Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2003-04-29
2004-03-30
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S264000, C310S179000
Reexamination Certificate
active
06713930
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to a power generator, and in particular to a reduction of heat dissipation and undesirable voltage differentials in a power generator.
Thermal issues are critical to the design of a high power electrical generator and can serve as limiting factors in generator operation. A typical design of a high power electric generator includes a rotor having rotor windings rotatably disposed inside of a stator having stator windings. The rotation of the rotor induces an electromagnetic field in the stator, which electromagnetic field in turn induces a current in, and voltage drop across, the stator windings. However, the electromagnetic field also induces eddy currents in the stator, which is magnetically and electrically resistive. The eddy currents cause the dissipation of energy in the stator in the form of heat and impose a thermal constraint on the operation of the generator.
In order to improve generator efficiency and reduce generator size, generator manufacturers are constantly endeavoring to improve the thermal performance of the generator. For example, a prior art design of a high power electrical generator
100
is illustrated in
FIGS. 1
,
2
, and
3
.
FIG. 1
is a cross-sectional view of generator
100
from an isometric perspective.
FIG. 2
is a cut-away view of generator
100
along axis
2
—
2
. As shown in
FIGS. 1 and 2
, electrical generator
100
includes a substantially cylindrical stator
102
having a stator core
104
and housing a substantially cylindrical rotor
110
. Multiple circumferentially distributed and axially oriented keybars
118
are coupled together at each of a proximal end and a distal end by one of multiple flanges
204
(not shown in FIG.
1
). Each keybar
118
is coupled to an outer surface of stator
102
. The multiple keybars
118
, together with the multiple flanges
204
, form a keybar cage around the stator
102
.
An inner surface of stator
102
includes multiple stator slots
106
that are circumferentially distributed around an inner surface of stator
102
. Each stator slot
106
is radially oriented and longitudinally extends approximately a full length of stator
102
. Each stator slot
106
receives an electrically conductive stator winding (not shown).
Rotor
110
is rotatably disposed inside of stator
102
. An outer surface of rotor
110
includes multiple rotor slots
114
that are circumferentially distributed around the outer surface of rotor
110
. Each rotor slot
114
is radially oriented and longitudinally extends approximately a full length of rotor
110
. An air gap exists between stator
102
and rotor
110
and allows for a peripheral rotation of rotor
110
about axis
130
.
Each rotor slot
114
receives an electrically conductive rotor winding (not shown). Each rotor winding typically extends from a proximal end of rotor
110
to a distal end of the rotor in a first rotor slot
114
, and then returns from the distal end to the proximal end in a second rotor slot
114
, thereby forming a loop around a portion of the rotor. When a direct current (DC) voltage differential is applied across a rotor winding at the proximal end of rotor
110
, an electrical DC current is induced in the winding.
Similar to the rotor windings, each stator winding typically extends from a proximal end of stator
102
to a distal end of the stator in a first stator slot
106
, and then returns from the distal end of the stator to the proximal end of the stator in a second stator slot
106
, thereby forming a stator winding loop. A rotation of rotor
110
inside of stator
102
when a DC current is flowing in the multiple windings of rotor
110
induces electromagnetic fields in, and a passage of magnetic flux through, stator
102
and the loops of stator windings. The passage of magnetic flux in turn induces an alternating current in each stator winding and eddy currents arid magnetic and resistive losses in stator
102
.
FIG. 3
is a side view of a cross-section of generator
100
and illustrates a coupling of magnetic flux
302
from rotor
110
to stator
102
as the rotor rotates inside of the stator. Magnetic flux
302
generated by a rotation of rotor
110
couples to and passes through the surrounding stator
102
. Magnetic flux
302
induces a flow of multiple eddy currents in the magnetically and electrically resistive stator
102
, which currents cause energy dissipation and heat generation in the stator that poses a thermal constraint on the operation and capacity of generator
100
. As a result, generator designers are always seeking improved methods of thermal management for power generator stators.
One known thermal management technique is the construction of stator core
104
from multiple ring-shaped laminations
402
.
FIG. 4
is a partial perspective of generator of
100
and illustrates a typical technique of constructing stator core
104
. As shown in
FIG. 4
, the multiple ring-shaped laminations
402
are stacked one on top of another in order to build up stator core
104
. Each lamination
402
is divided into multiple lamination segments
404
. Each lamination segment
404
includes multiple slots
120
(not shown in FIG.
4
), wherein at least one slot
120
of each segment
404
aligns with one of the multiple keybars
118
. Each keybar in turn includes an outer side
124
and an inner, or locking, side
122
that mechanically mates with one of the multiple slots
120
. Stator core
104
is then constructed by sliding each lamination segment
404
, via one of the multiple slots
120
, into the keybar cage formed by the multiple keybars
118
. The coupling of one of the multiple slots
120
of a lamination segment
404
with a locking side
122
of a keybar
118
affixes each lamination segment
404
, and thereby each lamination
402
, in position in stator
102
. By building stator core
104
from stacked laminations, as opposed to constructing a solid core, circulation of a current induced in stator
102
is limited to a lamination, thereby restricting current circulation and size and concomitantly reducing stator heating.
The above thermal management technique does not fully address thermal problems caused by a “fringing” of magnetic flux at each end of stator
102
. As illustrated in
FIG. 3
, the “fringing”
304
of magnetic flux at each end of stator
102
results in a number of flux lines
302
axially, or normally, impinging upon each end of stator core
104
and upon the multiple flanges
204
. A result of the fringing magnetic flux
304
is a greater flux density at each end of stator core
104
as compared to more centrally located portions of the stator core. The greater flux density at each end of stator core
104
results in increased eddy currents and greater heat dissipation in the laminations of stator core
104
near the ends of the stator, as opposed to more centrally located laminations. The fringing effect also results in increased eddy currents and greater heat dissipation in each flange
204
.
In order to combat a buildup of heat at each end of stator
102
due to fringing magnetic flux
304
, an inner surface of stator core
104
, at each end of the stator core, is radially stepped away
202
from rotor
110
, as shown in
FIGS. 2 and 3
. By increasing the distance between rotor
110
and stator core
104
at each end of the stator core, an amount of flux axially impinging upon each end of the stator core is reduced. However, the stepping of the ends of stator core
104
away from rotor
110
is only a partial solution to the stator core heat dissipation problem presented by “fringing” and does not address the problem of heat dissipation in the multiple flanges
204
.
A portion of the fringing magnetic flux
304
also impinges upon the ends of each of the multiple keybars
118
. The impinging of fringing magnetic flux upon an end of a keybar
118
can produce an uneven coupling of flux into each keybar, with a greater flux density at a keybar end than in more centrally located portions of the keybar. The uneven coupling of flux can p
Longwell Ronald Irving
Salem Sameh Ramadan
Shah Manoj Ramprasad
Banner & Witcoff , Ltd.
Cuevas Pedro J.
General Electric Company
Mullins Burton S.
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