ROTOR ASSEMBLY FOR AN ELECTRICAL MACHINE

Abstract: An alternator rotor assembly comprises two "claw pole" pole pieces (12, 14) and a field coil assembly (28) mounted between the pole pieces for rotation therewith. The field coil assembly comprises a bobbin (30) and a coil of wire (32) wound around the body of the bobbin. The bobbin further comprises two walls (34, 36) extending radially away from the body of the bobbin, between which the coil wire is wound. Projecting from the walls are flaps (38, 40, 42, 44) which extend radially outward. The flaps are designed to be bent over by the pole fingers (16, 18, 20, 22) as the rotor assembly is assembled. When so bent over, the flaps help retain the coil wire (32) on the bobbin and help prevent short-circuiting of the coil wire to the inner radius defined by the pole fingers. The ends of the flaps have notches (50, 52) that assist in self-alignment.

Description: ROTOR ASSEMBLY FOR AN ELECTRICAL MACHINE Field of the invention The present invention relates to electrical machines generally, and more specifically to rotor assemblies for electrical machines. Background of the invention The rotor assembly of an automobile alternator of the "claw pole" variety generally comprises two opposing "claw pole" pole pieces with intermeshing fingers. The two "claw pole" pole pieces define an inner space in which is disposed a field coil assembly. The field coil assembly normally comprises a bobbin around which is wound a coil of wire. The field coil assembly is then affixed to the two "claw pole" pole pieces for rotation therewith. In order to maximise the power output capability of an alternator, it is advantageous to incorporate as much wire into the field coil assembly as possible, to maximise the electromagnetic field generated by the field coil. However, maximising the amount of wire can cause two problems. First, the wire can tend to "spill" out of the bobbin as the rotor rotates during operation of the alternator. Second, the wire can tend to short-circuit against the inner radius defined by the pole fingers of the two "claw pole" pole pieces. One solution to the two problems is the incorporation of flaps which project radially outward from the walls of the coil bobbin. Typically, each wall of the bobbin would have as many flaps as the number of pole fingers on one "claw pole" pole piece. The flaps on each wall of the bobbin would further be disposed such that they would each be bent over the field coil by one pole finger of a "claw pole" pole piece when the rotor were assembled. The flaps thereby bent over help eliminate the two problems referenced above which are encountered when trying to maximise the wire in the field coil of the alternator. Specifically, the flaps when bent over help retain the field coil wire in the bobbin. Also, the flaps provide an electrically insulating barrier between the coil wire and the inner radius defined by the pole fingers of the two "claw pole" pole pieces. Although the bobbin flaps are quite effective in eliminating the two above-referenced problems, the flaps introduce one problem of their own. Because the flaps are bent over by pole fingers of the "claw pole" pole pieces, alignment between the pole fingers and the bobbin flaps must be maintained as the rotor is assembled. Because the bobbin flaps normally have significant stiffness, such alignment is often hard to maintain. The result is a rotor assembly process which can be difficult. Therefore, a rotor which is designed to facilitate alignment of the pole fingers with the bobbin flaps will provide advantages over the prior art. Specifically, assembly of such a rotor will be made considerably easier. Summary of he invention According to the present invention, there is provided a rotor for an electrical machine, comprising a bobbin having a body defining a centre and an axis of • rotation extending through the centre, first and second ■walls extending radially outwards from the body, at least one flap extending from the first wall radially away from the centre, and a notch in the radially outer end of each flap; and a coil formed of wire wound around the body of the bobbin between the first and second walls. Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which :- Figure 1 is an exploded view showing the principal components of an alternator rotor assembly 10 according to one embodiment of the present invention, Figure 2 is a perspective view of coil bobbin 30 of Figure 1, Figure 3 is a partial assembly drawing of rotor assembly 10 of Figure 1, Figure 4 is a side view showing the principal components of a rotor assembly 10' according to a second embodiment of the present invention, Figure 5 is a perspective view of silencer assembly 70 of Figure 4, and Figure 6 is a partial assembly drawing of rotor assembly 10' of Figure 4. Detailed Description of the Preferred Embodiments Referring to Figure 1, an alternator rotor 10 according to one embodiment of the present invention will be described. Rotor 10 comprises a first pole piece 12 and a second pole piece 14. Those skilled in the art will recognise pole pieces 12 and 14 as pole pieces from a "claw pole" rotor. First pole piece 12 and second pole piece 14 are mounted on a shaft 15. Shaft 15 is rotatably mounted within the alternator assembly as a whole, the mounting not being shown in Figure 1 because it is conventional in the alternator art. Pole pieces 12 and 14 each comprise fingers (such as fingers 16, 18, 20 and 22) . The fingers of first pole piece 12 intermesh with the fingers of second pole piece 14 when rotor 10 is assembled. In general, there are gaps between the fingers of first pole piece 12 and second pole piece 14 when the fingers of the pole pieces are intermeshed. Mounted between first pole piece 12 and second pole piece 14 is coil assembly 28 comprising a bobbin 30 and field coil 32 formed by wire wound around bobbin 30. Also mounted between first pole piece 12 and second pole piece 14 is noise ring 33. Noise ring 33 is a metal ring which bears against the inner surfaces of the fingers of pole pieces 12 and 14. Noise ring 33 helps prevent vibration of the fingers of pole pieces 12 and 14 as rotor assembly 10 rotates within the alternator assembly as a whole. That reduction in vibration reduces audible noise generated by the alternator assembly. Referring now to Figure 2, bobbin 30 will be described in further detail. Bobbin 30 is preferably made of an electrically insulating material such as a plastics material. Bobbin 30 comprises walls 34 and 36 between which field coil 32 (Fig. 1) is wound. Extending as projections of walls 34 and 36 are flaps (for example, flaps 38 and 40 extending from wall 34 and flaps 42 and 44 extending from wall 36) . The flaps further have notches (for example, notch 46 on flap 38, notch 48 on flap 40, notch 50 on flap 42, and notch 52 on flap 44) . Referring now both to Figure 1 and Figure 2, coil assembly 28 is mounted between first pole piece 12 and second pole piece 14. Flats 54 and 56 on bobbin 30 cooperate with corresponding flats on hub 17 of first pole piece 12 and the hub (hidden in Fig. 1) of second pole piece 14, to help affix coil assembly 28 to pole pieces 12 and 14 for rotation therewith. This method of affixing coil assembly 28 to pole pieces 12 and 14 is known and disclosed in 095/16297. The assembly of pole pieces 12 and 14 and coil assembly 28 is illustrated with reference to Figures 2 and 3. (Note that noise ring 33 (Fig. 1) is not shown in Figure 3 for simplicity.) Here, it is seen that the flaps on bobbin 30 (for example, flaps 38, 40, 42 and 44) are aligned with pole piece fingers (for example, fingers 16, 18, 20 and 22) . When pole pieces 12 and 14 and coil assembly 28 are fully mated, the flaps on bobbin 30 are bent over field coil 32 by the pole piece fingers. The bending over of the flaps helps hold the wire of field coil 32 between walls 34 and 36 of bobbin 30 and helps prevent electrical shorting between the coil wire and the pole piece fingers (for example, fingers 16, 18, 20 and 22) . The alignment between pole piece fingers (for example, fingers 16, 18, 20 and 22) and flaps of bobbin 30 (for example, flaps 38, 40, 42 and 44) is facilitated by the notches in the flaps (for example, notch 46 on flap 38, notch 48 on flap 40, notch 50 on- flap 42, and notch 52 on flap 44) . The notches cooperate with the pole fingers to assure alignment of the bobbin flaps (for example, flaps 38, 40, 42 and 44) with pole piece fingers (for example, fingers 16, 18, 20 and 22) as first pole piece 12, coil assembly 28 and second pole piece 14 are assembled. Without the notches in the flaps of bobbin 30 (for example, notch 46 on flap 38, notch 48 on flap 40, notch 50 on flap 42, and notch 52 on flap 44) , the assembly process of rotor 10 would be difficult. That difficulty would be due largely to the flaps on bobbin 30 being fairly stiff. As a result, alignment of the pole fingers (for example, fingers 16, 18, 20 and 22) with the flaps (for example flaps, 38, 40, 42 and 44) would be difficult to maintain as first pole piece 12, second pole piece 14 and coil assembly 28 are assembled. A second embodiment of the present invention is illustrated with reference to Figure 4. Here, in addition to first pole piece 12, second pole piece 14 and coil assembly 28, rotor assembly 10' comprises a silencer assembly 70. Silencer assembly 70 is more particularly illustrated with reference to Figure 5. Silencer assembly 70 comprises silencer insert 71 and noise ring 33. Noise ring 33 snap-fits into the inner diameter of silencer insert 71. Silencer insert 71 is generally cylindrical in shape. The walls of silencer insert 71 are formed by material (preferably a plastics material) intended to fill gaps between fingers of pole piece 12 and pole piece 14 when rotor 10' is assembled and the fingers of first pole piece 12 and second pole piece 14 are intermeshed (Fig. 4) . Silencer insert 71 therefore defines generally-triangular openings (for example, openings 72, 74, 76 and 78) which are shaped to accommodate the fingers of first pole piece 12 and second pole piece 14. By filling the gaps between the fingers of first pole piece 12 and second pole piece 14, silencer insert 71 is intended to reduce acoustic noise generated by the rotation of rotor assembly 10' . Additionally, noise ring 33 has the same purpose as has been previously described, namely reduction of vibration of the fingers of pole pieces 12 and 14. Silencer insert 71 includes a series of guide ribs, or projections, such as guide ribs 80, 82 and 84. The guide ribs project generally toward the centre of silencer insert 71. The function of the guide ribs is described with reference to Figure 6. Figure 6 is a partial assembly drawing of rotor 10' showing first pole piece 12, silencer assembly 70 and coil assembly 28. (Note that noise ring 33 is not shown in Figure 6 for simplicity.) During the assembly of rotor 10' , first pole piece 12 and silencer assembly 70 are first assembled. Next, coil assembly 28 is mated with the subassembly comprising first pole piece 12 and silencer assembly 70. As discussed previously, the flaps of bobbin 30 (such as flaps 42 and 44 in wall 36 of bobbin 30) are fairly stiff. That stiffness tends to discourage alignment of the flaps of bobbin 30 with the fingers of first pole piece 12 (for example, fingers 16 and 18) during the beginning of the assembly of coil assembly 28 to the subassembly comprising first pole piece 12 and silencer assembly 70. However, due to co-operation between the notches in bobbin 30 (such as notch 50 in flap 42 and notch 52 in flap 44 -- see also Fig. 2) and the guide ribs of silencer insert 71 (such as guide ribs 82 and 84) , alignment is facilitated as coil assembly 28 begins to be assembled to the subassembly comprising first pole piece 12 and silencer assembly 70. Second pole piece 14 (Fig. 4) is then assembled to the subassembly illustrated in Figure 6. As that assembly begins, the notches in the flaps of wall 34 of bobbin 30 (for example, notch 46 in flap 38 and notch 48 in flap 40) cooperate with the pole fingers (for example, fingers 20 and 22) of second pole piece 14 to assure alignment of the pole fingers with the flaps (for example flaps 38 and 40) of bobbin 30.

Claims: Claims 1. A rotor for an electrical machine, comprising a bobbin (30) having a body defining a centre and an axis of rotation extending through the centre, first and second walls (34,36) extending radially outwards from the body, at least one flap (38,40) extending from the first wall (34) radially away from the centre, and a notch (46,48) in the radially outer end of each flap (38,40); and a coil (32) formed of wire wound around the body of the bobbin (30) between the first and second walls (34,36). 2. A rotor for an electrical machine as claimed in Claim 1, wherein the bobbin (30) further comprises at least one flap (42,44) extending radially outwards from the second wall (36), each flap (42,44) having a having a notch (50,52) in its radially outer end. 3. A rotor for an electrical machine as claimed in Claim 2, wherein the flaps (38,40,42.44) extending from the first and second walls (34,36) are adapted to be bent over the coil (32) wound around the body of the bobbin. 4. A rotor for an electrical machine as claimed in Claim 3, wherein the bobbin is made of an electrically insulating material. 5. A rotor for an electrical machine as claimed in any preceding claim, further comprising a first pole piece ( 1 4) comprising a generally circular body defining a centre, an axis of rotation and a periphery, and a plurality of pole fingers (20,22) extending from the periphery parallel to the axis of rotation, each the pole finger having an end; wherein the bobbin (30) is coaxially coupled to the first pole piece (14) for rotation therewith; each notch is shaped to accommodate insertion of the end of one of the pole fingers (20,22) as the bobbin and the first pole piece are coupled; and the flaps(38,40) are bent over the body of the bobbin (30) by the pole fingers. 6. A rotor for an electrical machine as claimed in Claim 5 when appended to Claim 2, further comprising a second pole piece (12) comprising a generally circular body defining a centre, an axis of rotation and a periphery, and a plurality of pole fingers (16,18) extending from the periphery parallel to the axis of rotation, each the pole finger having an end; wherein the second pole piece (12) is coaxially coupled to the bobbin (30) and to the first pole piece (14) for rotation therewith, the pole fingers (20,22) of the first pole piece (14) are intermeshed with the fingers (16,18) of the second pole piece (12); and the pole fingers of the second pole piece (14) engage in the notches of the flaps (38,40) of the first radially extending wall (34) and act to bend the flaps over the body of the bobbin. 7. A rotor for an electrical machine as claimed in claim 6, further comprising at least one insert (70) comprising material shaped substantially as the shape of the spacing between the intermeshed pole fingers (16,18,20,22) of the first and second pole pieces (14,12) and located between the fingers of first and second pole pieces to at least partially fill the spacing between the fingers of the first and second pole pieces, said at least one insert further having at least one projection projecting generally toward the axis of rotation of the pole pieces and shaped to accommodate a notch of the bobbin as the first pole piece(14) the bobbin (30), and the insert (70) are assembled. 8. A rotor for an electrical machine as claimed in C laim 7, wherein the rotor comprises a single cylindrical insert.

Inventors: HARRIS RICHARD KENNETH, YORK MICHAEL TIMOTHY, HARRIS, RICHARD, KENNETH, YORK, MICHAEL, TIMOTHY
Applicants: FORD MOTOR CANADA, FORD MOTOR CO, FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD MOTOR COMPANY OF CANADA LIMITED
Application year: 1995
Main classification: H02K3
IPC codes: H02K1, H02K3