328 Ë 9 HTS Maglev bearing and flywheel energy storage system
between HTS stator and PM rotor is mainly axial for the ASB and radial for the RSB,
respectively. This is the basic distinction for these two types of HTSB. The HTS stator is
cooled down by cryogenic liquid or a cryocooler. The usual material for the HTS stator
is REBaCuO (RE denotes the rare earth elements), such as YBaCuO and GdBaCuO,
because REBaCuO has a high magnetic irreversibility field and can be grown as a
large-grain bulks [13].
ASB can reach 520,000 rpm [14] due to its simple structure and easy manufacture,
but its stiffness is low. The Boeing Company’s ASB prototype has a PM rotor with
diameter of about 220 mm, and its axial and radial stiffness is 144 and 69 N/mm,
respectively [16, 17]. The RSB stands out because of its compact structure, larger
carrying capacity, and higher stiffness, which is more suitable for heavy-load appli-
cations. In 2007, ATZ Company in Germany manufactured a RSB achieving an axial
force of 10.08 kN and stiffness of 4.5 kN/mm [18]. The diameter and length of the PM
ring are 200 and 120 mm, respectively. Nexans SuperConductors GmbH in Germany
developed a RSB with a radial force of 6.9 kN and radial stiffness of 5.1 kN/mm [19].
The inner cylindrical surface of the HTS stator is 325 mm in diameter and 305 mm
in length. During the actual design process, RSB can be classified into two types:
RSB with inner PM rotor (as shown in Fig. 9.1b) and RSB with outer PM rotor. A
RSB with an inner PM rotor has been chosen by ATZ and Nexans SuperConductors
GmbH. Compared to the outer PM rotor structure, a RSB with an inner PM rotor has
advantages, such as easy location of the PM rotor during field cooling, simple design
of touch-down bearing, high homogeneity of rotor fields, low rotational loss, and easy
assembly.
The good operation performance of the HTSB is fundamental to engineering
applications, so researchers have carefully investigated the primary characteristics
like stiffness, damping, rotor dynamics, stabilization, force creep, temperature depen-
dence, and rotational loss. The stiffness, rotational loss, and stability are particularly
important for the practical application of HTSB.
- Stiffness. Stiffness means the resistance ability to exterior force and torque for
a HTSB, which can be improved by lowering the working temperature [18] to
improve the material performance of HTS bulks and PMs. For example, ATZ found
that the radial stiffness of the RSB increases from 1.4 kN/mm at 79 K to 1.8 kN/mm
at 72 K and that the axial stiffness increases from 3 kN/mm at 78.5 K to 4.5 kN/mm
at 72 K. Moreover, the axial force doubles to 10,080 N with an axial displacement of
3.3 mm at 72 K compared with that at 82.3 K [18]. On the other hand, the stiffness
also changes during rotation. By rotating the HTSB to 1500 rpm, the correspon-
ding force decreases by almost 50% at 80 K. This is considered to be correlated
to the dependence of the critical current density on the temperature of HTS
bulks [20].
Cross stiffness is an interesting parameter for HTSB. This is just the influence of
axial displacement on radial stiffness or the influence of radial displacement on axial