Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-04-13
2002-06-11
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S208000
Reexamination Certificate
active
06404092
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of electric machines. It relates to a winding bar for the high-voltage winding, in particular stator winding, of an electric machine, comprising a plurality of conductors which are arranged above and/or next to one another, and a conductor bundle with a rectangular cross section, the conductor bundle being surrounded outside by an insulation.
In this case, the conductors can be arranged electrically in parallel (bar winding) or be interconnected in series (coil winding). However, in normal operation the voltage between the conductors is substantially smaller than that across the bar insulation.
The invention also relates to a method for producing such a winding bar.
2. Discussion of Background
Winding bars such as are used, for example, in stators of rotating electric machines frequently have the cross section represented by way of example in FIG.
1
. The winding bar
10
, which is inserted into a slot
11
, provided for the purpose, in the stator laminated core
12
, comprises a bundle of individual conductors
13
which are arranged running in parallel above and/or next to one another. The conductor bundle, which generally exhibits a rectangular cross section with edges
15
, is surrounded on all sides by an insulation
14
. As a result of the shape, the electric field at the edges
15
is stronger than on the flat sides of the winding bar
10
. Consequently the edge region is particularly susceptible to electric breakdown or electric long-term failure.
In order to achieve the best possible filling of the slot with conductor material, and the best possible transmission of heat via the bar insulation, an attempt is made to achieve the thinnest possible insulation, at least on the flat sides of the bar, which dominate in terms of area. The conventional production of the insulation
14
in the case of winding bars with a rectangular cross section is described, for example, in “Sequenz: Herstellung der Wicklungen elektrischer Maschinen” [“Sequence: Production of windings of electric machines”], Springer-Verlag 1973, pages 128-129. According to this, strips of mica paper, which is coated with glass fabric on a substrate for the purpose of increasing the tensile strength and tear resistance, are wound in the form of layers around the bar or conductor bundle, subsequently impregnated with synthetic resin, molded-in and cured at raised temperatures. The thickness (d
1
in
FIG. 1
) of the insulation
14
is approximately the same on all the flat sides of the winding bar
10
in this method. At the edges
15
, it theoretically exhibits the same thickness d
1
(see the enlarged partial section in FIG.
2
), but becomes smaller in practice because of the locally increased contact pressure (small supporting surface in the edge region) which is produced when the strip is wound around the conductor bundle at a constant rate with a constant winding tension. According to the formula for coaxial cylinders, the maximum electric field at the edges
15
can be specified as:
E
max
=
U
r1
·
ln
r2
r1
=
U
r1
·
ln
r1
+
d
*
r1
In this case, (in accordance with FIG.
2
), U is the on-load voltage, r
1
the inner radius of curvature of the insulation
14
, r
2
the outer radius of curvature of the insulation
14
, and d*the thickness (mostly reduced with respect to d
1
) of the insulation
14
in the region of the edges
15
. It is clear from this that the electric field which in the case of radii r
1
≦3 mm which are technically easy to realize is in any case already distinctly stronger at the edge
15
than in the region of the flat sides, will once again increase as a consequence of the reduced insulation thickness d*, as results in the case of many production methods.
The effects of the increased field strength can be considerable, particularly in the case of continuous electric loading, since the failure rate t
−1
increases with the electric field in a strongly superlinear fashion. In rough terms, a law of exponents holds between the lifetime t (in h) and electric field E (in kV/mm) in accordance with
t
t
0
=
K
·
[
E
E
0
]
-
n
,
t
0
=1 h and E
0
=1 kV/mm. For a service life exponent n=8 (this is a frequent value in the case of insulating materials for rotating electric machines in accordance with the prior art), this means, for example, that an increase in the field strength by 20% reduces the service life to less than ¼, but conversely that reducing the field by 20% increases the service life by a factor of approximately 6.
Lowering the edge field strength could now be achieved, for example, by going over from an insulation
14
with constant thickness d
1
(=d*), as is shown in FIG.
1
and
FIG. 2
, to an insulation with an angular outer contour (r
2
=0). This assumption, carrying out a computer model calculation based on the finite elements method for a value of d
1
=2.5 mm and r
1
=2.5 mm, produces a reduction in the maximum field strength E
max
in the edge region of 11%, which corresponds to a computational prolongation of service life by the factor 2.5 in the case of a service life exponent of 8. This factor increases disproportionately with a higher service life exponent (for example factor 4 for n=12).
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a novel winding bar in which a distinct reduction in the maximum field strength in the edge region is achieved with simple means without the need to increase the thickness of the insulation in the region of the flat sides, as well as to specify a method for producing said bar.
The object is achieved in the case of a winding bar of the type mentioned at the beginning by virtue of the fact that the thickness of the insulation on the edges of the winding bar is greater than the thickness of the insulation on the flat sides of the winding bar. The enlargement of the insulation thickness in the edge region produces a correction of the field lines of the electric field, which leads to the desired reduction in the edge field strength.
A first preferred embodiment of the winding bar according to the invention is defined by virtue of the fact that the insulation on the edges of the winding bar has a curvature with an inner curvature contour and an outer radius of curvature, and that the outer radius of curvature is smaller than the sum of the equivalent inner radius of curvature of the inner curvature contour and the thickness of the insulation on the flat sides of the winding bar. Maintaining an outer radius of curvature differing from zero produces a uniform change, favorable for the field distribution and for the mechanical stability, of the thickness in the edge region.
In principle, such a shaping of the edge regions can be achieved with the aid of a wound insulation when thermoplastic strips are used for the insulation. However, the shaping becomes particularly simple when, in accordance with a second preferred embodiment, the insulation consists of a thermoplastic polymer in which filler particles made from an insulant are distributed. In a preferred development of this, polyetherether ketone (PEEK) is used as the thermoplastic polymer, and mica platelets are used as the filler particles. Instead of PEEK it is also possible to make successful use of other substances such as, for example, polysulfone (PSU) or polyether sulfone (PES).
In accordance with a further preferred embodiment of the invention, the special shaping, used to reduce the edge field strength, of the insulation in the edge region is also extended to the conductors of the conductor bundle surrounded by the insulation in such a way that at least the conductors arranged in the region of the edges respectively comprise a bundle of individual insulated, in particular stranded wires. It hereby becomes possible to use compression molding to bring the conductors themselves into a shape which leads to a further in
Baumann Thomas
Fried Reinhard
Joho Reinhard
Oesterheld Jörg
ABB Research Ltd.
Burns Doane Swecker & Mathis L.L.P.
Dinh Le Dang
Ramirez Nestor
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