IEEE TRANSACTIONS ON MAGNETICS, VOL. 27. NO. 5. SEPTEMBER 1991 4355

In-Situ Magnetization of NdFeB Magnets for Permanent Magnet Machines

Liuchen Chang, Student Member, IEEE, Tony R. Eastham, Senior Member, IEEE, and Graham E. Dawson, Member, IEEE

Abstract-In-situ magnetizers are needed to facilitate the as- sembly of permanent magnet machines and to remagnetize the magnets after weakening due to a fault condition. The air-core magnetizer in association with the silicon steel lamination structure of the rotor has advantages over its iron-core coun- terpart. This novel method has been used to magnetize the NdFeB magnets in a 30-hp permanent magnet synchronous mo- tor. The magnetizing capability for different magnetizer ge- ometries was investigated for the magnetization of NdFeB ma- terial. The design, testing, and operation of this magnetizer are reported in this paper.

I. INTRODUCTION INCE neodynium-iron-boron (NdFeB) permanent S magnets were introduced in early 1980’s, their appli-

cations have spread rapidly over many sectors of industry, particularly to electric machines. NdFeB magnets offer lighter weight, stronger mechanical strength, and lower cost than other rare earth magnets. Due to their high remanence and high coercivity, radially magnetized sur- face-mounted permanent magnet poles for electrical ma- chines are made possible by using thin NdFeB magnets.

Except for very small machines, several individual magnet blocks are usually needed to form one pole of an electric machine. If NdFeB magnets are premagnetized before assembly, the attraction force between the magnets and the steel components and the repelling force between adjacent magnets of the same polarity exert mechanical stresses on the magnets. This can cause damage (chip- ping) of the magnet blocks and can leave hard-to-clean magnetic debris in the machine. Therefore, unmagnetized NdFeB magnets are preferred for assembly of permanent magnet machines if the individual magnet blocks can be magnetized conveniently after assembly.

During the operation of permanent magnet machines, the magnets may be partially demagnetized due to fault conditions such as high temperature or high short-circuit current. To rectify these situations, in-situ magnetizers are needed to avoid the time and difficulty of removing the magnets, remagnetizing them, and reassembling the rotor structure.

Several magnetizers have been designed for rare-earth magnet materials [ 11-13]. These magnetizers have used iron cores to reduce the magnetizing force. Usually the

Manuscript received November 14, 1991; revised February 21, 1991, The authors are with the Department of Electrical Engineering, Queen’s

IEEE Log Number 9144748. University, Kingston, Ont., Canada K7L 3N6.

iron core of a magnetizer is tapered at the pole tips in order to compress the flux into the permanent magnets. However, if high magnetizing flux density is required, as for some grades of NdFeB magnets, the iron ‘core of the magnetizer itself demands a high MMF as a result of its saturation. This limits the magnetizing flux density in NdFeB magnets 141. In additioq, the tapered pole tips ex- hibit prominent fringing effects as the iron core becomes saturated, causing an uneven magnetization of permanent magnet blocks.

A magnetizer with an air core is a linear device. High magnetizing force can be obtained by a large current, which can be achieved under transient conditions as a re- sult of much lower inductance of an air-core magnetizer as compared with that of an iron-core magnetizer. In ad- dition, an air-core coil is comparatively easy and inex- pensive to build.

An air-core in-situ magnetizer was designed and tested for magnetizing the NdFeB magnets of a 30-hp 4-pole permanent magnet synchronous motor. The requirements for magnetization are first described in this paper. The magnetizing capability of the magnetizer for different ge- ometries has been analyzed by the finite element method. The test results for the chosen design are presented. This magnetizer can also be used to magnetize other types of permanent magnets and to magnetize an individual mag- net positioned at the center of its air-core coil.

11. MAGNETIZATION REQUIREMENTS There are different grades of NdFeB magnets with quite

diverse magnetizing and demagnetizing properties. The magnetizing flux density required for NdFeB magnets ranges from 2.0 T [ l ] to 3.5 T [2]. In general, 3.0-3.5 T results in a satisfactory magnetization [4]. Even though reduced magnetizing requirements have been observed at higher temperatures [ l ] , 151, [6], this effect is becoming insignificant as the temperature characteristic of NdFeB material has been improved over the past few years. The remanence as a function of magnetizing force for a sample NdFeB magnet used in the 30-hp 4-pole permanent mag- net synchronous motor (PMSM) is shown in Fig. 1 , where the magnetizing force is represented by the magnetizing flux density in tesla, measured at the surface of the mag- net. This particular magnet was magnetized previously in the opposite direction. From Fig. 1, it may be seen that the magnet loses its remanence at a demagnetizing field of about 1.7 T , after which the remanence increases as

0018-9464/91$01.00 0 1991 IEEE

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:: 0.0 - 1 -

0.6 -

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0 1 2 3 4 5 6 Magnetizing Force poH (T)

Fig. 1. Magnetizing characteristic of the NdFeB magnets used for a 30-hp permanent magnet synchronous motor.

the magnetizing force becomes higher. Beyond 3.5 T , the remanence increases very little, showing a “saturation” phenomenon.

A high current is required to generate a flux density up to 3.5 T. For magnetizers, a pulsed current is usually used instead of a dc current. As a result of the finite conductiv- ity of NdFeB magnets, eddy currents are induced during the transient current pulse to resist the change of flux den- sity in the magnet materials. Therefore, the duration of the current pulse must be long enough to allow flux pen- etration into the magnets. In practice, a current duration of 20 ms is sufficient for sintered rare earth magnets.

A uniform distribution of the magnetizing field is also desirable. The uniformity of an air-core magnetizer is de- graded for thicker magnets due to fringing effects. Lam- inated silicon steel at the back of the magnets can improve the field uniformity, as illustrated in Section IV.

111. MAGNETIZER DESIGN The magnetizer, as shown schematically in Fig. 2, is

composed of a capacitor bank, a control thyristor, a free- wheeling diode, and an air-core coil. The air core of the coil has a cross-sectional area slightly larger than that of the magnet block. If desired, an individual magnet can be magnetized at the center of the air-core coil, resulting in a high degree of uniformity. An estimate of the field dis- tribution is obtained by the two-dimensional finite ele- ment method and is illustrated in Fig. 3(a). The flux den- sities across the top and bottom surfaces of magnets located i) at the center, and ii) at the lower end of the coil, are shown in Fig. 3(b).

For an ideal magnetizer, the flux densities at the sur- faces of the magnet should be equal and uniform. The following “uniformity” factor U is selected as a measure- ment of the effectiveness of the magnetizing field:

Im

IEEE TRANSACTIONS ON MAGNETICS. VOL. 21, NO. 5, SEPTEMBER 1991

Qac

I R= -t Fig. 2. Magnetizer circuit. For the test circuit, R = 500 a, C = 14 OOO

pF, L = 8.8 mH.

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Fig. 3. (a) Field distribution of an air-core magnetizer. The permanent magnet block is 50.8 mm wide X 6.35 mm thick. (b) Flux density distri- butions along the top and bottom surfaces of a magnet at two positions within the magnetizer. I&2-magnet at the center, 3&4- magnet at the lower end, 1&3-E along top surface, 2&4-E along bottom surface.

where m the number of measured points, Bf i , Bbi individual flux densities at the top and bottom

B,, Bb average flux densities over the top and bottom surfaces of the magnet, respectively.

surfaces of the magnet, respectively.

- -

4357

4.0 - 1.5 -

3.5 - 1.2 -

9.0 - 0.9 -

2.5 - 0.6 -

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CHANG er al.: MAGNETIZATION OF NdFeB MAGNETS

B (T) U L (ma) T (ma)

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3.0 -

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15

12

40

95

30

25

20

15

Fig. 4 . Effects of coil height on magnetization of an NdFeB block 25.4 mm x 50.8 mm x 6.35 mm thick.

0

0 Lo 10 20 90 40

1 . 5 j 0 . O I < , , , , , ,

No. of k p m

Fig. 5 . Effects of number of winding layers on magnetization of the NdFeB block.

The first two terms account for the flatness of field distri- bution over the surfaces of the magnet. The third term considers the difference in surface flux densities across the magnet. Clearly, a low U value indicates a good magne- tizing field. For an ideally uniform field distribution, CJ is zero.

In practice, the capacitance and dc voltage (see Fig. 2) are limited by both equipment availability and economic considerations. It is beneficial to investigate the influence of coil geometry on the magnetizing capability. The vari- ations of inductance L, magnetizing flux density Bmag, pulse duration T, and uniformity factor U with respect to changes in the height and the number of layers of the air- core coil are illustrated in Fig. 4 and Fig. 5 , respectively.

As the height and the number of layers of the coil in- crease, the field uniformity increases, and the current pulse duration increases due to the higher coil inductance. However, the current magnitude and thus the flux density magnitude decrease due to the higher inductance and re- sistance of the coil. It is noted that for very small induc- tance L and resistance R of the coil, the current pulse should be very high. In practice, the necessary ratings of the thyristor and diode and the increased heat dissipation must be taken into consideration. The latter effect in- creases the waiting time between cool-downs if no special

cooling measures are taken. All the above factors, includ- ing the magnetizing flux density, field uniformity, current pulse duration, and heat dissipation must be considered in selecting a suitable geometry for the air-core coil.

A coil with an air-core cross section of 35 mm x 55 mm was used to magnetize the magnet blocks of dimen- sion 25.4 mm X 50.8,” X 6.35 mm thick, with the field along the thickness axis. 1dAWG wire was used to form a 440-turn 13-layer 50-mm-high coil with an induc- tance of 8.8 mH and a resistance of 1.4 Q .

IV. EXPERIMENTAL RESULTS As mentioned in Section 11, the uniformity of the mag-

netizing field can be improved by the presence of silicon steel laminations at the back of the magnet. The field dis- tribution is shown in Fig. 6(a) and may be compared with Fig. 3(a). The silicon steel laminations, even though lo- cally saturated near the magnet, have a higher permeabil- ity and thus draw flux deep inside the ferromagnetic ma- terial, creating a flux distribution that is normal to both the upper and lower sides of the magnet block. A com- parison of magnetizers with and without the lamination backing is given in Table I. The silicon steel laminations increase the coil inductance and thus increase the current pulse duration under transient conditions. Both magnitude and the uniformity of the magnetizing field are improved.

When magnetizing NdFeB magnets in the permanent magnet synchronous motor, the silicon steel laminations form the rotor structure. The placement of the in-situ magnetizer in association with NdFeB magnets of the PMSM is seen in Fig. 7. The pulse current waveform and the flux density waveform measured at the surface of magnet block by means of a Hall effect probe are illus- trated in Fig. 8. The in-situ magnetizer was positioned over each individual block successively to magnetize all the 40 blocks of permanent magnets forming the four poles of the rotor.

V. CONCLUSION In-situ magnetizers are needed to facilitate the assem-

bly of permanent magnet machines and to remagnetize NdFeB magnets after fault conditions. The air-core mag- netizer, with the help of the steel lamination backing to improve the magnitude and uniformity of the magnetizing field, successfully magnetized the NdFeB magnets in a 30-hp 4-pole PMSM. Compared with their iron-core counterparts, air-core magnetizers are low in cost, small in size, light in weight, and, most advantageously, easy to assemble. Design considerations, including the mag- nitude and uniformity of the magnetizing field and the du- ration of the current pulse, have been examined in order to meet the requirements of the NdFeB magnetization. Analysis, based on the two-dimensional finite element method and a simple R-L-C model, is quite adequate. Details of the magnetizer and its transient waveform have been presented.

4358 IEEE TRANSACTIONS O N MAGNETICS. VOL. 27. NO. 5 . SEPTEMBER 1991

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-60 -60 -40 -20 0 20 40 d0 60 Distance (mm)

(b )

Fig. 6. (a) Field distribution of an air-core magnetizer with the magnet block mounted on silicon steel laminations. (b) Field distributions along the surfaces of a magnet at two positions within the magnetizer. l&2-magnet at the center. 3&4-magnet at the lower end, 1&3-E along top surface, 2&4-E along bottom surface.

TABLE 1 COMPARISON OF AIR-CORC MAGhFT17FR CHARACTEKlSTlCS U l T H A \ l l NI1 HOLT LAhfI“19TlO4 B4Chl\G FOR

THF MAG\ET BLOCK

Magnet at the Lower End Magnet at the Center Magnet Position

Magnetizers U B,,%,,,>,,, E,,,, U Bh,,,,>,,, E,,,,

With laminations 0.426 3.633 Without laminations 0.675 2.023

V,, = 800 V. C = 14000 pF.

1“ \-cl IWI &act probe

Fig. 7 . Location of the air-core magnetizer for NdFeB blocks on the rotor of a permanent magnet synchronous motor.

0 1bO 200 300

Time (mi)

Fig. 8. Current and f lux density waveforms of the magnetizer.

3.895 0.294 4.215 4 . IO3 2.571 0.264 3.61 1 3.600

While having been designed for magnetizing the NdFeB magnets of a permanent magnet synchronous machine, the air-core magnetizer can be used for other types of magnets as well as for premagnetizing individual magnet blocks positioned at the center of the coil.

REFERENCES

K. M. Richardson and E . Spooner, ”Magnetization procedures for Nd- Fe-B magnets in electrical machines.” in Proc. 3rd lnt. Conf. on Elec- rrical Machines and Drives. London. UK. Nov. 16-18. 1987. pp.

T . J . Harned, “Transient finite element analysis of permanent magnet magnetization,“ in Proc. 19th Ann. Symp. on Incremental Morion Control S w e m s und Derzicrs, San Jose, CA, Jun. 1990, pp. 52-61, D. C. McDonald. “Magnetizing and measuring B & H in high energy product rare earth permanent magnets,’’ IEEE Trans. Magn., vol. MAG-22, no. 5 . pp. 1075-1077, Sept. 1986. J . R. Place, “Magnetizing techniques for rare-earth magnets,” Power Conversion and Inrelligenr Morion, vol. 14, no. 7, pp. 26, 29. Jul . 1988. K. M. Richardson and E. Spooner, “The properties of permanent mag- net materials for the excitation of large electrical machines,” in Proc. Sym. on Elec. Drives, Cagliari. Italy, Sept. 15-17, 1987, pp. 13-17. M. A. Bohlmann, “Utilizing temperature effects in magnetizing rare earth magnets,” in Proc. 8th Int. Workshop on Rare-Earth Magneis and Their Applications and 4rh Inr. Symp. on Magnetic Anisotropy and Coerciviry in Rare-Earrh Transifion Metal Alloys, Dayton, OH, May

250-254.

6-8, 1985, PP. 321-325. [7] A. Higuchi’ and S. Hirosawa, “Sintered Nd-Fe-B permanent mag-

nets,” IEEE Trans. Magn., vol. 25, no. 5 , pp. 3555-3560, Sept. 1989.

CHANG el al . : MAGNETIZATION OF NdFeB MAGNETS 4359

Liuchen Chang (S’87) received the B.S.E.E. degree from Northem Jiao Tong University, Beijing, in 1982, and the M.S.E.E. degree from China Academy of Railway Sciences (CARS), Beijing, China, in 1984. From 1984 to 1987 he worked at CARS as a researcher on railway traction sys- tern, electrical drives, and microprocessor controls. He is currently work- ing on the Ph.D. degree at the Department of Electrical Engineering, Queen’s University, Canada.

His principal research interests and experience include analysis, design and drive of electric machines, power converters, vehicle traction systems, microprocessor controls and computer aided design.

and high-speed transportation, linear and rotary electrical drives, and elec- tromagnetic analysis. Administratively, he serves as Director of Research Services and of Intemational Programs at Queen’s University.

Dr. Eastham is a Registered Professional Engineer of the Province of Ontario, Director of IEEE Canada, and Past President of the High Speed Rail Association.

Tony R. Eastham (M’75-SM’83) received the B.Sc. degree in physics from the University of London in 1965, and the Ph.D. degree from the University of Surrey, England, in 1969.

After research positions at Plessey Telecommunications Ltd. and at the University of Warwick, he moved to Canada in 1972 where he joined the Canadian Institute of Guided Ground Transpon. He is now a Professor of electrical engineering at Queen’s University in Kingston, Ontario, having joined the faculty in 1978. His research activities include innovative urban

Graham E. Dawson (S’66-M’69) received the B.A.Sc., M.A.Sc., and Ph.D. degrees from the University of British Columbia in 1963, 1966, and 1970, respectively.

In 1969 he joined the Department of Electrical Engineering, Queen’s University at Kingston, as an Assistant Professor. He was promoted to Associate Professor in 1975 and to Professor in 1981. His electrical engi- neering research activities have been associated with urban and high-speed transportation systems where he has current interest in the design and per- formance of rotary and linear traction moton and their energy manage- ment.

Dr. Dawson is a Registered Professional Engineer in the Province of Ontario.