Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 78

providing a low magnetic eld in the bore of the magnet assembly. The average

magnetic fl ux density during high- eld operation was 1.25 T, whereas the average

low- ux density was 0.29 T [ 53 ].

Another problem, addressed by Arnold et al. [ 53 ], was related to the non-ideal
structure of the Halbach array. Namely, an ideal structure would represent an
in

eld magnetic fl

nite number of magnet segments in each ring. However, due to the nite number

of permanent magnet segments, the uniformity of the magnetic ux density varied,

especially near the interfaces of two segments. Therefore, fringing magnetic

fl

elds
appeared at the interfaces of the segments of the outer ring. Moreover, due to the
small distance between the rings, the interaction of the fringing elds was very

high, which led to very high changes of the amplitude of the torque. Arnold et al.
[ 53 ] proposed a larger number of segments or a increased distance between the
magnet rings to solve that problem.
In 2007 at the third International Conference on Magnetic Refrigeration at Room
Temperature, which was held in Portoro in Slovenia, Okamura et al. [ 54 ] presented

an optimized magnet assembly, which was serving a prototype for the Chubu
Electric Power Co. This magnet assembly represented an improvement over a
previous solution in which two magnet bars (with the maximum magnetic

ž

ux
density being 0.77 T) [ 55 ], attached to a soft iron core, were rotating and thus
providing a magnetic

fl

eld in the four beds of the AMRs. The AMRs were posi-
tioned between the rotating magnet bars and the static outer ring made of soft iron.
As denoted by Okamura et al. [ 54 ], the problem related to the magnet assembly was
in the eddy currents, which were produced along the outer soft iron ring. As a
solution the authors cut the soft iron ring on a number of thinner rings, attached to
each other with an electrical insulator in between. This measure drastically
improved the performance. Furthermore, the authors have introduced a new inner
rotating part, which provided a higher magnetic

ux density with a magnitude of
1.1 T. The cross-section of the new magnet assembly is shown in Fig. 3.37 .

fl

Fig. 3.37 The cross-section
of the magnet assembly
designed and constructed by
Okamura et al. [ 54 ]