Patent Application
20100109469
The technology disclosed herein relates generally to rotary machines and, more specifically, to wedges used for the retention of conductor (or stator) bars in the stator core slots of dynamoelectric machines.
Large dynamoelectric machines such as electrical generators employ a laminated stator core for transmitting induced voltages to the generator terminals through stator conductor bars. The cores are usually made by assembling already-slotted punchings or laminations in an annular housing for later enclosing the generator rotor. The slotted punchings, when assembled, define axially-extending, radially-oriented core slots which terminate at the radially inner-circumference of the stator annulus. The stator bars, or conductors, with ground insulation are laid in the radial slots and a wedging system is used to hold the bars in place against electromagnetic forces present when the machine is operating. If the wedging system is not effective, ground or conductor insulation may be damaged in the ensuing vibration, ultimately leading to a forced outage of the generator.
Electromagnetic fields in the generator induce forces on stators bars during normal operation or short circuit conditions that require wedges to support and restrain the bars within the stator core slots.
Currently fiberglass laminate material (such as, for example, National Electrical Manufacturers Association (NEMA) G11 is used in making the wedges, and while G11 provides good mechanical strength, it is abrasive to the stator laminations.
Cotton phenolic material has also been used as a wedge material, and while it is non-abrasive to the core, it has lower thermal and mechanical capability versus fiberglass laminates such as G11. The reduced mechanical strength and thermal capability of cotton phenolic thus limits the application of wedges made using this material. Other solutions such as low friction coatings have also been tried.
In U.S. Pat. No. 4,200,818, there is disclosed a stator wedge partially covered with a non-woven felt made of Kevlart, and in U.S. Pat. No. 4,607,183 there is disclosed a wedge with an abrasion resistant layer. In commonly owned, co-pending application Ser. No. 11/889,928, wedge bodies having surfaces in contact with the core are disclosed wherein at least the contact surfaces are covered with a woven aramid fabric material.
In one aspect, the present invention relates to a slot wedge for a stator adapted for use in a dynamoelectric machine comprising a wedge body having opposite side edges adapted to engage complimentary stator core slots, wherein at least the side edges are covered with an aramid paper material.
In another aspect, the invention relates to a slot wedge for a stator adapted for use in a dynamoelectric machine comprising a wedge body having opposite side edges adapted to engage complimentary stator core slots, the side edges each having a semi-circular shape, wherein at least the side edges are covered with a woven aramid fabric or an aramid paper material.
In still another aspect, the invention relates to a method of making a slot wedge for a stator adapted for use in a dynamoelectric machine comprising: (a) providing a fiberglass wedge body formed to a predetermined shape, including opposite side edges adapted for engagement within stator core slots; and (b) covering at least the opposite side edges of the wedge body with an aramid paper material.
Exemplary but nonlimiting embodiments of the invention will now be described in detail in connection with the drawings identified below.
In conjunction with the foregoing, a filler strip 24 may extend axially (longitudinally) along the slot radially inward of bar 22. A number of dovetail wedges 26 are introduced into the slot 14 (and spaced apart along the axial length of the slot 14) so as to bear radially against the insulating filler strip 24. More typically, a ripple spring (not shown in
The dovetail wedges are typically formed with oppositely-facing inclined surfaces 28 which engage inclined surfaces of the dovetail slot 16 to facilitate the assembly of the stator bar wedging system. The material used for the dovetail wedges 26 is preferably of high-strength insulating material which can be cut or molded to the desired wedge shape. The wedges are thus preferably formed of a molded resinous compound employing a suitable filler to add strength, or in the alternative, are formed of any suitable commercially-obtainable cotton phenolic materials such as Textolite® (a registered trademark of the General Electric Company). In some designs, however, and as noted above, cotton phenolic wedge by itself lacks the required mechanical strength for thinner and/or wider wedge configurations. It will be understood that the length of the wedges 26 may vary from what is shown in
With reference to
One commercially available aramid paper well suited for use in this invention is available from E.I. du Pont de Nemours and Company, and sold under the trade name NOMEX®.
In a first exemplary process, the wedge itself is made from a prepeg fabric, a bulk molding compound, or a liquefied resin (e.g., G11) poured into a mold cavity containing a woven glass roll the length of the wedge 30. The aramid paper 32 may be applied to the wedge by molding, pultrusion, extrusion or by gluing the paper to the wedge. Molding, pultrusion and extrusion, where the surface applied integrally to the part (or wedge), producing the part in one step, are preferred over adhesive due to better bonding which prevents surface layer separation.
In another exemplary but nonlimiting embodiment illustrated in
With reference now to
It will be appreciated that the invention is equally applicable to wedges having other dimensional proportions (e.g., length to width ratios, thickness, etc.), and/or different edge shapes (e.g., oval, square, etc.) which engage the core slots, and thus the above-described embodiments are intended to be merely exemplary and nonlimiting. In addition, the core slot engaging surfaces may be continuous or intermittent along the length of the wedge bodies. For example, the dovetail surfaces could be notched at spaced locations along their respective lengths to enhance air flow and cooling.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
