8.2 Dynamic characteristics Ë 269which can be attributed to the fact that the force stiffness in the vertical direction
is larger than that in the axial direction. Magnetic flux densityBzand its derivative
dBz/dzaround the on-board HTSC, which contribute to the levitation forces, are larger
than the fields and field gradients in the lateral and forward directions due to the
PMG’s joint disturbances. Thus, PMG’s optimization and high-precise assembly are
both effective in suppressing those dynamic motions as well as improving the other
quasi-static levitation properties.
When the HTS Maglev vehicle model suffers some undesirable disturbances,
different from other kinds of Maglev, the nonlinear repulsive forces from the on-board
bulk YBCO pull the vehicle back due to the inherent HTS flux-pinning property. The
more flux penetrate into the HTS bulk, the better anti-disturbance capability the
vehicle will have. According to the distribution of the PMG magnetic field density,
a lower FCH means a larger vertical magnetic flux density and its gradient. That is
why the maximum amplitude decreases with increasing FCH as shown in Fig. 8.8.
The maximum amplitude, 414휇m, happens when the bulks are cooled at the 60 mm
height and the vehicle moves at the speed of 90 mm/s. The dynamic phenomenon is
still slight because of the low speed.
However, Fig. 8.8 implied that the effect of the running speed, a conventional
factor resulting in the instability, on the dynamic motion exists. The vibration in-
tensity increases with the speed at the higher FCH. However, the effect is not so
strong with a decreased FCH. When the forward speed of the vehicle is considered,
the vertical magnetic field gradient of the PMG with time should be formulated as
(dBz(x,y,z)/dz)dt. The improvement of speed brings a much faster change of the
magnetic flux, then a derivative larger restoring force will be produced to pull the
vehicle back. Thus, the higher speed may not bring more danger in the operation of a
HTS Maglev vehicle over a PMG unlike other kinds of Maglev vehicles. However, this
phenomenon needs more experiments for verification.
Fig. 8.8:Maximum amplitude of point E under the forward speed at different FCHs.