Application of Polar Anisotropic NdFeB Ring-Type Permanent Magnet to Brushless DC Motor

授課老師：王明賢 教授學　　生：簡敏洲學　　號： M9929101

Hyo Jun Kim , Dong Hwan Kim , Chang Seop Koh , and Pan Seok ShinIEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007

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Outline Abstract I. INTRODUCTION II. EXPERIMENTS　　 A. Alignment of the NdFeB Powder　　 B. Evaluation of the Four-Pole Polar Magnet and Motor

III. RESULTS　　 A. Magnetic Field Analysis and Powder Alignment Behavior　　 B. Effects of Premagnetization

IV. CONCLUSION REFERENCES

Abstract A four-pole polar anisotropic sintered NdFeB permanent magnet (PM) w

ith high surface magnetic flux density is developed using a dry pressing process with pulse magnetizing fields.

The effects of powder-filling density, magnetic field intensity, distribution in the cavity, and premagnetization of the powder on the magnet performance are investigated.

Through an application of the developed PM to a brushless DC motor, it

is shown that the premagnetization of the powder is very effective for increasing the surface magnetic flux density.

Experimental results with a brushless DC motor adopting the developed PM shows that the surface magnetic flux density is increased up to 0.63 T, and the back emf at 1000 rpm measured 2.7 V, which is 41% higher than the conventional segment type PM.

I. INTRODUCTION

　　 In the design of brushless DC motors, permanent magnets (PMs) with high-energy density, such as NdFeB, are essential to have high power-to-volume ratio. Among the NdFeBPMs, plastic PMs, made by injection molding process of the mixture of NdFeB powder and binders, are widely being used mainly for low power applications. However, for higher power applications, sintered NdFeB PMs having higher energy density than the plastic one, are more attractive [1].

Until now, among the sintered NdFeB PMs, the segment-type anisotropic PMs and ring-type radial anisotropic PMs have been developed and applied to the design of motors. In general, the ring-type radial anisotropic PMs have slightly lower energy density than the segment-type anisotropic PMs because it is difficult to apply strong magnetic fields enough to align the NdFeB powder during the dry pressing process.

　　 In the viewpoint of the motor design, the polar anisotropic sintered NdFeB PM is expected to give stronger magnetic fields, and therefore a PMmotor with higher power density is expected to be designed. However, the polar anisotropic sintered R-Fe-B PM, where R represents rare-earth metals, as well as NdFeB sintered PM, is often noted but rarely studied because of their strong and complex dependence on flux distribution and orientation ratio, etc. [2].

　　 In this paper, the effects of powder filling density, magnetic field intensity and distribution in the cavity, and the premagnetization of powder are investigated for the development of high-performance polar anisotropic sintered NdFeB PM. This paper also compares the performances of three brushless DC motors which employ a radial anisotropic ring type sintered NdFeB PM (35SH), segment anisotropic sintered NdFeB PMs (39SH), and a polar anisotropic sintered NdFeB PM (39SH), respectively.

II. EXPERIMENTS

A. Alignment of the NdFeB Powder 　　 The magnetic properties of a sintered NdFeB PM, such as resi

dual magnetic flux density and intrinsic coercive force, are generally affected a lot by the filling density and alignment of the anisotropic NdFeB powder.

　　 In this paper, the alloy of Nd14Dy1B6Co1Al0.5Nb0.5Febal in

atom percentage composition was prepared through the strip casting process under an Ar atmosphere. After hydrogenation and dehydrogenation treatments, the alloy was crushed and milled in a jet mill to NdFeB powder; hereinafter, this powder will be referred to NP, with an average particle size of 3.5 m.

　　 In order to improve the powder alignment, the NP was premagnetized with a pulse field of 1600 kA/m; hereinafter, this premagnetized powder will be referred to PNP, and mixed in the blender.

　　 The NP and PNP are filled with a filling density in the range of 2000–3000 kg/m in a nonmagnetic mold. In the pressing process, to align the powder in polar anisotropic direction, a capacitor-discharge pulse magnetizer with four poles, of which the capacitor bank, capacitance, and initial charging voltage are 6.25 kJ, 2000 F, and 2500 V, respectively.

B. Evaluation of the Four-Pole Polar Magnet and Motor

　 The magnetic properties of the sintered magnets were measured by using a flux meter after magnetization. The commercial PM motors employing radial and segment PMs were selected as benchmarks against a new motor employing the developed polar anisotropic sintered PM. Comparisons are made under the condition of the same outer dimensions of all motors.

Fig. 1. Pulse magnetizer: (a) cross section of mold; (b) four-pole polar aligned NdFeB powder in cavity.

III. RESULTS A. Magnetic Field Analysis and Powder Alignment Behavior 　　 In order to apply strong magnetic field to align the powder, a pulse

current is applied instead of DC current. According to Rodewald, it has been reported that using a pulse magnetic field of 6400 kA/m, sintered Nd-Fe-B magnet with a maximum energy density of 451 kJ/m can be obtained [3]. A pulse current is an effective choice for a magnetic field source.

　　 In order to find a proper magnetic field strength in the cavity for making a high-performance polar anisotropic magnet, the alignment behavior of the powder with varying magnetic field strength has been investigated.

Fig. 2 shows the relationship between powder alignment degree and the applied magnetic field intensity.

　　 The virgin curves of NP were measured by putting the magnetic powder that is NP into a capsule and changing the filling density to 2000–3000 kg/m .

　　 It means it is needed to reduce the filling density for the

reduction of friction between powders in order to improve the alignment of powders.

Fig. 2. Powder alignment ratios of NP on various filling densities.Powder alignment ratios were normalized by alignment degree of NP at = 2000 kg/m3, Ha = 1200 kA/m.

B. Effects of Premagnetization 　　 The PNP is obtained by mixing in the blender after magnetization wit

h NP. Fig. 4 shows the virgin curves of PNP.

Fig. 4. Comparisons of powder alignment ratios between NP and PNP on various filling densities. Powder alignment ratios were normalized by alignment degree of NP at = 2000 kg/m ; Ha= 1200 kA/m.

　　 Depending on the filling density, the alignment behavior of PNP according to the applied magnetic field intensity shows distinctive difference from that of NP.

　　 It is shown that the alignment degree of PNP is higher than that of NP as filling density increases.

　　 since premagnetized powder has a bigger alignment torque than NP at the high filling density, PNP tends to get affected by external magnetic fields than NP.

　 On the other hand, when the magnetic field intensity is less than 240 kA/m, the alignment ratio of PNP is lower than that of NP due to the attraction among the magnetized powder.

　　 It seems that there exists critical value of the magnetic field intensity that has to be overcome towards the magnetic field direction according to the filling density.

C. Magnetic Performance of Four-Pole Polar Magnet and Its Application

It can be seen that the surface magnetic flux density from the PNP sintered magnet is 10% higher than that of NP magnet thanks to premagnetized effect.

Fig. 5. Comparison of surface flux density distribution between NP magnet and PNP magnet after magnetization.

Fig. 8 compares the measured back emf waveforms of the motors under 1000 rpm. Comparing the back emf constants, K = 0.0027 V /rpm is measured with the developed PNP magnet, which is around 41% higher than K = 0.00192 V /rpm of the segment NP magnet.

Similarly, we experimented an increase of torque constant from K = 0.0225 Nm/A to 0.0316 Nm/A (load condition 0.0294 Nm), that is again an increase of 40%.

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Fig. 8. Comparison of 1-phase back emf. between by using polar anisotropic magnet and using current commercial magnets.

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IV. CONCLUSION 　 In the fabrication of a polar anisotropic sintered NdFeB ringtype

PM using pulse magnetizing fields, the required field intensity, filling density, and premagnetization method have been investigated. Low-level filling density of magnetic powder in the mold is very favorable for the improvement of alignment.

　 In high-level filling density, the premagnetization is proven very effective to improve the powder alignment.

　 The performances of the PM motor employing the fabricated polar anisotropic sintered NdFeB magnet are also improved compared with a motor employing conventional segment type NdFeB magnets.

REFERENCES [1] V. S. Ramsden, “Application of rare-earth magnets in high

performance electric machines,” in Proc. 15th Int. Workshop REM and Thei

r Applications, 1998, p. 623. [2] Y. Sun, R. W. Gao, G. B. Han, G. Bai, T. Liu, and B.Wang,

“Effects of powder flowability on the alignment degree and magnetic prop

erties for NdFeB sintermagnets,” J. Magn. Magn. Mater., vol. 299, p. 82,

2006. [3] W. Rodewald, B. Wall, M. Katter, K. Ustuner, and S. Stein

metz, “Extraordinary strong Nd-Fe-B magnets by a controlled microstructure,” in Proc. 17th Int. Workshop REM and Their Applications, 2002,

p. 25.