An Improved Fractional Slot Concentrated Winding for Low-Poles Induction Machines

Transcription

1 An Improved Fractional Slot Concentrated Winding for Lo-Poles Induction Machines G. Dajaku, S. Spas, Xh. Dajaku, and D. Gerling Φ Abstract Recent investigations on IMs ith FSCWs sho that, due to the higher MMF harmonic content of concentrated indings, this machine type is characterized by lo torque capability and quality, as ell as, lo efficiency and thermal problems. This paper presents a ne fundamental ave FSCW ith sinusoidal MMF aveform and a non-overlapping inding arrangement. The proposed inding solution consists of to different FSCW types and a dual slot-layer stator structure. The first inding is located on the outer slot-layer region, hile the second one is located on the inner slot-layer. Additionally, both indings are shifted in space to each other for a specific angle. This inding configuration has several advantages like high quality MMF ave ith lo harmonic contents, shorter end-inding length, and applicability to lo poles IMs. Obtained results from the analysis of different IMs sho that the proposed FSCW provides high performances in different application areas. Index Terms Fractional slots concentrated inding, dual slot-layer stator, lo harmonic content, induction machine. U I. INTRODUCTION NTIL NOW, three-phase induction machines commonly use single or double layer, overlapping, distributed indings. This inding configuration results in more sinusoidal magneto-motive force (MMF) distribution, and hence, good machine performance. Figs. and illustrates to full-pitch single layer (SL) distributed indings ith different number of coils/pole/phase (qparameter), hile Fig. c) compares their corresponding MMF characteristics. As ell can be seen here, the MMF aveform quality depends strongly on q, hereby ith increasing q the MMF aveform becomes more sinusoidal. Hoever, the inding complexity and manufacturing process also increase. Further drabacks of this inding type are the large end-inding length, lo slot filling factor that results to higher Ohmic losses, packaging problems, and so on []. On the other side, fractional slot concentrated indings (FSCWs) are becoming more and more attractive solution for lo cost and high torque density applications. The use of FSCWs offers simple and full automatized manufacturing, shorter end-indings and consequently a reduction of the eight/torque ratio of the machine []-[5]. For many years, the FSCW are commonly used for permanent magnet (PM) machines, hoever recently the interest on this inding type is groing up also for other electric machine types, such as G. Dajaku is Senior Scientist ith FEAAM GmbH, D Neubiberg, Germany ( gurakuq.dajaku@unib.de). S. Spas and Xh. Dajaku are ith the Institute for Electrical Drives, Universitaet der Bundesehr Muenchen, D Neubiberg, Germany ( sachar.spas@unib.de, xhevat.dajaku@unib.de). D. Gerling is Full Professor at the Universitaet der Bundesehr Muenchen, Institute for Electrical Drives, D Neubiberg, Germany ( dieter.gerling@unib.de). synchronous reluctance machines (SynRM) [6, 7] and induction machines (IMs) [8]-[9]. While FSCWs are advantageously concerning manufacturing, packaging and torque density, they have the draback of a high harmonic content in the MMF ave. These harmonic components spread ith different speeds in opposite directions. As result, an inductive rotor ill produce oppositional forces ith a poor net torque and high rotor bar losses. In [8]-[], performances of different IMs ith FSCWs in the stator and a squirrel cage in the rotor are considered. Presented results sho poor performances mainly due to the MMF harmonics hich cause parasitic torque and high losses. To minimize the interaction of MMF harmonics ith the rotor, in [, ] a multi-layer tooth ound rotor is used as selector, so only the operating harmonic ave is effective. Other approaches to improve the performance of the IMs ith FSCWs have been addressed in [3]-[9]. In [3]-[5] the high harmonics are reduced by a special design of each stator coil of the inding. Reference [6] uses a doublesided FSCW stator arrangement ith a mechanical offset beteen the to stator components to suppress some highorder MMF harmonics. Furthermore, a dual slot layer stator construction in combination ith a multi-layer inding to minimize the space harmonics of an IM is presented in [7]. In addition to the three-phase solutions, reference [8] presents design considerations and trade-offs involved in applying FSCWs to multi-phase IMs. c) -.5 q = q = q = q = Harmonic Order Fig.. Winding layouts,. SL distributed inding ith q=,. SL distributed inding ith q=3, c). Corresponding MMF characteristics /6/$3. 6 IEEE 6

2 Of course, as ell mentioned earlier the IMs ith cage rotor are very sensitive on the air-gap flux density harmonics. For high efficiency applications this machine type requires an optimized inding ith high quality MMF distribution. Hoever, this demand is not fulfilled completely ith above solutions since their MMF spectrum still include some high-order space harmonics, despite the enormous improvements performed by them. Hence, their efficiency still to be lo compared ith the conventional IMs. In [9], a ne FSCW ith sinusoidal MMF aveform, but ith lo inding factor for the fundamental ave ( =6%) is investigated. Presented results sho high performances for high voltage IMs, hereby the lo inding factor has been compensated by increasing the stack length as ell as the slot filling factor. In this paper, an alternative FSCW ith sinusoidal MMF aveform and ith improved inding factor for the fundamental ave is presented. The ne inding design consists of to different FSCW systems that are integrated in a dual slot-layer stator design. Using this stator structure in combination ith the ne inding configuration the inding factor for the fundamental ave is increased for about 4% compared ith solution presented in [9]. To prove the functionality of this inding type, to IMs for different applications are investigated. Obtained results sho high performances for both machines types. II. FUNDAMENTAL WAVE FSCWS The use of FSCWs allos a high number of pole/slot combinations []-[5]. Until no these inding types have been utilized mainly in PM machines. A common FSCW configuration is the ell knon 3-teeth/-poles combination illustrated in Fig.. Due to the lo number of slots per pole and per phase it is characterized by a high MMF space harmonic content. The operating ave of this inding type is the st MMF harmonic (fundamental harmonic). To other inding types that operate also ith the fundamental harmonic are presented in Figs. and c). The second inding topology is the 6-teeth/-poles three-phase FSCW, hile the third one is the -teeth/-poles inding presented in [9], hich internally is a six-phase inding, hoever using the star&delta combination it could be supplied from a three-phase current system. The inding function can be ritten as, m ( S ) Θ ( S, t) N I e j ωt φ φ = () π +,, + 4, 5,... for 3T/P = +, 5, + 7,,... for 6T/P and T/P here, m is the number of phases, N the total number of turns per phase, I the phase current, and the inding factor for the v MMF space harmonic. For the first and the second inding, e have, π = sin () QS Hoever, for the third inding type connected in star&delta combination, the resulting inding factor is π N,Y,Y + ( N, Δ / 3), Δ cos ( ) = (3) N here,,y and, Δ are the corresponding inding factors for the star and the delta inding, respectively, hile N is the resulting total number of turns per phase. π = =,Y, Δ sin (4) N = N + N Δ / 3 (5),Y, Figs. to c) sho also the MMF characteristics of considered FSCWs. It can be concluded here that, ith increasing number of stator slots the MMF aveform becomes more sinusoidal, hoever the inding factor for the fundamental ave decrease. According to eqs. () to (4), the corresponding inding factors for the st MMF harmonic are 86.6%, 5%, and 5.8%, respectively. [p ] c) Space Harmonics Fig.. Fundamental ave FSCWs ith corresponding MMF characteristics,. 3-teeth/-poles,. 6-teeth/-poles, c). -teeth/-poles Space Harmonics Space Harmonics 7

3 III. PROPOSED FSCW CONFIGURATION The proposed ne FSCW is designed to provide a to pole fundamental MMF distribution to match the original three-phase double layer distributed inding ith q=. According to [], the ne inding topology consists of to inding systems that can be connected as star or star&delta combination. The first inding system has six concentrated coils hich are ound around six stator teeth, hile the second inding system has telve concentrated coils ound around telve stator teeth. In addition to that, the stator core has a dual slot-layer structure ith six slots at the outer region and additionally telve slots beside the air-gap region, Fig. 3. The inding layout for the first and the second inding system are different. The first FSCW is analogous ith the 6-teeth/-poles inding illustrated in the previous Fig., hile the second inding is the - teeth/-poles inding topology (index and denotes the first and the second inding system, respectively). In the exemplary inding topology given in Fig. 3 the first and the second inding are shifted in space for degrees and connected in star&delta combination, ith the second inding in delta, hile ith the first inding connected beteen the three-phase inverter and the delta terminal. Hoever, for only star or delta connection the mechanical shifting angle should be 9 degrees. A. Winding MMF Analysis Analogous to the previous inding analysis, the inding function for the proposed inding can be ritten as, m j( ωt φs ) Θ ( φs, t) = N I e π (6) = +, 5, + 7,,... For star&delta combination, the functions for the resulting inding factor and the total number of turns per phase are here, N = + ( N / 3) N (7) N = N + N / 3 (8) π = sin 6 π sin = In the above eqs. (7) to (9), ith N and N are denoted the number of turns per phase for the respective inding systems, hile and are the corresponding inding factors for the first Y and the second Δ inding, respectively. Otherise, for only star or delta connection, e have, π N + N sin 3 = () N (9) N = N + N () The MMF inding characteristics are very sensitive to the N /N turn ratio. Fig. presents the MMF aveform, as ell as, its corresponding space harmonics for the optimized case, hile Fig. 4 shos the variation of inding factors for the st (fundamental), the 5 th and 7 th harmonics. As ell mentioned earlier, the IMs are very sensitive to high-order MMF harmonics, therefore the reduction of these harmonics, such as the 5 th and 7 th, is of main interests. As can be seen from Fig.4, for the turn ration of.464 these higher harmonics has been completely canceled, hile the inding factor for the fundamental ave is about 36%. Thus, compared ith the reference FSCW presented in [9], ith proposed inding the inding factor of the operating ave is increased for about 4%, hile the MMF aveform quality has remained the same Harmonic Order Fig. 3.. Stator inding layout of the proposed FSCW [],. MMF characteristics for N /N =.464. Winidng factor [ % ] v= v=5 v= N /N [--] Phase-A Phase-A Fig. 4. Variation of inding factors as function of turns ratio N /N. 8

4 B. Modular Design Fig. 5 illustrates one possible method to realize the ne machine design using pre-ound coils. The stator structure could be divided into to modules; in the outer stator module ith six straight teeth, and the inner module hich has double of teeth and closed slot openings. Therefore, in the first step the pre-ound coils could be inset radially in the module and, then in the second step the both ounded modules have to be combined ith each other. IV. EXEMPLARY INDUCTION MACHINES In this section, the proposed FSCW has been applied on to IM designs for different application fields. The first machine design is a 6-poles high torque traction IM, hile the second machine is a -poles IM for high voltage applications and ith direct-on-line run-up capability. For the traction machine design a common aluminium cage rotor has been used, hile for the high-voltage IM a special copper cage rotor construction ith multi end-rings [] is considered. The higher MMF harmonics effect, e.g. th and 3 th, is decreased by selecting a proper number of rotor bars according to the relation for the rotor coupling factor, π ζr = sip QR here Q R is the number of rotor slots (bars). () With Desgn- and Design- are noted the first and the second IM, respectively. Table- shos the main specifications for the required machine torque, speed, and the supply voltage, and Table- gives the main design constraints. The performances of both machines have been investigated ith finite elements method (FEM). A. Six-Poles Traction IM According to given targets in Table-, the traction machine should be able to provide a high continuous torque of.5knm at 8rpm (base speed), and also to provide a constant poer for the higher speed (up to 3 rpm). Considering further the geometry constraints given in Table- the design and the optimization progression sho that the six-pole machine design ill be the optimal solution. Further, the rotor slot shape and the number of rotor slots have been optimized carefully to achieve desired torque and higher efficiency. To minimize the harmonic effect on the rotor bar losses the number of rotor slots (bars) has been selected to be 35, here the coupling factors for the th and 3 th are 6% and %, respectively. It is important to note that the presented results in folloing are obtained for the nonskeed rotor. Of course, the machine performances concerning the torque ripples and the rotor cage losses can be further improved by a proper skeed rotor design. Fig. 6 shos the machine s geometry ith field results at nominal operation point for the optimized traction IM. To prove the machine s overload capability, the torque and the efficiency results vs. rotor slip are presented in Fig. 7. Further Table-: Requirements Design- Design- Nominal torque,5 knm,45 knm Nominal speed 8 rpm 3 rpm Maximal speed 3rpm 3 rpm Voltage 4 V DC 6 kv Table-: The main machine specifications Design- Design- Active length 35 mm 648 mm Outer stator diameter 5 mm 695 mm Air gap length.5 mm 4. mm Number of stator slots 8 & 36 6 & Number of rotor slots Number of pole-pairs 3 Rotor material Aluminium Copper Rotor type Conv.cage rotor Multi end-ring cage rotor + Fig. 5. Modular design ith pre-ound coils. Fig. 6.. Traction IM geometry ith flux density lines at nominal operation speed,. Stator geometry sketch in 3D to visualize the end-turns length. 9

5 the effect of the rotor slot number on toque ripples is investigated in Fig.8. Based on the obtained results it can be concluded, that the proposed FSCW provide a high torque capability and high efficiency, as ell as, lo torque ripples. At nominal operation point machine s efficiency is 95%, hile the torque ripple lies by 3%. B. High Voltage To-Poles IM Different from the first application field, the second IM has been designed for high voltage application, constant operation speed, and for direct-on-line run-up capability. Considering that, the machine should run at 3 rpm by the fixed 5Hz grid frequency, a to-poles machine has been considered for the analysis. Also here a proper number of rotor bars is selected to minimize rotor cage losses resulting from the th and 3 th harmonics. Thus, in the first optimization phase the number of rotor bars is taken to be (Q R =), since ith this selection, according to () the th component can be completely cancelled, hile the coupling factor for the 3 th harmonic is 4%. Hoever, to achieve an optimal rotor core structure under the given geometry constraints the number of the rotor slots needs to be higher. Therefore for the final optimized rotor the number of rotor slots is selected to be 33 (n R x ), here according to [] a multi end-ring rotor cage construction has been applied to obtain the optimal rotor design ith the minimal cage losses. Fig. 9 shos the geometry for the to-poles ASM, as ell as, the flux density lines for the nominal operation point, the multi-cage rotor construction is illustrated in Fig. 9, hoever, the torque response results are given Fig.. Obtained results for the 6kV, 5Hz and 985 rpm under given load conditions are shon in Table-3. From obtained results it can be concluded that the proposed FSCW is very efficient also for high voltage IMs. Multi-cage Rotor: Three End-rings Conv. End-ring Fig. 9.. The IM geometry ith the flux density lines at the nominal operation speed,. Multi end-ring rotor design []. Fig.. Torque response for 6kV, 5Hz, and 985 rpm operation point. Fig. 7. Machine results for the torque/efficiency vs. rotor slip. Table-3: Simulation results for the to-poles IM Speed Torque 985 rpm,46 knm Torque ripple,9 % Iron losses 5,98 kw Copper losses, stator,6 kw Copper losses, rotor 5,98 kw Efficiency (including mechanical 96,7 % losses) Fig. 8. Torque ripples results for different number of rotor slots. V. CONCLUSIONS A FSCW ith high quality MMF aveform and ith improved inding factor for the operating ave, suitable for lo-poles induction machines ith squirrel cage rotor, has been presented. The proposed inding design consists of

6 Poered by TCPDF (.tcpdf.org) to different FSCW systems that are integrated in a dual slot-layer stator configuration. The first inding type is the common 6-teeth/-poles FSCW that operate ith the fundamental harmonic, hile the second inding is the - teeth/-poles inding that is a idely used inding for PM machines. In the proposed inding configuration the second inding has been used to compensate (eliminate) the 5 th and the 7 th MMF harmonics resulting from the 6-teeth/-poles inding. Thus, ith presented combination the most unanted higher harmonics up to the th are completely cancelled. Hence, its MMF aveform is very sinusoidal. To IMs ith proposed FSCW have been analyzed for different applications. The first IM is designed for high torque capability in traction applications, hile the second IM is designed for lo-poles and high voltage applications. For both machine types a proper number of slot bars have been selected to minimize the rotor cage losses resulting from some specific higher MMF harmonics. Based on the obtained results it can be concluded that the proposed FSCW provide a high torque capability and a high efficiency, as ell as, lo torque ripples, and is suitable for different application areas. It is important to note that the presented simulations are performed for non-skeed rotor, hoever better performances could be achieved by applying an optimized skeed rotor design. VI. REFERENCES [] J. Li, D. W. Choi, et al., Effects of MMF Harmonics on Rotor Eddy- Current Losses for Inner-Rotor Fractional Slot Axial Flux Permanent Magnet Synchronous Machines, IEEE Transactions on Magnetics, Vol. 48, No., pp , Feb.. [] F. Meier, J. Soulard, PMSMs ith Non-Overlapping Concentrated Windings: Design Guidelines and Model References, Ecologic Vehicles-Reneable Energy, Monaco, March 9. [3] J. Cros, P. Viarouge, "Synthesis of High Performance PM Motors ith Concentrated Windings," IEEE Transactions on Energy Conversion, Vol. 7, pp , June. [4] D. Ishak, Z.Q. Zhu and D. Hoe, Comparison of PM brushless motors, ith either all or alternative ound teeth, IEEE Trans. on Energy Conversion, Vol., No., pp.95-3, 6. [5] C. Shi-Uk et al., Fractional slot concentrated inding permanent magnet synchronous machine ith consequent pole rotor for lo speed direct drive, IEEE Transactions on. Magnetics, Vol. 48, No., pp , Nov.. [6] C. M. Spargo, B. C. Mecro, J. D. Widmer, Application of Fractional Slot Concentrated Windings to Synchronous Reluctance Machines, IEEE International Electric Machines and Drives Conference, IEMDC3, pp.68-65, Chicago, USA, May -5, 3. [7] Z. Azar, Z. Q., Investigation of Electromagnetic Performance of Salient-Pole Synchronous Reluctance Machines Having Different Concentrated Winding Connections, IEEE International Electric Machines and Drives Conference, IEMDC3, pp , Chicago, USA, May -5, 3. [8] A.M. El-Refaie, M. R. Shah, Comparison of Induction MachinePerformance ith Distributed and Fractional-Slot Concentrated Windings, in Proc. IEEE Industry Applications Society Annual Meeting, 8. IAS '8, pp. - 8, 5-9 Oct. 8. [9] T. Gundogdu, G. Komurgoz, B. Mantar, Implementation of fractional slot concentrated indings to Induction Machines, The 7 th International Conference on Poer Electronics, Machines and Drives (PEMD 4), Manchenster, UK, 8- April 4. [] J. P. Bacher, A. Mütze, Comparison of an induction machine ith both conventionally distributed and fractional-slot concentrated stator indings, e & i Elektrotechnik und Informationstechnik, February 5, Volume 3, Issue, pp [] Alberti, L., Bianchi, N., "Design and tests on a fractional-slot induction machine," Proc. IEEE Energy Conversion Congress and Exposition (ECCE), pp.66,7, 5- Sept.. [] F. Eastham, T. Cox, P. Leonard, and J. Proverbs, Linear induction motors ith modular inding primaries and ound rotor secondaries, IEEE Trans. Magn., vol. 44, no., pp , Nov. 8. [3] O. Moros, Gerling, D., Ne 3-Teeth / 4-Poles Concentrated Winding for Use in Induction Machines, 4. Tagung Elektrische Antriebstechnologie für Hybrid- und Elektrofahrzeuge, Haus der Technik, Vol. 3, pp.35-36, November 3, Muenchen, Germany. [4] Patzak, D. Gerling, Design of an Automotive 48 V Integrated Starter-Generator on the Basis of an Induction Machine ith Concentrated Windings, 7 th International Conference on Electrical Machines and Systems (ICEMS-4),.-5. October 4, pp , Hangzhou, China. [5] M.V. Cistelecan, F.J. Ferreira, M.Popescu, Three phase toothconcentrated multiple-layer fractional indings ith lo space harmonic content, Proc. IEEE Energy Conversion Congress and Exposition (ECCE),, pp.399, 45, -6 Sept.. [6] J. F. Eastham, T. Cox, and J. Proverbs, Application of planar modular indings to linear induction motors by harmonic cancellation, IET Elect. Poer Appl., vol. 4, no. 3, pp , Mar.. [7] V. M. Sundaram, H. A. Toliyat, A Fractional Slot Concentrated Winding (FSCW) Configuration for Outer Rotor Squirrel Cage Induction Motors, IEEE International Electric Machines and Drives Conference (IEMDC-5), pp. -6,.-3. May 5, Coeur d'alene (ID), USA. [8] A. S. Abdel-Khalik, S. Ahmed, Performance Evaluation of a Five- Phase Modular Winding Induction Machine, IEEE Transactions on Industrial Electronics, Vol. 59, No. 6, pp , June. [9] O. Moros, G. Dajaku et al., Ne High Voltage -Pole Concentrated Winding and Corresponding Rotor Design for Induction Machines, The 4 th Annual Conference of the IEEE Industrial Electronics Society (IECON-5), Yokohama, Japan, November, 5. [] G. Dajaku, Elektrische Maschine, German patent application No [] C.E. Linkous, High Efficiency Induction Motor ith Multi-Cage Rotor, United States Patent, US , 976. VII. BIOGRAPHIES Gurakuq Dajaku as born in 974 in Skenderaj, Kosova. He received the diploma degree in electrical engineering from the University of Prishtina, Kosova, in 997 and the Ph.D. degree from the Universitaet der Bundesehr Muenchen, Munich, Germany, in 6. Since 7 he has been a Senior Scientist ith FEAAM GmbH, an engineering company in the field of electric drives. Since 8 and he has been a Lecturer at the Universitaet der Bundesehr Muenchen, Germany, and the University of Prishtina, Kosova, respectively. His research interest is in the field of electrical machines and drives. He has published numerous technical papers in different IEEE journals and conferences and has several international patents and patent pending applications. Dr. Dajaku received the Rheinmetall Foundation Aard 6 and the ITIS (Institute for Technical Intelligent Systems) Research Aard 6. Sachar Spas Sachar Spas as born in Lviv, Ukraine, in 988. He got his MSc degree in Mathematical Engineering (Mechatronics) from the University of Federal Defense Munich, Germany in. Since he is a Research Scientist ith FEAAM GmbH and since 5 he is leading a research group on Modeling and Control of Electrical Drives at the Institute of Electrical Drives, Universitaet der Bundesehr Muenchen, Germany. Xhevat Dajaku as born in Skenderaj, Kosova, in 967. He graduated, in Electrical Engineering from the University of Applied Science in Mitrovica, Kosova in 7 ith the diploma degree. Since 8 he orks at the Universitaet der Bundesehr Muenchen, Munich, Germany. His research interests include design and analysis of special electric machines. Dieter Gerling as born in 96 in Menden/Sauerland, Germany. He received the diploma and Ph.D. degrees in electrical engineering from the Technical University of Aachen, Aachen, Germany, in 986 and 99, respectively. From 986 to 999, he as ith Philips Research Laboratories, Aachen, as Research Scientist and later as Senior Scientist. In 999, he joined Robert Bosch GmbH, Bühl, Germany, as Director. Since, he has been a Full Professor at the Universitaet der Bundesehr Muenchen, Munich, Germany.