Method of fixing unmagnetized magnetizable members to a rotor disc and a rotor disc
A rotor disc assembly 10 for an AC machine includes a rotor disc 20 , permanent magnets 30 and a spider 40 . The rotor disc 10 has a circumferential lip 22 projecting from a surface thereof. The permanent magnets 30 are positioned in a circular array on the surface of the rotor disc 20 , with constant angular pitch, and so as to lie against the lip 22 . The spider includes an annular base portion 41 , with a plurality of arms 42 projecting radially outwards therefrom. The spider 40 is fixed to the surface of the rotor disc 20 such that each arm 42 is between and abuts a respective juxtaposed pair of the permanent magnets 30 . The arms 42 and the members 30 are each provided with cooperating wedge-shaped cross sections such that each arm 42 constrains the respective two permanent magnets 30 both axially and tangentially on the surface of the rotor disc 20 .
1. A rotor disc assembly for an alternating current machine, the assembly comprising:
a rotor disc with peripheral structure projecting from a surface of the disc;
a plurality of magnetisable members that are trapeziform in cross-section; and
a fixing apparatus, comprising a plurality of arms that are trapeziform in cross-section to match with the members:
wherein the members are positioned on the surface in a circular array and adjacent the peripheral structure such that during operation of the machine the members are radially constrained thereby, and the fixing apparatus is attached to the disc; and further
wherein the plurality of arms of the fixing apparatus are arranged to abut each of the members so as to constrain them axially and tangentially during operation of the machine.
2. A rotor disc assembly according to claim 1 wherein, the fixing apparatus and the magnetisable members have mutually abutting surfaces that are substantially oblique to the surface of the disc.
3. A rotor disc assembly according to claim 2 wherein the fixing apparatus is a spider with a plurality of arms projecting radially from a center of the spider.
4. A rotor disc assembly according to claim 3 wherein, the spider is attached to the disc such that each member is separated from a neighborring member by an arm of the spider and each member is in abutment with two arms of the spider.
5. A rotor disc assembly according to claim 2 wherein the arms of the spider have a first substantially trapeziform cross section, a shorter of the two parallel sides of this first trapezium being adjacent the surface of the disc; and the members have a second substantially trapeziform cross section, a longer of the two parallel sides of this second trapezium being adjacent the surface of the disc.
6. A rotor disc assembly according to claim 5 wherein the perpendicular distance between the two parallel sides of the first trapezium is less than the perpendicular distance between the two parallel sides of the second trapezium, such that each member projects further form the surface of the disc than does each arm of the spider.
7. A rotor disc assembly according to claim 3 wherein the spider includes a locator between each pair of arms such that attaching the spider to the disc locates the members adjacent the circumferential wall.
8. A rotor disc assembly according to claim 7 wherein the locator is resiliently deformable structure integral with the spider.
9. A rotor disc according to claim 1 wherein the peripheral structure of the disc is a continuous circumferential lip.
10. A rotor disc assembly for an alternating current machine, the assembly comprising:
a rotor disc with peripheral structure projecting from a surface of the disc;
a plurality of magnetisable members that are trapeziform in cross-section; and
fixing means, comprising a plurality of arms that are trapeziform in cross-section to match with the members;
wherein the members are positioned on the surface in a circular array and adjacent the peripheral structure such that during operation of the machine the members are radially constrained thereby, and the fixing means is attached to the disc, characterized by the fixing means being arranged to abut each of the members so as to constrain them axially and tangentially during operation of the machine.