10-12-2012, 12:07 PM
AC Losses in a Toroidal Superconductor Transformer
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Abstract
In order to study the viability of coreless ac
coupled coils, a superconductor transformer based on BSCCO-
2223 PIT tapes was constructed. To achieve the minimum flux
leakage, a toroidal geometry was selected. Both secondary and
primary coils were wound around a glass fiber reinforced epoxy
torus, obtaining a solid system. The field inside the transformer,
the coupling factor, and the losses in the system were computed
and measured, providing suitable parameters for new
improvements in these systems.
INTRODUCTION
The high temperature superconductor transformer is
lighter, smaller and has a higher efficiency than the
conventional transformer [1]. The windings of most
superconducting transformer prototypes have been built with
Bi-2223 tapes [1-4]. These prototypes have used very
different geometries, but when the ferromagnetic material is
taken out and one wants to maintain a high coupling factor it
is necessary to look for a geometry to confine the magnetic
field lines. Examples are annular [5] and solenoidal
transformers [6].
DESIGN OF THE PROTOTYPE
The transformer were wound with Bi-2223 tape. In order to
reinforce the transformer structure, the Bi-2223 windings
were wound onto a glass fiber torus [3, 10]. The minimum
radius of the coils before losing their superconductor
characteristics has been evaluated by us [11]. In according
with it, we selected a torus with 30 cm inner diameter and 36
cm outer diameter. The cross section diameter of the torus is
thus less than 10% of its major diameter aiming at a ratio
close to ideal. Similarly, we tried to wind the tape in turns as
close together as possible to achieve a homogeneous current
layer. In this case, we needed 341 turns for the inner coil. For
the outer one we used 447 turns to get a transformation ratio
different from unity. The final structure consist of 5 layers
that we conected properly to get the coils sought.
CONCLUSION
AC losses in a transformer were measured by the electrical
method with a 4-channel lock-in amplifier. The theoretical
AC losses were evaluated and an alternative formulation
proposed. The experimental results show good agreement
with the proposed theoretical expression. For this reason, we
believe that the expression (2) adequately represents the AC
losses in a transformer working at low frequencies. Tests at
high frequencies were not carried out, and we can not affirm
that the expression will be valid for these frequencies.