Low Costs and High-Efficiency Electric Machines

Abstract — This paper present a novel method for reducing or complete canceling some space harmonics of low order in the air-gap flux density of electric machines with fractional slots, tooth-concentrated winding using magnetic flux barriers in the stator teeth region. Furthermore, the new method increase simultaneously the efficiency of the working harmonic and with this the power density of the electric machine. The presented technique is applied to a PM machine with 12-teeth/14-poles single layer concentrated winding during designing of an electric machine for automotive application. Compared with the conventional design it show enormous improvements on the machine performances; the new PM machine design is characterized with high efficiency and high power density.

Keywords: Tooth concentrated winding, permanent magnet machine, efficiency improvement, low manufacturing costs.

I. INTRODUCTION

Recently, for the application where high torque and high power density are required, permanent magnet (PM) synchronous machines with high energy magnet materials such as NdFeB or SmCo have gained increased popularity. A best example is the development of electric vehicles, where weight, volume and torque density are all at a premium. Furthermore, in traction applications a wide constant power region is also required. Over the past several years PM machines have emerged as the best option to satisfy all these demands. Generally PM synchronous machines with fractional slot concentrated windings (FSCW) are widely used. The use of concentrated windings offers the advantage of short and less complex end-winding, high slot filling factor, low cogging torque, greater fault tolerance, and low manufacturing costs. The stator coils may be wound either on all the teeth (double-layer winding) or only on alternate teeth (single-layer winding) and the manufacturing of these windings may be much cheaper because they contains simple coils that can be wounded automatically. Further, using FSCW different combinations of numbers of poles and numbers of slots are possible [1, 3]. However, the magnetic field of these windings has more space harmonics, including sub-harmonics. For the PM machines, the torque is developed by the interaction of a specific high stator space harmonic with the permanent magnets. On the other side, the rest of others sub- and high harmonics, which rotate with the different speed and also in opposite directions, lead to undesirable effects, such as localized core saturation, eddy current loss in the magnets [4 to 6], and noise and vibration [7, 8], which are the main disadvantages of these winding types.

G. Dajaku is Senior Scientist with FEAAM GmbH, D-85577 Neubiberg, Germany (e-mail: Gurakuq.Dajaku@unibw.de).

D. Gerling is Full Professor at the University of Federal Defense Munich, Institute for Electrical Drives, D-85577 Neubiberg, Germany (e-mail: Dieter.Gerling@unibw.de).

To improve the magneto-motive force (MMF) winding performances of the FSCW regarding to power losses and noise problems several methods and techniques are developed and investigated in the past [9 to 18]. References [9 to 16] show different methods for the reduction of winding sub-harmonics, however, in [17, 18] another new methods for reduction simultaneously the sub- and high MMF harmonics are presented. These techniques shows enormous improvements on the PM machine performances such as reduction of the sub- and high MMF harmonics more than 60%, reduction of radial force modes of low order and also reduction of the machine losses (magnet losses, iron losses and so on).

Of course, the use of PM synchronous machines with concentrated windings has not only advantages. Usually the permanent magnets are made of rare material and the price for this material increase tremendously in the last time. Rare earth share of total value add for motor increased from 6% to 41% for electric motor supplier between 2010 and 2011 [19]. On the other side, the demand for the PM machines in the automobile industry and also in the wind energy is enormous increasing which further increase the price for the magnet materials. It is important to point out that the machine price is also one of the main requirement for the industry, and the main motivation of this work was to present a novel technique for the PM machines with simple concentrated windings which simultaneously improves the electromagnetic performances of the machine (efficiency and also the torque capability) and also decrease the magnet material amount compared with the conventional design. Analogous to [14, 15], the new technique reduce the unwanted air-gap flux density harmonics using flux barriers in the specific stator teeth regions [20]. The presented method is used to improve the performances of a 12-teeth/14-poles PM machine with single-layer concentrated winding, however of course this method is also useful for any type of concentrated windings.

II. 12-TEETH/14-POLES CONCENTRATED WINDING

As well mentioned previously, there are many possible slot number and pole number combinations for PM machines with concentrated windings. The winding layout and the winding factor of a PM machine with concentrated winding depend on its combination of pole and slot number. Therefore, this combination should be carefully chosen in order to maximize the fundamental (MMF working harmonic) winding factor and thus the torque density. Single-layer windings are preferred to double-layer windings when a high fundamental winding factor and high fault-tolerance is required. Otherwise, double-layer windings are preferable to limit the losses and torque ripple.

Gurakuq Dajaku and Dieter Gerling

a) b)

Fig. 1: a). PM machine with the conventional 12-teeth /14-poles single-layer winding, b). Corresponding MMF winding harmonics.

a) b)

Fig. 2: a). PM machine with the conventional 12-teeth /10-poles double-layer winding, b). Corresponding MMF winding harmonics.

0 2 4 6 8 10 12 14 16 18 200

0.2

0.4

0.6

0.8

1

1.2

1.4

Space Harmonics

MM

F [

p.u

. ]

0 2 4 6 8 10 12 14 16 18 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Space Harmonics

MM

F [

p.u

. ]

Figures 1-a and 2-a show two PM machine with 12-teeth in the stator and 14-poles in the rotor and with different concentrated winding layouts. Single-layer winding have coils wound only on alternate teeth, whereas each tooth of the double-layer windings carries a coil. The corresponding MMF harmonics for these winding types are presented in figures 1-b and 2-b. As well is shown, for the both presented winding types the 1st, 5th, 7th, 17th and 19th are the dominant space harmonics and usually for different machine designs the 5th or the 7th harmonic are mostly used as working harmonics. For the 14-pole machine, however, only the 7th stator space harmonic interacts with the field of the permanent magnets to produce continuous torque. The other MMF space harmonics, in particular the 1st, 5th, 17th, etc., which have relatively large magnitudes, are undesirable and in some cases they limit the usefulness of this winding type in different specific applications.

Let’s remember that the main difference between the presented winding types is not only in the winding layout,

but, for the single-layer winding the winding factors for the 5th, 7th and so on are about 3.3% higher compared with the double-layer winding. However, as well shown from the figure 1-b the amplitude of the 1st MMF sub-harmonic is relatively too high which induce huge losses in the rotor core and magnets and thus it decreases the torque density and the efficiency of the machine. As results, the PM machines with the double-layer winding are mostly used in many applications. Of course, considering the main advantages of the 12-teeth single layer winding shown in figure 1-a, such as the simple and cheaper manufacturing (only six coils are required), and high winding factor (high power capability) for the working harmonics, the main motivation of work presented in [20] was to improve the MMF winding performances regarding to the sub-harmonics. Therefore, in [20] a simple and high efficiency technique is presented for reduction of the air-gap flux density harmonics using magnetic flux-barriers in specific stator tooth regions. Simultaneously, the new method increases the capability of

the working harmonics and thus the power density is increased compared with the convectional PM machine design.

III. NEW STATOR CORE WITH FLUX-BARRIERS IN STATOR TEETH

In order to maximize the flux-linkage and torque density of the PM machines with concentrated windings, it is desirable for the coil-pitch to be as close to the pole-pitch as possible. Therefore, number of stator slots NS and rotor poles 2p should differ by the smallest possible integer. The most popular combinations of slot number and pole number is S 2 1N p= ± and S 2 2N p= ± . For the first case, typical slot/pole combinations for three-phase electric machines are NS /2p=3/2, 3/4; 9/8, 9/10, etc. The main disadvantages for these winding types are the low winding factor for the fundamental harmonic and the unbalanced magnetic forces due to the diametrically asymmetric location of the stator phases. Hence, in many application the second case with the slot and pole number S 2 2N p= ± is mostly using. A good example is the 12-teeth/10-poles or the 12-teeth/14-poles. A simple sketch for these combinations is illustrated in the following figures 3-a and 3-b.The relation between the slot- (coil) and the pole pitch for the presented machine topologies are:

5

0.833T

P=

=τ

τ for the 12-teeth/10-poles

7 0.856P

T

= =ττ

for the 12-teeth/14-poles

Therefore, from the magnetic point of view the both stator slot and the rotor pole combinations show equal electromagnetic characteristics. However, as is shown from the figures 1 and 2 this winding type contains MMF sub-harmonics which mostly are responsible for the rotor iron and the magnet losses. For the double-layer winding two novel methods for reduction of the MMF winding sub-harmonics are developed and analyzed in [12 to 15]. According to the first technique, the sub-harmonics of the FSCW are reduced or completely canceled by using winding coils with different number of turns per coil side [12, 13], however, another new technique proposed in [14, 15] solves the same problem by modifying the stator yoke in specific locations by using magnetic flux-barriers.

Analogous to [14, 15], another new technique for reduction of winding sub-harmonics for the single-layer winding topologies is developed in [20]. Also here, additional magnetic flux-barriers are used for reduction the MMF sub-harmonics effect, however, different from [14, 15] in the new method the flux barriers are used in the stator teeth regions, figure 3-c. It is found in [20] that this novel method is very efficiency for reduction of air-gap flux density sub-harmonics (1st and the 5th), however, simultaneously it increase the capability for the 7th MMF harmonic. Comparing the relation between the slot- and the pole pitch for the new stator topology we have:

,

5

0.72T New

P=

≈ττ

for the 12-teeth/10-poles

7

,

1P

T New

= ≈τ

τ for the 12-teeth/14-poles

Therefore, from the above relation and also from figure 3-c it is shown that the new stator structure with magnetic flux-barriers in the stator teeth represents an ideal magnetic flux path for the 7th MMF harmonic, however, for the sub-harmonics represents a high magnetic resistance. As results, using a proper shape for the flux-barrier regions the air-gap flux-density sub-harmonics due to stator field efficiency can be reduced.

Fig. 3: a). 12-teeth /10-poles design with the conventional stator core, b). 12-teeth /14-poles design with the conventional stator core, c). 12-teeth /14-poles design with the new stator core topology.

To show the effect of the new method on the air-gap flux

density harmonics the single layer 12-teeth winding with the conventional and also the new stator core is investigated in the following. For the both investigated machines solid rotors are considered. Figure 4 show the field lines distribution under one-phase (phase-A) excitation, however, figure 5 compares the air-gap flux-density harmonics under three phase symmetrical excitation. As shown from the figure 4-a the considered 12-teeth single layer winding with the conventional stator behave as a two pole machines, however, using flux barriers in the stator teeth the flux lines are forced to close through the neighbor teeth, figure 4-b. As results the air-gap flux density sub-harmonics are reduced.

a) b) c)

Flux barriers

2 60T = °τ

52 72P= = °τ

72 51.4P= = °τ

72 51.4P= = °τ

2 60T = °τ

,2 51T New ≈ °τ

On the other side, comparing the air-gap flux density harmonics due to the reaction field under three-phase excitation and for the conventional and the new stator, it show enormous improvements on the flux density characteristics using the new technique:

− the 1st sub-harmonic is reduced by 73%, − the 5th sub-harmonic is reduced by about 19%, − On the other side, the 7th working harmonic is

increased by about 16%.

a) b)

Fig. 4: Field distribution with phase-A excited, a). Conventional stator core, b). New stator with flux-barriers.

Fig. 5: Comparison of air-gap flux density harmonics due to stator currents

(under three-phase excitation).

IV. A NEW 12-TEETH/14-POLES PM MACHINE FOR AUTOMOTIVE APPLICATION

Using the new stator structure with flux-barriers in the stator teeth regions a low cost (LC) 12-teeth/14-poles PM machine with inset magnets in the rotor and single layer concentrated winding is designed and analyzed in following. The main geometry data of the studied PM machine are presented in Table-I, however, figure 6 shows the geometry. The stator structure consists of six simple concentrated coils, six stator core modules and also of six additional stator teeth component of “T-shape”, which are used as flux-barriers and also for fixing (mounting) the complete stator structure. I is important to point out that the flux-barriers should be a non-magnetic and also a non-electric (non-conductivity) material. The rotor consists of a simple core structure with 14 rectangular magnets. Further, the machine is designed for maximal DC link voltage UDC,max=12 V and the maximal

rotor speed nMax=3000 rpm. Finite element methods (FEM) are used for the design and analysis of the considered PM machines.

TABLE I: MAIN GEOMETRY DATA

Outer rotor diameter 48 mm

Outer stator diameter 81 mm

Gap length 0.5 mm

Magnet length (magn. direction) 2.5 mm

Magnet width 7.5 mm

Active length 70 mm

Turns per coil & Parallel path 10 & 2

NS 12

p 7

Fig. 6: Geometry of the 12-teeth/14-poles PM machine with the new stator

structure.

Different simulations are performed in following to show the performances of the new machine compared with the conventional design. Under Ieff=70A load condition the electromagnetic torque results for new and the conventional stator design are presented and compared in figure 7. The obtained results show that with the new stator topology the torque capability is increased for about 16% compared with conventional design. Additionally, the torque ripples are also lower. Further, under the same load condition and for the rotor speed n = 1200 rpm the machine losses are compared in figure 8. Also here, huge improvements on the machine losses are achieved. As results of sub-harmonic reduction the iron rotor losses are reduced for about 50%, however, the magnet losses for about 60%. The stator copper losses are the same since the simulations are performed under the same excitation condition.

As well is shown, with the new stator topology, in one side the unwanted sub-harmonics are clearly reduced and with this the losses in the machine are reduced (efficiency improvements), however, on other side, simultaneously the torque density is increased. Compared with other techniques where any kind of modification in magnetic circuit or winding for improving the machine efficiency, torque ripple

0 2 4 6 8 10 12 14 16 18 200

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Harmonic Order

B [

T ]

Conv. Stator

New Stator

Flux barriers

-73% -19% +16%

and so on, always are related with negatively influencing the working harmonic and also the torque density, however, with the new invention the reverse occur.

Fig. 7: Comparison of torque results for Ieff = 70A.

Fig. 8: Comparison of machine losses for Ieff=70A and n=1200 rpm.

V. ADVANTAGES AND BENEFITS OF THE NEW STATOR STRUCTURE FOR THE PM MACHINES

In the previous section, the comparison for the two motor designs has been performed under the same geometrical constrains. However, if the machines should be designed for the same torque capabiliy, then according to figure 7 the new machine can be scaled in axial length, and for about 16% reduced active length the both machines provides the same electromagnetic torque. The following figures 8 and 9 compares the torque and machine losses results for the convectional machine with 70mm active length and the new scaled machine with 58.8mm active length. The other electrical and geometrical constrains are taken the same. As well shown, the both machines provides the same torque, however, with enormous loss reduction: 11.6% for copper losses, 17.5 for stator iron losses, 58% for rotor iron losses, and 65% magnet losses. Of course, if the comparison would be made for the same output power (mechanical power) the new machine design satisfies the requirements with an active

length for about 20% shorter than the conventional design. Therefore, the new machine is characterized not only with high power density and high efficiency, however, the material amount for the rear earth magnets, copper and iron core can be reduced more than 15%. This leads to reduction in the motor costs and also in the machine volume and mass. Considering the huge requirements on the PM machine for different industry applications such as automobile industry, wind energy and so on, the application of presented new machine design for such applications would bring enormous economic and ecological benefits for the industry and the environment.

Fig. 8: Comparison of torque results for Ieff = 70A.

Fig. 9: Comparison of machine losses for Ieff=70A and n=1200 rpm.

VI. REALIZATION – ALTERNATIVE SOLUTIONS

Of course the realization and optimization of electric machines with the new presented technique can be performed in different ways such as using stator core with different teeth width and flux-barriers in alternate teeth, flux-barriers in all teeth, different shape for flux-barriers, stator modules with radial laminated and U-shape iron core and so on [20]. These techniques enable the design and optimization and also the manufacturing (production) of the electric machines easily, simply and cheaper. The following figures 10-a to 10-c illustrate different possible alternative solutions. Of course, the presented techniques are applicable also for double layer tooth concentrated windings.

0

1

2

3

4

5

0 40 80 120 160 200

T [N

m]

rotor position [el. degree]

Electromagnetic Torque

Conv. StatorNew Stator

0

20

40

60

80

P [

W ]

P_Cu P_Stator P_Rotor P_MagnetsIron Iron a

LossesConv. StatorNew Stator

0

1

2

3

4

0 40 80 120 160 200

T [N

m]

rotor position [el. degree]

Electromagnetic Torque

Conv. Stator (Lstk=70mm)New Stator (Lstk=58,8mm)

0

20

40

60

80

P

[ W ]

P_Cu P_Stator P_Rotor P_MagnetsIron Iron v

Losses

Conv. Stator (Lstk=70mm)New Stator (Lstk=58,8mm)

VII. CONCLUSIONS

PM machines with fractional slot concentrated winding are increasingly used in various industry applications owing to their many advantages. However, they are characterized by high content of space harmonics in the air-gap MMF distribution which generate undesirable effects and in some cases they limit the use of these winding types. To overcome the drawbacks of concentrated winding, this paper presents a novel solution for reduction of air-gap flux-density sub-harmonics for concentrated windings using magnetic flux barriers in specific stator teeth regions. Simultaneously, the new solutions increase the efficiency of the working harmonic and with this the power density of the electric machine.

The new technique is used for designing a PM machine with 12-teeth/14-poles concentrated and single layer winding. Under the same electrical and geometrical constrains, the conventional stator and the new stator with flux barriers in stator teeth are consider and the obtained results are compared. It is shown here that the new stator structure improves enormously the performances of the PM machines with the concentrated windings. The electromagnetic torque is increased for about 16% however the rotor losses are reduced more than 50%. Therefore, considering the torque density and the efficiency of the new machine design, a reduction up to 20% in the material amount (rear earth magnets, copper, core) and in the machine volume and mass is possible compared with the conventional design. Thus, the application of presented new machine design for different industry applications would bring enormous economic and ecological benefits for the industry and the environment.

REFERENCES [1] D. Ishak, Z. Q. Zhu: “Comparison of PM Brushless Motors, Having

Either All Teeth or Alternate Teeth Wound”, IEEE Transactions on Energy Conversion, Vol. 21, No. 1, March 2006, pp. 95-103.

[2] F. Magnussen, Ch. Sadarangani: “Winding factors and Joule losses of permanent magnet machines with concentrated windings”. 2003 IEEE International Electric Machines & Drives Conference (IEMDC 2003), 01-04.06 Madison Wisconsin, USA.

[3] F. Libert, J. Soulard, “Investigation on Pole-Slot Combinations for Permanent-Magnet Machines with Concentrated Windings,” International Conference on Electrical Machines (ICEM 04), September 2004, Cracow, Poland.

[4] M. Nakano, H. Kometani: “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] N. Bianchi, E. Fornasiero: “Index of rotor losses in three-phase

fractional slot permanent magnet machines”. Electric Power Applications, IET, vol. 3, No. 5, September 2009.

[6] H. Polinder , M. J. Hoeijmakers, M. Scuotto : “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.

[7] G. Dajaku, D. Gerling: “Magnetic Radial Force Density of the PM Machine with 12-teeth/10-poles Winding Topology”. IEEE International Electric Machines and Drives Conference, IEMDC2009, Florida USA, May 3-6, 2009, pp.157-164.

[8] J. Wang, Zh. P. Xia, D. Howe, S. A. Long : “Vibration Characteristics of Modular Permanent Magnet Brushless AC Machines”. IEEE IAS Annual Meeting, 2006, Tampa, Florida, USA.

[9] H. Kometani, Y. Asao, K. Adachi: “Dynamo-electric Machine”, US Patent 6,166,471, Dec. 26, 2000.

[10] K. Ito, K. Naka, M. Nakano, M. Kobayashi: “Electric machine”, US Patent 7,605,514, Oct. 20, 2009.

[11] M. V. Cistelecan, F. J. T. E. Ferreira: “Three phase tooth-concentrated multiple-layer fractional windings with low space harmonic content”, IEEE on Energy Conversion Congress and Exposition (ECCE) 2010.

[12] G. Dajaku: “Elektrische Maschine”, German patent, DE 102008 057 349 B3, July 15, 2010.

[13] G. Dajaku, D. Gerling: “Eddy Current Loss Minimization in Rotor Magnets of PM Machines using High-Efficiency 12-teeth/10-poles Winding Topology”, International Conference on Electrical Machines and Systems (ICEMS-2011), 20.-23. August 2011, Beijing, China.

[14] G. Dajaku: “Elektrische Maschine”, German patent application No. 102008 054 284 A1.

[15] G. Dajaku, D. Gerling: “A Novel 12-Teeth/10-Poles PM Machine with Flux Barriers in Stator Yoke”, 20-th International Conference on Electrical Machines (ICEM’2012), 02.-05. September 2012, Marseille, France, (paper accepted).

[16] G. Dajaku, D. Gerling: “Different Novel Methods for Reduction of Low Space Harmonics for the Fractional Slot Concentrated Windings”, The 15th International Conference on Electrical Machines and Systems (ICEMS-2012), 21.-24. October 2012, Sapporo, Japan, (paper accepted).

[17] G. Dajaku, D. Gerling: “A Novel 24-Slots/10-Poles Winding Topology for Electric Machines”, International Electric Machines and Drives Conference (IEMDC-2011), Niagara Falls (Ontario), Kanada, May 15-18, 2011, pp. 65-70.

[18] G. Dajaku, D. Gerling: “Efficiency Improvements of Electric Machines for Automotive Application”, 26th International Electric Vehicle Symposium (EVS26-2012), Mai 06-09, 2012, Los Angeles (CA), USA.

[19] D. Zechmair, K. Steidl: “Why the Induction Motor Could be the Better Choice for your Electric Vehicle Program”. The 26th World Battery, Hybrid and Fuel Cell Electric Vehicle Symposium & Exhibition (EVS), May 06.-09. 2012, Los Angeles, California, USA.

[20] G. Dajaku: “Elektrische Maschine”, German patent application No. DE 102012103677.2, 04. May 2012.

a) b) c)

Fig. 10: Different stator topologies with flux barriers in teeth region.

radial laminated

axial laminated

Gurakuq Dajaku; Dr.-Ing. Gurakuq Dajaku is with FEAAM GmbH, Werner-Heisenberg-Weg 39, D-85577 Neubiberg, Germany, phone: +49 89 6004-4120, fax: -3718, e-mail: Gurakuq.Dajaku@unibw.de. Born in 1974 (Skenderaj, Kosova), got his diploma degree in Electrical Engineering from the University of Pristina, Kosova, in 1997 and his Ph.D. degree from the University of Federal Defense Munich in 2006. Since 2007 he is Senior Scientist with FEAAM GmbH, an engineering company in the field of electric drives. From 2008 he is a Lecturer at the University of Federal Defense Munich. His research interest is in the field of electrical machines and drives. Dr. Dajaku recive the Rheinmetall Foundation Award 2006 and the ITIS (Institute for Technical Intelligent Systems) Research Award 2006. Dieter Gerling; Prof. Dr.-Ing. Dieter Gerling is head of the Institute of Electrical Drives at the University of Federal Defense Munich, Werner-Heisenberg-Weg 39, D-85579 Neubiberg, Germany (phone: +49 89 6004-3708; fax: -3718; email: Dieter.Gerling@unibw.de). Born in 1961, Prof. Gerling got his diploma and Ph.D. degrees in Electrical Engineering from the Technical University of Aachen, Germany in 1986 and 1992, respectively. From 1986 to 1999 he was with Philips Research Laboratories in Aachen, Germany as Research Scientist and later as Senior Scientist. In 1999 Dr. Gerling joined Robert Bosch GmbH in Bühl, Germany as Director. Since 2001 he is Full Professor at the University of Federal Defense Munich, Germany (http://www.unibw.de/EAA/).