Direct Driven Axial Flux Permanent Magnet Generator for Small-Scale Wind Power Applications

Transcription

1 Euopean Association fo the Development of Renewable Enegies, Envionment and Powe Quality (EA4EPQ) Intenational Confeence on Renewable Enegies and Powe Quality (ICREPQ 11) Las Palmas de Gan Canaia (Spain), 13th to 15th Apil, 2011 Diect Diven Axial Flux Pemanent Magnet Geneato fo Small-Scale Wind Powe Applications A. P. Feeia 1 and A. F. Costa 2 1 School of Technology and Management, Polytechnic Institute of Bagança Campus de Santa Apolónia, Apatado 1134, Bagança, Potugal Phone: , Fax: , apf@ipb.pt 2 Depatment of Electical and Compute Engineeing, FEUP R. D. Robeto Fias, Poto, Potugal Phone: , Fax: , acosta@fe.up.pt Abstact. Small-scale wind powe applications equie a cost effective and mechanically simple geneato in ode to be a eliable enegy souce. The use of diect diven geneatos, instead of geaed machines, educes the numbe of dive components, which offes the oppotunity to educe costs and inceases system eliability and efficiency. Fo such applications, chaacteized by low speed of otation, the axial flux pemanent magnet geneato is paticulaly suited, since it can be designed with a lage pole numbe and high toque density. This pape pesents an axial flux pemanent magnet synchonous geneato, double sided with intenal oto and slotted statos. Such a stuctue gives a good compomise between pefomance chaacteistics and feasibility of constuction. The design pocess of the machine is descibed and validated by test expeiments. Key wods Wind enegy, axial flux, pemanent magnet geneato, diect diven, design. 1. Intoduction Thee is cuently significant inteest in the development of small-scale wind tubines fo the uban envionment, with both hoizontal and vetical axis being consideed [1-3]. Small-scale wind tubines ae also an attactive choice fo autonomous applications and fo ual aeas whee the installation of a distibution gid is not economically easonable [4]. The powe ange of those systems is usually about 500 W to 5 kw. Pemanent magnet excitation is favoued in this powe ange, fom the point of view that the equied volume of pemanent magnets is nowadays costly affodable. The main advantages of pemanent magnet synchonous geneatos ove wound-oto geneatos ae due to the fact the fist ones do not equie any extenal excitation cuent, which tanslates in a significant decease in oto losses and also allows the use of a diode bidge ectifie at the geneato teminals, with significant cost benefits in the powe convete topology [5-7]. Pemanent magnet excitation also allows a significant decease of the pole pitch, which tanslates in cost and mass eduction [8]. This assumption togethe with the axial flux geneato configuation favou the use of a diect diven geneato (gealess system) because it allows the use of highe pole numbe [9]. Futhemoe, axial flux pemanent magnet machines ae ecognized fo having highe toque density than thei countepats based on adial-flux, this being moe appaent in a design with a lage numbe of poles. Axial flux pemanent magnet machines may be designed in vaious configuations [10-12]. A double-sided axial flux pemanent magnet synchonous geneato with intenal oto intended fo small-scale wind enegy systems was poposed on [13]. In this pape, the analytical design of the machine is enhanced with finite element analysis (FEA). Expeimental test esults ae caied out. 2. Geneato concept Axial flux machines ae chaacteized by an axially diected aigap flux. The simplest stuctue uses one stato ing fom and a disk oto (Fig. 1), both having the same active inne and oute diametes which defines the active pat of the machine whee the electomechanical convesion takes place RE&PQJ, Vol.1, No.9, May 2011

2 Fig. 1. Single stato-single oto axial flux machine stuctue. The axial length is dependent on the flux density in the stato and oto yokes. Thus, in opposition to adial flux machines, both stato and oto yokes can be fully utilized with a pope yoke design. As a consequence, when the numbe of poles inceases the axial flux machine adial active pat emains almost unchanged, but the axial length can decease and the toque density inceases. Hence, axial flux machines ae paticulaly suited in low speed, high toque applications such as wind powe convesion systems. A dawback of the single stato-single oto stuctue is the unbalanced axial foce between the oto and the stato, which leads to a thicke oto disk and complex beaing aangements, this compaed to double sided stuctues, pesented in Fig. 2. These stuctues eliminate the axial foces between pemanent magnets and the stato ion with the benefit of inceasing the total ai gap suface. pemanent magnet machines [15, 16], due to shote end windings. Howeve they geneate both odd and even hamonics, and some, also poduce sub-hamonics in the EMF. All exta hamonics ceate additional flux in the machine which esults in high eddy cuent losses in a conducting oto and in the pemanent magnets, which may cancel the benefits of the shote end windings. A conventional distibuted winding with one slot pe pole and phase is pefeed. The flux path associated with this machine topology is shown in Fig. 3. The flux tavels axially in the oto stuctue and completes its path by etuning cicumfeentially aound the statos coes. Fig. 3. 2D plane of the axial flux pemanent magnet machine. (a) (b) 3. Design pocedue The main dimensions of the machine ae achieved iteatively via analytical appoach, based on some simplifying assumptions, as descibed on [13]. Fig. 2. Double sided axial flux machine stuctues. (a) intenal stato. (b) intenal oto. The double sided stuctue with intenal oto (Fig. 2 (b)) simplifies the manufactue pocess due to easie fixation of the stato ings to the fame; it also favous the cooling pocess because the main heat souce is located nea the suface. The axial flux pemanent magnet synchonous geneato developed is based in stuctue. Slotted statos incease emakably the amplitude of the aigap flux density due to shote aigap. This educes the equied amount of pemanent magnets, which yields savings in the geneato pice. On the othe hand, slotting may evoke undesied toque pulsations, but the adopted stuctue allows the otation of one stato ove one half of the slot pitch with espect to the othe, which esults in educed slot ipple and space hamonic components [14]. Cuent ipple in slotted statos may also be educed due to highe leakage inductance compaed to slotless statos. It s cuently assumed that concentated windings ae an effective way to educe Joule losses in low speed The equivalent magnet cicuit appoach and pemanent magnet load line chaacteistics ae used to estimate the magnetic loading, which detemines the stato yoke dimensions, and the specifications and dimensions of the magnets. This analytical appoach does not lead to exact magnetic field solutions. Howeve it often leads to analytic solutions that ae conducive to the fomulation of design equations. On the othe hand, finite element analysis (FEA) leads to much moe accuate magnetic field solutions because it models all flux finging paths, but it only povides a numeical solution. In a sense, magnetic cicuit analysis solves poblems fom a maco pespective, wheeas FEA complements the design fom a mico pespective. The stengths of one appoach ae the weaknesses of the othe. To estimate the electic loading (o the linea cuent density) a heat tansfe model is implemented, using the maximum tempeatue allowed in the coils and the suounding tempeatue (this study is not pesented hee). To account the vaiation of the magnet width to pole pitch atio, the modelling of the axial flux machine is pefomed in thee diffeent computation planes. The oveall pefomance of the machine is obtained by RE&PQJ, Vol.1, No.9, May 2011

3 summing the pefomances of thee sub-machines. By this way, concening the FEA analysis, 3D FEA is avoided. A. Magnetic cicuit analysis In each computation plane the flux densities in diffeent pats of the machine ae found based on a non-linea eluctance netwok ove a pole pitch so that symmety conditions ae fulfilled. The eluctance netwok in use is pesented in Fig. 4, whee,,, and ae y t s g m the eluctances fo the stato yoke, tooth, slot opening, ai gap and pemanent magnet, espectively. t I y s t I y s t I Fig. 5. Magnet flux density at the middle plane, unde no load condition. The ai gap flux density ove one pole is shown in Fig 6. The flux concentation effects nea the tooth tips ae highlighted, with an incease of the amplitude of the flux density below the tooth tip in the vicinity of the slot opening. g g g Fig. 4. Reluctance netwok ove one pole pitch. A paticula eluctance element is calculated as li, (1) i S 0 i whee l i is the length, is the elative pemeability and S is the coss-sectional aea of the element modelled. i The magnetomotive foces and m ae the MMF due I to pemanent magnets and stato phase cuents, espectively. The MMF poduced by the pemanent magnet, is calculated as:, (2) m m whee B Sm is the emanent magnetic flux. The eluctance netwok is solved iteatively using the matix equation 1. (3) The elative pemeability of the ion pats (stato yoke and tooth) is a function of the flux density, being the final esult of (3). Fo the laminated mateial in use, B is found using BH cuves. Theefoe, the magnetic flux vecto,, has to be found iteatively. The eluctance netwok is solved unde no-load and load conditions, with 0 in the fist case and NI in I I s the second case, whee N is the numbe of coil tuns in s a slot and I is the cuent though one coil tun. Fig. 6. Ai gap flux density unde no load condition. Unde load condition, in steady state opeation, a magnetostatic analysis is possible, due to synchonous speed of the geneato, feezing a paticula instant of its opeation, which coesponds to machine analysis in the oto efeence fame: fo a constant synchonous speed, the opeating electical fequency is p. The suface cuent densities in the diffeent slots ae detemined once the distibution of the winding is fixed, using the maximum value of the ated stato cuent and evaluating the thee-phase cuents fo a time instant t. Fo the distibuted winding with one slot pe pole and phase and a paticula time instant with t 2 ad, the magnet flux density distibution unde ated load condition is shown in Fig. 7. B. 2D finite element analysis (FEA) The no load flux density is obtained pefoming a magnetostatic analysis. Fig. 5 shows the magnet flux density distibution at the middle computation plane. Fig. 7. Magnet flux density at the middle plane, unde load condition RE&PQJ, Vol.1, No.9, May 2011

4 4. Expeimental geneato An axial flux pemanent magnet low-speed geneato pototype, double-sided with intenal oto and slotted statos has been constucted based on the analytical pocedue wheeas FEA analysis contibutions have not been included. Outputs of the design study ae given in Table I as well as the main paametes of the pototype. (a) (b) Table I. - Main paametes of the axial flux pemanent machine pototype. PARAMETER VALUE Axial moto length 5,62 cm Aigap thickness 0,5 mm Coe inne adius 6,5 cm Coe oute adius 9 cm Pemanent magnet axial length 1 cm Magnet occupation atio (at aveage adius) 0,617 Numbe of pole pais 10 Pole pitch (at aveage adius) 2,49 cm Pemanent magnet oveall weight 0,262 kg Rated speed 600 pm Rated toque 5,4 Nm Rated powe (at 600 pm) 340 W No-load phase voltage (ms value at 600 pm) 80,6 V Numbe of phases 3 Numbe of coils pe stato 60 Numbe of tuns pe coil 24 Numbe of slots pe pole and phase 1 Stato phase esistance (at 20ºC) 7 Ω Linea cuent density (at aveage adius) 8,28 ka/m The pototype machine is shown on laboatoy test in Fig. 8. Stato ings ae fixed to the oute fame. The theephase windings of the two statos ae connected in seies and the sta connection is used to avoid ciculating cuents. The inne oto consists in a holed paamagnetic aluminum disk to suppot the NdFeB pemanent magnets. Compaed to the suface mounted pemanent magnets in a non-holed oto disk, this solution involves the machination and the manipulation of half magnet pieces. Axially magnetized cylindical pemanent magnets NdFeB gade N30SH ae used to accomplish the excitation. Fig. 9 shows the geneato oto disk and one stato ing. Fig. 9: Geneato oto disk (a) and one stato ing (b). 5. Test esults and discussion The machine unde test has been diven by an induction geneato fed by a fequency convete and loaded with a vaiable esistive load. The shaft toque has been measued by a toque tansduce and the electical powe, cuent and voltage have been egisteed by a powe analyse. Fig. 10 shows the no load phase to neutal voltage wavefom at the ated speed 600 pm, fequency 100 Hz. This wavefom has a significant 23,1% thid hamonic (Fig. 11). Fig. 10: No load phase to neutal voltage at 600 pm (voltage scale: 40 V/div; time scale: 5 ms/div). Component of the hamonic fequency/fundamental component 1 0,231 0,098 0,019 0, Hamonic fequency Fig. 11: Hamonic components in no load phase to neutal voltage. Fig. 8: Pototype geneato (in laboatoy). The hamonic content of the no load phase to neutal voltage wavefom would be substantially educed if one stato was shifted one half of the slot pitch with espect to the othe. Obviously, it should be expected a decease of the ms value of the fundamental component as a RE&PQJ, Vol.1, No.9, May 2011

5 consequence of the electical phase angle esulting between the EMF s induced in the two active potions of each coil of the winding. Othe technical solution to achieve substantial eduction of the no load voltage hamonic content, based on magnet o slot skewing, inceases the complexity of machine manufactuing. AC esistive load tests wee caied out fo two diffeent speeds fom the expected opeating speed ange fo the geneato. The esults ae shown in Fig. 12 to 14. Voltage (phase to neutal) (V) Electic powe ((W) Efficiency (%) pm 300 pm 0 0,4 0,8 1,2 1,6 2 Fig. 12: Phase to neutal voltage vesus cuent pm pm 0 0 0,4 0,8 1,2 1,6 2 Cuent (A) Fig. 13: Electic powe vesus cuent. 600 pm 300 pm 0 0,4 0,8 1,2 1,6 2 Fig. 14: Efficiency vesus cuent. The efficiency at ated opeating condition was evaluated in 86%, which is acceptable fo a non optimized pototype. The losses in the geneato ae dominated by stato Joule losses. Ion losses of about 6,3 W and 16,3 W wee evaluated at 300 pm and 600 pm, espectively, while Joule losses at ated cuent ae about 41,5 W. The highe tem of Joule losses ae due to lage end-windings of the distibuted winding aangement used. In futhe investigation, it should be consideed the use of concentated windings instead of distibuted windings in ode to decease the end-windings and consequently the Joule losses. 6. Conclusion In this pape, the design and constuction of an ai efigeated axial flux pemanent magnet synchonous geneato have been pesented. Although initially designed as a diect diven geneato fo small-scale wind tubines, it has othe application aeas such as mico hydo powe plants. The geneato concept was selected specifically fo ease of manufactue, eliability and low cost. An iteative design pocedue has been developed using analytical and numeical appoaches. The pototype has been manufactued in a peliminay stage of the development of the design pocedue, thus it lacks the FEA analysis contibutions. Futhe investigation should be pefomed in ode to incease the efficiency, namely the use of concentated windings instead of distibuted windings in ode to decease the end-windings and consequently the Joule losses. Refeences [1] WePowe. (2010, Octobe, 2010). WePowe, Sustainable Enegy Solutions. Available: [2] QuietRevolution. (2010, Octobe, 2010). Available: [3] HelixWind. (2010, Octobe, 2010). Available: [4] A. N. Celik, "Enegy Output Estimation fo Small-Scale Wind Powe Geneatos Using Weibull-Repesentative Wind Data," Jounal of Wind Engineeing and Industial Aeodynamics, vol. 91, pp , [5] J. A. Baoudi, et al., "A eview of Powe Convete Topologies fo Wind Geneatos," Renewable Enegy, Elsevie, vol. 32, pp , [6] H. Polinde, et al., "Basic Opeation Pinciples and Electical Convesion Systems of Wind Tubines," EPE Jounal, vol. 15, pp , Decembe, [7] T. Senjyu, et al., "Wind Velocity and Roto Position Sensoless Maximum Powe Point Tacking Contol fo Wind Geneation System," in 35th Annual IEEE Powe Electonics Specialists Confeence, PESC 2004, Aachen, Gemany, 2004, pp [8] A. P. Feeia, "Poblemática e Pespectivas da Utilização do Geado de Ímanes Pemanentes na Podução de Enegia Eólica," Ms. C. Dissetation, Faculdade de Engenhaia da Univesidade do Poto (FEUP), Poto, Potugal, [9] J. R. Bumby and R. Matin, "Axial-Flux Pemanent- Magnet Ai-Coed Geneato fo Small-Scale Wind Tubines," IEE Poceedings - Electic Powe Applications, vol. 152, pp , Septembe, [10] M. Aydin, et al., "Axial Flux Pemanent Magnet Disc Machines: A Review," Wisconsin Electic Machines & Powe Electonics Consotium, Madison , [11] S. Huang, et al., "TORUS Concept Machines: Pe- Pototyping Design Assessment fo Two Majo Topologies," in IEEE Industy Applications Confeence, 36th IAS Annual Meeting, 2001, pp [12] A. Paviainen, et al., "Axial Flux Pemanent Magnet Geneato with Concentated Winding fo Small Wind Powe Applications," in IEEE Intenational Confeence RE&PQJ, Vol.1, No.9, May 2011

6 on Electic Machines and Dives, IEMDC'05, 2005, pp [13] A. Feeia, et al., "Pototype of an Axial Flux Pemanent Magnet Genaato fo Wind Enegy Systems Applications," in 12th Euopean Confeence on Powe Electonics and Applications, EPE 2007, Aalbog, Denmak, [14] F. Magnussen, et al., "Pefomance Evaluation of Pemanent Magnet Synchonous Machines with Concentated and Distibuted Windings Including the Effect of Field Weakening," in Intenational Confeence on Powe Electonics, Machines and Dives (PEMD 2004), 2004, pp [15] A. D. Gelando, et al., "High Pole Numbe, PM Synchonous Moto with Concentated Coil Windings," in Intenational Confeence on Electical Machines, ICEM 2004, Cacow, Poland, [16] F. Magnussen and C. Sadaangani, "Winding Factos and Joule Losses of Pemanent Magnet Machines with Concentated Windings," in IEEE Intenational Electic Machines and Dives Confeence, IEMDC'03, Madison, Wisconsin, USA, 2003, pp RE&PQJ, Vol.1, No.9, May 2011