8.3 Methods to improve Maglev performances Ë 291Fig. 8.32:Levitation (a) and guidance force (b) comparisons of the bulk levitation unit above the
PMG between the cases of direct 30- and 6-mm FCH re-magnetization height after ZFC. The inset in
(a) is the enlargement of the two force curves at gaps of 6–10 mm.
of 6 to 10 mm. The maximum levitation forces were 627 and 658 N, respectively, for
the case of the direct 30- and 6-mm FCH re-magnetization height. The corresponding
maximum guidance forces had a larger difference, 106.4 and 123.2 N, respectively. That
is to say, by a re-magnetization process at a gap of 6 mm above the PMG after ZFC, the
levitation capability and stability of the HTS bulk levitation unit had been improved
simultaneously with respect to the direct 30-mm FCH case. Although the increase ratio
was not very large, it had broken through the present defect in performance impro-
vement with the sacrifice of one important force. Furthermore, the re-magnetization
method after ZFC presents a possible working way for applications of HTS bulks to
levitation systems.
8.3.3An on-board double-layered HTSC array
At present, all HTS Maglev test vehicles employ a single-layered HTS bulk layout inside
the cryostat [5–15]. Previously, there were many reports on the dependence of the
magnetic levitation forces on the thickness or even layer number of the HTS bulks
[41–43]. It was reported that the levitation forces kept increasing with the thickness of
a HTS bulk and did not saturate until 12 mm [44]. This may be the reason why the bulk
samples in common use are around 12–15 mm in thickness which is also dependent on
the bulk processing technologies. If two layers of HTS bulk are applied over a PMG, the
question is whether there is still some magnetic field available above the upper surface
of the single-layered bulk samples to interact with the new upper bulk layer. Because
the levitation force is derived from the electromagnetic force, whereJis the induced
current inside the HTS bulk,Bex, the external magnetic field, andVis the volume of the
bulk body. If the answer is positive, some levitation forces could be further obtained by