01531489_PMG_AXIAL_SMALL_WT.pdf

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Axial Flux Permanent Magnet Generator with Concentrated Winding for Small

Wind Power Applications

Abstracti- An axial flux permanent magnet machine, designed

to operate as a generator in a small-scale wind-power

applications, is described in this paper. The machine is realized

by using consecrated stator winding with open slots and surface

mounted permanent magnets on rotor disk. Such a novel

generator structure is simple to construct and its performance is

good; it offers sinusoidal back-emf waveform, low torque ripple

and high efficiency. A 1.6 kW prototype machine is constructed

and is installed to a pilot power plant. Design results as well as

test result for the prototype machine are reported in this paper

I. INTRODUCTION

Small-scale wind power plants are an attractive choice togenerate electrical power on rural areas where the installation

of the distribution network is not economically reasonable. In

such locations, e.g. on small islands, wind power plants orsolar sells or both together can be used to charge batteries or

in direct heating purposes. Concerning stand-alone windmill

applications the rated power of which is below 10 kW, the

use of permanent magnet machines as a generator has been

studied intensively recently. In [1] a 2 kW permanent magnetmachine with radial flux structure was proposed and in [2]-

[4] studies related to the TORUS type axial flux machineswere presented. In these studies the common feature is the

use of direct-drive low-speed generators. The low-speedgenerator does not require step-up gearbox in power

transmission between the turbine and the generator, which is

typically required in more conventional windmill concepts.As a result, the direct-drive concept improves the reliability

of the system and reduces the maintenance costs.

In this paper a single sided axial flux permanent magnet

generator with double-layer concentrated stator windings is

described. Test result obtained from the prototype machine aswell as test-results obtained from the pilot power plant are

reported. The prototype machine has been on operation in a

pilot power plant since autumn 2003. Fig. 1 illustrates theused generator concept and presents the generator as it is

installed to a windmill structure.

(a) (b)

Fig. 1. (a) Structure of generator. (b) Generator as a part of turbine structure.

II. AXIAL FLUX PERMANENT MAGNET MACHINE WITH DOUBLE

LAYER CONCENTRATED WINDING

Concentrated stator windings are an effective solution toreduce Joules losses in electrical machines [5]. By

combining the concentrated winding and axial flux

permanent magnet machine, which is proven to offer a hightorque to volume ratio [6], a high performance electrical

machine is obtained. In a concentrated double layer winding,each of the coils is wound around one tooth. A layout of sucha winding is illustrated in Fig. 2. By introducing open slots it

is possible to manufacture the coils beforehand and just

install the coils around the teeth. This procedure makes the

winding to be realized with low price. However, some

concern must be paid how to ensure the fixing of the coilaround the tooth due to the lack of the tooth tip. The

concentrated winding makes the coil ends to be short in the

radial direction. In the case of axial flux machines this solves

the problem how to arrange the end-windings in the limitedspace between the shaft and the inner radius of the stator

core, which can be a severe problem for conventional 3-phase

windings. Short end-windings decrease also the overallexternal diameter of the axial flux machine and thereby the

overall space required by the machine.

A. Parviainen and J. PyrhnenLappeenranta University of Technology

Department of Electrical EngineeringSkinnarilankatu 34

Lappeenranta, Finland

P. KontkanenKylmtec Ky

Jokipellonkatu 13Outokumpu, Finland

B-

A+

A-

A-

A+

C-

C+

C+

C-

B+

B-

B-

B+

A-

A+

A+

A-

C+

C-

C-

C+

B-

B+

B+

Fig. 2. A winding layout on 2D plane for axial flux machine with double layer concentrated winding. Winding configuration is for 12 slots and 14 poles.

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As a drawback of the single sided axial flux machine

construction, illustrated in Fig. 1 (a), there appears quite a

large uncompensated attractive forceFbetween the rotor and

the stator and is given by

0

2agapagapp

2

BSF

, (1)

whereSagap is the air gap surface area

Bagap is the air gap flux density

p is the relative rotor pole surface area covered by the

permanent magnet

0 is the permeability of free space

The attractive force causes that the bearing system must be

capable of tolerating it. This demands using of a thrustbearing as a main bearing on the shaft. Rotor disk has to be

also thick enough in order to avoid excessive deflection. Thedeflection of the rotor disk due to the magnetic forces may be

calculated analytically by employing methods given by

Yuang in [6], for example. For double sided axial fluxmachine structures these mechanical concerns are cancelled

out during machine operation because the double air gapsystem causes that the total axial force affecting the rotor disk

is small. If compared to the TORUS type axial flux machines

realized with airgap winding [2], an advantage of the

proposed structure is the slotted stator. The slotting increasesremarkably the amplitude of the airgap flux density due to the

shorter airgap and consequently this reduces the required

amount of permanent magnets yielding savings in thegenerator price. As a drawback, the leakage and mutual

inductances are increased compared to the air-gap winding.

The slotting may also evoke undesired torque pulsations and

cause additional losses.

III.PROTOTYPE MACHINE

A. IntroductionThe design of the prototype generator was made under

geometrical constraints given by the turbine manufacturer.

The maximum allowed external diameter for the generator

was 300 mm including the end-windings and the inner

diameter including the end-windings had to be over 120 mm

because of the main thrust bearing is integrated to the space

available inside the stator. The required output power

(against resistive load) was 1.6 kW at the speed of 250 rpm.

Apart from the previous constrains the followingrequirements were given:

Machine structure must be simple and easy to

manufacture

Higher efficiency compared to commercially

available 1.5 kW DC-generators

Torque ripple below 5 % of the rated torque

Due to the first constraint, a concentrated winding structure

was considered. By realizing the machine 3-phase winding

with double layer concentrated winding, only certain stator

slot - rotor pole combinations are possible to use [7].

Obviously combinations, which have high fundamental

winding factor and low amplitude for cogging torque, arefavorable. In the reported application, the geometrical

constraints restrict the selection of slots and basically only

slot numbers below 24 were feasible to employ. Theemployed machine structure was selected among the

configurations presented in Table I. The selection was partly

based on the results of 3D finite element analysis (FEA),

which was performed for the reported machine

configurations.By taking into account the geometrical limitations and the

construction simplicity, efficiency and torque ripple

requirements a machine with 12 slots in stator and 14 poles in

the rotor was selected. The main parameters of the machine

are reported in Table II. The results of 3D FEA for the

selected machine configuration are reported in following

chapters. The stator and the rotor of the machine are shown in

Figs. 3-4. Such a structure utilizes the PMs effectively in the

case of open slots; the surface area of one PM is slightly

higher compared to area of one tooth. Higher number of slots

will decrease the relative length of the end-winding with

respect to the active part of copper but on the other hand the

number of the coils required increases increasing the

manufacturing costs.

Fig. 3. The stator and the rotor of the prototype machine.

Fig. 4. The prototype machine while assembling the parts together.

TABLE I

STUDIED MACHINE CONFIGURATIONS

Number of stator

slots QNumber of rotor poles

12 10

12 1418 22

24 22

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B. Modeling of the machine

The inherent 3D geometry of the axial flux machine causesthat the accurate modeling of the machine requires the use of

3D finite element (FE) software. Because of the concentrated

winding, one cannot model only one pole in the FE-model as

is usually done with conventional machines. This yields to

situation in which several poles have to include to the FE-model or even whole machine has to model in order to obtain

the symmetry. As a result the obtained 3D FE-model is verytime consuming to solve even by using present day efficient

personal computers. Fig. 5 illustrates the 3D FE-model of thestudied axial flux machine. Pre-dimensioning of the machine

is done via analytical approach and by using 2D finite

element analysis (FEA).

C. Stator connection

In order to avoid circulating currents the stator of machine

is connected to star. For 3-phase machines the third harmonicand its odd multipliers are in 360 degree phase shift with

respect to each other thus forming circulating currents if the

back-emf includes those harmonics and the machine isconnected to delta [8]. Among the studied machine

configuration the selected 12-14 structure produces the mostsinusoidal back-EMF waveform. In order to analyze weather

the circulating current is harmful or not in the case of this

particular machine, the machine was modeled in 3D in deltaconnection and the circulating current was determined from

no-load computations. Fig. 6 shows the observed circulating

current in delta connection according to 3D-FEA. In this casethe waveform of induced back-emf is very sinusoidal,

thereby the amplitude of

Fig. 5. 3D FE-model of axial flux machine.

Fig. 6. Circulating current in delta connection under no-load situation.

the circulating current is low, about 3 % from the rated

current of the machine. Even though the circulating current is

small compared to the nominal current, it is undesired and thestar connection is used.

D. Torque analysis

In direct-drive windmill applications smooth torque is

required from the generator, meaning low amplitude for the

cogging torque. This improves the starting capability of the

turbine at low wind speeds and reduces vibrations. Thecogging torque was computed by using 3D FEA. The

obtained peak value of the cogging torque was 1 Nm forthe prototype generator. The torque behavior under load

condition was analyzed by solving a time transient 3D FE-model with circuit coupling. Fig. 7 shows the obtained torque

from the 3D FEA.

Fig. 7. Calculated torque waveform.

IV. MEASUREMENT RESULTS

Several test runs were performed for the generator under

load and no-load conditions. The obtained efficiency for the

machine was 82 % in the test run at nominal load. Fig. 8shows the measured no-load phase voltages for the prototype

machine while the temperature of the magnets is 20 C.Highly sinusoidal voltage waveforms are observed. In Fig. 9,

phase currents are reported. Fig. 10 shows the obtained

temperatures in the stator winding during a test run. One has

to notice that the machine is designed to operate outdoors, so

TABLE II

PARAMETERS OF PROTOTYPE GENERATOR

Parameter Definition Value

P Nominal output power at speed 250 rpm 1.6 kW

Uph Output voltage (RMS) at speed 250 rpm 75 Vhm Thickness of PM

5.0 mm

g Physical length of airgap 1.0 mmQ Number of stator slots 12

p Number of rotor poles 14Dout Stator outer diameter 258 mm

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the operating temperature of the PMs and the stator winding

is in practice lower that the ones obtained in the laboratory

environment. When the generator produces maximum power,

the wind speed is also high and thereby the cooling of themachine is better than in the laboratory environment without

external cooling fan.

0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055 0.06

-150

-100

-50

0

50

100

150

t[s]

Eph

[V]

3D-FEA

Measured

Fig. 8. Measured and calculated no-load phase voltages. The difference in

amplitudes is due to the physical air gap length. Due to the manufacturingtolerances it was slightly larger in a prototype than in FE model.

0.02 0.025 0.03 0.035 0.04 0.045 0.05

-10

-5

0

5

10

t[s]

Iph

[A]

3D-FEA

Measured

Fig. 9. Measured and calculated phase currents.

0 60 120 180 240 300 36020

30

40

50

60

70

80

90

100

110

120

130

t[min]

T

[C]

Fig. 10. Measured phase windings temperatures in a test run at nominal load.

IV. PILOT POWER PLANT

The prototype generator is installed to the pilot power plant,Fig. 11. The power plant has been in operation since

November 2003 successfully. The pilot power plant is used to

study the performance of the proposed generator under actual

conditions. In Fig. 12 the test results are reported under actual

wind conditions. As can be noticed, the wind-speed requiredto achieve the rated power lies 12 m/s. A comprehensive

report related to the results obtained from the pilot plant willbe reported separately.

Fig. 11. Small-scale wind power plant.

0 30 60 90 120 150 180 210 2400

250

500

750

1000

t[min]

P

[W]

0 30 60 90 120 150 180 210 2402

4

6

8

10

12

t[min]

v[m/s]

0 30 60 90 120 150 180 210 2400

250

500

750

10001250

1500

t[min]

P

[W]

0 30 60 90 120 150 180 210 2402

4

6

8

10

12

t[min]

v[m/s]

Avarage electrical power (per minute)

Average wind speed (per minute)

Instantaneous electrical power

Maximum wind speed (per minute)

Fig. 12. Measurement results obtained from the pilot power plant.

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V. CONCLUSIONS

A 1.6 kW axial flux permanent magnet generator for direct-drive wind-power applications was described. The

proposed concentrated winding concept with open slots

makes the generator manufacturing easy and it offers good

performance. The obtained phase voltage and current is

practically pure sine wave and the torque ripple is low. Theefficiency of the machine is also good. As an authors

opinion, the mechanical structure of the machine proposedis simpler and cheaper to construct than the other axial flux

machine configurations with laminated stator cores.

ACKNOWLEDGEMENT

The work presented was a part of DENSY-program(Distributed Energy Systems Technology Programme),

financed partly by TEKES, national technology agency of

Finland. Authors would like express their gratitude for theprogram personnel due to resources provided by the program.

REFERENCES

[1] A. Schmidhofer and H. Weiss, "Optimisation of Power Electronics for

Small Stand Alone Wind Power Stations," in Proceedings of 10th

European Conference on Power Electronics and Applications(EPE2003), Toulouse, France, on CD-ROM, September 2-4, 2003.

[2] B.J. Chalmers, W. Wu, and E. Spooner "An Axial-Flux Permanent-Magnet Generator for A Gearless Wind Energy System," in IEEE

Transaction on Energy Conversion, vol. 14, 1999, pp. 251-257.

[3] J. Azzouzi, G. Barakat, and B. Dakyo, "Analytical Model for a

Magnetic Design Approach of an Axial Flux Permanent MagnetSyncronous Machine for Wind Energy Application," in Proceedings of

10th European Conference on Power Electronics and Applications(EPE2003), Toulouse, France, on CD-ROM, September 2-4, 2003.

[4] L. Sderlund, J-T. Erikson, J. Salonen, H. Vihril, and R. Perl, "APermanent Magnet Generator for Wind Power Applications," in IEEE

Transaction on Magnetics, vol. 32, 1996, pp. 2389-2392.[5] F. Magnussen and C. Sadarangani, "Winding Factors and Joules Losses

of Permanent Magnet Machines with Concentrated Windings," in

Proceedings of IEEE International Electric Machines and Drives

Conference, IEMDC03, Madison, United States,pp. 333-339, 1-4 June,2003.

[6] Z. Zhang, F. Profumo, and A. Tenconi, "Axial flux versus radial flux

PM motors," in Proceedings of SPEEDAM, Capri, Italy, pp. A4-19 -

A4-25, 5-7 June, 1996.

[7] P. Salminen, Fractional Slot Permanent Magnet Synchronous Motorsfor Low Speed Applications. Dissertation, Lappeenranta University of

Technology, 2004, p. 150.

[8] E. Nipp, "Examinations of Torque Interruptions and Circulating

Currents in PM Synchronous Motors with Switched Stator Windings,"Proceedings of the International Conference on Electrical Machines,

ICEM'98, Istanbul, Turkey, pp. 273-278, September 2-4, 1998.[9] W. C. Yuang, Roarks formulas for stress and strain, Singapore:

McGraw-Hill Book Co., 1989, p. 763.

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