8.3 Methods to improve Maglev performances Ë 287Fig. 8.28:Levitation forces vs. height on the different trapped-field conditions.
magnet during the levitation force measurement loops because the guidance forces
mainly depended on the trapped flux inside the HTS bulk.
Figure 8.28 shows that the higher the trapped field of the HTS bulk magnet was,
the smaller the repulsion levitation force became and the bigger the attractive force
got. The reason was that the magnetized YBCO bulk behaved more like a “magnet”
with the “S pole” surface close to the nearby concentration flux pole, as an “N
pole” of the PMG. Then the attractive forces were generated between the HTS bulk
magnet and the PMG as the negative part of the eventual levitation forces. Moreover,
the bigger external magnetization field caused a stronger HTSC “magnet”, which
further produced a larger attractive force. Thus, the levitation forces decreased with
the stronger HTS bulk magnet. In contrast, the levitation forces would increase by
changing the negative effect into positive effect only if the “S pole” orientation of the
HTSCM was placed in reverse over the PMG, which was feasible and easy to put into
effect in practice for load enhancement.
On the other hand, the levitation forces did not continue the above negative trend
of the HTS bulk magnet with field higher than 0.349 T, like the 0.487-, 0.616-, and
0.720-T curves in Fig. 8.28. This was attributed to the limitation of the maximum flux-
trapping capacity of the YBCO sample. Similar to the guidance force trend as shown
in Fig. 8.29, it will be of little help to improve the Maglev performance with a stronger
HTS bulk magnet. At 15-mm WH, the interaction vertical forces began to change from
a repulsive force to an attractive force when the trapped field was more than 0.210 T.
The attractive forces achieved a maximum 7.4 N at the 0.349-T trapped field. Thus, the
HTS bulk magnet will be more suitable to realize stable suspension in the future.
On the other hand, Fig. 8.29 shows all the guidance forces at the 15-mm WH and
the guidance stiffness was negative like the conventional stable HTSC-PMG levitation.
The guidance forces increased with the increase of the trapped field of the bulk magnet
above the PMG due to the more flux in the bulk magnet from the external static