Design of Polar Anisotropic Sintered NdFeB Ring-Type Permanent-Magnet for Brushless DC Motor

Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

Design of Polar Anisotropic Sintered NdFeB Ring-Type Permanent-Magnet for Brushless DC Motor Hyo Jun Kim!, Dong Hwan Kim!, Andrew S. Kim!, Sang Myun Kim!, Chang Seop Koh 2, Gyu-Hong Kang 3 (I. Jahwa Electronics Co. Ltd., Cheongju, Chungbuk 361-763, Korea; National University, Korea;

2. School of ECE, Chungbuk

3. Motor-Net International Co. Ltd., Puchon, Kyunggi, Korea)

Abstract: A Four polar anisotropic sintered NdFeB PM with high surface flux density was developed using dry pressing process with pulse magnetizing fields. In this paper, effects of filling density, magnetic field intensity, distribution of the cavity, and of the way of pre-magnetized powder for the high performance polar anisotropic sintered NdFeB PMs were studied. It can be seen that magnets produced by pre-magnetized powder process shows magnetic surface flux's maximum value that is 6,300 G due to pre-magnetized effects and that the back e.m.f. of Brushless motor, is 5.4 Vp/p per 1000 rpm, an increase about 35% above the conventional segment magnet. Key words: polar; pre-magnetized; anisotropic sintered magnet; pulse field; BLDC motor

1 Introduction

but rarely studied because of their very complex

In the design of Brushless DC motor, permanent magnets (PMs) with high energy density, such as

dependence on flux distribution and orientation ratio etc[2 J• In this paper, effects of filling density, of

NdFeB, are essential to have high power to volume

magnetic field intensity, distribution of the cavity, and

ratio. Among the NdFeB PMs, plastic PMs, made by

of the way of pre-magnetized powder for the high

injection molding process of the mixture of NdFeB

performance polar anisotropic sintered NdFeB PMs

powder and binders, are widely used mainly for low

were studied. This paper also presents comparative

power applications. However, for the high power

results of the performance of three Brushless DC PM

motors, sintered NdFeB PMs, which have higher

motor, the first one employing a current radial

energy density than the plastic NdFeB PMs, are more

anisotropic ring sintered NdFeB PM, the second one

attractive!". Till now, among the sintered NdFeB PMs,

employing arc shaped anisotropic sintered NdFeB PMs,

the segment-type anisotropic PMs and ring-type radial

and the third one employing a polar anisotropic

anisotropic PMs have been developed and applied to

sintered NdFeB PM which was developed using dry

the design of motors. In general, the ring-type radial

pressing process with pulse magnetizing fields.

anisotropic PMs have slightly lower energy density

2 Experiments

than the segment-type anisotropic PMs because it is

The magnetic properties of a sintered NdFeB PM,

difficult to apply enough strong magnetic fields to align

such as residual magnetic induction and intrinsic

the NdFeB powder during the dry pressing process. In the viewpoint of the motor design, the polar

coercive force, are greatly affected by the alignment of

anisotropic sintered NdFeB PM is expected to give

the anisotropic NdFeB powder. In this paper, alloy of

stronger magnetic fields at the air-gap of the motor, and

NdI3Dy\B6Co\Alo.sNbo.sFebal in atom percentage com-

therefore, a PM motor with higher power density is

position was prepared by the strip casting process

expected to be designed. However, this important polar

under an Ar atmosphere. After hydrogenation and

anisotropic sintered R-Fe-B, where R-rare-earth metals,

dehydrogenation treatments, the alloy were crushed

and Nd-Fe-B sintered PMs in particular, is often noted

and milled in a jet mill to NdFeB powder (NP) with an

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Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

average particle size of 3.5 urn. If it was necessary to

step. Even during the pressing step, DC current

improve the alignment, NP was mixed in the blender

provides uniform magnetic field, an advantage, but if

after magnetizing it (PNP-Pre-magnetized NdFeB

the DC current is compared to the actual zone, the

Powder) with a pulse field of 20 kOe. The NP or PNP

equipment becomes an economical issue. Depending

filled up to a filling density in the range of 2.0-3.0

on the equipment composition, the pulse current's

g/cnr' in a non-magnetic mold, and a capacitor-

magnetic field direction is bigger than the DC current,

discharge pulse magnetizer, of which capacitance and

therefore according to W. Rodewald, it has been

~,

and 2,500 V,

reported that aligning the powder in high pulse mag-

respectively as shown in Fig.l (a), is used to apply

netic field of 80 kOe will obtain over 98% of alignmenr",

stronger magnetic field than 8 kOe in the pressing

Also, in the case of multi-polar magnets, formation of

1 [2 •

simple magnetizing fixtures could be an advantage for

Fig. I (b) shows the aligned status of the anisotropic

lasting-magnet production that has multiple poles. Pulse

powder in a non-magnetic mold.

current is an effective choice for the magnetic field

initial charging voltage are 2,000

process to have more than 90 % of alignment degree

source, but due to the structure of the magnetic circuit, having to use non-magnetic mold puts a limit on the strength of the field acting on NP. Thus, it is important for magnetic circuit design of the mold for the maximum magnetic field to be revealed. First, to establish the goal magnetic field strength in the cavity, the powder alignment behavior of the magnetic field powder with varying magnetic field strength has been studied. Fig.2 Fig.l

Cross section arrangement of (a) designed mold (b) 4.pole polar aligned NdFeB powder in cavity

shows the formation of Nd13DY1B6COIAlo.5Nbo.5Febal by putting the magnetic field powder into a capsule and changing the filling density 2.0-3.0 g/cm ' to

The compacts were sintered at 1060-1100 'C for

measure the alignment ratio that varies due to approved

four hours and then annealed at 500-600 'C for two

magnetic field. NP's alignment ratio increases as

hours. The magnetic properties of the sintered magnets

applied magnetic field increases, and filling density of

were measured using a flux meter after grinding and

2.6 g/cnr' for 8 kOe of applied magnetic field shows

magnetization. The commercial PM motor employing

about 93% alignment, and over 98% of alignment was

radial anisotropic ring sintered NdFeB PM and arc

effectively seen in magnetic fields greater than 15 kOe.

shaped anisotropic sintered NdFeB PMs are optimized

As magnetic powder's filling density decreases, the

for maximum efficiency and used as a benchmark

alignment of the NP was increased since it becomes

against the other motor, whose polar anisotropic sintered NdFeB PM have a sinusoidal surface flux density distribution. Comparisons are made when all motors are designed to have the same outer dimension. It will be shown that the motor with polar anisotropic

sintered NdFeB PM has a higher air gap flux density and torque density than the current commercial PM motor with radial anisotropic ring sintered NdFeB PM

or with arc shaped anisotropic sintered NdFeB PM.

3 Results There are two types of magnetic field sources, DC current and pulse current where NP is permitted to

Fig. 2 Measurement of microstructure alignment

align with the magnetic flux flow during the pressing

ratio of NP on various tilling density

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Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications

easier for the magnetic field direction to change due to

as it approaches fine particles, and each particle will

the decrease friction among NP.

have an easy direction of magnetization, broken up into

The magnetic field's contour line of the designed

domains and spontaneously magnetized parallel to this

mold and analysis results of the magnetic field strength

direction. In the magnetization process, there are three

variation in radial direction and tangential direction of

categories: single domain particles, multi-domain

the flux contour line on various radii is shown in Fig.3.

particles and saturable multi-domain particles". The

As shown on the picture, mold's internal radius was 15

almost saturable multi-domain particles never return to

mm and 5 mm where each were 1, 0.3 T that showed

multi-domain state after the magnetization in contrast

maximum and minimum values, and the average IBI

to single domain particles or multi-domain particles,

showed about 0.7 T of magnetic field intensity and pole

and they show an obvious coercivity.

to pole flux contour lines were formed, However, as

As experimented in Fig.2, PNP, after blending,

shown in Fig.2, sufficient alignment could not be

was varied from 2.0-3.0 g/cnr' to measure the

expected from 0.7 T of magnetic field intensity that

alignment ratio versus the applied magnetic field and is

was especially true when alignment becomes harder as

shown in Fig.4 and to check the pre-magnetized effect

filling density increases. In general, magnetic field

it is compared with that ofNP. Depending on the filling

against normal direction and tangential direction

density, the pre-magnetized powder's alignment versus

contraction rate of sintered part were 20% and 30% and

applied magnetic field intensity behavior shows dis-

the difference in contraction rate becomes smaller as

tinctive difference from that of NP. As filling density

the magnetic field's strength is smaller. This can be

increases it can be seen that the alignment of PNP is

also confinned in the contraction rate after the sintering

higher than that of NP. Based on 8 kOe of applied

process as shown by this paper.

magnetic field intensity, it can be noted that filling

In order to enhance the powder alignment, we

density of 2.0 to 2.4 g/crrr' did not show a big

studied pre-magnetization effect of the NP. Fine pow-

difference in alignment, but 2.8 to 3.0 g/crrr' showed

der that finished the jet-mill process will be pulverized

2% to 4% of alignment. This is due to saturable multidomain particle'S interior that magnetizes many domain walls, and those walls during magnetization magnetize from multi-domain to single domain, and during the blending process few grains form closed chains where the grain is affected by external magnetic fields due to pre-magnetized powder having a stronger alignment torque than that of NP. On one side, in areas

100

p = 1.4g'cc

p = 1.0g/cc

NP NP PNP p

= 2.8gi cc

8

101214161820

Applied magnetic field, Ha (kOe)

Fig.3 Finite element model (a) 3D and (b) 2D (c) magnetic field intensity of normal direction

Fig. 4 NP and PNP magnetic hysteresis loops of as

and tangential direction against flux line

function of mling density

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Proceedings of 19lh International Workshop on Rare Earth Permanent Magnets & Their Applications

5,.....--.,..--.----.--.__

where magnetic field intensity is less than 3 kOe, pre-

....,

~ Rlllthd llillooti'opll! lDagnet

4

magnetized powder's alignment is lower than NP due

~.-.-. S~eiU tbRpell

3

to magnetized powder attraction that exists, and there is

-i--<>-c Polar 1l1~1.01·t!lpl. lD"lPlell

a critical value that has to be overcome towards the magnetic field. Fig.5 shows the PNP magnet and NP magnet that have finished blending and have been produced with magnetically aligned four pole polar magnet at filling density of 3.0 g/crrr' that compares its surface magnetic

15

30

45

60

75

90

Tim .. (m sec )

flux. It can be seen that magnets produced with NP with

Fig.7 Comparison of Ivphase back e.m.f. of using

PNP process done shows magnetic surface flux's maxi-

polar anisotropic magnet by this paper with

mum value that is 10% higher due to pre- magnetized

radial anisotropic ring magnet and segment

effect. Fig.6 shows variation of the PM during fabrica-

magnet

tion, where it is distorted a lot after sintering due to

4 Conclusions

excessive shrinkage. A four pole brushless DC motor is constructed

In the fabrication of Polar Anisotropic Sintered

using the fabricated polar anisotropic sintered NdFeB

NdFeB Ring-type PM using dry pressing process with

PM, and the back emf has been measured. Fig.? com-

pulse magnetizing fields, the analysis of magnetizing

pares the measured back emf waveforms under 1000

fixture, filling density, and powder pre-magnetization

rpm. It can be seen that the back e.m.f., 5.4 Vp/p per

effect has been studied to improve the alignment of

1000 rpm, increased around 35% above the segment

Nd-Fe-B sintered magnets. The magnetic field strength

magnet.

in the mold using a capacitor-discharge pulse magnetizer is about 8 kOe which is not sufficient for full alignment, where field intensity needs to be over than 15 kOe in the pressing process. Low level filling density of magnetic powder in the mold is very favorable for the improvement of alignment. In order to escape the decrease of the alignment in high level filling density, pre-magnetization effect, which is effective for the improvement of

.{l.6 L..-""-..................L-...-=...................................--L.

o

~

100

1~

200

2~

---L...;;..;.--l.J

300

J~

alignment at high level filling density, was studied. As a result, the back emf of the fabricated polar

RD1adon angle (deg.)

anisotropic sintered NdFeB PM is increased around Fig. 5 Comparison of surface flux density distribution

35% above the current conventional segment magnet.

between NP magnet and PNP magnet after magnetization

References: [I]

V.S. Ramsden, Proceeding of Rare-earth Magnets and Their Applications, 15th, pp. 623, (1998).

[2]

Y. Kaneko, K.Tokuhara, and N.1shigaki, Jpn, l. Powder and Powder Metallurgy, 42, pp.695, (1994).

[3]

W. Rodewald, B. Wall, M. Karter, K. Ustuner and S. Steinmetz, Proceeding of the 1i

Workshop on Rare Earth

Magnets and Their Applications, pp. 25, (2002).

Fig. 6 Variation of PM during fabrication after pressing with magnetic field (a), after sintering (b),

h

[4]

K.Kobayashi, T Proceeding of High Performance Magnets and Their Applications, 18th, pp. 499, (2004).

after grinding (c)

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