Transverse Flux Machines with Distributed Windings for In-Wheel Applications

Transcription

1 PEDS009 Transverse Flux Machines with Distributed Windings fr In-Wheel Applicatins Salwa Baserrah*, Ken Rixen, Bernd Orlik* *Institute fr Electrical Drives, Pwer Electrnics and Devices University f Bremen Bremen, Germany Abstract Transverse flux machine (TFM) useful fr in-wheel mtr applicatins is presented. This transverse flux permanent magnet mtr is designed t achieve high trque-t-weight rati and is suitable fr direct-drive wheel applicatins. As in cnventinal TFM, the phases are lcated under each ther, which will increase the axial length f the machine. The idea f this design is t reduce the axial length f TFM, by placing the windings arund the statr and by shifting thse frm each ther by electrically 10 r 90, fr three- r tw-phase machine, respectively. Therefre, a remarkable reductin n the ttal axial length f the machine will be achieved while keeping the trque density high. This TFM is cmpared t anther similar TFM, in which the three phases have been divided int tw halves and placed ppsite each ther t ensure the mechanical balance and stability f the statr. The crrespnding mechanical phase shifts between the phases have accrdingly been taken int accunt. The mtrs are mdelled in finite-element methd (FEM) prgram, Flux3D, and designed t meet the specificatins f an ptimisatin scheme, subject t certain cnstraints, such as cnstructin dimensins, electric and magnetic lading. Based n this cmparisn study, many recmmendatins have been suggested t achieve ptimum results. Keywrds- Transverse flux; in-wheel mtr; distributed windings; flux-cncentrated; surface permanent magnet I. INTRODUCTION Direct-driven mtrs are the new trend f the electric mtrs, which are cnsidered t imprve the efficiency f the electric vehicle (EV) drive system. This type f mtrs is directly munted inside the wheel, called als in-wheel r hub mtr. Such mtr will eliminate transmissin gears r mechanical differentials with their assciated energy lss; and this is the main reasn behind the imprvement in efficiency. Varius cmparisn surveys f merits and demerits have been already reprted the types f electric mtrs that are suitable fr EV. Inductin mtr drives are preferred fr EV prpulsin purpse in [1]. Alternatively, permanent magnet brushless dc mtr featured cmpactness, lw weight and high efficiency. DC, inductin, permanent magnet synchrnus, switched reluctance and brushless DC are cmpared in [] and it is cncluded that amng these mtrs, PM and brushless DC mtrs are attractive chice fr EV applicatins. Different cmpact cnstructins f different electric mtrs, such as synchrnus, PM and switched reluctance mtrs, f relatively high trque density and achieving imprvements n the verall efficiency f the electric vehicle are cnstructed in [3-5] and still there are mre electric mtr design ptimisatin schemes appearing in electric vehicle research. In cmparisn with cnventinal radial-flux machines, axial-flux permanent magnet (AFPM) machines will allw explitatin f a higher percentage f statr winding fr trque prductin [6-7]. Several AFPM wheel mtrs designed fr electric cars are cmpared in [8] and multi-stage AFPM cnstructed in [9], and cncluded that machines f interir PM give the best cmprmise in terms f pwer density, efficiency, cmpactness and lng-term verlad capability characteristics. Permanent magnet (PM) mtr fr slar-pwered in-wheel mtr is demnstrated and examined in [10], an axial field air gap winding is utilized. TFM with flux cncentrated cnfiguratin is designed fr in-wheel applicatins and reprted in [11]; hwever, nly ne phase has been cnstructed and tested. The idea f TFM with tw-phase windings lcated arund the statr fr small pwer range is patented in [1] with axial permanent magnet (PM), hwever, n thrugh study r design imprving investigatins have been carried ut. Nevertheless, a cmparisn study f pwer density fr axial flux machines with varius tplgies have been cnducted in [13], general purpse sizing and pwer density equatins are being presented. A sectr-wise, three phase surface PM-TFM with distributed windings as an inner rtr machine is described and analytically mdelled in [14]. Althugh the pwer factr is stated t be imprved t 0.7; the trque density is reduced. Fllwing the suggestins f previus experiences, a nvel design methdlgy n permanent magnet transverse flux fr in-wheel mtr applicatins will be presented, cmpact design f small axial length and maximum explitatin f radial space with high trque density is the design target. II. STRUCTURE OF NEW DESIGN Transverse flux machine underlines the high trque mtrs, which can be classified as surface permanent magnet 10

2 TFM (SPM-TFM) and flux cncentrated TFM (FC-TFM). Permanent magnets in SPM-TFM are magnetised in directin perpendicular t the directin f rtatin, as the permanent magnets in FC-TFM are f parallel magnetisatin directin t the rtatin. Since the trque density achieved by FC-TFM is higher than the SPM-TFM, it is preferred t use FC tplgy fr cnstructing in-wheel mtrs, thugh it is mechanically difficult in cnstructin. In this descriptin, flux cncentrated cnfiguratins will be cnsidered in details, ther cnstructins fr SPM-TFM will be pinted ut t. In rder t lcate the windings arund the statr, certain steps shuld be managed s that the mechanical shift f the windings with respect t the rtr will cp with the phase f the electric lading. Specifying the number f ples in the statr fr each phase is the starting step and it shuld be selected as an even number, n each layer f the statr, s that the winding can be wunded feasibly arund them. Fig. 1.a shws the 3-phase FC-TFM f 56 ples with distributed three full phases. The ple number represents the number f the PMs r the rtr ples. The ple pitch, τ p is defined in millimetres as the air gap diameter times divided by the ple number r simply 360 divided by the number f ples, which is equal t 6.43, expressed in mechanical degrees. The PMs are marked with different clurs indicating ppsite tangential magnetisatin directin. Each statr phase has sixteen ple pitches, 16τ p. Each tw cmplete statr ples crrespnd t τ p. The statr ples are divided int tw parts, ne is lcated in the centre f the statr axial length, and the ther part cnsists f statr ples that are shifted by 1τ p and are divided and placed in tp and bttm layers s that each side f the winding will be sandwiched by the statr ples. As the ple pitch pair number fr each phase is specified fr this machine t be 8, the crrespnding number f rtr ples fr each phase is sixteen, since each 1τ p is equivalent t the mean distance between tw rtr ples. By placing the first phase f the statr and its crrespnding rtr phase span; the first step f the design will be accmplished. In the same way, the secnd statr phase which is identical t the first phase but displaced by a mechanical shift that crrespnds t 10 electrically is added. The mechanical shift, θ 3, is equivalent t /3 τ p and calculated by (1). PEDS009 The machine is designed fr q =, cnsequently, the number f rtr ples fr the tw phases can be easily deducted as 16 ples (i.e. phase 1) + ples (i.e. τ p ) + mechanical shift + 16 (i.e. phase ), this will result in 34 rtr ples and a fractin that crrespnds t /3 τ p. Fllwing the same manner, the third phase f the statr will be placed, which will lag phase ne by 4/3 τ p i.e., 40 electrical degrees. The resulting number f rtr ples fr the whle machine will result in: 3 16 ples (i.e. 3 phases) + 3 (i.e. τ p ) + ples (i.e. 3 θ 3 = 3 (/3 τ p )) = 56 rtr ples. As a matter f fact each statr phase crrespnds t 10 mechanical degrees i.e., 1 16 (i.e. phase1) + 1(i.e. τ p ) +1(i.e. θ 3 = /3 τ p ). Thus, 18τ p + /3 τ p = (56/3) τ p = (56/3) 6.43 = 10. The mechanical phase span, 3, can be calculated by (3) F 3- F = (3) By setting q = 1 and cnsidering nly ne-half f the mechanical shift in (), the inner distances between the phases can be reduced and it is calculated t be 1 τ p θ 3. This will require reversing the winding terminals. As reducing the distance between the phases will be superir t the interactin trque, the effect f interactin mutual flux between the phases will rise and cnsidering the phases t behave independently, will nt be t a certain aspect crrect. TFM with distributed windings arund the circumference can be cnstructed nly with a certain number f ples in the rtr in rder t cnfirm the mechanical shift with the balance f the cnstructin. The great advance in pwer electrnic devices technlgy and cntrl schemes allw us t design efficient pwer supplies with different phase shifts e.g., f 30 r 60, thus makes the cnstructin f TFM with distributed windings with mechanical shift ther than 10 pssible. Fig. 1.b shws the allwable number f ples suitable fr building TFM with distributed windings arund the circumference in relatin t number f statr ple number n each layer fr each phase. This figure has been cnducting with cnsidering the mechanical distances between phases fr q =. θ = (1) 3 10 P P = number f ples In rder t achieve sufficient areas between the phases arund the statr, a quantity f multiple f τ p will be added t θ. Therefre; the mechanical displacement between the 3 statr phases fr full winding arrangement, 3, can be calculated as in (). 3 = θ 3 + q τp () q = 0,, 4, 6,..., P 1.a. 1.b. Figure 1. Three-phase FC-TFM with full distributed windings 1.a. Machine structure 1.b. PMs number selectin 103

3 In rder t cnsider the mechanical balance f the machine, each statr phase has been divided int tw parts and placed ppsite each ther s that when ne part f phase is feeding the machine, its facing part will als be n peratin, therefre, this will emphasise the axial mechanical stress balance symmetrically. This scheme fr lcating split phases arund the statr is shwn in Fig..a fr TFM with 56 ples. The same methd that was carried ut with full winding distributin, thrugh placing the windings arund the statr, is applied here with taking int cnsideratin a distance f 4/3 τ p between the split phases, i.e., half the distance f distributed full winding structure as shwn in (4). 1 = θ + τ q q 3-S 3 p q =, 4, 6,..., The mechanical shift is cnsidered t be half that f the full three-phase windings, i.e., 1/3 τ p. In this way, the phases can be supplied with reverse currents. Therefre, the phase sequence will be as each phase lags the ther by 10 fr full distributed winding machine, while reverse rder f phases will be expected fr the split phase TFM. This can definitely be explained in terms f the mechanical shift, since 1τ p + 1/3 τ p crrespnds t electrically 40 and similarly τ p + /3 τ p crrespnds t electrically 10 i.e., the phase shift is f 10 electrical degrees. The suitable ple number fr split phase three-phase FC-TFM is shwn in Fig..b each phase is divided int tw equal parts. The actual mechanical phase span fr each phase part can be calculated by (5) q 3 -S 3- S = (5) 3q The same rtr is used fr tw different statr structures with full distributed and split distributed windings. The mst difficult part t be cnstructed is the rtr since the permanent magnets are difficult t be fixed arund the frame. Hwever, the cnstructin f statr is simplified by making use f SMC material, which permits flexible machine design and features additinally, very lw eddy current lss and pssibilities t imprve thermal characteristics. Despite the lw permeability f the SMC material, it is cnsidered t be mst apprpriate P (4) PEDS009 fr PM-machines as the magnetic reluctance f the magnet dminates the magnetic circuit; hence, the mtr will be insensitive t the permeability f the cre [15]. III. OTHER POSSIBLE ALTERNATIVE STRUCTURES Tw-phase TFM can as well be cnstructed with distributed full and split windings arund the statr. Fig. 3.a shws tw-phase TFMs with full windings f 56 ples viewed as a tp sectin. Apparently, the tw-phase TFM with full windings distributin will exhibit additinal nise prblems since there is mechanical instability, due t unequal mechanical distance between the tw phases fr any chice f P. The mechanical shift fr -phase winding is 50% τ p and can be calculated by (6). = (6) θ 90 P The unequal mechanical distances between the phases can be calculated by using (7) and (8). = θ + q τ (7) 1 p = q τ θ (8) q p = 0, 1,,..., P The mechanical phase span,, can be btained frm (9). ( ) = (9) In Fig. 3.b, the pssible number f statr ple pitch pairs fr each phase has been calculated fr different ple numbers. By cmparing Fig. 1.b and Fig. 3.b, the suitable rtr ple number fr 3-phase and -phase windings cnfiguratin, respectively, are nt the same. Obviusly, the ple number arrangement, which has been applied fr -phase machine can nt be applied fr the 3-phase machine and vice versa. Dividing the tw phases int parts will eliminate the mechanical instability and insure equal distances between all.a..b. Figure. Three-phase FC-TFM with split distributed windings.a. Machine structure.b. PMs number selectin 3.a. 3.b. Figure 3. Tw-phase FC-TFM with full distributed windings 3.a. Machine structure 3.b. PMs number selectin 104

4 the phases divisins. The split -phase distributed windings cnstructin is shwn as tp view f 58 ples in Fig. 4.a, each phase is divided int tw parts. The suitable number f PMs fr this cnstructin is shwn in Fig. 4.b The distances between the phases have been chsen t allw feasible spaces between the windings as fund by (10). -S= θ + q τp (10) PEDS009 q = 0, 1,,..., P The phase span fr each part f -phase machine can be fund ut by applying (11). This equatin assumes that each phase f the tw phases has been divided int tw parts S -S = (11) 4 SPM-TFM can as well be cnstructed with distributed windings. Fig. 5 shws 3-phase SPM-TFM with full and split distributed windings. The end-windings will add a significant effect t the leakage inductance f SPM-TFM. Nte that the red and green parts n the rtr represent the PMs, f ppsite magnetisatin directin, which are placed next t each ther in a circle. The magnetisatin directin f the PMs in SPM-TFM is in axial directin, while in FC-TFM is in tangential directin. Rtating magnets can be mdelled with a cmplex current sheet and using the cnvectivediffusin equatin fr the translatin mtin, thrugh which a fast steady-state mdel is btained [16]. IV. CONSTRUCTION OF SMALL FC-TFM A. Test Operatin In rder t insure the peratin f the machine, tw small FC-TFM machines have been simulated with number f ples f 80. The best selectin f the number f ples will be stated thrugh running several finite element (FE) simulatins and examining the trque density prductivity thrugh calculating the trque cnstant [13]. In rder t achieve strng magnetic fields fr small machine vlume and weight, rare earth permanent magnets are used (e.g. NdFeB) in the designs. Fig. 6 and Fig. 7 shw tw 5.a. 5.b. Figure 5. Three-phase SPM-TFM with full and split distributed windings 5.a. Full winding 5.b. Split winding different FC-TFMs with full and split distributed windings, respectively. B. FEM Investigatin The tw mtrs underg 3D magnetstatic simulatins. Flux density distributins are displayed in Fig. 8 fr bth the machines, while the rtr at a mechanical psitin f 50% f τ p. The phases are excited with 3-phase currents in each mtr. Figure 6. Three-phase FC-TFM with full distributed windings 4.a. 4.b. Figure 4. Tw-phase FC-TFM with split distributed windings 4.a. Machine structure 4.b. PMs number selectin Figure 7. Three-phase FC-TFM with split distributed windings 105

5 PEDS009 Full phase span = 1τ p Split phase span = 6τ p Three full phases excited with 3-phase currents Three split phases excited with 3-phase currents 8.a. 8.b. Figure 8. FE-simulatin f three-phase FC-TFM with full and split distributed windings 8.a. Full winding 8.b. Split winding The split phase cnstructin perated with the same electric lading that is applied t full winding machine. Fig.8.a shws flux density fr full winding variant and Fig.8.b fr split winding case. The flux density f the statr ples in ne phase will be f a higher value than thse f the ther tw phases since the peak value f the magnetic field f the current fr this phase that supprts the magnetic field f the PMs presents at the mechanical psitin f 0.5τ p. Cgging trque due t attractin frces between active PMs and irn parts in the statr, reluctance trque, due t saliency facing the air gap, interactin trque due t interactin between the magnetic fields f PMs and armature current are the trque cmpnents f the TFM, which are shwn in Fig.9. Fig. 9.a shws the average trque fr full winding structure is almst the same as its crrespnding value fr split phase structure (.75 Nm), which is demnstrated in Fig. 9.b. This is bviusly the result, because the same electric lading f full winding case is applied in split winding case study. Other simulatins have been carried ut fr nly τ p fr each phase with Flux3D Sftware frm Cedrat [17]. Since the sftware will calculate the trque fr nly τ p and then it will apply a factr f peridicity, it is pssible then t divide the Average Trque 9.a. 9.b. Figure 9. Trque cmpnents f 3-phase FC-TFM with full and split distributed windings 9.a. Full winding 9.b. Split winding 10.a. 10.b. Figure 10. FE- simulatin f three phases individually including inbetween distances f FC-TFM with full distributed 3-phase windings 10.a. Output trques btained directly frm Flux3D simulatins 10.b. Output trques derived after mathematical manipulatins utput trque frm simulatin by number f ple pairs t get a resultant trque fr nly τ p. Fig.10.a. plts the trque cmpnents ver 1τ p f ne phase fr cmplete 80 ples as the machine has n distances between the phases. Since each phase in full winding structure cvers 4τ p, therefre, the trque prduced frm simulating a peridical TFM f τ p will be scaled by a factr f 1/40 t get the trque fr each phase. This prcess is repeated fr each phase, taking int accunt the mechanical phase shift between the phases. The simulatins f in-between distances have als been taken int accunt and pltted in Fig.10.a; hwever, it shuld be scaled. Adding the trques fr the three phases including the trques due t distances between the phases after scaling gives average trque f 3.5 Nm, which appears t be similar t that generated fr the whle machine simulatin as shwn in Fig. 9. It is wrth t be mentined here that the distances between the phases have n big influence n the generated utput trque, which can be clearly deduced frm Fig.10.b. The effect f these distances n the trque is nly additin f 106

6 1% f peak value f ttal average trque. As it shwn the summatin f phases trques and the summatin f the phases trques with the in-between phases trques are almst f similar values. The interactin phase trque cmpnents are calculated and pltted in Fig. 10.b. V. FC-TFM FOR IN-WHEEL MOTOR PEDS009 Tp view f τ p Statr prtin f τ p Magnetisatin directin (Tangential directin) Rtr prtin f τ p A. Cnstructin The small TFM that has already been displayed in Fig. 6 is re-designed fr a bigger size f TFM suitable fr munting as in-wheel mtr. The machine is cnstructed f a material f linear BH characteristics and has a ttal diameter f 40 mm, D ut, τ p f 11.7 mm and an air gap length f 0.7 mm. The machine is shwn in Fig. 11.a. Cmputer memry f 8 GB will nt facilitate the simulatin f the whle machine. Therefre, nly τ p -simulatins have been cnducted and the mathematical peratins have been applied t get the utput trque f the whle machine. Tw ple pitch segment is shwn in Fig. 11.b. B. FEM Investigatin The flux density distributin f τ p is shwn in Fig. 1. The machine is excited with 3-phase sinusidal current. The statr ples face the rtr ples at a psitin, the current value has been kept t small as it is shwn in Fig. 1.a. At this psitin the statr ples reach high values f flux density f.1 T. The maximum current is applied after a mechanical displacement f 0.5τ p. At the psitin f the high current that appears in Fig. 1.b, the statr ples flux density reaches values arund 3.0 T. The shes f statr ples are f trapezidal shape t increase the statr ple area facing the air gap which causes a remarkable increase in statr cre back flux and that will in cnsequence imprves the trque cnstant, K T, which is a functin f peak value f armature current, I Peak, csine f trque angle, cs ( θ t ) and average trque, T av, given by (1) [10]. Tav K T = (1) I Peak cs( θt ) r in ther frmat, p K T = λ mpm = p( p Φ) Kl (13) p = number f ple pairs; Φ = flux per ple; K l = leakage flux cefficient. The trque cnstant is a factr which directly reflects the trque prductin capability determined by PM flux and gemetry f mtr. The value f K l is rati f flux entering the bttm part f the C-cre and the flux in the air gap per ple. It is basically a functin f the number f ple pairs and gemetry f the mtr. An interesting remark in this equatin is that if the ple number has predminant effect n the trque prductin capability then the trque density will increase with the increasing ple number and if K l has the predminant effect, then increasing the ple number will result 11.a. Half axial length 11.b. Figure 11. Three-phase FC-TFM as in-wheel mtr 11.a. Cmplete cnstructin 11.b. Tw ple pitch n decrease n K l, therefre, reductin n average trque will be a cnsequence result. Thus, a specific number f ple exists which will allw maximum trque prductin capability fr a given vlume f a machine. Nte this value shuld als match thse values f ple number that appears n Fig. 1.b fr 3- phase full winding cnstructin. Magnetstatic simulatin has been extended t btain the trque prduced by the τ p segment as shwn in Fig. 13. The trque cmpnents ver 1τ p perid are pltted in Fig. 13.a. These trques cmpnents are pltted as there are n distances between the phases and as the machine is a peridical machine and symmetrical arund the axial axis. The trque cmpnents are calculated fr ne segment f τ p. After applying numerical scaling and eliminating the effect f the distances between the statr phases, Fig. 13.b is btained. The interactin phase trque is f sinusidal wavefrm. The interactin phases trques are shwn, each phase winding cvers mechanically 16τ p. The average 3-phase trque is calculated t be 37 Nm as shwn in Fig. 13.b. This value fund t be almst the same as when split winding structure is utilized. Nte that that x-axis 1.a. 1.b. Figure 1. Flux density distributin f τ p f 3-phase FC-TFM with full distributed windings as in-wheel mtr 1.a. Flux density at n current psitin 1.b. Flux density at maximum current psitin 40mm 107

7 Fllwing mathematical scaling applied t the results f these simulatins, the trque wavefrm fr the whle machine can be derived. Such apprach will reduce the simulatin run time and therefre, it has efficiently been used t get the utput trque wavefrm fr the big-sized in-wheel mtr. It is recgnized thrugh simulatin f the distances between the phases that they will nt have a big influence n the value f the average dynamic trque. Therefre, the inter-distances between the phases are suggested t be cnstructed f nn- PEDS009 magnetic material such as aluminium and might be utilized t lcate the cling system f the machine. ACKNOWLEDGMENT The authrs wuld like t thank Gustav Matthies and the wrkshp in IALB at University f Bremen, Germany fr the suggested ideas t imprve the verall munting and cnstructin f the machine. Als, the authrs wuld gratefully like t thank Cedrat fr the use f their FEM sftware Flux3D. 13.a. 13.b. Figure 13. Trque f three-phase FC-TFM with full distributed windings as in-wheel mtr 13.a. Trque cmpnents f peridical τ p machine (withut in-between distances) 13.b. Interactin trque wavefrms f full winding TFM (with in-between distances) in the figures is pltted as a percentage f the ple pitch which is labelled as tau. VI. CONCLUSION Tw different structures f 3-phase FC-TFM based n distributed windings have been described and mdelled via FEM. Other different pssible structures have been pinted ut such as -phase FC-TFM and 3-phase SPM-TFM with full winding and split phase structures. Thrugh lcating the windings arund the statr, the axial length f the machine will be reduced and as a cnsequence, the trque density can be increased. The mechanical stability can be assured by splitting the phase int several segments; as the same trque will be achieved by full winding cnstructin as well as by split winding structure. Since it is difficult t simulate a whle structure f a big machine, therefre, a small scaled machine has been develped and simulated. It has been verified that Flux3D sftware can be used t simulate big-sized machines f nn-peridical structures. The trque prduced frm the whle structure can be deduced via simulating nly tw ple pitches, and a distance between the phases. REFERENCES [1] L. Chang, Cmparisn f AC drives fr electric vehicles A reprt n experts pinin survey, IEEE AES Systems Magazine, Vl. 9, Issue 8, pp. 7-11, Aug [] N. Hashemnia and B. Asaei, Cmparative study f using different electric mtrs in the electric vehicles, Prceedings f the 008 Internatinal Cnference n Electrical Machines, pp , Sept [3] M. Terashima, T. Ashikaga, T. Mizun, K. Natri, N. Fujiwara and M. Yada, Nvel mtrs and cntrllers fr high-perfrmance electric vehicle with fur in-wheel mtrs, IEEE Transactins n Industrial Electrnics, Vl. 44, N. 1, pp.8-37, Feb [4] K. Ohyama, M.N.F., Nashed, K., As, H. Fujii and H. 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