Abstract- Due to their high efficiency and reliability, permanentmagnet synchronous machines are widely used in low speeddirect-driven applications. The present paper deals with thedesign and analysis of a special topology of a PMSG suited forsmall rating, direct-driven wind turbine. A preliminary designprocedure is presented and the results are introduced in FEMbased software in order to analyze the performances of themachine. The theoretical approach is validated thoughtexperimental measurement and methods adopted to improveenergy performance.

Index Terms—permanent magnet claw pole synchronousgenerator, design, 3D magnetic field analysis, experimentalanalysis.

I. INTRODUCTION

In wind power applications different types of electricalmachines have been proposed and used. Permanent magnetmachines are widely used in most of the low speed windturbine generators, due to their high efficiency andreliability.Synchronous PM generators can be divided into radial, axialand transversal flux machines. The availability of modernhigh energy density magnet, such as NdFeB, has madepossible designing special topologies.

The replacement of the rotor excitation winding withpermanent magnets in a synchronous machine brings the wellknown advantages of a simple rotor design without fieldwindings, slip-rings and exciter generator, lower heatdissipation and higher overall efficiency. The development inpower electronics, enabling energy efficient drives, hasincreased the interest in permanent magnet synchronousmachines, as attractive rivals to the common asynchronousgenerators, in small and medium power systems, autonomousand parallel, connected to the electrical grid. Permanentmagnet synchronous machines use three rotor topologies:surface mounted permanent magnets, buried permanentmagnets and claw-pole configuration. Claw pole machinesare commonly used as automobile, wind and hydrogenerators due to their simplicity and low manufacturingcost. The higher number of poles makes it an alternative invariable speed systems. The permanent magnet replaces thecircumferentially wound global excitation coil, increasing theoverall efficiency of the machine [10, 11].

The present paper approaches the analysis of claw polepermanent magnet generator performances, by means of the3D finite elements method. Four claw-pole solutions wereconsidered in order to study the influence of differentparameters of the rotor on the machine performances, i.e.magnetic field density, saturation, induced emf.

Technical University of Cluj Napoca, Department of ElectricalMachines, Cluj, Romaniaflorin.jurca@mae.utcluj.ro

II. CLAW-POLE PERMANENT MAGNETSYNCRONOUS GENERATOR

Claw-pole topology is a special one and could be appliedto different classes of electrical machines with either rotor orstator built using this structure. Some well knownapplications are the claw-pole alternators, the eddy-currentcoupling and brakes and permanent magnet synchronousactuators [1], [2].

Fig. 1. A general view and the rotor claw-pole structure of the analyzedgenerator

The automobile alternator, using a claw-pole rotorstructure, employs a homopolar winding current, apermanent magnet or both solutions for producing themagnetic field. The permanent magnets are placed aroundthe shaft, between the claws and core, on the claw surfaces orinto the space between the claws. This topology allows theincreasing of the pole number, without reducing the

Design and Development of a Three-PhasePermanent Magnet Claw Pole SynchronousGeneratorPreparation of a Formatted Technical Work for

the ICEM

Florin Nicolae Jurca, Claudia Marţiş, Karoly Biro, Claudiu Oprea

XIX International Conference on Electrical Machines - ICEM 2010, Rome

978-1-4244-4175-4/10/$25.00 ©2010 IEEE

magnetomotive force per pole. This feature makes it suitablefor low-speed direct driven wind turbine.

The proposed claw-pole topology for wind conversionsystems is developed as 8-poles permanent magnetsynchronous one, with the configuration presented in figure1.

III. PRELIMINARY DESIGN

An electrical machine design problem is to find a setconsisting of topological structure, materials, and geometryfor a specific application. The selection of the machineproper topology for a specific application is a difficultproblem to be solved during the design process.

Considering the proposed topology, a very challengingtask is the selection of the proper combination of the numberof stator slots and the number of rotor magnetic poles. Thenumber of stator slots is given by:

Zs=mi (1)

for a m-phase machine, i being an integer. There are someother important criteria for the selection of the number ofslots, as: harmonic content and amplitude of the back-emf,corresponding rotor pole number, torque quality, acousticbehavior, etc. The selection of the proper number of rotorpoles is based on: operational speed of the motor in theapplication, torque and power density aspects, correspondingstator slot number, cogging torque constraints, etc.

The analyzed topology is 3-phase, 36 stator slots/8 rotorclaw-pole rotor with Alnico axially permanent magnetmounted on the shaft, between the steel plates carrying theclaw-poles. The dimensioning procedure was applied for thefollowing set of key parameters: apparent power Sn – 150(VA); rated voltage Un – 100 (V); rated speed nn – 750rpm;pole pair number p – 4 [11].

For stator leakage inductance and resistance neglected,the output power of a synchronous generator is given by:

3n

nis nC

p2S60D

(2)

with Dis - the stator inner diameter, C- Esson constant, λ –geometrical form factor. The outer stator diameter and thestack lenght result from:

isdos DKD (3)

BAnDkS60

lpN

2is

2ca

n

, i

wca

22k

k

(4)

with l - the stack length, , kw – winding factor, αi - polecovering coefficient (with a value between 0.6 and 0.9, as afunction of the permanent magnet position and location), Ap -electrical loading, Bg - magnetic loading (usually between0.4 and 0.8 T).

Special attention has to be paid to the rotordimensioning. The structure of one steel plate is presented infigure 2. The outer diameter results as the difference betweenthe stator inner diameter and the double of the air-gap length(0.8 mm):

Dor=Dis-2g (5)The rotor claw pole width results as:

rotpol )8.06.0(b (6)with τrot – rotor pole pitch. The height of the rotor pole tip isgiven by ghspg )42( , and the height of the rotor baseresults from:

rp

rpp B

Blh

(7)

with the length of the rotor pole, smaller than the rotorlength, lrp = lr – (5..15%)lr , lr – the rotor length, Bg – air-gapmagnetic field density, Brp – rotor pole magnetic flux density,σ – leakage coefficient. [3], [11].

Fig. 2 One steel plate

Fig. 3 Claw-pole

The resulted main dimensions are given in Table 1.

Table I. Main dimension of the permanent magnet claw pole synchronousgenerator

Stator inner diameter 0.095 [m]

Stator outer diameter 0.145[m]Stack length 0.110[m]Rotor diameter 0.0924[m]Rotor pole width 0.029[m]Rotor pole length 0.105[m]Rotor pole base height 0.015[m]Steel plate radius 0.031[m]Rotor pole tip height 0.021[m]Permanent magnet diameter 0.06[m]Shaft diameter 0.022 [m]

IV. MAGNETIC FIELD ANALYSIS

The finite element method (FEM) is a powerful tool fordesign of electrical machines and others electromagneticdevices. FEM is a simple, robust and efficient widely usedmethod of obtaining a numerical approximate solution for agiven mathematical model of the machine. Due to thecomplex three-dimensional structure of claw-poles, themagnetic field inside the claw-pole generator needs to betreated as a 3D problem, using Flux 3D software,considering the geometry details and non linear magneticproperties of the materials [2].

The magnetic field analysis using FEM involves threestages: pre-processing, field solution and post-processing.

Development of the geometry, problem definition (choiceof the material, ferromagnetic iron non-linear characteristicintroduction, magnetizing the magnets in the requireddirection) and mesh generation are done in the pre-processing phase.

Once the basic structure of the machine was introduced,the mesh was generated and the field equations solved, forno-load regime. Figure 3 presents the generated mesh of themachine. In order to obtain more accurate results, a highquality domain discretization will be applied in the importantsections of the machine, as air-gap and rotor poles.

The results of the computation, in terms of magnetic fielddensity, induced emfs or cogging torque can be extracted inthe post-processing stage. For the proposed machine, themaps of the magnetic flux density, its distribution in the air-gap and in the stator tooth are interesting.

Fig 4. Generated mesh

As it can be noted, the magnetic field density value atthe basis of the claw-pole, where the direction of themagnetic field path is changing, is close to the saturationvalue. Since, due to its complex three-dimensional structure,the rotor claws are usually press-formed out of solid metal orpressed in a die of iron powder, magnetic field distributionand harmonic content are very useful to develop an optimumdesign in terms of low eddy-currents losses.

The wave form presents maximum values when the air-gap is minimum and minimum values when the air-gap ismaximum. As it can be noted in figure 6, the induction undereach rotor pole has the same value, the peaks correspondingto the four stator teeth overlapped with one rotor pole [7],[8].

Fig 5 Magnetic field density maps

0 10 20 30 40 50 60 70 80 90-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

[o]

B [T

]

Fig 6. Magnetic field density in the air-gap along a pole-pair.

The emf in the stator windings of the claw polesynchronous generator is given by:

12 wbkpfE

(8)

The emf value results 57V, when the pole flux p wascalculated within 3D-FEM analysis and the series number ofturns per phase 750w1 .

V. EXPERIMENTAL APPROACH OF THE CLAW-POLESYNCHRONOUS MACHINE

An experimental set-up was realized in order to measurethe value of rated voltage of the claw-pole generator. A DCmachine was connected to the generator to provide thenecessary torque. The major parameters of DC motor are:rated voltage-110V, nominal speed-1450 rpm.

Fig 7 Experimental set-up. The theoretical results, obtained through the magneticfield analysis using Flux 3D, are validated by measurementson the developed test bench. The synchronous generator wastested, for no-load conditions, at three driven speed: 500,750, 1000 rpm. The induced emf is presented in Fig. 8.Table II presents the rms value of the induced emf for thethree tested driven speeds [11].

Table II. Induced emfrated speed Field analysis Experimental

measurements500 rpm 35 38750 rpm 56 57

1000 rpm 70 72

Working as a stand–alone power plant or as a powerplant connected to a grid, an important problem to solve inrenewable energy sources plants is to produce energy withoptimal quality parameters. One of the parameters thatdefine electrical energy from the quality point of view is thecontent of harmonics, given by the consumers and thegenerators. For the claw pole synchronous generator thefrequencies of the voltage spectrum are given as:

rotstator ff (9)where ,...2,1,0k,1k2 represents the space harmonicsof the rotor mmf in the air-gap. Fig. 9 depicts the harmoniccontent of the induced emf for 500, 750 and 1000 rpm drivenspeed, respectively [4], [6].

Fig 8. Claw-pole generator voltage for 500, 750 and 1000 rpm

As it can be noted in Fig. 9, the third harmonic is themost important component of the spectrum. The magnitudesof first and third harmonic for each considered situation arepresented in Table III.

Table III. First and third no-load induced emf harmonicsFirst harmonic Third harmonic

ratedspeed Frequency Magnitude Frequency Magnitude

500 rpm 33.3Hz 29 99.9Hz 7750 rpm 50Hz 57 150Hz 9

1000rpm 66.6Hz 69 199.8Hz 13

For DC isolated consumers, small wind turbinesequipped with permanent magnet claw pole synchronousgenerator could be a solution. A constant level of the

generated voltage for a wide range of speed and load, witha high efficiency, will be obtained by using a capacitor.

Fig 9 Stator voltage harmonic for 500, 750 and 1000 rpm.

The claw pole generator was tested for analyzing itsperformances as an insulated power source for feeding DCconsumers. The generator was driven by a DC motor and theAC induced emf was rectified by a rectifier. A capacitorbank was used for keeping a constant level of the DC voltagefor a wide range of speed and load. As it can be noted fromFig. 10c, the number of harmonics is increasing, and thelevel of third harmonic is also very high, due, mainly to thearmature reaction phenomenon.

Reducing the harmonic magnitude and content istherefore an important issue. These could be achieved byintroducing in the electrical circuit two diodes as it can beseen in Fig. 11. The two diodes are mounted between thenull of the generator and the rectifier bridge. To obtainconclusive results, the measurements have been made forthree levels of speed - 500, 750 and 1000 rpm. Table IVpresents the DC voltage for each situation [5].

Fig 10. DC voltage (a), AC voltage (b) and harmonic spectrum (c) for therated speed

Table IV. DC voltageRated speed [rpm] 500 750 1000

Ucc [V]-without saving diodes 29.6 49 75

Ucc [V]-with saving diodes 36 63 92

Fig11. Layout diodes AD+ and AD- saving

Fig 12 DC voltage (a), DC voltage (b) and harmonic spectrum (c), withwith saving diodes for the rated speed

VI. CONCLUSIONS

The paper presents the theoretical and experimentalapproach of a claw pole topology for synchronousgenerators, suitable for wind conversion systems. Thepreliminary design model of the machine was developed,followed by a simulation carried on by implementing thetopology in Flux 3D. The experimental results validate thetheoretical approach.

The harmonic content of the voltage spectrum is animportant item in the evaluation of the energy quality in windsystem with claw pole synchronous generator. In order toanalyze the harmonic behavior of the machine, the spectrumof the stator voltage of the machine was developed, based onthe measured voltage.

As an insulated power energy generator, the claw polesynchronous generator can be used for feeding DCconsumers. For reducing the harmonic content of thegenerated voltage, the saving diodes are used.

The harmonic behavior of claw pole synchronousgenerator could be improved. The next step in the research

of the harmonic behavior of this type of electrical machineswill approach an optimization method in order to reduce theharmonic content of the stator voltage.

VII. REFERENCES

[1] Rameshol, I., Henneberger, G., “Calculation and measurement of timecharacteristics of local field quantities of local in the air-gap of claw-pole alternators” IEEE Transaction on Magnetics, vol:33, Issue: 5, Sept1997 Pages:4200-4202.

[2] P.P. Silvester, Finite elements for electrical engineers, CambridgeUniversity Press, 1990

[3] F. Jurca, C. Martis, C. Oprea, K. Biro “ Claw-Poles Machines in thePower Systems based on Renewable Resources,” InternationaleConference on Power Electronics, Drives and Motion (PCIM),Nürnberg (Germany), 2006, CD 123_PP_64_Jurca.pdf,

[4] C. Martis, B. Tataranu: Analytical Description of the SynchronousMachine Frequency Response For Diagnosis Purposes, VolumulConferintei Internationale Power Electronics, Drives and Motion(PCIM), Nürnberg (Germany), 2006, on CD 122_PP_62_Martis.pdf.

[5] S. Risse, G. Henneberger – “Increasing the power of alternator for thenext car generation: A simulation approach” International ConferenceElectrimacs’ 1999, Vol II, pp:273-278.

[6] C. Martis, F. Jurca, C. Oprea, C. Nicula, K. Biro: Harmonics Analysisin Renewable Energy Sources Based on Induction and SynchronousGenerators, International Conference MicroCAD, 2006, Miskolc(Hungary) Section J: Electrotechnics and Electronics, pp. 41 – 47

[7] F. Jurca, C.Martis, I. Birou, K. Biro - “Analysis of a claw-polesynchronous machine for wind power module” InternationalConference on Electrical Machines, ICEM 2008, Portugalia, ISBN:978-1-4244-1736-0.

[8] F Jurca., C.Martis., Emil Trifu., K .Biro, “Permanent magnet claw-polesynchronous generators behavior in wind conversion- “ InternationalPCIM Europe 2006, Nurnberg, Germany, on CD, may 2007.

[9] F. Jurca, I. Birou, C.Martis.,” Finite element magnetic field analysis ofa claw-pole synchronous generators for wind conversion systems”ISEF’-Praga 2007 pp 61-63,

[10] Hennberger, G., Kupperes, S., Ramesohl, I.: “Numeral calculationsimulation and design optimization of claw-pole alternators forautomotive application”, Proc. Of the 7th Int. Conf. on ElectricalMachines and drive, 1996.

[11] F. Jurca, “Claw pole generator for small electric systems” Ph.D ThesisTehnical University of Cluj-Napoca, 2009.

VIII. BIOGRAPHIES

Florin Nicolae Jurca graduated Technical University of Cluj Napoca,Romania in 2004 and got his PhD in Electrical Engineering from the sameuniversity in 2009. He published 26 scientific papers, most of them inConference proceedings and 1 textbook. He is now assistant lecturer at theDepartment of Electrical Machines, Technical University of Cluj Napoca,Romania. His main field of interest is in permanent magnet synchronousmachine.Claudia Martis graduated Technical University of Cluj Napoca, Romaniain 1990 and got her PhD in Electrical Engineering from the same universityin 2001. She published 64 scientific papers most of them in Conferenceproceedings and 3 textbooks. She is now professor at the Department ofElectrical Machines, Technical University of Cluj Napoca, Romania. Hermain field of interest is in permanent magnet and switched reluctancemachines for energy efficient fault tolerant applications.Karoly Biro graduated Technical University of Timisoara, Romania in1962 and got his PhD in Electrical Engineering from the same university in1978. He published 124 scientific papers, most of them in Conferenceproceedings and 14 textbooks. He is now professor at the Departament ofElectrical Machines, Technical university of Cluj Napoca, Romania. Hismain field of interest is in: experimental parameter determination ofelectrical machines, special electric machine design, numeric modeling oftransient behavior of electrical machines, study and numeric modelingconverter- machine systems, motor fault detection and diagnosis.Claudiu Oprea graduated Technical University of Cluj-Napoca, Romaniain 2004 and got his PhD in Electrical Engineering from the same universityin 2010. He published so far 21 scientific papers, most of them inConference proceedings. He is now assistant lecturer at the Department ofElectrical Machines, Technical University of Cluj Napoca, Romania. Hismain fields of interest are permanent magnet linear generators and brushlessmotors.