Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 77

Fig. 3.35 A double Halbach array, which was for instance applied in magnetic refrigeration by
Tura and Rowe [ 49 , 50 ](a ON eld position, b OFF eld position)

Halbach array (presented in Fig. 3.35 ). The problem that characterized the double
Halbach solution was in the actual magnetic

fl ux density at low eld, which was not

zero (having an average high- eld magnetic fl ux density of 1.34 T and an average

low- ux density of 0.57 T [ 53 ]). This, of course, is not desired. As

stated further by Arnold et al. [ 53 ], the total

eld magnetic fl

eld vector orientation inside the AMR
volume rotated, which could induce additional rotational forces and eddy currents.
The new magnet design of a triple Halbach array with the increased number of
magnet segments in each ring (each ring comprised 12 segments of magnets

— not

shown in Fig. 3.36 ) improved the homogeneity and the magnetic ux density. The

two outer magnetic rings rotated in the counter direction with respect to each other,
while the inner magnetic ring was static. In this way a sinusoidal

fl

eld waveform
was produced. In analogy with the previous solution, the three rings, when their
vector of the magnetic

fl ux density was aligned, lead to the maximum magnetic fl ux

density in the bore. When the two outer rings were rotated by 180 ° C with respect to

the inner static ring, the magnetic fl ux density vectors cancelled each other, thus

Fig. 3.36 A triple Halbach array with two outer magnetic rings rotating in the counter direction
and with the inner static magnetic ring (see also Arnold et al. [ 53 ])