Patent Application
20150214820
The present invention is directed to improved reliability and manufacturability of an electric machine and, more particularly, to welded interconnections of a stator winding.
Dynamoelectric machines in automotive applications include alternators, alternator-starters, traction motors, and others. The stator of an electric machine typically includes a cylindrical core formed as a stack of individual laminations and having a number of circumferentially spaced slots that extend longitudinally through the stator core. A rotor assembly includes a center shaft and is coaxial with the stator core. The stator core has wires wound thereon in the form of windings that extend axially through ones of the core slots. End turns are formed in the windings at the two axial ends of the stator core, whereby a given winding forms an end loop as it extends circumferentially to a different slot.
Stator windings may be formed by inserting and then connecting together individual “hairpin” conductors each having a crown or apex portion and having two legs that extend in a same general direction. For example, hairpins may be formed from a heavy gauge copper wire with a rectangular cross section, into a predetermined shape for insertion into specific rectangular slots in the stator core. Hairpin conductors are typically coated with an insulating material prior to insertion, so that adjacent hairpin surfaces within a slot are electrically insulated from one another.
Typically, the apex portions of the hairpins protrude from one axial end of the stator core and the leg ends of the hairpins protrude from the opposite axial end. After insertion, the portions of the legs protruding from the stator core are bent to form a complex weave from wire to wire, creating a plurality of adjacent wire end pairs. Adjacent paired wire ends are typically joined to form individual electrical connections, such as by a welding operation. In a given electric machine, it may be desirable to join together the cross-sectionally short sides of rectangular hairpins. Such short hairpin sides may also include rounded corners, so that the engagement surfaces of the adjacent pair that form the faying surfaces of a weld are difficult to align. As a result, the joinder of adjacent pairs of hairpins may result in a number of connections having increased resistance and/or defective joints. For example, the faying surfaces may slide laterally and become misaligned, and/or the contact surface area at a welded joint may be insufficient for reducing electrical resistance and improving electrical performance.
It is therefore desirable to obviate the above-mentioned disadvantages by providing a method and structure for joining hairpin type conductors.
According to an exemplary embodiment, a method of forming a stator winding includes providing a stator core having a plurality of longitudinally extending slots formed about a circumference thereof, providing a plurality of hairpin conductors each having a substantially rectangular cross-section and each having an apex portion and a pair of legs that terminate at respective ends, cutting a bevel at each leg end, inserting the hairpin legs into respective ones of the slots so that the leg ends extend from an axial end of the stator core, bending the hairpin legs to form a plurality of adjacent pairs of leg ends with beveled cuts facing one another, compressing the beveled cuts of each pair together, and resistance welding the pairs to form a plurality of welded joints.
The foregoing summary does not limit the invention, which is defined by the attached claims. Similarly, neither the Title nor the Abstract is to be taken as limiting in any way the scope of the claimed invention.
The above-mentioned aspects of exemplary embodiments will become more apparent and will be better understood by reference to the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding or similar parts throughout the several views.
The embodiments described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of these teachings.
Two resistance welding electrodes, discussed further below, respectively contact conductor ends 21, 22 at engagement locations 30, 31. The electrodes may be angled so that when they are moved toward one another during a welding operation, they push tapered surfaces 23, 24 toward one another. For example, when the gap between tapered surfaces 23, 24 is approximately one-quarter inch to one-half inch, and when the electrodes press toward one another with a force of approximately fifty to six hundred pounds of force, conductor ends 21, 22 are bent so that upper edges 32, 33 are made contiguous. When edges 32, 33 touch one another, welding current flows through such contiguous portions. As the welding current is gradually increased, the copper of conductor ends 21, 22 is annealed and softens. In order to prevent lateral movement of conductor ends 21, 22 during the welding, the electrodes may be provided with forked or notched surfaces that keep conductor ends 21, 22 in alignment with one another. Peak welding current may be 5,000 to 10,000 amperes, or any other suitable current. Typically, engagement locations 30, 31 are as close as practical to respective conductor ends 21, 22. As the welding current and compressive force of the electrodes are maintained, the remaining portions of tapered surfaces 23, 24 are made flush and mated. The gradual increase in welding current allows the copper conductor material to soften and be easily compressed. A brazing alloy may be provided. For example, a brazing alloy may be a SIL-FOS composition primarily containing copper, silver, and phosphorous (SIL-FOS is a registered trademark of Handy & Harmon Corp., White Plains, N.Y.). The brazing alloy may be placed to direct welding current there-through. In particular, the electrical resistance of the brazing alloy may be about five or ten times that of the hairpin conductor, whereby the alloy becomes much hotter than the hairpin during the initial welding period and creates a brazed joint, and where continued welding current directs the joinder of hairpin legs along the intersection of tapered surfaces 23, 24. For example, brazing alloy may be formed or placed along outer edge surfaces 32, 33 so that the joinder of surfaces 23, 24 begins at an intersection of edges 32, 33 and proceeds along such joined surfaces toward location 29. When surfaces 23, 24 are properly joined to be flush, the welding current is removed while the electrodes remain in place until the weld cools and is mechanically stable. It may be necessary to pull the brazing alloy tape to disengage it from the weld after a portion of the brazing tape has melted. The brazing alloy may be provided as a tape having lateral perforations that allow the tape to be easily broken away from the portion of the brazing tape being applied. In such a case, a measured, consistent amount of brazing alloy may be applied.
Welding parameters such as time, incremental (e.g., 0.5 milliseconds) current levels, rise and decay times, pulse width, duty cycle, cooling time, and others may be accurately controlled with a mid-frequency resistance welding machine. For example, the various welding parameters may be controlled according to profiles based on any number of criteria. The non-destructive softening effected by controlled application of welding current allows the copper of conductor ends 21, 22 to soften and be more easily bent and compressed. Generally, longer weld periods having a more gradual rise in current and associated heat may allow use of a lesser compressive force. For example, when welding current rise time is increased, a small compressive force of approximately thirty to eighty pounds may be sufficient and this lower compressive force may reduce the possibility of misalignment respecting conductor ends 21, 22. Similarly, the compressive force may be modulated to optimize the level of applied heat. For example, the electrical resistance at a welding target location may be increased by using a relatively smaller amount of compressive force. This increased electrical resistance provides increased localized heat during the application of welding current. The brazing alloy also facilitates rapid heat transfer, so that the softening of copper and the compression of softened copper into a joint may be performed without excessive melting and without effecting a destructive welding process. In particular, the conductivity of brazing alloy is low, for example eighteen percent, and such low conductivity acts as an electrical resistance that generates heat during the resistance brazing/welding.
In an exemplary embodiment, a large initial compressive force acts to bend the copper so that edges 32, 33 come into contact; compressive force may then be reduced to assure that sufficient heat is then maintained for melting the brazing alloy and softening the copper, and where the compressive force may again be modified to assure that the weld is accurately formed. Typically, a fillet (not shown) is formed in a weld region having the highest peak temperature. In another example, a small compressive force can quickly provide sufficient heat to begin softening the copper in joint portions that are already in contact. A relatively large initial compression may bend conductor ends 21, 22 until the distal ends of edges 32, 33 come into contact; after coming into contact, a greatly reduced compressive force allows the electrical resistance between faying surfaces to remain high, whereby welding heat is maintained. By comparison, if the compressive force exerted by the electrodes is too great, the electrical resistance at the joint becomes too small for maintaining proper welding heat. In another example, a medium-sized initial compressive force, such as about two hundred pounds, and a small welding current, such as about 1,000 amperes for thirty to ninety milliseconds, may be utilized for engaging edges 32, 33 and then softening the copper conductor ends 21, 22; the compressive force may then be increased to about 300 pounds and the welding current may be increased to about 5,000 amperes until faying surfaces 23, 24 are fully flush and mated. The brazing alloy melts during this 5,000 ampere period. The associated temperature rise is nonlinear, as the increased heat causes the electrical resistance of the copper to increase. The welding current is turned off while the electrodes remain in their final position until the weld cools. Typically, the welding electrodes have a fluid cooling system in close proximity to the work surface, so that the cooling of the electrodes and weld requires only a small amount of time, such as one-quarter second to one second or more.
Compressive forces 47, 48 may each be applied at an angle in order to optimize and direct the compression of surfaces 23, 24 toward one another. Such angle may be changed during a welding operation. For example, the angle may initially be chosen to press edges 32, 33 into contact in order to direct the welding current therethrough. Once edges 32, 33 are in contact and have an electrical resistance between them, localized heating begins and the angle(s) may be changed for more efficiently and precisely pressing surfaces 23, 24 together. In another exemplary embodiment, electrode heads 38, 39 may be stepped into position and then used to apply respective compressive forces 47, 48 in a first compression, and may then be stepped into another position for applying compressive forces 47, 48 in a second compression, etc. Various shapes may alternatively be implemented in forming electrode heads 38, 39, although the respective sizes of electrode heads 38, 39 may not exceed available circumferential working space for each conductor pair being joined. For example, respective contacting surfaces of electrode heads 38, 39 may each be formed as single plane surfaces each having a groove formed therein for capturing the associated conductor wire targets 40, 41. The shapes of electrode heads 38, 39 may be chosen for holding/retaining and/or aligning conductor ends 10, 11.
Even when the planes defined as tapered surfaces 23, 24 are properly aligned for being joined together, lateral movement may be possible, and such lateral movement may be reduced or prevented by forming mating feature(s) in conductor ends 21, 22, discussed further below, and/or by utilizing electrodes having shapes that retain engagement locations 30, 31 and prevent lateral misalignment. In an exemplary embodiment, the welding electrodes may have a contact area of approximately one-quarter inch by one-quarter inch, for welding together each of 108 adjacent pairs of conductor ends 21, 22 in a fully populated six inch diameter stator where adjacent welds are approximately one-eighth inch apart.
Compressive forces 47, 48 may each be applied at an angle and/or may be moved axially in order to optimize and direct the compression of surfaces 23, 24 toward one another. Such angle may be changed during a welding operation. Electrode heads 49, 50 are moved axially into position and are then used to apply compressive forces 47, 48 in opposed radial directions. A brazing alloy tape 51 may be applied to the intersection of edges 32, 33 after they are bent to be contiguous. As shown by placement direction arrow 52, brazing tape 51 is moved axially into the brazing location and is pulled away from the brazing location in order to break brazing tape 51 away from the target surface when sufficient brazing alloy has been applied. Brazing tape 51 may alternatively be applied at location 29, and the brazing alloy may alternatively be utilized in the form of preformed brazing clips rather than as a tape.
By the disclosed embodiments, it can be seen that the surface area of a welded joint is substantially increased by creating tapered faying surfaces 23, 24. This increased joint surface area provides improved electrical performance of an electric machine, including a larger current path, improved machine efficiency, reduced operational temperatures, and reduced power losses. For example, typical electrical currents through a given conductor end joint of a hairpin conductor may be more than 300 amperes, and any improvement in current capability at hairpin joints results in substantial overall machine performance. In a worst case, a thin hairpin connection may act as a fuse and cause an electric machine to stop operating.
While various embodiments incorporating the present invention have been described in detail, further modifications and adaptations of the invention may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.
