8.2 Dynamic characteristics Ë 275“force-displacement” measurement), except for case 2 because the forces in case 2
oscillated with the rotating PMG. Case 1 and 2 curves experienced a bigger hysteresis
area with a smaller force in the returning part when compared to the “directly return”
curve. This further validates that the shielding current decrease was caused by
irreversible energy loss.
Further exploration will improve the preliminary understanding of the dynamic
equilibrium of an HTS Maglev vehicle. It is believed that these observed phenomena
are of great importance in the future running and design considerations for the HTS
Maglev system.
8.2.3Levitation performance at different working temperatures
The relationship between critical current density (Jc) and working temperature (T) is
given by the following equation:
Jc∝ [ 1 −(T/Tc)^2 ]^3 /^2. (8.3)According to the Eq. (8.3),Jcof the YBCO bulk will be increased with decreasing tem-
perature. Eq. (8.4) below gives the common solution of the levitation force produced
byJcinside a HTSC bulk in magnetic field,
FLev=XJsdV=X[Jc(Bx(훿 0 )−Bx( 0 ))/훿 0 ]dV, (8.4)whereJcis the critical current density at a certain temperature, andVrepresents
the volume where the induced supercurrent flows in the superconductor.Bxis the
horizontal component of the original magnetic field, and훿 0 is the penetration depth
of the magnetic field which is given by the following relation:
훿 0 =(Bz−BFC)/휇 0 휆Jc, (8.5)whereBzis the vertical component of magnetic field at the smallest levitation gap
during the levitation measument,BFCis the vertical component of magnetic field at
FCH,휆is the coefficient determined by the configuration of the HTSC sample, and휇 0
is the permeability of vacuum.
Eqs. (8.3) and (8.4) indicate that the levitation performance of HTS bulk will beco-
me better at lower working temperature in an applied magnetic field. In this section,
the specific effects of temperature on some significant levitation performances are
reviewed for the HTS Maglev vehicle application. Then, a ring-shaped YBCO bulk
above a two-pole Halbach array’s PMG [25]. (Halbach PMG, Section 8.4.1) was set up
as the basic model for an HTS Maglev vehicle. The ring-shaped YBCO superconductor