332 Ë 9 HTS Maglev bearing and flywheel energy storage system
to 2700 N (1500 rpm) when the temperature of the HTS bulk was increased from 68 to
72 K [35]. Synthesizing the reported research results, it is clear that the inhomogeneity
of the PM rotor field causes loss and heat inside the HTS bulks. The heat increases
the temperature and decreases the critical current density within HTS bulks. This will
reduce the stiffness of the RSB. In the RSBs of ATZ and Siemens, HTS bulks are cooled
by copper conduction cooling. The cooling efficiency is lower than the case of direct
liquid nitrogen cooling shown in Fig. 9.3. HTS bulks of Fig. 9.3 are immersed in liquid
nitrogen, then heat inside the bulks can be easily dissipated. Finally, the decrease of
the axial stiffness of RSB is only 8%.
It has been proved that the liquid nitrogen cooling method has better cooling
efficiency, which assures the stiffness stability of the RSB at different speeds. On
the other hand, although the copper cooling method has a lower cooling efficiency,
introduction of a cryocooler to the copper cooling method will make the cooling
operation convenient because no Dewar cover is needed between PM rotor and HTS
stator (while the liquid nitrogen cooling method needs a Dewar which covers the
whole HTS stator).
Overall, the temperature rise problem can be solved by the following methods:
reducing the working temperature of HTS bulk, optimizing the cooling system, in-
creasing cooling efficiency, improving the manufacture technique to increase the
homogeneity of the PM rotor, etc. All these methods can reduce the influence of
rotation of the PM rotor on the stiffness of RSB. In practice, the selection criterion
should be based on cost, technical requirements, and difficulty of liquid nitrogen
cooling method vs. the copper conduction cooling method.
9.2.2Calculation method of radial stiffness
Radial stiffness of the RSB is usually obtained by experiment or calculation (Q. X. Lin,
Z. G. Deng, D. H. Jiang, G. T. Ma, J. Zheng, J. S. Wang, and S. Y. Wang, Calculation
method for radial stiffness and deflection angle stiffness of radial high temperature
superconducting magnetic bearing, unpublished data). The experimental result can
provide a precise value, but it costs much time and effort. The calculation is simple,
but its precision and effectiveness are sensitive to the selected parameters.
KEPRI developed a method to obtain the radial stiffness of a RSB. It applies a
calculation equation based on the experimental test to obtain the radial stiffness.
The error of this testing and calculation method was reported to be 12.14% [22]. This
calculation method may be able to be improved for higher precision.
Considering that the influence between HTS bulks is small [22], the radial stiffness
of a RSB can be equal to the sum of a radial stiffness between the PM rotor and each
HTS bulk. In Fig. 9.5a, the center circle represents a PM rotor, and the arrow represents
the radial movement direction of the PM rotor. The surrounded tile shaped objects
are HTS bulks in different positions. When the PM rotor moves, the distance between