recent advances in materials for use in permanent magnet machines -a review

8/10/2019 Recent Advances in Materials for Use in Permanent Magnet Machines -A Review

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Recent Advances in Materials for use in Perman ent Magn et Machines -A Review

Uday S. Deshpande

Black Decker US.),nc.

701

East Joppa Road

Towson, MD 21286

Abshuct - A

review of the statwf-theart in materials

used

io

permanen t magnet machines is presented.

On going

research

lor new materials

is

also discussed.

Io

the end, the impact of

these and the

upcoming

materials on m otor design

ls disenssed.

I. INTRODUCTION

With the increasing use of moto rs in the automotive sectors

and the wide variety of applications that are involved, the

demands on the materials have changed.

In

the automotive

industry today, there is an increasing trend tow ard sa more

electric car. Increasing numbers of features in the m odem

automobile

are

being motorized. It

is

said that there are an

average of 30 electric motors per car today with the number

likely to increase to over

100

by the end of

this

decade. The

applications

run

the gamut from window-lift actuators, power

seats, power doors, antilock brakes, electric power steering,

integral starter-altemator to the main traction motor in

electric vehicles.

In

meeting the various performance and

cost requirements, the materials used in the motors need to

evolve as well.

This paper reviews the recent developments n materials

for use in permanent magnet machines. In patticular,

permanent magnet materials and soft magnetic m aterials

are

discussed

The permanent magnets are broadly classified as

ferrites, AlNiCo,

or

rare earths (including

Samarium

Cobalt

(SmCo) and Neodymium-Iron-Boron (NdFeB)).

This

paper

ignores AlNiCo and only briefly

looks

at SmCo because of

their very limited use in automo tive applications . The typica l

range of properties for ferrite and %eo grades is shown in

Fig.

1.

F e m t a are typically Barium Femte or Strontium

Fem te and can be made by injection

or

comp ression molding

or sintering with the properties typically increasing to

remnant flux density B, of 4kG, intrinsic coercivity H of

4kOe and

maximum

energy prod uct BH- of - 5MGOe.

These magnets have poor low temperature properties and a

reasonably high Curie temperature

of

- 450C. The magnets

are low cost and the technology is well estab lished.

Rare-earth magnets (NdFeB) can similarly be made by

injection or compression molding

or

sintering. This broadly

classifies them as %bonded neo

or

sintered neo magnets.

The bonded neo magnets have typical properties of 6.8kG

(B3, 15kOe HJ nd

-10

MGOe ((BH)-). They have

poorer properties at higher tem peratures but perform well at

lower tempe ratures compared to the ferrite magnets. Sintered

0

10

15

2

25

30

HciW)

Fig. I Rangeofpmpcrtics f

he diffeml

-el grades

ne0 magnets have typical properties of 12kG

BJ,

20kOe

H

and - 30MGOe

((BH)-)

and like the bonded neo types

have poorer properties at higher temperatures They typically

can handle temperahues higher th n the bonded ne0 magnets

but are limited by the relatively lower Curie temperature of

-320OC.

Neo

magnets are more expensive th n the ferrite

magnets.

Note: It may

be

a little misleading

to list

the typical

properties

as

above because of the range that these magnets

cover as

is

evident from Fig.

1.

Soft magnetic materials have been fairly unchanged over the

years. Cold

rolled

magnetic

lamination

( C W ) is

still

widely used, as is silicon-based

iron

with the various

additives. These are characterized primarily by

core loss

Wkg)and permeability. Typicalproperties are - W k g

for

core loss and a permeability of - 2000 at 15kG and 50 60 Hz.

II.

NEW

DEVELOP-

In

the opinion

of

the author, most

of

the major

developments in recent times have been in permanent

magnets. Softmagnetic materials have seen improvements

in

processing for ease of m anufacturing and corrosion resistance

but tittle has changed at the fundam ental property leve ls.

(Materials like Co-Fe and

Vanadium

Permandur are not

considered due

to

their lack of use in automotive

applications). One change is in

Soft

Magnetic Composites

(SMC) and this will

be

discussed shortly.

0-7803-78

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2 3

IEEE 509

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A.

Permanent Magnets

From Fig. 1 we see that there is a gap between ferrite and

bonded ne0 magnets and again between bonded ne0 and

sintered neo m agnets. Fem te m agnets are the lowest c ost but

also have the lowest properties of the magnets under

consideration here. Bonded n e0 magn ets address some of the

concems of fem tes but are more expensive and

are

limited at

higher temperahues. Sintered neo h ave highest properties

as

well

as

cost and are also limited by temperature. Ne0

magnets are also affected by corrosion, which necessitates

special corrosion resistant coating prior to use. Most of the

development

has

been

to

bridge th ese gaps.

I. Ferrite M agnets

The low cost ferrite magnets have been the ones

primarily used in automotive app lications but bonded ne 0

magnets are making in-roads into

t h i s

domain by focusing on

enhancing their higb temperature pmpenies and lowering

cost

The main issue with ferrite magnets has been the

relatively lower properties

(B,,

a

nd BH& and the

tendency to face demagnetization at lower temperatures.

This l st feature is a concern in automotive applications

where operation at -40C

is

routinely required. In response,

the femte magnet developers have focused on increasing

magnet strength and low temperature capabilities. One

approach

has

been the use of additives such as Cobalt (CO)

and Lanthanum-Cobalt (LatCo) to the base powder.

Addition of COessentially increa ses

B,

while maintaining

H

while the addition of

La

CO ncreases both B, and

H,. his

is

typically done for sintered fem tes. Fig. 2a shows range of

ferrite magnets fiom Groupe Carbonne LorraineilJgimag.

The figure shows the grade where only CO is added and the

grades where both

La- is

added. The sintered fem tes with

CO or L a K o additives reach a B,of 4300

4500

Gauss and

H

Of

4000

-

5000

Oe.

U

Bonded Nea Magnets

The primary issue with bonded ne0 is achieving full

densification and goo d higb tempera ture performance. The

use of a plastic binder material causes a reduction in the

achievable density and a limitation of the m aximum operating

temperature. Ad d i t i~ ~ l l y ,onded ne0 magnets fall between

sintered femtes and sintered neo. The effort then

has

been to

address both ends of the spectrum reduce cost to compete

with ferrite magnets and increase the operating temperature

rating as well as magnetic properties to compete with the

sintered neo magnets.

Magneqnench, the cbief producer of the bonded ne0

powder has been working in

both

these regards.

Improvements in processing the powder as well as other

strategic decisions have helped address cost issues and

improvements in processing and advancements in coating

technologies have helped address performance issues.

Magnequench in conjunction with Daido Steel Co. recently

announced the development of new anisotropic powders

using the Magnequench rapid quen ching process along with a

special plastic d eformation process that resulted in magnets

with BH,, of 22M GO e with maximum operating

temperature of 100C and BH, of 17MGOe with maximum

operating temperature of 1 2 5 T [l], [9].

Another candidate for making bonded neo magnets is

the so-called

HDDR

process initially developed by

Mitsubishi Materials Co. NdFeB powder is subjected to

hydrogen under pressure (hydrogenation), which causes the

powder

to

become very brittle (disproportionation)and thus

allows milling to fmer particles. The hydrogen is then

desorbed and the NdFeB recombined to produce anisotropic

bonded ne0 magnets with energy product of around

ISMGOe. Recently Aichi Steel

Corp.

reported a modified

HDDR

process called the d-HDDR w here they co ntrolled the

pressure at which the hydrogenation takes place.

This

allowed them to achieve greater anisotropy and make bonded

ne0 ma gnets with a BH, of

-

5MGOe [l].

Group h o l d uses a d ifferent approach in their bonded

magnets offering where a femte-neo blen d is used.

In

hese

ferrite-neo hybrids, NdFeB powder i s blended w ith the

strontium (or barium) ferrite.

This

results in isotropic

mag nets in which the ferrite and NdF eB com pensate each

others temperature characteristics and provide a more

temperature stable magnet. The cbaracteristics are affected by

the relative percentages of the blends. This

is

shown in Fig.

2b. These magnets address both the increased

flux

requirement and increased temperature resistance

requirement but are weaker than the bonded ne0 magnets

describe d above [4].

III. Sintered Ne0 Magnets

Developments

in

sintered ne0 m ap et s have focused on

improving strength and

high

emperature capability. With the

increasing use of high powerhigh performance automotive

applications like eleceic power steering, integrated starter

alternator, mc tion motors for

EV/HEV,

the demand for high

temperahue performance has increased. With a Curie

temperature of - 320C, this

has

been a challenge. Recently

Sumitomo reported magnets capable of op eration up to 2ZOT

with 250C capable magnets also being developed [I],

[8].

From a strength point of view, the theoretical

energy product for a single NdlFel.BI crystal

is

64MGOe.

This

gives a B, of about 1.6T. Recently Kaueko (Sumitomo)

and Rodewald (Vacum schmelze) reported magnet properties

in the neighborhood of 15.19kG B 3 , 9.8kOe&) and

56MG Oe (BH-) [l] [2], [3], [SI. They achieved

this

by

optimizing the alloy composition, improved domain

alignment by the use of alternating pulsed orienting field and

optimizing the sintering conditions to optimize the

microstructure of the magnets. These values are close

to

the

practical limit for sintered NdFeB m agnets.

A major thrust in research has been in processing to

improve dom ain alignment and pow der purity to achieve the

high performance magnets. Another area

of

focus has been

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-_.

_..

Fig. 2b Farite-NCOmapel material from rouph o l d

141.

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to

improve the temperature capability. The use of

dysprosium is cmmon to increase

H

but this is an

expensive element. Materials processing to use other

eleme nts in place of

Dy

are being studied to add ress this issue

as

well. Fig. 3 shows the range of properties for sintered neo

magnets

from

ShinEtsu

Magnetics [7]. The l ine is the l i t

on properties that can be reasonably achieved in the opinion

of th e author.

Corrosion has been a m ajor issue for NdF eB magnets.

Various coatings have been studied and developed and

has

largely ameliorated

t h i s

concern. In recent times, the use of

hydrogen fuel cells bas created a new concern for corrosion

protection. Hydrogen is extremely corrosive toNdFeB, a fact

that is exploited in the HDDR process. Outside of that, the

effect

is

detrimental.

ShhEtsu

Magnetics

bas

repoIted

development of a new coating that has shown promise in

protecting against hydrogen [7]. More details on this new

coating are forthcoming. Typical coatings

used

are epoxy

coating, nickel plating, a l u chromate ion vapor

deposition. The relative merits are based on

the

application.

In the

opinion

of

the author, the aluminum ion vapor

deposition with chromate coating works very well for

automotive application offering good corrosion resistance,

good adhesion properties and good dimensional control.

Most voice coil m otor magnets tend to use nickel coating and

the epo xy coating tends to be acceptable for g eneral industrial

applications.

W.

ther

Materinls

Other developments have be en to get away fkom NdFeB

base to counter supply issues as well

as

extend the

l i t s

hat

are inherent. In this regard, work is being done using

Sm2Fe17N3

Samarium IronNitride

(SmFeN)). Its properties

can theoretically surpass those of NdFeB but the processing

is

much more complex and not yet suitable for commercial

production. Sumitomo Metal Miniig CO

has

produced

injection molded isotropic SmFeN magnets with

an

energy

product of - 15MG Oe by a reduction diffusion process.

Work is on going in the study of magnets made with a

combination of SmFeN/FeN and it

bas

been reported that

properties of SmFeNFeN combmation

can

theoretically

reachanenergyproductof I5OMGOeandaB.of2.IT [SI.

Nanocomposites (combmation of h a d and soft

magnetic materials) are another development where the

potential for very high-energy product ex ists.

Soft

magnetic

material is added to the h ard phase to reduce dependence on

the rare-earth elements. The high saturation magnetization of

the soft phase and the high anisotropy of the hard phase

combine to offer the poten tial for BH, appro aching

-90MGOe. Prof. Sam Liu of the University of Dayton

reported making powder level samples with a BH, of

93MGOe [I],

[Z].

Recently (February

2003),

Prof. Liu

reported making nanocom posite magnets with a BH- of

35MG Oe 161. The research seem s

to

hold the promise of

living up to its potential but more d evelopment is needed

Samarium

Cobalt magnets are briefly mentioned for

their inherent ability to operate at high temperatures.

Curr ently the strongest SmCo magnet is -28MGOe and can

operate up to -3OOOC. SmCo magn ets capable of operating up

to

500C

have been reported by Electron Energy Magnets

having

hear

2 quadrant B-H characteristics up to the

operating t e m p h u e limit. On going research is focussed

on increasing the maximum energy product over 30MGOe

and developing temperature compensated

SmCo

magnet

grades for operation up to 50OoC

[l] [Z].

Fig.

4

shows the

historical progress of rare ea rth magnets.

Years

Fig.4DNClopmntsinrarc-earthma%nctsovertheycars(CaunesyOfPmf:

S. is.

University

of

D q

B. SopMagnetic M aterialr

As mentioned earlier, there

bas

been no fundamental

change in

soft

magnetic material other t h nprocessing. Most

of the development in this area

has

been to improve

production process to increase consistency, develop better

coatings, and reduce costs. The

limts

on

he steel

are

the

same -peak saturation flux densities for the CRML

rades

of

-

2T

and peak

permeability

of

2000-3000 at

1.ST.

he core

loss

is a fimction of composition, thickness, processing and is

typically 5W kg. As mentioned earlier, Co-Fe steels and the

like are not considered here due to their relatively specialized

use.

The one new development has been in

soft

magnetic

composites where iron particles of -150pm in size coated

with a thin inorganic surface insulation along with various

organic additives are pressed

in

a die and then annealed and

cured to form the desired

pari

(e.g. stator of the mo tor).

This

material has inherently lower permeability

-

500) and

saturation flux density

1.8T) t h n

lamination steel and

slightly bigber core loss

(-

10WKg). It also

has

a lower

mechanical strength compared to

lamination

steel

[lo],

[ I l l .

Fig. 5compares typical B-H curves for lamination steel and

SMC material.

The lower initial permeability and the lower saturation is

easily seen from Fig. 5 Despite the seeming drawbacks of

this material it offers some interesting features and

possibilities for making motors. Due to its manufacturing

512



5/7

process, it is easy to make comp lex shapes while ma intaining

good dimensional tolerances. The

3-D

nature

offers

the

possibility of s u e and weight savings by allowing shapes that

are optimized for the application.

Lamination

stee 6

SMC material n

cum*

Compar ison

2.5

7

material

Fig. 5 Camparison oftypicalB-H

CY

for l a

steel a d

MC

Prof. Alan Jack of the University of Newcastle upon

Tyne is heavily involved in research using this material and

had published several papers sho wcas ing the capabilities

of

the

soft

magnetic composite ma terial [IO] [ l l ] , [12]. Recent

reports by other authors have discussed the use of SMC

in

various applications in automotive, home appliances,

industrial applications. Use of SMC in different motor types

has also been reporte d [12].

There are certain applications where the material use

offers some benefits not obtained from conventional

lamination material. While

this

material holds promise, it is

by no means a replacement for the conventional lamination

steel. A lot of research is ongoing to exploit the properties

of

this material.

It was mentioned

earlier

in the paper that no major

developments have occurred in conventional lamination

steels. By

this

it is meant that nothing has happened that has

allowed low carbon, low silicon steel to have saturation flux

density of 2.5T or have an an -hysteretic B-H loop. This s at

least not

in

the knowledge of the author.

Most of the work has been to improve processes to

increase consistency in steel properties, reduce core

loss

by

improved purity, development of new and improved surface

coatings and in general to improve the usability of the steel.

A lot of development has t ken place in these

areas.

European Electrical Steels has reported activities in the abov e

areas

n developing low

loss

steels that they market under the

Polycor brand

[13].

Following the foregoing discussion, Fig. 6 shows

the

new

map showing the ranges of the various permanent magnet

materials.

60

50

10

0

Distribution of Magn et grades and their relative properties

I

0

5 10 15 20 25 30

Hci kOe)

Fig.6:Ranges of various m gnet p d e s

the size of the motor for a given power size or provide more

power for the same motor size. An example is given for a

11. IMPACT ON M OTOR DESIGNS

surface

PM

motor. The baseline numbers

are

for a motor

wth

35Mme

intered

N ~ ~agnets p,

1 , 2 3 ~ ,

,

21koe,

BH, = 35MGOe). The best sintered NdFeB magnet

The basic impact

of

magnet properties on mo tor design is

well known

-

a stron ger magnet offers the potential to reduce

513

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reported to date (Rodewald, B. = 1.519T, H = 9.8kOe,

BH,

=

56.7MGOe) is

used

and the impact on motor size

and performance compared in per unit numbers.

It

is seen from Table I that the

23%

increase in B,offers

a

- 20%

increase in

output

or a -20% reduction in

size

(length). In the above study the diameters and winding were

unchanged. The data is at m m emperature (25C). The flux

densities are obviously higher and

so

will be the saturation

effects. This is reflected in the higher torque ripple num bers.

TABLE I

COMPARISONofMOTOR PARAhETER5

With the advent of the new materials there are lot of

choices for the motor designer . At the same time, it is

important that the material characteristics be properly

understood so as to optimize the design. In addition to

material, new processing techniques have been developed

that offer additionalpossibilities.

With the new m aterials, new process can

be

developed to

take full advantage of their capabilities and perhaps simplify

and economize the production process. Conventional bonded

magnets

have been used

as

ringsmade by ex husion, injection

or compression molding.

Matsushita

Electric Industrial Co.,

has

reported the use of rolled flexible bonded magnets for

small motors,molding magnet material

directly

on

to

the

rotor back iron or even molding the magnet material into

pockets in a rotor core for IPM motors

[l]

121.

In doing

so

they report new techniques for manufacturing motors with the

new

types

of magnets that offer advantages in size process

and perhaps cost over the conventional techniques of m otor

manufacturing.

From a design aspect, the temperature and the B-H

characteristics of the new magnets have

to

be considered but

also the manufacturing process. Manufacturing a mo torho tor

where the magnet material is directly bonded on to the rotor

iron for example, results in end magnet propelties beiig

different from magnet powder properties. This needs to be

taken into consideration during the motor design process to

ensure proper m otor design.

The use of SMC m aterials is a very good example of the

above comments. Due to its inh erent isotropic,

3-D

nature,

conventional design method ologies will not provide the

best

design for a motor using S MC materials.

The higher strength magnets (flux output and high

temperame capability) are key elements in providing

reasonable motor designs to facilitate the automotive

applications needs. These magnets enable high power

density, compact motor designs for electric power steering,

traction motor for EV/HEV as well as other applications

where the tight c o nf ie s of the au tomotive under-the-hood

dictate the package size. The new bonded neo mag nets will

help provide means

to

address applic ations that need similar

compact motors for applications

that

are not

as

high power.

In the

end,

it all boils down

to

cost.

For automotive

applications, the cost for the neo magnets would have to

continue to decrease. Bonded ne0 magnets need

to

approach

the cost of femte magnets and sintered neo magnets need

to

approach the current cost of bonded ne0 magnets. There is a

txend along these lines but the costs have

to

drop more

to

ensure wide acceptance and use.

N ONCLUSIONS

This digest has attempted

to

provide an insight into

the new m aterials availab le for PM mo tor designers. Some

discussion and thoughts on the impact o f these m aterials has

been provided. Some thoughts on magnets costs have also

been made. The new materials c n be helpful in facilitating

motorized applications for a wide range of au tomotive needs.

ACKNOWLEDGMENTS

The author gratefully ackn owle dge s James Krajczynski

of Globe M otors for his help and d iscussions.

REFERENCES

[I]

G.C. Hadjipanayis, M.J. Bonder, Eds. h o c . Of

I T h

Intemational workrhop on Rare Earth Magnets and their

Applications, Delaware USA Aug. 2002,

[2]

H.

Kaneko, M. Homma, M.

Okada,

Eds. Proc. Of

Is

Intem ational workrhop onRare Earth Magnets andtheir

Applications,

Aug.

ZOM), Sendai, Japan

[3]

Gorham Advanced Materials,

Permanent Magnet stem

Power Electronics for Motion

Control,

September

2002, Cincinnati, on

USA

[4] P. Campbell, Magnetics

Tutoriol,

magnetweb.com

[ 5 ]

Magnet Catalogs, Websites of Sumitom o Ugimag

OlnD://www.sumitomosma.co~,

(hitu://www.carbonelorraine.com/urrimag),

002.

2002

a] S.Liu, Private Communications, University of Dayton,

[ 7 ]

T. Minowa, Private C ommunications, ShinEtsu 2002

[ 8 ]

Y. Matsuura, Y.Kaneko, Private Communications,

Sumitomo, 2002

191 G. Riley, A. Albers, Private Communications,

Magnequench,

2002

[IO] P. Jansson, A. Jack, Magnetic Assessment of SMC

Materials,

Proc. Of the

21' Annual

Conf. On Properties

and Applications of Magnetic Materials, IIT Chicago,

May 2002

[I I] A.G. Jack, Experience with the use

of

soft magnetic

composites in electrical

machines,

Proceedings

of

ICEM-98, Istanbul,

pp.

1441-1448.1998.

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[12] A. Jack, B. Mecrow, P. Dickinson, C. Madison,

D.

Stephenson,

T. Evans, J

Burdess, Soft

Magnetic

Composites -

A n

Examination of Potential,

courtesy

Hoganas.

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ga des , processing

and

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Phys.

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pp.

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recent advances in materials for use in permanent magnet machines -a review

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