IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 27, NO. 2, JUNE 2012 403

A Combined Wye-Delta Connection to Increase thePerformance of Axial-Flux PM Machines With

Concentrated WindingsHendrik Vansompel, Peter Sergeant, Luc Dupre, and Alex Van den Bossche

Abstract—In this paper, a combined wye-delta connection is in-troduced and compared with a conventional wye-connection of aconcentrated winding. Because the combined wye-delta connec-tion has a higher fundamental winding factor, the output torque ishigher for the same current density when a sinusoidal current isimposed. As the combined wye-delta connection has only a minorinfluence on the losses in the machine, the efficiency of the machineis also increased. The combined wye-delta connection is illustratedin detail for an axial-flux permanent-magnet synchronous machinewith a rated power of 4 kW at a fixed speed of 2500 r/min, us-ing finite element computation and measurements on a prototypemachine.

Index Terms—Axial-flux machine, combined wye-delta connec-tion, concentrated winding, permanent-magnet machines, windingfactor.

I. INTRODUCTION

COMPARED to distributed windings, concentrated wind-ings have the advantage of shortened end windings, higher

torque density and efficiency, ease in winding process andmounting and some configurations enable a low torque ripple.However, a general disadvantage of the concentrated windingsis a lower winding factor [1], resulting in a lower output torque,and high magnetomotive force (MMF) harmonic content, whichhas a major impact on the rotor losses [2].

In [3]–[5], a combined wye-delta connection is used to in-crease the winding factor, and, thus, the output torque, and de-crease the MMF harmonic content for machines with distributedwindings. In this paper, the use of such a combined wye-deltaconnection is investigated for machines with concentrated wind-ings. As the electromotive force (EMF) of such a concentrated

Manuscript received August 9, 2011; revised November 22, 2011; acceptedJanuary 9, 2012. Date of publication February 13, 2012; date of current versionMay 18, 2012. This work was supported by the Research Fund of the GhentUniversity Project BOF-associatieonderzoeksproject 05V00609, Fund of Scien-tific Research Flanders Projects G.0082.06 and G.0665.06, the GeconcerteerdeOnderzoeksacties Project BOF 07/GOA/006, and the Interuniversity AttractionPoles Project P6/21. Paper no. TEC-00386-2011.

H. Vansompel, L. Dupre, and A. Van den Bossche are with theDepartment of Electrical Energy, Systems and Automation, Ghent Uni-versity, B-9000 Ghent, Belgium (e-mail: Hendrik.Vansompel@UGent.be;Luc.Dupre@UGent.be; Alex.VandenBossche@UGent.be).

P. Sergeant is with the Department of Electrical Energy, Systems and Automa-tion, Ghent University, B-9000 Ghent, Belgium, and also with the Departmentof Electrotechnology, Faculty of Applied Engineering Sciences, University Col-lege Ghent, B-9000 Ghent, Belgium (email: Peter.Sergeant@UGent.be).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TEC.2012.2184114

Fig. 1. Considered AFPMSM topology: the YASA or SAT topology, whichconsists of two rotor disks within a stator.

winding has a zero-sequence component, circulating currents inthe delta-connected windings will cause additional losses. Onthe other hand, the zero-sequence flux in the delta-connectedwindings is suppressed, which will have an influence on theflux density pattern in the teeth and consequently on the ironlosses. Therefore, this paper focuses on the aspects which maylimit the use of the combined wye-delta connection for a con-centrated winding.

It will be shown that the rate of improvement of the wind-ing factor strongly depends on the combination of pole and slotnumbers. An overview of the winding factors for some com-monly used combinations is given. Finally a 16-pole–15-slotcombination is chosen and examined extensively. To illustratethe benefits of the combined wye-delta connection, a wye con-nection is compared with the combined wye-delta connectionfor an axial-flux permanent-magnet synchronous machine (AF-PMSM) with concentrated windings. Although an axial-fluxmachine is used, the same theory applies to radial machines.

A simplified overview of the used AFPMSM is given in Fig. 1.This topology, a yokeless and segmented armature (YASA)or segmented armature torus (SAT) topology discussed in [6]and [7], and like AFPMSM’s, in general, is very suitable, fore.g., wheel motors and direct drive wind energy applications[8]–[11].

As a concentrated double layer winding is used for this topol-ogy, each individual winding is wound around one tooth. In thisway, a modular stator construction is introduced: individual sta-tor teeth are made in advance, and are then combined to form asolid stator as shown in Fig. 2. This way allows easy comparisonbecause measurements take place on the same machine; just by

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404 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 27, NO. 2, JUNE 2012

Fig. 2. YASA or SAT topology allows to make an AFPMSM topology witha modular stator construction: individual stator teeth are made in advance, andare then combined to form a solid stator. Each module can be easily replacedwhich makes this topology very suitable for this research topic.

TABLE ICHARACTERISTICS AND PARAMETERS OF THE CONSIDERED

AFPMSG-PROTOTYPE

rearranging and replacement of the stator modules, a changefrom wye connection to combined wye-delta connection isperformed.

This construction method is not only advantageous for easymanufacturing, but has general advantages quoting [12]: 1)shortened end windings leading to higher torque density andefficiency, 2) ease of winding process and high winding fill fac-tor, 3) reduced mutual inductance between the machine phasesresulting in improved phase independence and fault tolerance,and 4) reduced stator core weight due to the absence of the statoryoke.

The comparison between a wye connection and a combinedwye-delta connection is performed by finite element compu-tation as well as by measurements on a prototype YASA AF-PMSM [13], of which the main characteristics are summarizedin Table I.

II. COMBINED WYE-DELTA CONNECTION

In [1]–[14], the winding factor kwY 1 for a conventional wyeconnection for different combinations of slots and poles, whichallow the realization of a balanced winding, is determined andsummarized in Table II. The phasorial diagram for a conven-tional wye connection is represented in Fig. 3 for the examplewith 15 slots and 16 magnets. As phase differences between

Fig. 3. Phasorial diagram of one phase for the conventional wye connectionfor the example with 15 slots and 16 magnets. The EMF of one coil is indicatedby Ec , and the resulting EMF by E .

the EMFs of the different coils Ec exist, the total phase EMFE has an amplitude which is only a fraction kz1 of the sum ofthe amplitudes of the contributing coil EMFs. This fraction, theso-called zone factor, is given by

kz1 =sin

(q′ α2

)

q′ sin(

α2

) (1)

where q′ is the number of neighboring slots assigned to onephase, and α is the electric angle between two slots.

As each coil consists of two wires lying in two neighbor-ing slots, a phase difference between these neighboring EMFsexists, which reduces the coil EMF with a factor kp , the pitchfactor, given by

kp = cos(α

2

). (2)

The winding factor kwY 1 , which is calculated in Table II fordifferent slot and pole numbers can then be calculated by

kwY 1 = kz1kp . (3)

In the considered example of 15 slots and 16 magnets, thezone factor kz1 equals 0.9567 and the pitch factor kp equals0.9945 leading to a winding factor kwY 1 of 0.9514.

As for easy construction, the individual winding is woundaround one tooth, the pitch factor cannot be changed. However,by using the combined wye-delta connection, the zone factorcan be increased and thus the winding factor. The phasorialdiagram for a combined wye-delta connection is represented inFigs. 4 and 5 for the example with 15 slots and 16 magnets. Byassigning the coils to the wye or the delta connection, dependingon whether the phase of the coil best suits with the phase of thewye or delta connection, a higher zone factor can be obtained. Inthe 15-slots–16-magnets configuration, the zone factor increasesfrom

kz1 =1 + 2 cos (12◦) + 2 cos (24◦)

5= 0.9567 (4)

VANSOMPEL et al.: COMBINED WYE-DELTA CONNECTION TO INCREASE THE PERFORMANCE OF AXIAL-FLUX PM MACHINES 405

TABLE IIWINDING FACTORS FOR DIFFERENT SLOT–POLE COMBINATIONS WHICH REALIZE A BALANCED THREE-PHASE SYSTEM1

Fig. 4. Phasorial diagram of one phase for the combined wye-delta connectionfor the example with 15 slots and 16 magnets. The EMF of one coil is indicatedby Ec . The resulting EMFs of the wye- and delta-connected coils are indicatedby EY and EΔ , respectively.

to

kz1 =1 + 2 cos (12◦) + 2 cos (6◦)

5= 0.9891 (5)

which is an increase of 3.4%. The corresponding winding di-agram and actual connections between the individual coils aregiven in Fig. 6. The same can be done for all combinations ofslot and pole numbers. The obtained winding factors are sum-marized in Table II.

In order to have the same output voltage in the wye con-nection as well as in the combined wye-delta connection, the

Fig. 5. Three-phase phasorial diagram for the combined wye-delta connectionfor the example with 15 slots and 16 magnets. The EMF of one coil is indicatedby Ec . The resulting EMFs of the wye- and delta-connected coils are indicatedby EY and EΔ , respectively.

delta-connected coils should have√

3 times the numbers of thewye-connected coils. As the slot width is the same for wye-and delta-connected coils, the wire section should be reducedby

√3. In this way, Joule’s losses in both the wye- and delta-

connected coil windings are equal. In the prototype machine,the winding consists of two coils per tooth, one on each sideof the tooth (see Fig. 1), and are put in parallel. For the wye-connected teeth, on each coil 72 turns are placed in four layersof 18 turns, while on the delta-connected coils 125 turns areplaced in five layers of 25 turns. The wire diameter of the wye-and delta-connected teeth is 1.05 and 0.8 mm, respectively. This

406 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 27, NO. 2, JUNE 2012

Fig. 6. Winding diagram and actual connections between the individual coilsin the combined wye-delta connection for the example with 15 slots and 16magnets.

combination of turn number and wire diameter results in thesame output voltage and Joule’s losses in the windings. Findingthe number of turns per layer, the number of layers and wind-ing diameters in order to have a good copper filling factor forboth wye- and delta-connected coils becomes more complicatedas for the wye-connected machine. However, enameled copperwires are offered in many diameters. Moreover, the modularstator concept simplifies the winding process as each tooth canbe wound outside the machine. Therefore, it is still possible toobtain good copper filling factors.

The combined wye-delta connection will also have an influ-ence on the losses in the machine. Due to the zero-sequence fluxin the delta-connected coils, circulating currents are present andwill result in additional Joule’s losses in the copper windings.As will be shown in the following paragraphs, the losses cor-responding to the circulating currents in the copper windingsare minor compared to global losses. As the circulating cur-rent in the delta-connected coils mitigates the zero-sequenceflux, the iron losses in these teeth are smaller comparing to thewye-connected teeth and result in global lower iron losses.

Next to the Joule losses in the copper windings and the ironlosses in the teeth, the combined wye-delta connection also hasan impact on the eddy current losses in the permanent NdFeBmagnets as the airgap magnetic field harmonic content is chang-ing. The magnitude of the eddy current losses is limited to ±3and ±9 W for no-load and load, respectively. The increase inmagnet eddy current loss due to the wye-delta connection forthe examined prototype is only 3.5%, i.e., ±0.3 W, which isnegligible to the global losses and is, therefore, not taken intoaccount in the comparison.

III. FINITE ELEMENT MODEL (FEM)

The theory explained in previous paragraph is evaluated usingan FEM. The FEM is the same for both wye and combined wye-delta connections, but differs in the way in which the currentsare imposed.

Because AFPMSM’s have an inherent 3-D structure, 3-DFEMs should be used in simulations. As these 3-D FEMs arevery time consuming, “quasi-3-D” [15], [16] approximationsusing multiple 2-D FEMs at different radii are often used (seeFig. 7). The global solution is found as a weighted summationover the different 2-D FEM solutions. In this way, the com-putation time can be reduced. In such a 2-D FEM, the wholecircumference of the machine is modeled, i.e., 15 teeth and 16magnets. As the magnets are in an North-South (NS) topol-ogy [17], only half of the machine needs to be modeled by

Fig. 7. Part of one of the six 2-D FEM solutions in which the circumferenceof the AFPMSM is modeled at a given radius. Left boundary is the symmetryaxis of the vector-potential problem. Flux density levels are in Tesla.

applying the Neumann boundary condition at the center partof the teeth. In circumferential direction, obtaining the rightsolution requires the use of periodic boundary conditions. Anexample of such a 2-D FEM is shown in Fig. 7.

The torque of the generator is calculated by evaluating theMaxwell stress tensor along a line in the center part of the airgap.As the Maxwell stress tensor is very sensitive to numerical noise,a very fine mesh around the tooth tip and the airgap is requiredto obtain the right torque values.

As this paper focuses on the stator teeth, an anisotropic ma-terial model based on the magnetic energy (variant on [18]) isapplied. The anisotropic material model calculates the magne-tization vector M as a function of the induction vector B. Theequation for the magnetic potential A equals

∇×(

1μ0

∇× A)−∇× M (∇× A) = Je (6)

where Je is the external current density.The magnetic potential equation is solved statically for dif-

ferent rotor positions using ±170.000 second-order elements.The voltage waveform is calculated a posteriori based on thecalculated flux waveforms of the individual coils. The currentwaveforms, that are imposed by specifying Je , are calculatediteratively based on the voltage waveform in case of a resistiveload. The circulating current in the delta-connected coils, forno load as well as full load, is done by iteration: for each staticsimulation, the current is modified until the zero-sequence fluxis mitigated.

The iron losses are calculated a posteriori, based on the sim-ulated flux density patterns. The used model for the iron lossesis based on loss separation [19], [20] and is explained in detailin [21].

The parameters in the anisotropic material model and theloss model are fitted from data retrieved by measurements onan Epstein frame. As the used material is grain oriented (GO),magnetic properties vary with respect to the direction in which

VANSOMPEL et al.: COMBINED WYE-DELTA CONNECTION TO INCREASE THE PERFORMANCE OF AXIAL-FLUX PM MACHINES 407

Fig. 8. Prototype AFPMSM: stator view without rotors.

the field is applied. Therefore, strips in seven different direc-tions, i.e., 0◦, 15◦, 30◦, 45◦, 60◦, 75◦, and 90◦, were cut outof a insulated GO steel sheet. The complete determination ofthe material required measurements for various magnetic fieldamplitudes and different frequencies.

IV. SIMULATION RESULTS AND EXPERIMENTAL VERIFICATION

To check the validity of the simulations, a prototype AFPMSGshown in Fig. 8 was built. In Table I, the main characteristics ofthe suggested AFPMSM are listed.

As only an increase of 3.4% in winding factor is expected,simulations and especially measurements should be done withgreat accuracy. However, when both wye and combined wye-delta-connected generators are connected to the same resistiveload at the same speed, the power is equal to the square ofthe terminal voltage. This means that the output power of thecombined wye-delta connected generator will be 1.069 times theoutput power of the wye-connected generator when connected toan equal resistive load at the same speed. This increase of 6.9%should be detectable in simulations as well as in measurementson a prototype.

For simulations as well as for the measurements, a three-phaseresistive load with a phase resistance of 30 Ω is chosen, and thereference speed is set to the nominal speed of 2500 r/min. Thecorresponding current delivered by the generator will in bothcases not exceed the nominal current.

To perform measurements on the suggested prototype AF-PMSG, an experimental setup was built. In this setup, the AF-PMSG is connected to a two-pole 7.5-kW induction motor via atorque sensor. This induction motor is fed by a 11-kW inverterthat is controlled by laboratory virtual instrumentation engi-neering workbench. An optical position sensor is used to obtainthe shaft speed, and a voltage measurement on the terminals ofthe AFPMSG is performed. Although each phase consists offive teeth, the winding of each individual tooth is accessible.The data retrieved from voltage, torque and speed measurementare sampled by a National Instruments data-acquisition systemwith a sampling speed up to 250 ksamples/s.

Fig. 9. Simulated and measured back EMF waveforms of two consecutiveteeth. For the 15-slot–16-pole combination, a shift of 12◦ exists between adjacentcoil back EMFs.

Fig. 10. Simulated and measured back EMF waveforms of three wye-connected teeth and two delta-connected coils. Circulating currents in the delta-connected teeth mitigate zero-sequence components in the back EMF of thedelta-connnected coils.

In all subsequent figures, simulated and experimental datawill be represented in the same axes to allow easy comparison.

In Fig. 9, the no-load EMFs of two consecutive teeth are pre-sented. Very good correspondence between simulated and mea-sured data is found. As illustrated in the example in Section II,a shift of 12◦ exists between adjacent coil EMFs. Furthermore,it can be noticed that apart from fundamental harmonics, thecoil EMFs contain additional harmonics. In the delta-connectedteeth, the triple-harmonics in the flux will give rise to circu-lating currents. These currents strongly mitigate zero-sequencecomponents in the coil EMFs (see Fig. 10).

Simulation results of the wye and combined wye-delta con-nection are given in Table III. As predicted by the theory, ahigher average torque output is achieved by the combined wye-delta connection: an increase of 7.8% and 7.2% for simulationsand measurements, respectively, which is slightly above the ex-pected 6.9%.

408 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 27, NO. 2, JUNE 2012

Fig. 11. Simulated and measured waveform of the circulating current in thedelta-connected coils. Corresponding Joule’s losses are limited due to the smallresistance of the delta-connected coils.

TABLE IIICOMPARISON SIMULATED DATA FOR WYE AND COMBINED WYE-DELTA

CONNECTION: MACHINES CONNECTED TO A RESISTIVE LOAD OF 30 Ω

In Fig. 11, the circulating current present in the delta-connected coils is shown. Due to the relatively high inductanceof the winding, the amplitude of the circulating current is lim-ited. As the multilayer 2-D FEM does not take into accountthe effects of the end windings on the inductance, the simu-lated circulating current is higher than the measured one. Asthe resistance of a delta-connected winding is calculated to be0.2 Ω, the Joule losses in the copper losses are very limited.Table III also shows that the no-load iron losses are slightly re-duced. This small decrease can be explained by the absence ofthe zero-sequence components in the flux-density distributionin the delta-connected teeth.

The full-load phase voltage and current waveforms of thewye and combined wye-delta connection are shown in Figs. 12and 13, respectively. Good correspondence between simulationsand measurements is found. Note that the full-load phase volt-age waveforms are deformed due to the armature reaction. Ascan be seen in the current waveform of the delta-connected teethin Fig. 13, the waveform is a superposition of the fundamentalsinusoidal component and the circulation current. However, theeffect of these circulation currents in the total Joule’s losses inthe copper windings are limited and the losses in both connec-tions are nearly equal.

For both wye and combined wye-delta connection, the electricpower output was measured using a three-phase power analyzer.Power outputs of 3776 and 4048 W were measured, which arein good correspondence with the simulated ones in Table III.

Fig. 12. Simulated and measured phase voltage and phase current for the wyeconnection of the coils.

Fig. 13. Simulated and measured phase voltage and phase current for thecombined wye-delta connection of the coils. Values for the wye- and delta-connected coils are indicated separately. Note that the current in the delta-connected coils is a superposition of the load current and the circulating current.

Moreover, the power ratio of the combined wye-delta connectionis 7.2% higher than those of the wye connected, which meansthat the fundamental winding factor is increased with 3.5%.

VANSOMPEL et al.: COMBINED WYE-DELTA CONNECTION TO INCREASE THE PERFORMANCE OF AXIAL-FLUX PM MACHINES 409

Fig. 14. Simulated phase voltage and phase current for the combined wye-delta connection of the coils. Values for the wye- and delta-connected coilsare indicated separately. Note that the current in the delta-connected coils is asuperposition of the load current and the circulating current.

Subtraction of the input mechanical power of the prime moverand the electrical power is used to estimate the losses in the ma-chine. Losses at no and full load are significantly higher thanthe simulated ones: ±132 and ±250 W, respectively for bothmachines. However, next to the Joule losses in the copper andthe losses in the iron of the stator teeth that are evaluated inthe simulations, also bearing, windage and Joule’s losses due toeddy currents in the magnets are present when performing mea-surements on the prototype. Therefore, the measured efficiencyof both machines is only 94%. As the output power increases forthe same active mass, i.e., mass of copper windings, stator teethiron, magnets, and rotor back iron, the power density increases.

V. HIGHER EFFICIENCY

In previous section, the increase in torque due to the higherwinding factor of the combined wye-delta connection was il-lustrated. Good comparison between simulations and measure-ments proved the validity of the FEM. In this section, as in [22],the influence of load on the efficiency is studied. Therefore,simulations in which a fundamental sinusoidal current with anrms value of 6.7 A is imposed in phase with the back EMF aredone for both wye and combined wye-delta connection. For thecombined wye-delta connection, the voltage and current wave-forms are shown in Fig. 14. However, this time the focus willbe on the efficiency of the machine during operation.

Comparison of the data in Table IV shows that the no-loadlosses due to the circulating current in the wye-delta-connectedonly result in small Joule’s losses. No load as well as full-load

TABLE IVCOMPARISON SIMULATED DATA FOR WYE- AND COMBINED WYE-DELTA

CONNECTION: FUNDAMENTAL SINUSOIDAL CURRENT WITH AN RMS VALUE

OF 6.67 A IS IMPOSED IN PHASE WITH THE BACK EMF

Fig. 15. Simulated full-load torque waveform of the wye- and combined wye-delta connection. Although a higher average torque output is achieved by thecombined wye-delta connection, the torque ripple is higher than that for the wyeconnection.

iron losses are even lower in the combined wye-delta machinedue to the absence of triple harmonics in the delta connectedteeth. As the losses remain the same while the output powerincreases, the efficiency of the machine improves.

Despite the increase in torque and efficiency, Fig. 15 showsthat the combined wye-delta connection has higher torqueripple.

VI. CONCLUSION

In spite of the many advantages of concentrated windings,an important drawback is that most configurations have a lowerwinding factor. In this paper, it was illustrated theoretically thatin most configurations the winding factor can be increased byusing a combined wye-delta connection instead of a commonwye connection.

The effectiveness of such a combined wye-delta connectionwas illustrated for a 16-magnet–15-slot YASA AFPMSM, byusing finite element computations and measurements on a pro-totype machine. The increase in output torque is comparablewith the increase in winding factor, as theoretically expected.On the other hand, the losses in the combined wye-delta con-nection were the same as those for a common wye-connectedmachine. As the power for the same machine was increasedwithout supplementary losses, the machines efficiency isincreased.

410 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 27, NO. 2, JUNE 2012

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Hendrik Vansompel was born in Belgium in 1986.He received the Bachelor’s and Master’s degrees inelectromechanical engineering from Ghent Univer-sity, Ghent, Belgium, in 2008 and 2009, respectively,where he is currently working toward the Ph.D. de-gree at the Department of Electrical Energy, Systemsand Automation.

His research interests include electrical machinesmodeling and design, particularly for sustainable en-ergy systems.

Peter Sergeant received the M.S. and the Ph.D. de-grees in electromechanical engineering from GhentUniversity, Belgium, in 2001 and 2006, respectively.

He is a Postdoctoral Researcher for the Fund ofScientific Research Flanders since 2006, and a Re-searcher in the University College Ghent, Ghent,Belgium, since 2008. His current research inter-ests include numerical methods in combination withoptimization techniques to design nonlinear elec-tromagnetic systems, in particular, electromagneticactuators.

Luc Dupre was born in 1966. He graduated in elec-trical and mechanical engineering in 1989 and re-ceived the degree of Doctor in applied sciences in1995, both from the University of Gent, Belgium.

Currently, he is full professor at the Faculty ofEngineering and Architecture of Ghent University,Belgium. His research interests mainly include nu-merical methods for electromagnetics, modeling andcharacterization of soft magnetic materials, micro-magnetism, inverse problems and optimization in(bio)electromagnetism.

Alex Van den Bossche received the M.S. and Ph.D.degrees in electromechanical engineering from GhentUniversity, Ghent, Belgium, in 1980 and 1990, re-spectively.

He was with Electrical Energy Laboratory, GhentUniversity, where since 1993, he has been a Professorin electromechanical engineering. He is an author ofthe book Inductors and Transformers for Power Elec-tronics (Boca Raton, FL: Taylor & Francis, 2005). Hewas a starter of the spin-off companies Inverto n.v.(1990) and recently Alenco n.v. (2009). His research

interests include electrical drives, power electronics on various converter typesand passive components and magnetic materials. He is also interested in renew-able energy conversion.