9.2 Characteristics of HTSB Ë 327before, HTSB can work well in vacuum even with a short-time failure of the cooling
system. Thus, there is still enough time to stop the flywheel safely before the HTSB
failed.
9.2 Characteristics of HTSB
The stable HTS levitation comes from the unique magnetic flux pinning property of
HTS bulk (see Chapters 1 and 3). It is interesting that the stable levitation can be
realized when the PM is located below or beside the bulk HTSC. If the PM is a cylinder
with an axial symmetric magnetic field, it can levitate above the HTS bulk and rotate
along its axis smoothly regardless of the direction of its axis. This is the basic principle
of HTSB.
The smooth rotation of the cylindrical PM is not affected by its initial position or
the direction of its axis. Researchers have developed an axial (ASB) and a radial high-
temperature superconducting magnetic bearing (RSB). ASB consists of a HTS stator
(with disc or ring shaped) and a PM rotor. The PM rotor just levitates above the HTS
stator (Fig. 9.1a). Before and after field-cooling activation, there is a gap difference
between the original cooling position and levitation position of the PM rotor. The ratio
of weight and gap difference of the PM rotor is defined as its average axial stiffness.
The RSB consists of a HTS stator (pipe-shaped) and a PM rotor. The PM rotor levitates
at the center of the HTS stator (Fig. 9.1b). After field-cooling activation, there is also
a height difference of the PM rotor. The definition of its average axial stiffness is the
same as for the ASB. When a radial force is exerted, the PM rotor will move along the
force to a new equilibrium position. The ratio of radial forces and radial displacement
of the PM is defined as the average radial stiffness for both the ASB and the RSB. The
PM rotor for both the ASB and the RSB is usually composed of PM rings and iron rings.
The PM rings are the source of magnetic field, and their polarity directions are usually
opposite, as shown by the arrows in the PM rings in Fig. 9.1. Iron rings are employed
to concentrate the magnetic field. The direction of magnetic field in the working gap
Fig. 9.1:Schematic diagrams of two types of HTSB: (a) ASB and (b) RSB. The arrows in the PM rings
show the magnetization directions.