8.2 Dynamic characteristics Ë 271adjacent permanent magnets (PMs), assembly errors such as PM cracks, junctions
between PMs, etc. Consequently, the on-board YBCO bulks experience a spatially
varying external magnetic field when traveling along the PMG, this moving field
depends on the running speed, and it may be cyclical. Figure 8.10 plots the vertical
component of the central magnetic field density of the PMG circumferential field at
different heights. The smallest field position was assigned to zero degrees and the hig-
hest circumferential field appeared at 221.52 degree which was chosen as the cooling
position of the onboard HTS bulk over the PMG. During the running experiment, the
dynamic levitation forces at gaps of 10 and 15 mm were of most concern.
Figure 8.11 shows the schematic diagram of the experimental YBCO bulks array
above the PMG in SCML-03 (see Section 5.6). It is seen that the circular PMG assembly
in SCML-03 belonged to the “Century” PMG type and that there were four pieces of
three-seeded melt-textured YBCO bulks (64 mm in length, 32 mm in width and 13 mm
in thickness) employed as the HTSC array.
With the above experimental set-up for SCML-03, the dynamic levitation forces of
the YBCO bulks were systematically studied under different linear equivalent speeds
from 0 to 238 km/h, in acceleration and constant speed conditions. First, Fig. 8.12
describes the actual levitation transition during the experimental process as the PMG
was accelerated from 0 km/h to 238 km/h and then decelerated back to 0 km/h in
steps. The steady speeds at every step were 1.5 km/h, 60 km/h, 119 km/h, 159 km/h,
179 km/h, 208 km/h and 238 km/h. The staying time at each speed step was 4 minutes.
In Fig. 8.12, the dark zone corresponds to the oscillation range of the levitation forces
while the light grey average line indicates the general tendency of the levitation
forces. This dark zone in Fig. 8.12 shows the sharp rise and decline behavior of the
levitation force with speed change which most likely was caused by interference. The
average line in Fig. 8.13 shows the true attenuation tendency of the levitation forces
with respect to speed. The average levitation forces decreased from 179 to 170 N after
65 minutes in operation. The static levitation forces decreased from 201 N before the
PMG rotated to 191 N after the PMG rotated. Meanwhile, there was no significant force
Fig. 8.10:Vertical central magnetic field component among the circular PMG.