Design and Analysis of an Axial-Field Permanent Magnet Generator with Multiple Stators and Rotors

Erol Kurt1, Member, IEEE, Serdal Arslan2, Mehmet Demirtaş1 and M. Emin Güven1 1Department of Electrics, Faculty of Technical Education, Gazi University, 06500 Teknikokullar Ankara Turkey

2Department of Electrical and Electronics Engineering, Faculty of Engineering, Hakkari University, 30000 Hakkari ekurt@gazi.edu.tr, serdarelk@hakkari.edu.tr, mehmetd@gazi.edu.tr, meguven@gazi.edu.tr

Abstract-An innovative axial-field permanent magnet generator (AFPMG) with multiple stators and double rotors is designed and the structural and electromagnetic features are investigated. This generator has two permanent magnet (PM) rotors at the left- and right-hand sides of a double-sided stator. In order to get benefit from the magnetic flux produced at the outer parts of the generator, two other stators are also positioned near the PM rotors. AFPMG is mainly considered for wind energy applications and it has three phases. The electromagnetic features are characterized by finite element method (FEM) and the transient solutions are analyzed based on a proposed equivalent magnetic circuit model using three phases of induced emf at the coils having two different geometries. New AFPMG generates 80V at each phase for a constant velocity of 125 rpm and it has an efficient dynamics with low cogging torque values.

I. INTRODUCTION

Recent technological improvements on the production of permanent magnet (PM) materials and power electronics have enabled new design, construction and applications of permanent magnet synchronous generators (PMSGs). The PMSGs are generally preferred over other generators and motors in the sense that they have high efficiency, high torque, compact structure, and fast dynamic response [1,2,3,4]. If an appropriate air gap between rotor and stator and a suitable design shape are provided, one can use them in many applications which require minimal torque ripple, acoustic noise and reduced vibration [1,4,5]. In the literature, there exist a number of PMGs, however, one can classify them in two groups [2,6]: Axial and radial field PMGs. Among them axial-field permanent magnet generators (AFPMGs) have advantages of lower cost, minimal cogging torque and medium speed operation over the axial field ones [6,7]. Therefore new control approaches can be used for position control in machine tools, robotics and torque control. Although these generators have the most promising nature, they also face with temperature rise problem [6]. In order to overcome this problem, one needs to consider a better design to enable a promising cooling. Therefore a new AFPMG design is proposed and the electromagnetic field analysis has been carried out since it is very significant to calculate the characteristics of the generator, accurately. Within this context, we aim to have higher torques in medium speeds, a better air cooling mechanism with respect to the positioning of magnets between the multiple stators and a minimal cogging torque with this new design.

This paper includes the structural features of a new PMSG and magnetostatic and electromagnetic transient simulations as well. While Sec. II includes the design process, the results of the magnetostatic solutions are presented in Sec. III. Time-dependent electromagnetic simulations together with the three-phase circuitry are given in the next section. Finally the concluding remarks are presented in the last section.

II. DESIGN PROCESS

The components of an axial-field permanent magnet generator (AFPMG) with multiple stators and double rotors are indicated in Fig. 1. This design includes double PM rotors attached to one axis and each PM has an axially oriented flux in an N-S-N-S arrangement.

Apart from the earlier studies [1,2,6,7], PMs have two distinct types: Trapezoidal magnet Mtra and triangle magnet Mtri (Fig. 2).

This feature gives an advantage to prevent the heat density

on PMs, since their locations are farther inside the volume of rotating band. Two magnet sets have a 15 degree shift azimuthally in order to facilitate the initial movement apart

Fig. 1. The components of the axial-field permanent magnet generator(elongated along z-axis): PM rotors, stators and coils are indicated by R, Sand C, respectively. While red, green and blue denote A, B and C phases; yellow and brown denote the non-magnetic insulating and core materials respectively.

Fig. 2. The geometries of axial flux PMs. Triangle (left) and trapezoidal (right) magnets have bended from corners in order to minimize the flux inhomogeneties. The thickness is 1 cm.

Proceedings of the 2011 International Conference on Power Engineering, Energy and Electrical Drives Torremolinos (Málaga), Spain. May 2011

978-1-4244-9843-7/11/$26.00 ©2011 IEEE

from the other studies in literature. The detailed geometries of PMs are given in Fig. 2. The magnets can be fixed in any non-magnetized material such as polyester, Teflon, etc… These two types of PMs are located side by side to construct two different circular materials labeled as rotor R1 and R2 and their diameters are 176 mm and 300 mm (Fig. 3). R1 and R2 have 12 magnets.

The stators include two types of coil systems: Trapezoidal

coil Ctra and triangle coil Ctri. The coil wires have 1 mm diameters. While the stators S1 and S3 have coils on only one side, S2 have two sides (Fig. 4).

Each coil system has cores made by M19. In order to minimize the heat and current density effects on the core S2, the locations of active stator areas at the middle of coils are arranged in to different circular regions having different diameters. Such an orientation provides a large area in order to cool down the core for high speeds. While the stators S1 and S3 have 12 coils in trapezoidal and triangle shapes, S2 has 24 coils at two sides (Fig. 4). This configuration enables the magnetic flux to pass inside the core radially from the peripherical region to the center or vice versa depending on the poles. The stators S1 and S3 are put opposite the magnets being at the left and right-hand sides of S2. Therefore, flux can continue along the axial direction. Note that any non-magnetic material can be used in order to attach S3 to the stator yorke. Note also that there exist an air gap of 2 mm between stators and rotors.

According to the above geometry, the active areas of two types of coils are ACtra= 1225 mm2 and ACtri = 336 mm2. Considering the entire active area, one arrives at Atot = 0.037 m2 for 48 coils. A rough estimation on the power output can be calculated by using;

)./(2INS>B<)+/(2NIS>B< P ''core''

core (1)

Here, primes indicate the values for Ctri. <B>, , I and N denote flux density, angular velocity, winding current and number of turns, respectively. When one uses large number of pole pitches (i.e. coil), that limits the active area of the stator. Therefore a nominal number for poles should be used. According to Eq.(1), for an average field value of B=0.6 T for the air gap, one roughly arrives at a value of P=2kW. Note that the numbers of coils are 24 for trapezoidal and triangular shapes, Eq. 1 should be multiplied by 24.

III. MAGNETOSTATIC SOLUTIONS

In order to identify the magnetic features, the flux lines can be taken from different regions of the complicated structure. In Fig. 5, flux density lines on y=0 plane is shown. While blue denotes the magnets (i.e. rotators), the cores (i.e. stators) are denoted by brown.

It is obvious that there exist maximal flux values around

the magnets. It may have a magnitude between B=1.6 T and B=1.0 T near the coils. Note that the flux density is higher at the triangular coils positioned above due to the small active area of Ctri. For such an axial permanent magnet generator, the magnitude is quite high and the lines are smooth. That ensures very low cogging torque since the lines intensify around the coils. One can clearly see the effect of double-sided core at the middle of figure. The lines are radially directed to the center of core till it passes through triangle coil. There exist only very little flux loss around the right edge of Mtri. Since the right part of figure is out of z=0 plane, there exist no field lines. Fig. 6 shows some samples of flux density lines near the edges of magnets and coils. Due to the N-S-N-S arrangement of PMs, tunnel-like magnetic flux densities are observed at the left- and right-hand sides of coils. The flux density of B=0.7 T nearly moves radially towards the triangle coils inside the double-sided core. There exist certain separatrix points on this core, that is, zero magnetic flux density is

Fig. 3. Geometries of two types of circular orientations of PMs (dark colored). Radial positions of corresponding coils are also same with these values.

Fig. 5. Flux lines on vertical plane (i.e. on x-z plane along the radius).

Fig. 4. Geometries of three stator sets S1, S2 and S3.

observed. However these points situate far from Ctri, although 15 degrees of shift from the angular position of Mtra is used.

Fig. 7 gives a better look at the magnetic flux density over the triangle coils. For the clarity, the cores are not indicated. It is obvious that nearly the flux density of B=1.6 T occurs inside the triangle coils and the flux density lines having B=1 T magnitude move out of one coil and travel to an adjacent coil inside the core at S3. Note also the flux paths traveling from the trapezoidal coil to the triangular one as also discussed in Fig. 6.

For a complete picture of design process, one can

summarize some advantages of the double-sided, radially and angularly different-positioned cores (DSRADPCs) as follows:

1) The initialization of rotor to the movement can be simplified; therefore the effect of mechanical damping at the initial time is overcome.

2) The effect of heating is minimized by locating the active areas of cores farther.

3) Much space is left between the rotators and stators for the cooling process of generator.

4) Higher magnetic flux densities are calculated.

IV. SIMULATION RESULTS Time-dependent simulations have been performed for various angular velocities in order to determine the performance of AFPMG. However we only give the results of 125 rpm in this paper for now. Initially, the circuitry is shown in Fig. 8.

Here A, B and C phases and with relevant coil sets and their resistances are indicated. In order to determine the output voltages of phase terminals, a series of voltmeters are used. In addition current can be determined for each winding in an individual coil. For the clarity, resistances of each 4 coil wires are indicated as 80 ohm for one phase branch. In order to generate a raw ac power with unregulated voltage, we assign each phase with the summation of 2 trapezoidal and 2 triangular coils. Since each stator includes 12 coils, one can then make 4 distinct sets indicated as CA1, CA2, etc… from one phase as a serial circuit. Within this circuitry, phase voltages for each branch of star configuration can be increased fourfold. While a sampling phase voltage is shown in Fig. 9, a flux waveform is presented in Fig. 10.

Fig. 8. External circuit with three phases A, B and C. See also Fig. 1 for the configuration of phases.

Fig. 7. Flux lines around the triangle coils. Low magnitudes of flux loss are denoted by blue.

Fig. 9. Phase voltage w.r.t time.

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Fig. 6. Flux lines inside the entire generator. For the clarity, fields near the edges of two coil and magnet systems are shown.

Fig. 10. Magnetic flux variation over the coil system CA1 in phase A.

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Note that the raw output signals are in ac form. This

waveform is an advantage in order to obtain a better rectified form. Therefore one can easily produce a constant dc voltage and can step up and down using a basic transformer for multiple levels of voltages if desired. Current in one phase is found to be around 15A at this speed (Fig. 11). Therefore an appropriate geometry for the cooling process is vital for such a system. Since current strictly depends on the resistances of wires, one can also use special windings in order to avoid the damages for continuous operations.

In order to determine the torque fluctuation of the

proposed generator, a sample waveform of cogging torque is presented in Fig. 12.

This waveform indicates that a low cogging torque value

(i.e. 0.08kNm) is obtained for the scale of a few kW power generator. Since this result is taken from the transient solution we expect a lower torque for the steady-state case. This result proves the efficiency of the generation system in this new generator. Note that the angular velocity is set to be 0.075 degrees per second. Power of the generator is calculated to be 1.5 kW at this angular velocity.

V. CONCLUSIONS

A new axial-field permanent magnet generator (AFPMG) is designed and analyzed electromagnetically. This generator has multiple stator and rotor and a double-sided, radially and angularly different-positioned core (DSRADPC) is positioned at the middle of the generator. This configuration provides a higher magnetic flux density inside the coils and assists to decrease heat produced at high speed continuous operations. The flux lose is minimized by adjusting appropriate geometry

and the initial movement of rotor is simplified by positioning the magnets with 15 degrees shift in angular direction. Proposed AFPMG model has 48 coils and the circuitry has 3 phases, thus each phase is produced by the output voltage of 16 coils. The simulations have shown that the power of 1.5kW can be obtained at 125 rpm with a low cogging torque value. We propose this generator as an alternative solution to the wind power applications.

REFERENCES

[1] S.P. Barave, and B.H. Chowdhury, “Optimal design of induction generators for space applications”, IEEE Trans. Aerospace Electron. Sys., vol. 45(3), pp. 1126-1137, July 2009.

[2] M.A. Rahaman, “Permanent magnet synchronous motors- a review of state of design art”, Proceedings of ICEM 1986, Athens, 1980, pp. 312-319.

[3] G.B. Kliman, “Composite rotor lamination for use in reluctance homopolar and permanent magnet motor”, US Patent no 4916346, April 1990.

[4] B. Singh, “Recent advantages in permanent magnet brushless dc motors”, Sadhana, vol. 22(6), pp. 837-853, December 1997.

[5] J.R. Bumby, and R. Martin, “Axial-flux permanent-magnet air-cored generator for small-scale wind turbines”, Proc. IEE- Electrical Power Appl., vol. 152(5), pp. 1065-1075, September 2005.

[6] B. Singh, B.P. Singh, and S. Dwivedi, “A state of art on different configurations of parmenent magnet brushless machines”, IE (I) Journal – EL, vol. 78, pp. 63-73, June 2006.

[7] G. Duan, H. Wang, H. Guo, and G. Gu, “Direct drive permanent magnet wind generator design and electromagnetic field finite element analysis”, IEEE Tran. Appl. Superconductivity, vol. 20(3), pp. 1883-1887,

June2010.

Fig. 11. Phase current w.r.t. time.

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Fig. 12. Cogging torque waveform.

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