296 Ë 8 New progress of HTS Maglev vehicle
is closely related to the intensity and the gradient of the applied magnetic field, as
follows,
휇 0 J = dBex/dr, (8.6)
FLev = XJ×Bxdv, (8.7)FGui = XJ×Bzdv, (8.8)where the induced currentJis dependent on the gradient of the external magnetic
fieldBexand levitation forceFLevand guidance forceFGuicorrespond to the Lorentz
force between the induced current and the horizontal component of the magnetic
fieldBxor vertical component of the magnetic fieldBz, respectively. This relationship
implies that we can use the magnetic field configuration to design a reasonable laying
mode of HTS bulks. As shown in Fig. 8.36b, the laying mode of a HTS bulk with its
c-axis parallel to the horizontal applied field is more efficient to excite a larger induced
current in thea-bplane than the counterpart in Fig. 8.36a. At present, the HTS bulk
is put inside the cryostat with itsa-bplane parallel to the surface of the PMG in
HTS Maglev prototypes [5–15]. In this layout, the bulk gains the maximum levitation
force at the maximum vertical field position and the maximum guidance force at the
maximum horizontal field position [46]. It means that it will be feasible to improve
the levitation forces or guidance forces by changing thec-axis laying direction of
the HTS bulk. Thus, the on-board HTS bulk appears in two different laying modes.
One is a traditional horizontal laying mode in which thec-axis of the bulk directs to
the vertical direction, the other one is a vertical laying mode in which thec-axis of the
bulk is along the horizontal direction.
Figure 8.37a and b depicts the levitation force curves of the bulk sample with
different laying modes measured at the PMG middle and field-pole positions in the
cases of ZFC and 30-mm FCH, respectively. The bulk shows a similar levitation force
curve in the two cooling conditions. When the bulk was put in a horizontal mode, the
levitation forces in the field-pole position were larger than that at the middle of the
PMG. In contrast, when the bulk was laid in a vertical mode, the levitation forces in
the PMG middle were larger than that at the field-pole position. Furthermore, at the
middle of the PMG, the bulk with a vertical laying mode got a larger levitation force
than that with a horizontal laying mode. These results verified again that the bulk
with a horizontal mode can obtain the best levitation force performance at the field-
pole position. At the same time, it also implied that it is possible to improve the bulk’s
levitation forces at the middle of the PMG by changing the laying mode from the
horizontal to the vertical one. By laying the bulk as a vertical way, the width of the
bulk facing the PMG surface was 13 mm rather than the former 32 mm (see Fig. 8.36).
At the same available space in width for the horizontal bulk, at least two bulks can be
laid in the vertical way. The two bulk’s levitation performance can be estimated by an
approximate superposition method [47].