designs at 3 MW due to a number of factors such as variations in their PM volumes and relative differences in their saliency ratios. Moreover, due to the optimal behaviour of key design parameters in the MDO environment, the superior performance of the 12/14 machines at 10 kW are reversed at 3 MW. Disappointingly, based on the same design requirements, the nominated rare–earth–free designs such as ferrite PMs and wound–field (WF) coils do not produce better torque ripple effects compared to rare–earth PMs, as should be expected for such flux–focusing machines. However, an inherent tradeoff was found between their aspect ratios and armature current densities which influence the active mass, especially for industrial– scale designs. Consequently, to ensure the feasibility of the optimal design, it may be needful to appropriately restrict the boundaries of the aspect and split ratios before engaging them in any MDO procedure. In another instance, it is found that it may be better to pursue MDO problems e.g., of WF– FSMs, by concentrating more on the performance (torque ripple and power factor) than on the cost of the machines. Interestingly, it was also found that the cheapest MS generator, even when compared with similar conventional wind generators at 3 MW, is the WF–FSM. Eventually, to validate the series of FEA prediction made during the study, a 10 kW WF–FSM generator prototype is selected, manufactured and tested, with certain novel implementations. Based on measured no–load, short– circuit, thermal, uncontrolled–normal and overload resistance, as well as current–controlled tests, the design feasibility as well as the conceptualisation of the proposed wind generator drivetrains has been proven beyond reasonable doubt. In agreement with Chen et al (2011) [44], there is, indeed, a bright future for FSMs. The study is concluded with a general conclusion and recommendations for the future.

AFRIKAANSE OPSOMMING: Soos hedendaagse wind turbines toeneem in grote, neem die koste van kragopwekking deur so ‘n turbine ook toe weens die aandryfstelsel wat gebruik word (ratkas of direk–gedrewe sisteme). As gevolg hiervan word daar gekyk na nie–konvensionele, medium–spoed (MS) turbines wat toegerus is met ‘n ratkas. Dit rede hiervoor is die goeie kompromi aangaande die generator en ratkas kostes. In hierdie studie word ‘n nuwe benadering tot multi–objektiewe ontwerp–optimering (MOO) vir twee verskillende geratte MS vloed–omskakelende masjiene (VOM), wat in hul mees eenvoudige radiale–vloedstrukture––die 12–stator gleuf/ 10 rotor pole (12/10) en 12/14 masjiene, met ‘n groot klem op skaars–aarde–vrye ontwerpe vir industriële toepassings. Gebaseer op eindige element ontledings (EEO), waardeur wringkragdigtheid, wringkrag–rimpeling en arbeidsfaktor geprioritiseer word, word verskeie ontwerpopsies verskaf vir die ontwerper om ingeligte keuses te maak, in verskillende Pareto kaarte. Vanuit ‘n voorlopige vergelykende studie van die 12/10 masjien, wat gebaseer is op ‘n skaars–aarde permanente magnete (PM) en toepassing vind in verskillende 10 kW turbines, is gevind dat die MS ontwerp die beste oplossing gelewer het in van gemiddelde wringkragdigtheid per generatorkoste. Gevolglik is die fokus op MS–turbines versterk om die 12/10 en 12/14 skaars–aarde PM–VOMs te evalueer, soos hul kraguitset van 10 kW na 3 MW vermeerder. ‘n Aanduiding dat skaars–aarde–vrye ontwerpe gebruik kan word in pleks van die huidige standaard, is die verbeteringe in terme van verhoogde wringkragdigtheid en verminderde wringkrag rimpelwaardes, wat verkry is vir die skaars–aarde ontwerpe by 3 MW, weens ‘n aantal faktore soos die variasies in hul PM volumes en die relatiewe verskille in hul speek verhoudings. Verder, weens die optimale gedrag sleutel–ontwerpsparameters in die MOO, word daar gevind dat die voortreflike vermoë van die 12/14 masjien by 10 kW omgekeerd is by 3 MW. Dit was egter teleurstellend dat, as gevolg van die selfde ontwerpskriteria, die benoemde skaars–aarde–vrye ontwerpe soos die ferriet– PMs en bewikkelde veld (BV) spoele nie beter wringkrag–rimpel–effekte in vergelyking met die skaars–aarde–vrye ontwerpe lewer nie; soos verwag word vir sulke vloed–gekonsentreerde masjiene. Nogtans, daar was ‘n inherente uitruiling gevind tussen die aspekverhoudings en ankerstroomdigtheid wat ‘n invloed het op die aktiewe massa, veral binne industriële–skaal ontwerpe. Gevolglik, om die haalbaarheid van die optimale ontwerp te verseker, kan dit nodig wees om die grense van die aspek en verdeelverhoudings behoorlik te beperk voordat hulle in enige MOO–prosedure betrek word. In 'n ander geval word gevind dat dit beter kan wees om MOO–probleme, bv. Van BV– VOMs, te volg, deur meer op die prestasie (wringkrag rimpel en arbeidsfaktor) te konsentreer as op die koste van die masjiene. Interessant genoeg is gevind dat die goedkoopste MS–generator, selfs as dit vergelyk word met soortgelyke konvensionele wind generators teen 3 MW, die BV–VOM is. Uiteindelik, om die reeks EEO–berekeninge van die die studie te bevestig, word 'n 10 kW BV– VOM–generator prototipe gekies, vervaardig en getoets, met sekere nuwe implementerings. Gebaseer op die gemete geen–las, kortsluit, termiese, onbeheerd–normaal– en oorlas–weerstand, sowel as die stroom–beheerde toetse, is die ontwerpsuitvoerbaarheid sowel as die konsepsie van die voorgestelde wind turbine dryfstelsels buite redelike twyfel bewys. In ooreenstemming met Chen et al (2011) [44] is daar inderdaad 'n blink toekoms vir VOMs. Die studie word afgesluit met met ‘n algemene gevolgtrekking en aanbevelings vir die toekoms.

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