PM soft magnets in new applications

MAGNETS IN PM S NEW APPLICATIONS Both hard and soft magnetic materials were discussed in several presentations and posters at the EURO PM 2001 conference in Nice. The present article summarises some of the key aspects relating to soft magnetic properties of ferrous PM materials and composites for automotive and other applications.

T

he

proliferation

both opportunities for increased PM magnetic

The full proceedings of the PM2001 conference are available from the EPMA, Old Bank Buildings, Bellstone, Shrewsbury SYl IHU, United Kingdom. Tel 01743 248899; Fax 01743 362968; www.epma.com

of elec-

tromagnetic devices in modern cars has provided new or applica-

tions as well as a variety of challenges. In a special interest seminar on PM Applications, Joan Morato and colleagues from AMES SA, based near Barcelona

Sintered ferrous PM materials offer a number of advantages for the mass production of soft mag-

activate

high permeability (p) to provide quick and effective response to flux variations. magnetic

netic components: materials and energy savings in the manufacture of complex shaped parts; dimensional accuracy; excellent surface finish; and an environmentally friendly process capable of achiev-

Sufficient yield strength and elongation was also required, to allow sensor rings to be press-fitted on the hubs without fracture.

ing tailor-made alloy compositions, purity, homogeneity, and especially, isotropic magnetic properties.

netic properties that are required for components of sensors, linear and rotary actuators, ignition coils, and electric motors. Among the factors driving the increased use of electro-magnetic compo-

As the authors showed (Table l), ferrous-based PM materials can provide a wide range of magnetic characteristics suitable for a variety of applications. The authors went on to discuss in

steel have been found suitable for this application. Iron and ironphosphorus parts exposed to

nents, the authors cited market forces involving improved comfort and security, as well as envi-

more

coatings

these materials in selected automotive applications. Sensors for

The authors went on to discuss the types of sintered materials suit-

ronmental tions, and

ABS brake systems, engine speed and position sensors to generate electrical impulses for fuel injec-

able for the fabrication of armatures (Fe-3% Si) and other parts of linear and rotary actuators (Fe, Fe-

tion

P)

and cost considerathe current trend to

detail

control,

the

suitability

and steering

3000

7.2 7.6

stainless steel

20

to

in Spain, presented a very clear survey of the different soft mag-

replace traditional mechanical and hydraulic systems.

Iron - 0.45% Phosphorus

sensors

movement

power steering - all need highsaturation induction (Bs) and

MPR January

2002

Plain iron, iron-phosphorus, and AISI type 434 ferritic stainless

of

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for

environments need to with zinc, organoor nickel-chromium (Figure

1).

valve-opening

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0026.0657/01/$-see

front

matter0 2001Elsevier

systems,

Science Ltd. All rights reserved.

stepper

linear alternator capable of generat-

motors. On the other hand, the soft magnetic cores of ignition

ing a power output of 3OkW, a project initiated by Sandia National

coils require

Laboratories

electric

switches,

and

low loss levels,

so

and funded by the

insulated (non-sintered) iron is a suitable material for this application. For electric starter motors, the pole shoes can be made from

(US) Department of Energy. The tubular design incorporates stator windings on the outside diameter and a combustion-driven piston

sintered iron-phosphorus, while encapsulated iron materials will

inside fitted with radially-oriented Nd-Fe-B (MQ31”) ring magnets.

find increasing application in electric motors and alternators for electric and hybrid vehicles. The complexities of the inter-

Peak piston velocity in its 200 mm reciprocating stroke corresponds to an (electrical) operating frequency of 400 Hz. Iron-resin soft magnetic

related magnetic properties for AC

ring-shaped pole pieces make use of

applications were aptly illustrated in two new developments described by

the isotropic characteristics to provide a flux density essentially the

Jack Hamill and Sim Narasimhan of Hoeganaes Corp., Cinnaminson, New Jersey, and David Miller of Magnequench International Inc.,

same as would be obtained using AISI 1010 steel. In the second example, the familiar flywheel magneto ignition

Research Triangle Park, North Carolina. These authors noted that

system for starting up lawn mowers and other internal-combustion-

for soft magnetic composites based on insulated iron powder, the goal of combining high permeability and

engine driven garden equipment has been redesigned for improved output and efficiency. Instead of

saturation induction with low core

the magnet mounted on the fly-

losses has remained elusive. While the resin insulation layer minimizes eddy current losses, depending on

wheel in conventional designs, giving a single voltage pulse per revolution, the arrangement has been

coating thickness or volume, it has a negative effect on density and per-

reversed, with a single Nd-Fe-B sintered magnet with pick-up coils on

Figure 1: Examples of ABS sensor rings (After 1. Morato et al.)

it::: 3

4

5

6

7

the stator and a unique flywheel

increase density) mechanical strain

assembly consisting of a non-magnetic hub with iron-resin compos-

micron DWP200

induced in the iron particles, resulting in higher coercive force and hys-

ite pole pieces fixed to the circumference (Figure 3). In this example the composite flywheel will produce eight voltage pulses per revo-

acrylic polymer-based dielectric, also with Iv/o silane + 2v/o of 5micron CaC03. DC magnetic hysteresis mea-

lution in a more cost-effective manner than putting additional magnets on the flywheel’s original

surements showed magnetic permeability decreasing and coercive force increasing as the dielectric

compressibility”, and oxidising the

design. The new design was said to have been tested to rotational speeds of 12,500 rpm “without substantial eddy current generation”.

content was increased, with better properties being obtained with the acrylic polymer. The electrical resistivity was found to be lower

iron particle surfaces to increase resistivity. Evidence of the improve-

The difficulties in optimising the various electro-magnetic prop-

for composite materials based on acrylic polymer, but the authors

ment in core losses by the use of a proprietary coating able to withstand heating to a temperature high

erties in PM soft magnetic composites was echoed in a presentation by three authors from Cluj-

said this was compensated

enough for stress relief is given in

Napoca, Romania: I. Chicinas and L.A.Sorcoi of the Technical University of Cluj-Napoca, and E.Holczer of S.C. Naposint SA. They studied the magnetic, electrical and mechanical properties of

cal properties. For AC applications, the composite with minus 100 micron iron powder and 5v/o acrylic polymer was said to be preferred, from the viewpoints of both processing and properties. n

pressure increases

(to the

teresis loss unless the material is given a stress-relieving heat-treatment. The latest iron-resin composite grades attempt to combat these effects by reducing the volume of resin, “combining the insulating material characteristics with powdered lubricant additions to improve

Figure 2. The authors went on to show how combining soft magnetic ironresin composites with high-energy Nd-Fe-B permanent magnets provides a solution to some of the difficulties. The first example concerns the development of a prototype

www.metal-powder.net

9

Figure 2: AC core loss at 60 Hz versus induction for a new soft magnetic composite stress-relieved at various temperatures. (After J. Hamill et al.)

Hoganas NC100.24 iron powder with a dielectric based on epoxy resin + iv/o silane + 2v/o of 5-

meability. Raising the compaction

6

Maximum induction, (kGauss)

CaC03, and Ductil iron powder with an

for by

shorter processing time and improved magnetic and mechani-

soft magnetic composites based on two types of iron powder and two multi-component dielectrics:

Joseph M. Capus Consulting Editor

MPR January 2002

21