9.2 Characteristics of HTSB Ë 335In comparison, the experimental value of radial stiffness of RSB is 140 N/mm while
the former calculated value is 157 N/mm (with an error of 12.14%) [22]. Thus, the new
calculation method gives an error of only 2.86% [Eq. (9.8)].
9.2.3Deflection angle stiffness
Deflection angle stiffness represents the capacity to resist an external torque, which is
an important parameter for dynamical operation of a HTSB.
When a torque is exerted on a PM rotor as shown in Fig. 9.7, the axis of the PM
rotor will deflect along the horizontal line which passes through the center point
A. If the torque isM, the deflection angle is휃. The deflection angle stiffness is
expressed as
k휃=M
휃. (9.10)
Because the effective gap between stator and rotor of RSB is small (usually 1–3 mm),
the deflection angle is small too. In the calculation, only the radial displacement of
each part of the PM is involved, and the axis of each part is considered to be vertical.
In Fig. 9.7, the axial distance between pointsBandAisl, and the radial displacement
ofBis
Dr=sin휃l≈휃l. (9.11)Whenl>0, pointBis located above pointA, and its radial displacement is to the right.
Whenl<0, pointBis located below pointA, and its radial displacement is to the
left. At pointB, there is a fraction of the PM rotor whose length is dl, with a radial
force:
dFR=dl×kR
L ×Dr=휃lkR
L dl, (9.12)Fig. 9.7:Schematic force diagram of the PM rotor when a torque is
exerted on.