8.3 Methods to improve Maglev performances Ë 285For the pre-load case, the change of the guidance forces was fluctuated. The initial
value was 9.794 N, and the final value was 0.825 N, larger than the initial value. At the
same time, Fig. 8.26b shows the guidance forces variation for the S2 sample moved
across the PMG2. In the no pre-load case, the first and third values were 7.613 and
9.042 N, respectively. Then the guidance forces changed circuitously. The final force
was 9.379 N, 23.2% larger than the initial value. In the pre-load case, the curve could
be divided into three phases: increasing phase, middle phase, and decreasing phase.
The last guidance force was just 0.346 N larger than the initial one.
By comparison, the guidance forces became larger after the pre-load procedure,
too. The increase ratio was 127.43% for sample S1 and 104.56% for sample S2. At
the same measure height, the guidance forces increased with the decrease of FCH
due to the increase of the trapping magnetic field. After the implementation of the
pre-load method, the HTS bulk trapped more magnetic flux, so the guidance forces
increased, too.
8.3.2Magnetization process
8.3.2.1Pre-magnetization
It is noted that the present HTS Maglev vehicle and related optimization studies 28–34
are conducted under well-recognized field-cooling magnetization (FCM) conditions,
which is a convenient way to realize the stable superconducting levitation. During the
FCM process, the HTS bulk stays in the same magnetic field as the magnetization field.
That is, the applied magnetic field is the magnetization field for the initial levitation
stability. Compared with the best trapped field capability of about 3.5 T at the working
temperature (liquid nitrogen temperature, 77 K), the actual utilization level of the
HTSC material is low and inefficient because the FCM field source is generally from
PM materials. Taking the first manned HTS Maglev test vehicle [5] (see Chapter 6) as
an example, the on-board bulk YBCO trapped only 0.174 T (the maximum magnetic
field density) at the applied 0.243 T PMG surface magnetic field at the 40 mm FCH.
On the other hand, it has been found that a higher trapped field brings a better
levitation performance of the HTS Maglev system (the HTSC-PMG system). Thus, it
is potential to introduce the HTS bulk magnet (HTSCM) with a higher trapped field
into the HTS Maglev system for better levitation. That means the HTSCM should be
first magnetized in a higher magnetic field rather than in the relative low applied
field and then put into the actual applied field. One more advantage of the pre-
magnetization method that can be predicted is that more magnetic field energy from
the outside field will be imported to the HTS Maglev system without changing its
working conditions.
Usually, a static magnetic field was regarded as the most efficient magnetization
condition for a HTSCM. Here, the static magnetization field was produced by an
electromagnet pair (Model EM4-CV, Lakeshore) which was charged by a magnet