A Novel 24-Slots/10-Poles Winding Topology for Electric Machines

International Electric Machines and Drives A Novel 24-Slots/10-Poles Winding Topology for Electric Machines Gurakuq Dajaku FEAAM GmbH D-85577 Neubiberg, Germany Tel: +49 89 6004 4120, Fax: +49 89 6004 3718 E-mail: Gurakuq.Dajaku@unib.de Homepage: http://.unib.de/eaa Dieter Gerling Universitaet der Bundesehr Muenchen D-85577 Neubiberg, Germany Tel: +49 89 6004 3708, Fax: +49 89 6004 3718 E-mail: Dieter.Gerling@unib.de Homepage: http://.unib.de/eaa Abstract- The fractional slots tooth concentrated indings are characterized ith high space MMF harmonics hich results to undesirable effects on electric machine, such as localised core saturation, eddy current loss in the rotor and noise and vibration. A ne and high efficiency method is presented in this paper to reduce simultaneously the sub- and high MMF harmonics of these inding types. The method is based on doubling the number of stator slots, using to identical inding systems connected in series and shifted to each other for a specific angle, using stator core ith different tooth idth and using different turns per coil for the neighbouring phase coils. With the proposed technique the unanted sub- and high inding MMF harmonics can be reduced or completely canceled. I. INTRODUCTION Permanent magnet synchronous machines (PMSM) ith non-overlapping concentrated indings are becoming more and more attractive solutions for different industry applications. The concentrated inding machines have potentially more compact designs compared to the conventional machines ith distributed indings, due to shorter and less complex end-indings. With such indings, the volume of copper used in the end-indings can be reduced in significant proportions, in particular if the axial length of the machine is small. In modern applications here multi-pole machines are needed, fractional slot inding arrangement ith number of slots per phase and per pole less than one (q<1) have become an attractive alternative for traditional solutions. The machines ith a number of slots per pole and per phase beteen 1/2 and 1/3 such as the 12-slots per 10-poles machine, generally present higher performances and is idely used in many industry application. A PM machine ith 12-slots and 10-poles is illustrated in the folloing figure 1. Its stator inding differs from that of conventional PM machines in that the coils hich belong to each phase are concentrated and ound on adjacent teeth, as illustrated in figure 1, so that the phase indings do not overlap. Using fractional slot tooth concentrated indings (FSCW) different combinations of numbers of poles and numbers of teeth are possible [1-3]. Hoever, the magnetic field of these indings has more space harmonics, including sub-harmonics. These unanted harmonics lead to undesirable effects, such as localized core saturation, eddy current loss in the magnets [4 to 5], and noise and vibration [6 to 9], hich are the main disadvantages of these inding types. There have been several orks in the last decade devoted to improve the performances of the FSCW, regarding to reduction of inding sub-harmonics [10 to12]. By using these techniques proposed in these orks, the sub-harmonics of the considered inding can be reduced or even canceled, hoever the other higher MMF harmonics still are presents in the machine and represents an serious problem further for the rotor losses and especially for the noise and vibrations. Therefore, the aim of this ork as to presents a novel method for simultaneously reducing the sub- and high inding space harmonics of the tooth concentrated indings. In the folloing ork the proposed technique is implemented for the 12-slot/10-poles inding, hoever it is available also for different m-phases FSCWs. Fig. 1: PM machine ith the conventional 12-slots /10-poles inding topology. II. ARMATURE REACTION MMF ANALYSIS In machines ith fractional slot indings, the indings are not sinusoidally distributed and the resulting air-gap flux density distribution may be far from being sinusoidal. By analysing the stator inding magnetomotive force (MMF) and its space harmonics gives the main information about the 978-4577-0059-0/11/$26.00 2011 IEEE 65

electric machine characteristics. As ell knon, the air-gap flux density, electromagnetic torque and torque ripple, magnetic radial forces and so on are directly related to the stator MMF characteristics. For the 12-slot/10-poles tooth concentrated inding the MMF distribution and corresponding space harmonics are shon in figure 2. It is shon from figure 2-b that the 1 st, 5 th, 7 th, 17 th and 19 th are the dominant space harmonics for this inding type. For the 10-pole machine, hoever, only the 5 th stator space harmonic interacts ith the field of the permanent magnets to produce continuous torque. The other MMF space harmonics, in particular the 1 st, 7 th, 17 th, etc., hich have relatively large magnitudes, are undesirable and in some cases they limits the usefulness of this inding type in different specific applications. a) MMF [p.u.] ν here, 8 iˆ N ν Θ m = ξ πν 5 1 ν cos π sin π ξ = ν ν 6 2 6 2 ν Θm is the amplitude of the v-th MMF space harmonic, ν ξ is the inding factor, î is the phase current amplitude, δ is the load angle, ω is the angular frequency, N is the number of turns per coil, and ν is the space (MMF inding) harmonic. III. A NOVEL 24-SLOTS/10-POLES WINDING TOPOLOGY To use the advantages of the 12-slot/10-poles permanent magnet machines, but to avoid the disadvantages described briefly in the previous section, a ne inding topology is presented in [13] using the folloing modification steps: - the 12-slots / 10-poles conventional concentrated inding is divided in to separate inding systems, - both indings are identical, are connected in series and are supplied by the same inverter, and - the second inding is shifted mechanically for a specific angle referred to the first inding. Therefore, using to identical inding systems shifted in space for a given shifting angle α the resulting MMF for the ne inding is [13]: b) MMF [p.u.] 3 π α (2) ν ν Θ (x, t) = ξz Θm cos ωt ν x + δ ν 2 τp 2 here, ith Q ν ξ Z is denoted the distribution factor, ν α ξ Z = cos ν 2 α = k α τ (3) Fig. 2: MMF distribution and the corresponding MMF spectrum for the conventional 12-slots /10-poles inding topology. Using Fourier series function the MMF distribution for the 12-slots/10-poles inding shon in the above figure 2-a can be described using the folloing equ. (1). ith, 3 π Θ (x, t) = Θ cos ωt ν x + δ ν m (1) ν 2 τ p It is important to underline here that, if the same stator topology is used for the ne inding, then according to fig. 1 and equ. (3) the space displacement beteen the first and second inding can t be performed continuously but stepise for an angle kq ατ here k Q =1,2,3, Q S, α τ represents the tooth pitch angle for the 12-slots / 10-poles concentrated inding and Q S is the number of stator slots. The distribution factor for the 1 st, 5 th and the 7 th space harmonics for different stator slot shifting angles are shon in the folloing figure 3. As shon, an effective reduction (about 75%) of the first sub-harmonic can be obtained for 5 ατ or 7 ατ shifting angle hereas the 5 th (orking harmonic) and the 7 th harmonics are just a little bit effected (about 5%). Of course, according to [6] the reduction of the 7 th harmonic is also of main interest because of radial forces, therefore from the figure 3 it is shon 66

that the reduction of the 7 th space harmonic is also possible if the inding systems are shifted for a multiple of one-half of the tooth pitch 1). Therefore, the realization of the ne inding according to [13] requires doubling the number of stator slots and ounding the coil indings around to stator teeth. Figure 4 shos the ne inding topology ith reduced 7 th harmonic according to proposed technique. Hence, as is shon the realization of a ne inding ith the main objective reducing of the 7 th harmonic makes the motor design a little bit more complicated (requires a doubled number of stator slots), but the short end-indings of the initial 12-slots/10-poles design remain unchanged even for this ne 24-slots/10-poles design. Fig. 3: Distribution factors vs. number of stator slots. Of course, even the 7 th high harmonic is completely reduced using the above proposed techniques, the 1 st sub-harmonic still is present. According to the figure 3, this harmonic component is reduced just about 20% but it still is a serious problem for the rotor losses (especially for surface magnets rotor). Therefore, in [13 and 14] different methods are proposed for an effective reduction of this component: 1). using different turns per coil for the neighbouring phase coils [13], or 2). using coil indings ith different turns per coil side [14]. Figure 5-a shos the inding distribution for the proposed solution-1 (for case of the simplicity only the phase-a is illustrated). With n1 and n2 are denoted the turns per coil for the first and the second coil, respectively (neighbouring coils of one phase). Further, A1 and A2 represent the coils of phase-a for the first and the second inding system, respectively. The corresponding MMF spectrum for the ne inding topology is presented in figure 5-b. As ell shon, using the ne techniques the main unanted harmonics such as the 1 st and 7 th are completely canceled. Further, it is shon that the 17 th harmonic is also reduced more than 50%. Therefore, having such improvements in the MMF spectrum leads to huge improvements on the PM machine performances, and make the ne inding type attractive for designing also of other AC machines such as asynchronous machines or electrically excited synchronous machines. a) Fig. 4: Ne 24-slots/10-poles inding according to [10]. The above figure 3 shos also that it is possible to reduce the 7 th harmonic don to zero if the shifting angle is chosen just a little bit larger than 2.5α τ. The realization of such inding type can be performed using a stator core ith different tooth idth as is illustrated in the folloing figure 5-a [13]. By selecting the tooth idth in the proper ay the specific MMF harmonics further can be reduced. 1) Of course the same procedure can be used also for reduction of the 5 th harmonic if the 7 th harmonic is used as orking harmonic. MMF [p.u.] b) Fig. 5: a). Ne 24-slots/10-poles inding ith solution-1, b). Corresponding MMF spectrum. 67

IV. DESIGN OF TWO PM MACHINES FOR HEV APPLICATION A standard distributed inding topology ith q=2 and ith a short pitch inding of one slot, and the ne 24-slot/10-poles inding are used in folloing for designing of to PM machines for hybrid electric vehicle (HEV) or battery electric vehicle (BEV) applications. As typical requirements for the automotive traction drive application, the folloing data are used: maximum DC-voltage U DC =300V, maximum short-time torque T max =250Nm @ 4700min -1, maximum speed n max =12000 min -1 and lo torque ripple T ripple <5% (at lo speed). As the ne motor design shall be compared to the most commonly used existing design (in folloing this ill be named standard design ), some additional boundary are fixed to ensure a fair comparison: same active volume (D Out =220mm, L Stack =130mm), air-gap length δ=1mm, same magnet volume, same maximum rotor mechanical stress, same demagnetization safety, and same operation temperature for the magnets and stator indings. The folloing figure 6 shos the geometry of the designed motors. For the both investigated motors buried rotor topologies ith tangentially insetted magnets in the rotor core are considered (8-poles rotor for the standard design, 10-poles rotor for the ne motor design). A first comparison is performed concerning the torque ripple of the to motor designs. Figure 7 compares the torque vs. rotor position characteristics of both motor designs under high load operation condition. Regarding to the torque ripple, the ne motor design is very advantageous compared ith the standard design; the ne motor design delivers a very smooth torque compared ith the standard design. It is shon from figure 7 that the standard motor produces a torque ripple of about 16%, hich is far beyond the acceptable limit (T ripple <5%). This ripple can be reduced by skeing, but then simultaneously the production costs ill rise and the mean torque ill deteriorate. Against that, the ne motor design delivers a very smooth torque ith a ripple of about 3%. Therefore, skeing is not necessary applying this design. III.1 AIR-GAP FLUX DENSITY COMPONENTS Fig. 7: Torque vs. rotor position characteristics for the standard and the ne design. a) b) Fig. 6: a). Standard 48-slots/8-poles motor design, b). Ne 24-slots/10-poles motor design. The impact of the ne motor design on the entire electric poer train is investigated in [15 to 17]. As an example, the characteristics of a typical future battery electric vehicle are taken. The efficiency of both machines are compared in the entire torque-speed plane. Figures 8 and 9 sho these characteristics (also called efficiency map ) for the standard motor and the ne developed motor, respectively. The efficiency map is calculated using FE methods. It can be deduced from these figures that the maximum efficiency is advantageous for the ne motor design (97% against 95%). Even more important is that the efficiency at lo-load operation is considerably better for the ne motor design. The outstanding features for lo-load operation become even more obvious, if the efficiency difference of both motors is plotted in a torque-speed-plane, see the folloing figure 10. Driving cycle simulations performed in [15-17] sho that the ne motor design leads to considerably loer losses (depending on the regarded driving cycle up to 20% in the motor) compared ith the standard design, resulting in prominent advantages for the entire electric poer train. 68

torque [Nm] 0 50 100 150 200 250 Fig. 8: Efficiency versus torque and speed of the standard motor. torque [Nm] 0 50 100 150 200 250 0 2000 4000 6000 8000 10000 12000 speed [rpm] 0 2000 4000 6000 8000 10000 12000 speed [rpm] Fig. 9: Efficiency versus torque and speed of the ne motor. 0 10 20 30 40 50 60 70 80 90 100 efficiency [%] 0 10 20 30 40 50 60 70 80 90 100 efficiency [%] V. CONCLUSION The fractional 12-slots/10-poles tooth concentrated inding is characterized ith high space harmonics hich result to undesirable effects on electric machine, such as localised core saturation, eddy current loss in the magnets and noise and vibration. A ne method is presented in this paper to reduce simultaneously the sub- and the high-harmonics of the inding MMF. The method is based on doubling the number of stator slots, using to identical inding systems connected in series and shifted to each other for a specific angle, using stator core ith different tooth idth and using different turns per coil for the neighbouring phase coils. To PM machines for HEV or BEV application are designed using the ne inding and a conventionally distributed inding. Comparing the machine characteristics for the designed motors, e.g. torque ripple and efficiency, the ne inding topology shos clearly better performances compared ith the standard distributed inding. The exemplary calculation in this paper as performed for a PM machine. Nevertheless, this ne inding is applicable also to other machine topologies, like induction machines and electrically excited synchronous machines. In addition, the described principles to reduce the MMF harmonics are applicable to any concentrated inding topology. REFERENCES [1] Magnussen F., Sadarangani Ch.: Winding factors and Joule losses of permanent magnet machines ith concentrated indings. 2003 IEEE International Electric Machines & Drives Conference (IEMDC 2003), 01-04.06 Madison Wisconsin, USA. [2] Ishak D., Zhu Z.Q., Hoe D.: Comparison of PM brushless motors, having either all teeth or alternate teeth ound. IEEE Transactions on Energy Conversation, vol. 21, 2006. torque [Nm] 0 50 100 150 200 250 0 2000 4000 6000 8000 10000 12000 speed [rpm] Fig. 10: Efficiency difference of both motor designs versus torque and speed (positive values mean: the ne motor design is better than the standard motor design in that operating point). -15-10 -5 0 5 10 15 efficiency difference [%] [3] Gerling D.: Analysis of the Magnetomotive Force of a Three-Phase Winding ith Concentrated Coils and Different Symmetry Features. International Conference on Electrical Machines and Systems (ICEMS), Wuhan, China, 2008. [4] Nakano M., Kometani H.: A study on eddy-current losses in rotors of surface permanent magnet synchronous machines. IEEE Transactions on Industry Application, vol. 42, No. 2, March/April 2006. [5] Polinder H., Hoeijmakers M. J., Scuotto M.: Eddy-Current Losses in the Solid Back-Iron of PM Machines for different Concentrated Fractional Pitch Windings. 2007 IEEE International Electric Machines & Drives Conference (IEMDC 2007), 3-5 May Antalya, Turkey. [6] Dajaku G., Gerling D.: Magnetic Radial Force Density of the PM Machine ith 12-teeth/10-poles Winding Topology. IEEE International Electric Machines and Drives Conference, IEMDC2009, Florida USA, May 3-6, 2009, pp.157-164. [7] Wang J., Xia Zh. P., Hoe D., Long S. A..: Vibration Characteristics of Modular Permanent Magnet Brushless AC Machines. IEEE IAS Annual Meeting, 2006, Tampa, Florida, USA. [8] Boesing M., Kasper K. A., Doncker R. W.: Vibration Excitation in an Electric Traction Motor for a Hybrid Electric Vehicle. 37 th International Congress and Exposition on Noise Control Engineering, Inter-Noise 2008, 26-29 October 2008, Shanghai-China. 69

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