Inductor devices – Coils of special configuration
Reexamination Certificate
2000-12-29
2003-03-18
Mai, Anh (Department: 2832)
Inductor devices
Coils of special configuration
C336S200000, C336S223000, C029S602100
Reexamination Certificate
active
06535100
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to transformer windings and, more particularly, to insulated transformer windings having insulated breakouts and methods for forming the same.
BACKGROUND OF THE INVENTION
Certain safety regulations relating to insulation between transformer windings require that the transformer be designed such that a stipulated winding creepage distance is allowed without contact between respective windings and such that a stipulated clearance between respective windings is provided. The creepage distance is the shortest distance between adjacent conductors following a surface without going through insulation (rather, the distance is measured as going around and/or along insulation). Additionally, regulations may require certain minimum air distances and distances through insulation between windings. It is desirable to meet such regulations while nonetheless reducing the cost and size of the transformer.
One method that has been used to meet the foregoing regulations is to provide substantial margins as illustrated by the transformer
10
as shown in
FIG. 5
, which may be referred to as a “margin coil design”. The transformer
10
has a core
12
with a core center leg
12
A and opposed end legs
12
B. The transformer
10
also includes a first, foil winding
30
that is wound about the center leg
12
A, and a second, wire winding
20
that is wound about the first winding
30
. Alternatively, the second winding
20
may be a foil winding also or the positions of the windings
20
and
30
may be reversed. The core
12
may include an insulating cover layer to prevent direct electrical contact with either of the windings
20
,
30
. Insulation layers
14
,
16
,
18
(which may be reinforced) are inserted between the center leg
12
A and the first winding
30
, between the first winding
30
and the second winding, and between the second winding
20
and the end legs
12
B of the core
12
.
In order to meet the above-mentioned required creepage distance and clearance (RCDC), primary margins M
1
and M
2
are provided above and below the winding
30
and secondary margins P
1
and P
2
are provided above and below the winding
20
. The required margins may depend on the voltage class of the transformer, the class of the insulation employed and/or other parameters. Typically, the sum of the shortest primary and secondary margins M
1
, M
2
, P
1
, P
2
should be greater than or equal to the RCDC. That is (regarding the core as electrically conductive), the margins and the RCDC should be related as follows:
M
1
+P
1
≧RCDC
M
2
+P
2
≧RCDC
M
1
+P
2
≧RCDC
M
2
+P
1
≧RCDC
The combined width of the winding
20
and the margins P
1
, P
2
and the combined width of the winding
30
and the margins M
1
, M
2
are each limited by the length L of the core center leg
12
A. The widths of the margins M
1
, M
2
, P
1
, P
2
may be substantial as compared to the widths of the windings
20
,
30
. Hence, a large portion of the available winding width may be consumed by the margins M
1
, M
2
, P
1
, P
2
, thereby necessitating the provision of a larger core and, accordingly, a larger transformer.
In order to provide better utilization of the available winding space, a transformer as described above may be formed without margins, i.e., with the widths of the windings being of nearly the same dimension as the length of the core center leg
12
A. An exemplary margin free coil transformer
10
′, which may be referred to as a “margin free coil design”, is shown in
FIGS. 6 and 7
. The transformer
10
′ has windings
20
′,
30
′, reinforced insulating layers
14
′,
16
′,
18
′, and a core
12
′ having a center leg
12
A′. Each winding
20
′,
30
′ has a breakout on each end thereof. The breakouts
34
′ of the foil winding
30
′ are shown in cross-section in FIG.
7
.
Notably, means must be provided in the margin free coil transformer
10
′ to address the creepage distance and clearance regulations discussed above. One method of solving this problem is to insulate the first (foil) winding
30
′ and its breakouts in their entireties such that the requirements for creepage distance and clearance, as well as distance through insulation, are met by the insulation about the first winding
30
′ alone.
For example, a winding foil strip
40
as shown in
FIG. 8A
may be provided. The strip
40
has a width that is approximately the same as the length of the center leg
12
A′. The strip
40
is covered with an insulator
40
A and then folded once to create a breakout
42
of the same width as the strip
40
, as shown in FIG.
8
B. However, in many transformers the width of the center leg
12
A′ is substantially less than its length and the breakout should be close to the width of the core. For example, in ferrite EE-cores the length to width ratio of the center leg is typically approximately two. To achieve the appropriate breakout width, the breakout
42
is folded again to form a narrow breakout
44
as shown in FIG.
8
C. The breakout
44
corresponds to one of the breakouts
34
′ (see FIG.
7
), for example. Notably, this method of folding creates substantial increases in thickness in certain areas as a result of the stacking of four layers of foil, as well as the insulation, on each layer. Additionally, the insulation may be damaged by the folding steps. If holes are formed in the insulation, the transformer may no longer meet the creepage distance, clearance and distance through insulation requirements. The existence of small holes in the insulation may be hard to detect.
According to a further prior art method, a triple insulated wire which is approved by safety agencies for use where reinforced insulation is required may be used for the winding
20
′ without additional insulation. The wire in the winding
20
′ itself provides the required insulation and there are therefore no requirements on the insulation of the winding
30
′ other than functional requirements. This method suffers from several drawbacks in practice.
As an alternative to using a folded foil winding, the winding
30
′ may be formed using an insulated winding foil strip
50
and a joined breakout
52
as shown in
FIGS. 9A and 9B
. The breakout
52
corresponds to one of the breakouts
34
′ (see FIG.
7
). The breakout
52
and the strip
50
are each covered with an insulator
50
A,
52
A except on end portions
50
B,
52
B. The end portions
50
B,
52
B are exposed to allow electrical contact between the strip
50
and the breakout
52
over most of the width of the strip
50
. According to some prior art methods, one or more supplemental insulation members may be provided covering the exposed portions of the winding foil strip and the breakout. However, such constructions may not in fact provide a true margin free coil design while still meeting applicable safety requirements and, accordingly, margins are still required.
SUMMARY OF THE INVENTION
The invention is directed to winding assemblies for use in transformers and methods for forming the same. The winding assemblies include one or more foil strips and insulation covers arranged to provide electrically insulated winding and breakout or breakout tap portions. The winding assemblies may be constructed so as to meet the aforementioned creepage distance and other requirements.
According to method embodiments of the invention for forming an insulated winding assembly for an electrical transformer, an integral foil strip having a lengthwise axis is provided. The foil strip includes a winding portion and a breakout portion extending from the winding portion along the lengthwise axis. The breakout portion is folded about the winding portion to form a fold between the breakout portion and the winding portion. Thereafter, an insulation cover is secured to the foil strip.
The step of securing an insulation cover to the foil strip may include securing a first insulation cover to the
Mai Anh
Myers Bigel & Sibley & Sajovec
Powerware Corporation
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