Abstract-The aim of this study was to assess the basic magnetic properties of a mixture composed of a commercial lubricant and soft magnetic iron particles, where the particle content is significantly higher than in conventional magnetic fluids, due to the fact that the particles verge on the sedimentation level. This so called magnetic slurry can, then, be regarded as a highly ductile magnetic conductin material for tentative use in magnetic circuit that can cgange shape under operation. The high particle content results in high saturation field densities that extend the application field towards electric power en ineering. This paper, therefore, focuses on properties life relative permeability, losses, the Q-factor and the efficiency for the power fre uencies 50 and 400 Hz. It shows that the relative permea%ilities are low (below 10). The values of the efficiency and the Q-factor decrease considerably above 0.5 T. That limit$ the application of the studied slurry in electric power engineering. In such applications, the powder properties must be improved

Power Engineering, G Engdahl

CiteSeerX

IEEE TRANSACTIONS ON MAGNETICS. VOL 32, NO 5 ,  SEPTEMBER 1996 5 109 
Experimental Study of Magnetic Slurries Regarding Magnetic Properties 
G. Engdahl, Power Engineering, ABB Corporate Research, S-72178 Vasterh, Sweden 
B. Lerebourg, Ecole Nationale SupCrieure d’Hydraulique et de Mkcanique de Grenoble, F-38400 St Martin d’ Hbres, France 
Abstract-The aim of this study was to assess the basic 
magnetic properties of a mixture composed of a commer- 
cial lubricant and soft magnetic iron particles, where the 
particle content is significantly higher than in conven- 
tional magnetic fluids, due to the fact that the particles 
verge on the sedimentation level. This so called magnetic 
slurry can, then, be regarded as a highly ductile magnetic 
conductin material for tentative use in magnetic circuit 
that can cgange shape under operation. The high particle 
content results in high saturation field densities that ex- 
tend the application field towards electric power en i- 
neering. This paper, therefore, focuses on properties l ife 
relative permeability, losses, the Q-factor and the effi- 
ciency for the power fre uencies 50 and 400 Hz. It shows 
that the relative permea%ilities are low (below 10). The 
values of the efficiency and the Q-factor decrease consid- 
erably above 0.5 T. That limit$ the application of the 
studied slurry in electric power engineering. In such ap- 
plications, the powder properties must be improved. 
I. INTRODUCTION 
The conventional magnetic fluids [ 11 have low magnetic 
saturation levels and can not, for that reason, be used in elec- 
tric power engineering, for example in controllable reactive 
components. To make them usable in such applications, the 
particle density can be increased so that the mixture becomes a 
very ductile material with a considerably higher magnetic sat- 
uration level. This study, therefore, is devoted to the assess- 
ment of the magnetic properties of such mixtures. 
11. THE SLURRY 
A. The Choice of the Slurry 
Powders for use in slurries in electric power engineering 
have to be made of a soft magnetic material, to be relatively 
non expensive, to be electrical non conducting and, finally, to 
have an ability to form dense and ductile slurries. Thus, two 
insulated iron powders from Hoganas AB were selected: the 
ABM.100.32 and the MHM.300.27 where the particle sizes 
are in the ranges [30 pm ; 200 pm] and [5 pm ; 50 pm], re- 
spectively. These powders are insulated which means that a 
thin iron oxide layer provides a very good internal electric in- 
sulation of the powder. 
B. The Choice of the Powder 
To get a homogeneous slurry, the powder has to be mixed 
with a more or less fluid matter, in this paper called 
“solvent.” Its principal role is to enhance the particle motion 
and join the particles together forming a slurry rather than a 
dust cloud. Lubricants were, therefore, an appropriate choice 
regarding the “solvent.” Different types of both commercial 
non expensive oils (mineral, synthetic and silicon oils) and 
mineral-oil-based greases have been tested regarding ductility 
and stability. 
C. The Preparation of the Ilrlixture 
The results of the mixing tests showed that a homogeneous 
slurry with a particle densiity higher than 35 volume percents 
could not be obtained with any kind of grease. Furthermore, 
the mixing operation resullted in the entry of a huge amount 
of small air bubbles that could not be removed. This problem 
might be solved by heating, the grease during the mixing. 
With oils, the preparation procedure performed began by 
pouring the iron powder into a big amount of lubricant. The 
mixture was, then, carefully stirred to avoid air bubbles being 
trapped. After that, one let the mixture sedimentate and finally 
the oil surplus was removed. The particle density was, then, 
about 45 volume percents, independently of the type of oil. 
High-viscosity polyalphaolefine and silicon oils associated 
with the roughest powder (ABM. 100.32) gave the best results 
regarding homogeneity and consistency of the slurry. The 
specification of the slurry whose experimental results are pre- 
sented in this paper is the following: 54 volume percents of 
the polyalphaolefine oil MOBIL SHC 626 (170 cSt at 20 “C) 
and 46 volume percents of the powder ABM.100.32 The dens- 
ity of the slurry was equal to 4.2 g/cm3. 
111. THE TESTING EQUIPMENT 
The sketch of the testing equipment is represented in Fig. 
1. It was composed of a 0.3 mm-thick-strip wound core that 
has been truncated to make place for the sample, a cylindrical 
container of Plexiglas full of slurry, and an induction coil. 
Two 1 mm-thick circular discs were placed on the top and the 
bottom of the container, respectively, to prevent the slurry 
from pouring out. These cliscs were made of a soft magnetic 
material, PERMEDYN [2 1, so as not to amount to an air gap 
regarding the magnetic field quantities. Finally, a centring de- 
vice surrounded the sampk with the aim of lining up the core 
and the sample and carrying the magnetic field intensity 
gauge. 
A sinusoidal-voltage soiirce supplied the induction coil. To 
prevent from big magnetic field distortions around the gap, a 
smaller sample (1 0 mm-high) also was designed. Thus, the 
general features of the testing equipment were: 
Core cross-section: 32 1 mm x 32.1 mm 
Container dimensions: [nner diameter: 32.1 mm 
‘Wall thickness: 0.1 mm 
Height: 48 mm for B < 0.6 T 
1 0 m m f o r 0 . 6 T < B . :  I T  
001 8-9464196S05.00 @ 1996 IEEE 
5110 
Fig. I .  The testing equipment 
disc 
I v .  THE MEASUREMENT EQUIPMENT 
A. The Magnetic Field Quantity Gauges 
To measure the magnetic field intensity inside the sample, 
a Rogowsky coil was placed beside the sample, see Fig 2. 
The number of turns of the coil, N, was approximately 1800 
C1. Contribution of the Rogowsky coil 
shape to the closed path 
C Closed path 
coil length 
A: coil cross-section area 
Fig. 2 Position of the Rogowsky coil beside the sample 
The relationship between the voltage, e ,  at the Rogowsky 
coil terminals and the magnetic field intensity inside the sam- 
ple, Hint, was calculated by means of the Equation (1). 
(1) 
Knowing that-H=B/po outside the sample and, therefore, in  
the coil, the Lenz's law gives the relationship between the 
contribution of the coil to this integral and the voltage at its 
terminals. Finally, one get: 
B was measured by means of a very thin coil with 8 turns 
that surrounded the sample at his half height. 
B. The Recording of the Signals 
To get the B-H hysteresis loop, the magnetic field quanti- 
ties were simultaneously recorded at several instants in a tran- 
sient recorder, during at least one period, T. 
V THE DATA PROCESSING 
The recorded signals were, then, processed in order to obtain 
losses, etc 
A.  The Relative Permeability Computation 
For each test, pr was estimated as the minimal value of the 
differential relative permeability function dB/dH, that is, 
graphically, the slope of the tangent of the B-H loop at its 
peak points. 
B. The MagneticLosses 
In Fig. 3, a typical anomaly of a hysteresis loop caused by 
the lack of measurement precision is shown That appeared 
especially at high induction. 
~ 
the hysteresis loops, the relative permeabilities, the magnetic the reactive power: 
Intersection points 
Fi,o 3. Example of a loop with two intersection points 
The intersection points of the loops appeared i n  the neigh- 
borhood (H = rt16,OOO A/m , B = -i.O 3 T), independently of 
the magnetic field intensity top-value during the tests As a 
consequence, the higher the magnetization, the bigger the sec- 
ondary loops compared to the main loop Theretore, the mag- 
netic losses were computed as the absolute summation of all 
the areas, though this computation may have overestimated 
them 
C. The Q-Factor and Efficiency Calculations 
The Q-factor represents the number of periods necessary to 
dissipate by losses the maximum reactive energy in the sam- 
ple: - -  - -  u top -  u top Q= 
Losses under one period JBdH 
T 
To compute it, the reactive volume energy, U, has been ap- 
proximated. In our case, 6U = H6B = H dB/dH 6H. Then, by 
neglecting the second order term of the mathematical expan- 
sion 6( 1/2H2 dB/aH) = H dB/dH 6H+ 1/2H%(dB/dH), one 
get: 
U = 1/2H2 dB/dH 
The efficiency, q ,  compares the magnetic power losses to 
S I I I  
10 - 
9 -  
8 -  
7 -  
6 -  
5 -  
4 -  
3 -  
2 -  
I -  
PT kBdH 
1 j ( a u l d t  
q = 1- 
p~ T at 
VI THERESULTS 
0 0.2 0.4 0.6 0.8 I 
Btop (TI 
Fig. 4. Relative permeabilities at 50 Hz (black line) and 400 Hz (gray line) 
The difference between the two curves can be ascribed to the 
imprecision of the graphic estimation of the slope at 0.17 T 
at 400 Hz. The corresponding magnetization curve, see in 
Fig. 5 ,  also showed the same shape for both frequencies: 
0 50 100 1 50 200 
Htop (kA/m) 
Fig. 5. Relation between Btop (peak value of magnetic field density) and 
Htop at 50 Hz (black line) and 400 Hz (gray line) 
In Fig. 6 and Fig. 7, the magnetic losses are compared with 
the soft magnetic material PERMEDYN. 
The drop observed near 0.6 T corresponds to the mentioned 
sample change. Just below 0.6 T, the magnetic field intensity 
was probably distorted resulting in an overestimation of the 
losses. Despite that, the losses of the studied slurry are con- 
sidered too high compared to the PERMEDYN which is made 
of a similar iron powder from the same supplier, Hoganas. 
0 0. I 0.2 0.3 0.4 0.5 
h o p  (TI 
Fig. 7. Losses comparison at 400 Hz between PERMEDYN (black line) 
and the studied slurry (gray line) 
50 , 100 , 
. ^  I 
‘” I 
A 99 I 
0 0.25 0.5 0.75 I 0 0.25 0.5 0.75 1 
h o p  (TI Btop (TI 
Fig. 8. (a) Q-factor at SO Hz (b) Efficiency at 50 Hz 
The sudden jump of Q and 17 around 0.6 T in the Fig. 8(a) 
and 8(b) can be explained in the same way as the drop of the ’ 
magnetic losses in the Fig. 4. Thus, the Q-factor and the effi- 
ciency below 0.6 T may be a little higher than these shown. 
VII. CONCLUSIONS 
The optimal use of the studied slurry is in the neighbor- 
hood of B top = 0.4 T. This value is considerably higher than 
those in conventional magnetic tluids, which indicates that 
use of magnetic slurries can be feasible in electric power ap- 
plications. However, the powder quality, regarding hysteresis 
losses at higher field densities, must be improved. 
REFERENCES 
[ I ]  B. D. Moskowitz and K. Raj, Ferrofluidics Corporation, Nashua, NH, 
[2] T. Cedell, “Magnetostrictive materials and selected applications,” 
USA, personal communication, 1995. 
doctorcil rhesis, Lund University, Sweden, 1995, pp. 165-188. 
0 0.2 0.4 0.6 
b o p  (TI 
0.8 I 
Fig. 6. Losses comparison at 50 Hz between PERMEDYN (black line) and 
the studied slurry (gray line) 


Experimental Study of Magnetic Slurries Regarding Magnetic Properties

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=CCFD15A67E3463EEFBD987ED052B2E03?doi=10.1.1.1069.5081&rep=rep1&type=pdf

Experimental Study of Magnetic Slurries Regarding Magnetic Properties

Abstract

Similar works

Full text

Available Versions

CiteSeerX