Transformer core

Details Transformer core Transformer core

2000-09-02

2004-01-27

Mai, Ahn (Department: 2832)

Inductor devices

Wound core

C336S212000, C336S215000

Reexamination Certificate

active

06683524

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to transformer cores and especially to three-phase cores comprising three frames of rings of transformer plate defining yokes in, for example, horizontal triangles and vertical legs extending between corners of the yokes. The invention also relates to single-phase shell cores having many rings, frame cores having two frames and two yokes, inductors, and components for the foregoing and transformers.
2. Description of the Prior Art
Transformer cores are almost solely made of transformer plates laid edge to edge to an EI form or ring. Some of them are made of cut rings and called C-cores. Others are wound to two rings inside a larger ring and cut to two E-core parts used for three-phase transformers.
Toroid transformers have a ring core which is not cut and is the only practical exception to the EI form. Small shell single-phase transformers and large three-phase transformers use the EI form of core.
A common three-phase transformer core will now be described. Virtually all have six coils, which by means of wires are wound on a cylinder forming three rod-shaped windings. The cores are composed of a multitude of thin, rectangular plates of electromagnetic material, which are stacked side-by-side with their long sides in alignment to form each of the legs.
The E-shaped plates form one yoke, and three short legs each extending its body into one of the transformer coils. Each leg faces a leg from a yoke at the opposite end of the coil. Thus, there is a core leg extending through each of the three sets of wound coils encircling the legs. The coils are bridged by one yoke at each side of the coils.
The plates of the core are thin sheets of metal, assembled in place, one sheet at a time, until an entire core is put together. This is a slow, labor intensive process.
The EI cores are inefficient to operate, and electrical losses occur at the juncture of many mating edges between the plates. That is, EI stacked cores in general have the drawback that the magnetic field has to cross small gaps between the edges from plate to plate.
There are further losses in the four outer corners of a complete three-phase transformer where field radiation occurs, since there is no ready path for the magnetic fields to flow. Further, the yokes are made of core material that is not encircled by coils and therefore does not contribute to the efficiency of the transformer, but to the contrary the consequence is that material and labor to form the yokes can be regarded to be wasted.
When the operation of a standard three-phase transformer is commenced, there are very high current losses. There are thus high losses during start-up as well as under load for standard three-phase transformers due to the conventional three-phase transformer cores.
The EI cores used in three-phase transformers have numerous other shortcomings. They vibrate and hum during operation. They set up electromagnetic radiation that is easily detectable at about five feet from typical three phase transformers. Due to e.g. electromagnetic forces in the space between the edges of the plates, there [is] will be noise in the core. Conventional three-phase transformers generate excessive amounts of heat, and means must be employed to cool them, requiring an excessive amount of cooling fluid.
As noted above, the inefficiencies of three-phase transformers requires each of them to have a large size. This requires them to be larger in both width and height. Large transformers are difficult and expensive to transport, due both to their size and height. In addition, shipping large transformers sometimes results in damage due to their instability. For example, large transformers have at times been unable to be shipped to offshore installations, and such shipping if possible is expensive. Designers of transformer cores have striven to obtain legs with an essentially circular cross-section because that gives the best efficiency of the final transformer. That is, transformer windings are nearly always cylindrical, having an interior void within the windings with a circular cross section. The core designer wants to fill that void. This is true for both three-phase and single-phase transformers. However, there is always a trade-off between efficiency and production requirements, leading to non-optimal transformer cores with non-circular legs.
U.S. Pat. No. 4,557,039 (Manderson) discloses a method of manufacturing transformer cores using electrical steel strips having approximately a linear taper. By selecting a suitable taper, a hexagonal or higher order approximation of a circular cross section for the legs of the cores is produced. However, the tapered strips are extremely difficult and time-consuming to produce, and the design is particularly not well adapted to large-scale commercial production.
In
FIGS. 1
,
1
a-b
is shown a prior art three-phase transformer core according to Manderson, generally designated
10
. The core has a general delta-shape, as is seen in the isometric view of
FIG. 1
, with three legs interconnected by yoke parts. In
FIG. 1
a
, a cross-sectional view of the core is shown before final assembly. The core comprises three identical ring-shaped parts
12
,
13
and
14
, the general shape of which appears in
FIG. 1
Each ring-shaped part fills up one half of two legs with hexagonal cross-sections, see
FIG. 1
a
, thus totaling the three legs of a three-phase transformer. The ring-shaped parts are initially wound from constant width strips to three identical rings
12
a
,
13
a
,
14
a
with rhombic cross-sections comprising two angles of 60° and two angles of 120°. These rings
12
a
-
14
a
constitute the basic rings. The orientation of the strips also appears from
FIGS. 1
a
and
1
b.
Outside of the basic ring in each ring-shaped part there is an outer ring
12
b
,
13
b
,
14
b
of a regular triangular cross-section. The outer rings are wound from strips with constantly decreasing width. When the three ring-shaped parts
12
-
14
are put together, see
FIG. 1
b
, they form three hexagonal legs on which the transformer windings are wound.
A drawback with this solution is that every size of transformer requires its own cutting of the strips. Also, the outer rings
12
b
-
14
b
are made of strips with decreasing width, leading to waste and also making the transformer according to Manderson very difficult to manufacture. Another drawback is that the design is not self-supporting, i.e. the ring-shaped parts tend to move in respect of each other.
U.S. Pat. No. 2,544,871 (Wiegand) discloses a transformer core with three legs, each leg being made from two ring-shaped parts and an auxiliary ring shaped part. There are thus nine ring shaped parts, three of which are used in an inefficient way, making Wiegand an expensive and impractical device.
Transformer cores are also described in the following documents: Swedish Patent No. 163797, U.S. Pat. No. 2,458,112, U.S. Pat. No. 2,498,747 and U.S. Pat. No. 2,400,184. However, the above mentioned problems are not overcome by the cores described in these documents.
The difficulty and expense of making transformer cores having a ring with a decreasing width has rendered this proposed transformer core construction totally impractical, and no such cores are known to exist in commercial use. Despite the recognition that transformer cores should nearly fill the circular interior of transformer windings, none has hitherto been proposed which is practical and economical. The design of such a transformer core would be of tremendous importance with respect to both three-phase and single-phase transformers. In addition, the provision of similarly designed inductors would be a most significant contribution to the art.
The electromagnetic and mechanical shortcomings of present three and a single-phase transformer core is very significant considering the number of transformers sold and in use throughout the world. The following figures demonstrate this. They were taken from the “The World Market for Transformers

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