Known commutated DC dynamoelectric machines typically include a motor housing, a stator, and a rotor assembly. The stator includes a magnet frame and a plurality of field poles including a bore extending therethrough. The rotor assembly is rotatably mounted in the housing, extends through the stator bore, and includes an armature core and a rotor shaft extending through the core. The armature core includes a plurality of slots wherein armature coils are wound. The rotor shaft includes a commutator and a plurality of radially arranged commutator risers. The commutator includes a plurality of commutator bars. The risers typically are copper strips with an inner end and an outer end. Each copper strip is welded or brazed to a commutator bar at the inner end and welded or brazed to an armature coil at the outer end. Typically, the inner weld is fabricated by welding or brazing with a silver- phosphorus alloy. During machine operation, the riser strips are subjected to mechanical, magnetic and centrifugal forces that sometimes produce fractures or fatigue cracks that can lead to a machine breakdown.

Known methods for reducing fatigue cracks include bundling the risers such that the middle portions of the risers are held together forming a triangular arrangement. Another method is to provide a bend in the risers as exemplified in Canadian Patent No. 1, 301, 820 issued May 26, 1992 and assigned to General Electric Canada Inc. Although the current methods have been successful in reducing fatigue cracks, the current methods have not eliminated fatigue cracks in commutators with very short risers. The method of bending the risers has decreased fatigue cracks near the outer weld, however, fatigue cracks near the inner weld may still occur.

Accordingly, a need exists for a method for reducing fatigue cracks in commutator risers near an inner weld, particularly for commutators with very short risers.

BRIEF SUMMARY OF THE INVENTION A rotor for a dynamoelectric machine includes a rotor shaft, a commutator, an armature, and a plurality of risers. The commutator includes a plurality of commutator bars and a plurality of insulator segments arranged alternately with each other in a circumferential direction of the rotor shaft.

The armature carries a plurality of armature coils extending in an axial direction along the armature. The armature coils include end windings. The risers are secured to outer ends of corresponding commutator bars and extend longitudinally in a radial direction of the rotor shaft. Each riser includes an opening therethrough and is connected to a corresponding end winding such that each armature coil is electrically connected to a corresponding commutator bar.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view of a prior art rotor assembly ; Figure 2 is a side view of a flexible riser assembly in accordance with one embodiment of the present invention ; and Figure 3 is a cross sectional view of a motor including the flexible riser assembly shown in Figure 2.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 is a side view of a prior art rotor assembly 10 including a rotor shaft 12 with a commutator support 14 and an armature core 16 mounted thereon. A commutator bar 18 is mounted on commutator support 14 opposite rotor shaft 12.

Commutator bar 18 includes a bar extension 19 including a slot 21. Rotor assembly 10 further includes a riser 20 that is inserted into slot 21 and attached to commutator bar 18 by bottom welds 22 typically provided by Tungsten inert gas brazing with a silver phosphorus alloy. Armature core 16 extends radially from rotor shaft 12 and forms a plurality of alternating slots (not shown) and teeth 24 around the periphery of armature core 16. A plurality of armature coils (not shown) are wound in the slots

and include end windings 26 protruding therefrom. End windings 26 are connected to riser 20 at a top weld 28. Riser 20 includes a width 30 defined by a first edge 32 and a second edge 34 substantially parallel to first edge 32. Riser 20 further includes a height 36.

During operation of a motor (not shown) including riser assembly 10, rotor shaft 12 bends producing one per revolution cyclic relative axial deflections between top weld 28 and armature core 16. Since end windings 26 are attached substantially inflexibly to teeth 24, which are integral with core 16, and core 16 itself, which, in turn, is attached substantially inflexibly to rotor shaft 12, at least a portion of the relative axial displacement is imposed on riser 20. Accordingly, an edge-wise bending of riser 20 occurs due to the one per revolution cyclic relative axial deflections.

Additionally, a relative thermal expansion produces an edge-wise bending of riser 20. Riser assembly 10 is typically utilized in highly transient duty cycle applications and a high electrical current energizes the armature coil associated with end windings 26 for brief periods of time. Since a thermal time constant of the armature coil is substantially less than the thermal time constants of armature core 16 and rotor shaft 12, an axial displacement occurs. As explained above regarding inflexibility, at least a portion of the axial displacement is absorbed by riser 20 producing an edge-wise bending of riser 20.

An additional cause of edge-wise bending is a centrifugal force on riser 20 during operation of a motor (not shown) including riser assembly 10. When rotor shaft 12 is rotating at a high angular velocity, end windings 26 experience a centrifugal force radially outwards and, since end windings 26 are attached only to riser 20 at top weld 28, the centrifugal force is transmitted to riser 20 producing an edge-wise bending of riser 20.

A stress in riser 20 resulting from a fixed edge-wise bending displacement is a function of height 36 and width 30 of riser 20. Given an equal axial displacement, a taller riser is stressed less than a shorter riser is stressed. If height 36

is relatively large, as is the case in large diameter DC machines, a corresponding riser stress is less than a corresponding riser stress in shorter risers with the same axial displacement. However, lengthening height 36 is usually not a viable solution for DC machines with relatively small diameters.

The stress in riser 20 is also a function of riser width 30. Given a fixed axial displacement, a shorter width 30 results in a lower stress level since the stress is inversely proportional with the width. Although a shortening of riser width 30 will reduce stress due to axial displacement caused by the relative thermal expansion and the one per revolution cyclic relative axial deflections, the shortening will increase stress due to the centrifugal radial force transmitted to riser 20 by end windings 26.

Stress to a shorter width riser due to direct radial centrifugal loading of end windings 26 is increased due to a decrease in riser cross sectional area. Accordingly, decreasing width 30 may not reduce fatigue cracks in riser 20.

Figure 2 shows a rotor 37 with a flexible riser assembly 38 including a rotor shaft 40 with a commutator support 42 and an armature core 44 mounted thereon. A commutator bar 46 is mounted to commutator support 42 opposite rotor shaft 40. Commutator bar 46 includes a bar extension 43 including a slot 45. Flexible riser assembly 38 further includes a flexible riser 48 including a riser top 50 and a riser bottom 52 attached to commutator bar 46 by a plurality of bottom welds or brazings 54. Armature core 44 extends radially from rotor shaft 40 and forms a plurality of alternating slots (not shown) and teeth 56 around the periphery of armature core 54.

A plurality of armature coils (not shown) are wound in the slots and include end windings 58 protruding therefrom. End windings 58 are connected to flexible riser 48 at a top weld 60. Flexible riser 48 includes a first width 62 defined by a first edge 64 and a second edge 66 parallel to first edge 64. Flexible riser 48 further includes a second width 68 defined by first edge 64 and a third edge 70.

Flexible riser 48 still further includes a third width 72 defined by second edge 66 and a fourth edge 74. An opening 76 extends through riser 48 between third edge 70 and fourth edge 74. In an exemplary embodiment, third width 72 is substantially equal to

second width 68, and opening 76 is substantially centered on flexible riser 48. In one embodiment, opening 76 is elongated radially. In an alternative embodiment, opening 76 is substantially oval. It is contemplated that the benefits of flexible riser 48 accrue to all risers with openings, including, but not limited to, openings with at least one line of symmetry, and openings without a line of symmetry.

During operation of a motor (not shown in Figure 2) including flexible riser assembly 38 axial displacements occur as described above. However, a stress of flexible riser 48 is less than one-half a stress of riser 20 given an equal axial displacement. First width 62 is equal to width 30 (shown in Figure 1) and opening 76 <BR> <BR> <BR> provides an effective width of less than one-half width 30 for flexible riser 48. The<BR> width62-opening76<BR> <BR> <BR> <BR> <BR> effective width is , which is less than one-half width 30. As 2 stated above, with a fixed axial displacement, stress is proportional to the width of each riser part. Therefore, stress is reduced by a factor greater than two. In an exemplary embodiment, flexible riser 48 includes a second opening (not shown) and stress is reduced by a factor greater than three. It is contemplated that flexible riser 48 includes any desired number of openings to obtain any corresponding reduction in stress.

Opening 76 can be any desired length to achieve any desired flexibility. In an exemplary embodiment, opening 76 extends between top weld 60 and at least one bottom weld 54. In an alternative embodiment, opening 76 extends approximately between riser top 50 and riser bottom 52. Riser 48 is fabricated from sheet stock and opening 76 is stamped or punched. Alternatively, opening 76 can be machined with a cutting tool.

Figure 3 is a cross sectional view of a motor 78 including flexible riser 48. Motor 78 includes a housing 80, a stator assembly 82, a rotor assembly 84, and a commutator assembly 86. Stator assembly 82 is located within housing 80 and includes a stator core 88 including a stator bore 90 for receiving rotor assembly 84.

Stator core 88 further includes a plurality of wound field poles 92. Rotor assembly 84 includes rotor shaft 40 carrying commutator assembly 86 and armature core 56

including a plurality of end windings 58. Commutator assembly 86 includes a plurality of commutator bars 46, a brush holder 94 including a plurality of brushes (not shown), and a plurality of flexible risers 48. Each flexible riser 48 is attached to a respective commutator bar 46 by at least one bottom weld 54 and attached to a respective end winding 58 with a top weld 60. Commutator assembly 86 further includes a plurality of insulator segments (not shown) arranged alternately with commutator bars 46 in a circumferential direction of rotor shaft 40.

During motor operation, flexible riser 48 has an effective width of less than one-half width 62 (shown in Figure 2) and, accordingly, stress is reduced by a factor greater than two, as explained above. By reducing stress, fatigue strength is improved which reduces fatigue cracks. Accordingly, fatigue cracks in commutator risers near an inner weld are reduced. It is contemplated that the benefits of a flexible riser assembly 38 accrue to all types of commutated motors, such as, for example, but not limited to, motors with risers of substantial height and motors with bent risers.

While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.