Rotor for an electric machine, retainer for retaining a winding in a slot in a rotor, and method of manufacturing a rotor

A rotor for an electric machine, the rotor comprising: a rotor body having a plurality of axially extending, circumferentially spaced slots therein; at least one winding conductor disposed in each said slot; and a retainer located in each said slot radially outwardly of the or each said winding conductor for limiting radial outward movement of the or each winding conductor; wherein each retainer has a radially inwardly directed surface configured so as to locate the or each winding conductor in a predetermined position in its respective slot. The invention further resides in a retainer and in a method of manufacturing a rotor.

This invention relates to a rotor for an electric machine, a retainer for
 retaining a winding conductor in a slot in a rotor, and a method of
 manufacturing a rotor. The invention relates in particular, but not
 exclusively, to the retention of winding conductors in high speed rotors
 for aircraft generator applications.
 FIG. 1 shows a known form of rotor for a rotary electric machine in which
 axially extending slots carrying rotor winding conductors 103 are
 partially closed to retain the conductors by retaining lugs 104, 105
 formed as integral parts of the rotor. The lugs 104, 105 are relatively
 heavy and have been found to have a detrimental effect upon the magnetic
 properties of the machine. Furthermore, during manufacture of the rotor
 101, it is necessary to thread the winding conductors 103 axially through
 their respective slots 102, which increases the time and effort required
 to produce the rotor and is potentially damaging to the winding conductors
 103 and any insulating lining 106 used inside the slots 102.
 As shown in FIG. 2, rotors for electric machines are also known which have
 longitudinal slots 201 in which conductor bars 202 are held against
 centrifugal acceleration by dovetail wedges 203 of high strength metal
 alloy. However, the wedges 203 of rotor 204 shown in FIG. 2 are relatively
 heavy, and have stress concentration points 205, 206 at the edges of the
 wedge and in the body of the rotor, which tend to weaken the rotor.
 The invention seeks to overcome or mitigate at least some of the above
 mentioned problems.
 Accordingly the invention provides a rotor for an electric machine, the
 rotor comprising: a rotor body having axially extending, circumferentially
 spaced slots therein; at least one winding conductor disposed in each said
 slot; and a retainer located in each said slot radially outwardly of the
 or each said winding conductor for limiting radial outward movement of the
 or each winding conductor; wherein each retainer has a radially inwardly
 directed surface configured so as to locate the or each winding conductor
 in a predetermined position in its respective slot.
 This predetermined position may be centred within the slot in the
 circumferential direction of the rotor.
 Each retainer is conveniently substantially crescent-shaped, viewed in
 lateral cross-section through the rotor and the inwardly directed surface
 of the retainer is the concave surface of the crescent. The or each
 winding conductor may be substantially circular in cross-section.
 Conveniently, the circumferential extent of each slot is such as to
 accommodate only one winding conductor.
 The invention also includes a rotor for an electric machine, the rotor
 comprising: a rotor body having axially extending, circumferentially
 spaced slots therein; at least one winding conductor disposed in each said
 slot; and a retainer located in each said slot radially outwardly of the
 or each said winding conductor for limiting radial outward movement of the
 or each winding conductor; wherein the retainer presents no axially
 extending corners to the wall of the slot in which it is located.
 Each slot may conveniently be defined by a continuous surface, which
 surface is curved where it mates with the retainer. Such a construction
 facilitates the avoidance of acute, stress inducing angles which tend to
 weaken the rotor.
 The invention also includes a rotor for an electric machine, the rotor
 comprising: a rotor body having axially extending, circumferentially
 spaced slots therein; at least one winding conductor disposed in each said
 slot; and a retainer for limiting radial outward movement of the or each
 winding conductor; wherein said slots each have a radially outwardly
 directed mouth for permitting radial insertion of the or each winding
 conductor therethrough, the retainer and a surface of the respective slot
 each being configured so as to permit radial insertion of the retainer
 into the slot and secure location of the retainer in a position radially
 outwardly of the or each said winding conductor within the slot.
 Preferably, the retainer is securely located in said position by rotation
 about its longitudinal axis following insertion into said slot. The
 retainer may be substantially kidney shaped in lateral cross-section and
 may be made from a substantially lighter material than the rotor body.
 The invention also includes a retainer for retaining a winding conductor in
 a slot in a rotor, the retainer being elongate and substantially kidney
 shaped in lateral cross-section. The retainer may conveniently be made of
 plastics material.
 The invention also includes a method of manufacturing a rotor for an
 electric machine, comprising the steps of: preforming a winding conductor;
 disposing the winding conductor in a slot in the rotor; and fitting a
 retainer in said slot in a position radially outwardly of the winding
 conductor.
 Conveniently, the or each retainer is fitted by inserting it radially
 inwardly into its respective said slot, then securely locating it in said
 position, preferably by rotating it about its longitudinal axis.

A rotor 1, part shown in lateral cross-section in FIG. 3, comprises a rotor
 body 2 having 84 closely circumferentially spaced, radially directed,
 axially extending slots 3 therein. Only one of the slots 3 is shown in
 FIG. 3. The rotor body 2 conveniently takes the form of a stack of
 appropriately shaped elements which are laminated together, for example by
 welding or using an appropriate adhesive. Each of the slots 3 contains two
 conductive windings, a radially inner winding conductor 4 and a radially
 outer winding conductor 5. Each winding conductor 4, 5 comprises a
 single-turn copper coil. A layer of insulating material, for example a
 polyimide film, or an appropriately shaped insulating member 6 is provided
 between the rotor body 2 and the winding conductors 4, 5.
 Each slot 3 comprises a mouth 3A opening to the outer periphery of the
 rotor body 2, a winding-receiving portion 3B, and an enlarged,
 retainer-receiving portion 3C intermediate the mouth 3A and the
 winding--receiving portion 3B. The intermediate portion 3C contains a
 retainer 7 disposed radially outwardly of the winding conductors 4, 5. The
 retainer 7 is elongate, of generally the same length as its respective
 slot, and is substantially crescent shaped in cross-section. The retainer
 presents its part-circular concave surface 8 to the radially outer winding
 conductor 5, such that when the winding conductors are subjected to
 centrifugal force caused by high speed rotation of the rotor, the retainer
 7 limits radially outward movement of the winding conductors and,
 moreover, tends to centre the radially outer winding conductor 5 within
 the slot 3 in the circumferential direction of the rotor. This may
 facilitate the passage of cooling fluid around the conductors within the
 slot, and improves the balance of the rotor.
 The retainer 7 is of plastics material, for example glass fibre filled
 PEEK, and presents no axially extending pointed corners internally of the
 slot 3 in which it is located, the co-action of the convex outer surface
 of the retainer with the correspondingly shaped region 3C of its
 respective slot holding the retainer in position in the rotor. The
 avoidance of sharp corners minimises the risk of damage to the face of the
 slot and the winding conductor 5 during insertion of the retainers 7 into
 the slots, and the relative softness of plastics material enhances this
 effect.
 As clearly seen in FIG. 3, the mouth 3A of the slot 3 is wide enough in the
 circumferential direction of the rotor 1 to permit radial insertion of the
 winding conductors 4, 5 therethrough. This permits an improved method of
 manufacture of the rotor 1, wherein each winding conductor 4, 5 is
 preformed on a jig and subsequently dropped radially into place in the
 respective slot 3 after having located the insulating members or formed
 the insulating layer 6 in the slots 3. Each retainer 7 is normally
 thereafter inserted axially into the location shown in FIG. 3. As clearly
 shown in FIG. 3, the larger diameter part-circular convex periphery of the
 retainer 7 engages the intermediate portion 3C of the slot 3 such that a
 centrifugal force exerted on the radially outer winding conductor 5 tends
 to force these mating faces of the slot and the retainer together in self
 locking fashion.
 The use of a lighter, plastics retainer 7 instead of the lugs 104, 105
 shown in FIG. 1 provides a lighter rotor 1 which is particularly
 advantageous for use in aircraft applications. The use of the
 part-circular cut out surface 8 in the retainer 7 permits a shorter
 winding receiving portion 3B of the slot 3 to be used.
 FIG. 4 shows another rotor 41 which is similar in most respects to the
 rotor 1 shown in FIG. 3, except that a particularly advantageous form of
 retainer 47 is shown. Features of the rotor 41 which are identical to
 features already described with respect to FIG. 3 are given the same
 reference numerals in FIG. 4. As clearly seen in FIG. 4, the retainer 47
 is more kidney shaped than the retainer 7 of FIG. 3, having a more rounded
 profile at the tips of the crescent than the retainer of FIG. 3. Moreover,
 retainer 47 is adapted to cooperate with the mouth 43A and
 retainer-receiving portion 43C of the slot 43 so as to allow insertion of
 the retainer 47 radially into the slot 43. Thereafter the retainer is
 located securely in position by rotating it about its longitudinal axis.
 The exemplary rotors 1, 41 are provided with field windings 4, 5 and are
 suitable for use in high speed generators for normal operation at about
 24000 rpm, with occasional overspeed situations. However, it is to be
 understood that the invention is not limited to generators or to high
 speed machines, and that electric motors and lower speed machines are
 specifically contemplated.