6.6 Levitation forces vs. trapped flux Ë 165Fig. 6.16:Temperature dependence of levitation forces
at the gaps of 5, 7, 10, 15, 20, and 25 mm [32].Fig. 6.17:Levitation forces vs. trapped flux at
different gaps [31].6.6 Levitation forces vs. trapped flux [31]
In the experiment of the dependence of levitation forces on the trapped flux, the same
YBCO bulk was used. First, YBCO bulk was cooled by liquid nitrogen in applied field,
which was provided by a permanent magnet. Trapped flux of YBCO bulk was control-
led by changing the gap between the sample and the permanent magnet. Secondly, the
levitation forces of the YBCO bulk above the PM guideway were measured by the HTS
Maglev measurement system (see Fig. 5.7). The central surface concentrating magnetic
field of the PM guideway was 1.2 T and the field distribution in space was symmetrical
about its central vertical plane.
Figure 6.17 gives the experimental results of levitation forces of a single YBCO
bulk with different trapped flux above the PM guideway. Figure 6.17 shows that the
levitation force decreases almost linearly with increasing trapped flux. It is well known
that the trapped flux ensures stable levitation of YBCO bulk in applied field, but it
weakens the levitation force simultaneously. This Phenomenon can be seen clearly
from Fig. 6.17. Thus, an optimum trapped flux is very important in practical levitation
systems. This Phenomenon allows YBCO bulk to provide both a larger levitation force
and a larger guidance force.