9.2 Characteristics of HTSB Ë 331PM rotor was fixed to the axial force sensors through a transmission mechanism and
an epoxy resin board. In this case, the HTS stator could move horizontally and PM
rotor could move axially with a precision of 0.05 mm. Force sensors could measure
the axial forces with a precision of 0.02 N. The PM rotor was located at the center
of the HTS stator with liquid nitrogen for cooling. After field cooling, the DC power
was switched on to drive the PM rotor, through a 280-mm-long stainless steel spindle.
The rotational speed was measured by a non-contact tachometer and is controlled by
voltage of DC power. The long spindle was used for reducing heat loss from the motor
to liquid nitrogen and eliminating influence of motor field on the PM rotor. When the
PM rotor moves axially, the axial forces of the RSB at different rotational speeds were
measured by SCML-02.
As is well-known, the experimental results of axial stiffness of a HTSB at different
rotational speeds are helpful for practical applications. Figure 9.4 shows these results
of the axial forces. The maximum axial force was 178.7 N (at 0 rpm) at an axial displa-
cement of 1.05 mm, and the relevant axial stiffness was 170.2 N/mm. When speeds of
the PM rotor were 645, 1827, and 3801 rpm, the maximum axial forces were 174.5, 176.2,
and 164.4 N with an axial stiffness of 166.2, 167.8, and 156.6 N/mm, respectively. The
maximum decrease of axial force was 14.3 N from 178.7 N (0 rpm) to 164.4 N (3801 rpm),
and the maximum decrease rate of axial stiffness was about 8%. This result indicated
that the rotational performance of this RSB was good compared with former reports in
Ref. 19. It was found that the radial force was reduced by about 50% at 1500 rpm and
80 K, which was thought to be correlated to the critical current density of HTS bulk
and its temperature dependence [20].
The large decline of force and stiffness influences the normal operation of HTSB.
Siemens found that the radial force of the RSB was reduced from 3700 N (at 0 rpm)
Fig. 9.4:Axial forces of RSB at different speeds.