6.9 Influence of HTS bulk geometry on the properties Ë 175Tab. 6.7:Influence of HTS bulk shape on the levitation forces.
Sample Cylinder 1 Hexagon 1 Square Cylinder 2
Shape
Weight (g) 45. 35 38. 89 29. 37 23. 3
Diameter (mm) 29. 5 29. 5 22. 0 22. 0
27
Thickness (mm) 10. 8 11 – –
Area (cm^2 ) 6. 83 5. 65 4. 84 3. 80
Trapped flux (T) 0. 168 0. 163 1. 39 1. 04
f 0 (N/cm^2 ) 10. 10 8. 67 6. 53 3. 00
F 5 (N) 69. 0 52. 7 31. 6 11. 4
F 10 (N) 52. 7 40. 1 23. 9 9. 5
F 15 (N) 22. 0 21. 1 18. 2 7. 6
F 20 (N) 15. 3 16. 3 14. 3 6. 7
Diagonal length, side length, side to side,F 15 (N) is the levitation force of gap of 15 mm.
The research results are listed in Tab. 6.7. It is well known that the magnetic levitation
forces are related to the critical current densityJc, the diameter of screening current
dand magnetic field gradient dB/dz. These measurement results also revealed the
physical essence of the HTS Maglev. Both diameter of cylinder 1 and the diagonal
length of hexagon 1 were equal, and the levitation forces at a gap of 15 mm [F 15 (N)]
on cylinder 1 and hexagon 1 were 22.0 and 21.1 N, respectively. Both the diameter of
cylinder 2 and the side length of the square were equal, and the levitation forces at
a gap of 15 mm [F 15 (N)] on cylinder 1 and hexagon 1 were 18.2 and 7.6 N, respectively.
The two groups of data above indicated that the path size of the ring shielding
current depended on the levitation forces on the different shaped bulks, for the same
conditions of the critical current densityJcand magnetic field gradient dB/dz. In
order to fully utilize the magnetic field of the PMG and not allow the magnetic field
energy leakage, the HTS bulks should generally be arranged to be gapless. Thus, the
hexagon and square were thought to be the best shapes, but experimental results
showed that the levitation forces on the hexagon and square HTS bulks over the PMG
did not increase, or it even was smaller sometimes. The experimental results implied
that a cylinder is better than a hexagon or square, because the cylinder has a longer
shielding current path than the hexagon and square. The longer the shielding current
path, the greater is the levitation force.
Song et al. [41] proved experimentally the correctness of this choice. He measured
the levitation forces of cylindrical and rectangular shaped HTS YBCO bulks over the
PMG (see Section 6.14) using the HTS Maglev measurement system SCMI-01 (see