9.2 Characteristics of HTSB Ë 329stiffness. Experimental results demonstrated that a radial displacement of 1.2 mm
leads to a decrease of the axial force by 25%, whereas an axial displacement of 2 mm
causes an increase of radial force by 20% [18].
As for the stiffness calculations, Takashi Hikihara introduced a deflection angle
stiffness equation for ASB, but the dimension parameters were simplified [21]. Korea
Electric Power Research Institute (KEPRI, Korea) developed a calculation method for
radial stiffness with the error of 12.14% [22].
- Rotational loss. Masaie assumed that rotation loss comes from the Lorentz force
between the irregular magnetization of HTS bulks and the current induced by
the PM rotor [23–26]. Researchers of International Superconductivity Technology
Center (ISTEC, Japan) pointed out that rotation loss can be reduced by improving
the magnetic field distribution of the HTS stator and adding insulators into the
multi-layer PM rotor [27]. Rotational loss is also related to other factors. Boeing
Company explored the rotational loss of ASB of a 5-kWh FESS with a PM rotor
accelerated to 14,500 rpm [28]. They found that the rotation loss decreases with
decreasing temperature and the loss with conduction cooling is not significantly
higher than that with direct liquid nitrogen cooling. They also measured the loss
of a 15.4-mm-diameter, 150.3-mm-long rotor with two ASB stators at a maximum
speed of 100,860 rpm [29]. It was found that the loss increased to the maximum
value at about 16,440 rpm when the speed was higher than 4800 rpm. - Stability. Stability is the guarantee of the normal and reliable operation for a
HTSB. In ISTEC, the decay of the levitation forces due to flux creep was reported
to be suppressed by pre-loading and super-cooling methods, and electromagnetic
bearings were applied to increase the rotational stability [27]. The axial displace-
ment of the rotor was less than 0.06 mm, and the radial amplitude was less than
0.03 mm in normal operation and as small as 0.15 mm at the critical rotational
speed [27]. ATZ developed an optimal design for the dynamic structure of rotor
(stiffer) and added two dynamic damping systems at both ends of the rotor shaft
to increase the stability [30]. KEPRI found that the thermal insulating bolts can
increase the damping of RSB and discussed two damping mechanisms in details
[31]. The damping coefficient of RSB as optimized by KEPRI was 12 times larger
than the estimated value [32]. Boeing found that inserting a secondary damper
between the HTS stator and the foundation ground increased the stability [33].
In Sections 9.2.1–9.2.3, the basic stiffness characteristics of HTSB are significantly
discussed.
9.2.1Axial stiffness characteristics of HTSB
The first-generation RSB prototype in our group was designed and developed as
shown in Fig. 9.2. It is made up of a PM rotor with 52 mm in diameter and 66 mm in
length (Fig. 9.3) and a HTS stator with six HTS bulks. Each HTS bulk had the dimension