3896 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 5, SEPTEMBER 1989

FORKATION OF TULTI-POLAR CRYSTALLINE

IUNUFACTURING ONE-PIECE ROTOR KAGI'ETS AND L1AGHETIC G R A I N ORIENTATION POR

FRON Nd-Fe-B ALLOY AND biONOCRYSTALLINE Fe -CO-Cr

A.I.Gridnev, V.S.Rastegaev and 1.P.Stadnik NPO Nagneton, Vladimir, USSR

The paper deals with the problems rela- ted to formation of orienting magnetic field when compacting Nd-Fe-B powder and during thermomagnetic treatment of Fe-CO-Cr mono- crystalline magnets.

Optimum orientation of magnetization vector that provides maximum magnetic flux has been calculated for quadripolar magnets as an example. Ways to obtain optimum orien- tation have been found.

It has been shown that application of one-piece multipolar magnets makes it possible to increase magnetic induction in operational gap by 20 - 40%. are very effective when used in electric motors and instruments. They make assembly work more simple end are stronger mechani- cally. Process of manufacturing multipolar permanent magnets is rather simple for iso- tropic hard magnetic materials.

In case of modern crystalline anisotro- pic hard magnetic materials, such as inter- metallic compounds of rear-earth netals with cobalt and iron, composite structures are being widely used where rectilinearly- magnetized prismatic permanent magnets are interconnected by sections made of soft magnetic steel.

from being less strong and more difficult to assemble, they are also less effective in terms o f their integral magnetic properties. For instance, in accordance with (I) it is possible to evaluate mean magnetic energy of multipolar permanent magnet by the fol- lowing criterion:

One-piece multipolar permanent magnets

The authors have found out that, apart

Their specific magnetic energy may be found through equation below:

"*ere: 4- mignctic flux in magnetic system with operational gap 3 ;

S- pole area; V- volume of multipolar permanent

magnet ; Assessment of known composite structures

shows that (BH)max value for multi- V by A

polar permanent magnets is 30 - 50% less than (BH)max of its hard magnetic material, the reason being the fact that the rotor volume is not fully utilized for accomodat- ing hard magnetic material in it and direc- tion of its magnetization is far from optimum.

magnets made of SmCo5 compound m d Ticonal single crystals are described in

Structures of one-piece quadripolar

[2,3] .

They provided magnetic flux that was 20-40% more than that of composite structures of the same size and hard magnetic material. Increasing demand for manufacturing one- piece multipolar permanent magnets from new materials, such as Fe-Nd-B and single crys- tals of Fe-CO-Cr, made it necessary to research deeper into formation of a multi- polar magnetic texture.

Powder metallurgy is the most widely- applied technique for producing permanent magnets from compunds of Fe-Nd-B type. Crystal grain orientation is done here by orienting alloy powder in magnetic field. To obtain maximum flux in a multipolar per- manent magnet it is therefore required to form a multipolar field of a certain topog- raphy with intensity of about 800 kA/m.

field should have may be found by using results of 14,5 1 , where-optimum topography of magnetization vector F A is determined for plane-parallel isotropic n-polar magnet placed into cylindrical cavity of ferromag- netic material at constant permeability.

In this case, magnetic flux c$ per unit length versus arc L of a stator cross- section may be calculated for quadripolar permanent magnet having cross-section S through the following formulae:

A n answer to what topography magnetic

Where: P and Q are points belonging to S and L accordingly.

The problem of finding optimum angle between direction M and X-axis in point P of area S may be solved both analytically and by simple exhaustion of its values from 0 to 2 3 with the aid of computer.

Construction of magnetization vector topography for isotropic hard magnetic ma- terial (in terms of possibility to obtain the same magnetization at any & ) shows that its topography is substantially curvi- linear.

It is interesting to compare this to- pography with the one of a field formed by conventional multipolar electromagnets used for magnetizing multipolar permanent mag- nets. Such electromagnets usually have a number of soft magnetic poles with DC wind- ings thereon. (Pig. 1 )

represented as: Field intensity in such a system may be

- = '0 i- 'charge ( 3 )

- Where: Ho - intensity created by current in

windings without magnetic circuit;

Hcharge - intensity created by magnetic charges formed on the surface of magnetic circuit;

-

0018-9464/89/O900-3896$01 .WO 1989 IEEE

3897

Fig. 1 In accordance with (3) let us introduce

magnetic potential.

(4)

Taking into consideration configuration of the system from (4) we can get an integ- ral equation of the first kind:

x)

Where: d (p) - distribution of a charge density on internal boun- dary of magnetic circuit cross-section;

the center of electro- magnet ;

MO- zero potential point at

Q - point of observation; I siw - aggregate winding

current ; Calculations in (5) show that if a pole

has the shape of a circumference, a mean- root square deviation of the field topog- raphy from optimum will be 1.2576, while mag- netic flux value will differ from its maximum by less than C..Ol%. Minor variations in shape and width of electromagnet pole do not change the value and topography of the field. Nd-Dg-Fe-B magnets

such material as (Ndo.8Dyo.2) (Feoe9* Bo,08)5,5 were compacted in special instal- lation 1 6 3 , that provided orienting field of a high intensity. This installation (Fig.2) is furnished with steering magnets placed between the poles.

Quadripolar cylindrical magnets from

Fig. 2

3xperiments have shown that field topo- graphy in this installation is also close to optimum.

hleasurements of magnetic properties Eade with the help of vibration magnetometer on small disks, cut out f r o m quadripolar mag- net produced from such installation, have shown that properties of hard magnetic Irate- rial differ very little over cross-section of the permanent magnet.

Degree of orientation of obtained sintered permanent magnets was estimated by relation between magnetization values & of blocks,cut out from the permanent magnet, and gS value of initial powder oriented in bipolar magnetic field. They were determined in 2500 kA/m field. Results of experiments showed that dS ratio for samples cut out from polar ar%a was 93% and the one for sam- ples cut out from interpolar section was 90%.

Cylindrical Fe-IJd-B magnets of various internal R and external R radii were com- pared for !heir dispositiog to cracking with SmCo5 samples of similar shape.

magnetic material such as Nd-Fe-B, as com- pared to magnets from SmCo , enables to considerably reduce &-valuas.

z-

It is shovm that application of hard

- Fe-CO-Cr magnets

Nonocrystalline magnets made of alloy having Cr - 21$, CO - IT$, EJo - 5 % , Ti -0.57~ and the rest Fe were grown in installation called llCrystallizer - 203tt. Large oriented single crystals (up to 60 mm in diameter and up to 200 m long) were obtained by adding up t o 25 of other elements such as A l , Be, Sb, Si, Ti. Such doping is necessary to re- duce IJ interaction value, i.e... the portion of alloy crystallized through diffusional interaction of liquid with already formed crystals. Single crystals grown at a speed of 2 - 5 mm/min. had blocks disoriented within the range of 0.5 - lo.

in monocrystalline Fe-CO-Cr with cubic lattice is difficult when using it f o r D I U - facturing multipolar permanent magnets. Let us take the most conventional quadri- polar magnet.

thermomagnetic treatment in traditional installations ensures-l/ value being only 70 - 75% of (B1l)max of bipolar permanent magnet treated along crystallographic direc- tionL100> that may be explained by non- coincidence of H and CIOO>directions in such installations.

Calculations of quadripolar monocr s- talline magnet flux through formulae (29 with experimentally-received dependence of M versus angle between direction of magnetic field at a given point and crystallographic direction<100> show that in order to in- crease its magnetic properties its field must have topography given in Fig. 3.

polar permanent magnet will approach (BH)max of its hard magnetic material, while its magnetic flux will be only 6% less than "idealtf, i.e.. . .the one calculated through formulae ( 2 ) with the use of 17 value for

Full utilization of crystal anisotropy

Application of curvilinear field during

PCr

In this case mean (BH)max of quadri-

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thermomagnetic treatment of permanent mag- net along direction coinciding with easy- magnetization axis <loo?-.

1 1 1 1 I 1 1 1 1 1 @ =Tie

IV '- - - - - I l ' I l l 1 1 1 1 1

S Fig. 3

Kevertheless, it is rather difficult to produce an ideal installation for ther- momagnetic treatment of monocrystalline magnets. Magnetizing installation r73 that has several windings with current o f diffe- rent direction on each pole enables to get closer to required field topography.

lionocrgstalline hard magnetic materials were used for manufacturing quadripolar ro- tors 50 mm in diameter and 15 mm long operating at a speed of up to 70000 rpm.

Properties of rotor material used f o r commercial production were as follows: Br =1.4 - 1.5 T; Hcb =70 - 75 kA/m; (BHjmax =72 - 80 kJ/m3;

Magnetic properties of quadripolar and hexapolar permanent magnets made of Fe-Nd-B alloy and quadripolar monocrystalline Fe-Go-Cr-lilo were compared to those of com- posite permanent magnet structures manufac- tured by securing rectilinearly-nagnetized plates,made of the same material, onto a soft magnetic sleeve. Application of one- piece permanent magnets makes it possible to increase induction in operational gap by 20 - 40%) thus considerably enhancing per- formance of electrotechnical products.

References

V.A.Chubrin and A.I.Gridnev. Kain parameter of Dermanent mamets. klectrotechnika, 1984 , NoT8 , pp. 38-40.

N.A.Kelin and A.I.Gridnev. Process of single-crystal permanent magnets

V.S.Rastegaev. Quadripolar one-piece rear-earth-Co rotor magnets. Elec tro t e chnical Indudry . Series Electrotcchnical materials, 19'1'1 edition 8 ( 8 5 ) 9 PP.11-12. I.P.Stadnik, N.I.Klevets, A.I.Gridnev and A.V. Baev. Calculation of optimum magnetic orientation for auadripolar

151 I.P.st adnik, N.A.Klevets and - A.I.Gridnev. Analytical solution of a task of finding optimum magnetic orientation for multipolar permanent magnets. Izvestiya Vuzov. Electro- mechanika, 1986, No.10, pp.13-20.

K.L.Uovogrenko, V . S .Rastegaev et al. Installation for compacting powders - - of hard magnetic alloys. A.C. Iio.540697 USSR, Information Bulletin 1976, Wo.45, p.25.

I.P.Stadnik, N.I.Klevets, A.V.Baev and A.I.Gridnev. Installation f o r magnetizing and thermomagnetic treatment of multipolar magnets. A.C. No.1027782 USSR, Information Bulletin, 1583, No.25.

permanent magnets. Izvestiya VUZOY. Electromechanika, 1986, No. 1 , pp. 21 -28.