6.3 Levitation forces of HTS bulk above PMG Ë 155Fig. 6.1:Levitation forces of single-NdFeB PM cylinder and a NdFeB PMG in ZFC at 77 K [24].
single-NdFeB PM cylinder (its surface magnetic flux density was 0.38 T), respectively
[24]. Clearly, the levitation forces of the NdFeB PMG were much higher than that of the
single-NdFeB PM cylinder.
Table 6.1 lists the levitation forces at different position above the surface of the
magnets.FLGis the levitation force over the PMG magnet andFLSis the levitation
force over the single-NdFeB PM cylinder. The levitation forceFLGbetween YBCO
(휙 30 ×17 mm thickness) and the PMG is 25 N, but the levitation forceFLSbetween
the same YBCO and the single-NdFeB PM cylinder is 0.1 N when the levitation gap is
25 mm. It is clear that the levitation force over the PMG is much larger than that over a
single-NdFeB PM cylinder. It is interesting that there is still a large levitation force at
higher levitation gaps.
Another melt-textured HTS YBCO bulk sample made in the Beijing General
Research Institute for Nonferrous Metals was subsequently measured. The YBCO bulk
sample had a diameter of 30 mm and a thickness of 14 mm. In this case, there was a
103.4-N (14.6 N/cm^2 ) levitation force at the 5-mm gap between the YBCO sample and
the NdFeB PMG. At that time, the levitation force was relatively high at the same.
Tab. 6.1:Levitation force vs. levitation gap over the single-NdFeB PM [FLS(N)]
and the PMG [FLG(N)] [24].
Levitation gap (mm) FLG(N) FLS(N)
5.0 94.0 18.4
10.0 68.9 7.0
15.0 47.9 2.8
20.0 34.4 0.5
25.0 25.0 0.1
30.0 18.1 0.0