Method for fixing permanent magnets on rotors of brushless motors and relevant product

Abstract: The magnets (11) on the surface of brushless motor rotors (10) are divided into groups. Each group is clad by a sheet (12), the edges of which are turned and inserted in radial slots (15) of the rotor , where they are fixed by bolts (16) or by longitudinal tenons inserted in the slots (15).

Description: The present invention relates to a method for fixing permanent magnets on rotors of brushless motors and relevant product. The permanent magnets of a rotor usually consist of bars in various sizes which are to be glued onto the surface of the rotor and then magnetized on the spot. Thereafter they are wrapped in a band of fiberglass or similar material that prevents them from coming off when the motor is on. Magnetizing the bars, once they are glued on the rotor, is a rather complex operation. Furthermore, magnetic field are generated, which interfere in the areas where positive and negative polarizations are close. These magnetic field are obviously harmful to the motor operation in that the bars are magnetized in the best way if they are magnetized one by one or, anyway, if just one pole is magnetized. The assembly operation, that is already expensive having to magnetize the bars on the rotor, is made even more costly by the need of band-wrapping the magnets to prevent them from coming off owing to the centrifugal force which they are subject to during operation. A magnetic field rotor is known from JP-A-60200753. This document discloses a rotor having four permanent magnets secured by an adhesive to the outer periphery of a drum. A sheetlike binder is wound around the magnets also covering peripheral axial slots in which the binder is secured by longitudinal holding members. It is an object of this invention to propose a method for fixing the magnets on the rotor that can overcome the above mentioned drawbacks. Therefore, the present invention proposes that the bars should be magnetized before fixing them to the rotor, glueing them in groups to sheet metal parts (thus forming positive or negative poles) and only afterwards to the surface of the rotor. The first advantage offered by this method is to be found in the previous automatic magnetization of the bars which is much simpler and quicker. Maximum magnetic saturation of the bars is obtained owing to the single-pole system. Moreover, band-wrapping becomes unnecessary as the outer sheet parts fix the magnets to the rotor in a permanent way, and are much more resistant and reliable in time than any other band-wrapping system now in use. With the above and other objects in view which will appear as the description proceeds, this invention provides a method and apparatus according to claims 1 and 4. The invention is described hereunder referring to the enclosed drawings, in which: Fig. 1 is a partial front view of a rotor made according to this invention; Fig. 2 is a partially sectioned side view of the rotor of fig. 1; Fig. 3 is an enlarged view of part A, fig. 1; Fig. 4 is a partial front view of a rotor obtained according to a slightly different method proposed by this invention; Fig. 5 is a side view with a partial axial section of the rotor of fig. 4; and Fig. 6 is an enlarged view of part B, fig. 4. In a rotor finished according to this invention, referring to the embodiment of figs. 1-3, a plurality of bars 11 made out of a magnetizable material are fixed to the circumferential surface of rotor body 10. The bars are divided in groups and each group is contained within turned up longitudinal edges 13 of a sheet part 12, preferably metal, that covers them. The longitudinal edges 13 of sheet parts 12 have turned tips 14 that are inserted in longitudinal peripheral slots 15 obtained in the circumferential surface of the rotor body 10. The sheet parts 12 are as wide as together to extend around the circumferential surface of the rotor, each between two adjacent slots 15. Keys 16 are inserted in the slots 15 so as to lock the tips 14 of sheet parts 12 in position and prevent them from sliding both radially and axially whereas. According to the method proposed by this invention, at first groups of magnetizable bars 11 are fitted adjacent eachother between the longitudinal edges 13 of sheet parts 12. Then each group of bars 11 is magnetized separately and homogeneously with either positive or negative polarity. Afterwards, the groups of bars with their sheet parts are positioned around the rotor body 10 to be fixed thereto. A thin layer of glue is then applied between the circumferential surface of the rotor 10 and each group of bars 11 to prevent the bars from sliding. The tips 14 of the edges 13 are then inserted in the slots, and finally, the keys 16 are inserted in slots 15 to lock the tips 14 of sheet parts 12 permanently and prevent them from coming off the rotor because of the centrifugal force that occurs during rotation. In an alternative embodiment (see figs. 4-6), the slots 15 are square rather than round and so are the tips 14 of sheet parts 12 and keys 16. In this case, keep plates 17 are fitted to each end of the rotor. The periphery of plates 17 is provided with a plurality of axially disposed slots 18 in which small section longitidinal ends 19 of the keys 16 are inserted. The keep plates 17 are fitted to the ends of rotor body 10 to lock the keys 16 in their seat and prevent the bars 11 from sliding out axially. Both of the above alternative solutions permit a highly resistant and reliable fixing of the magnets which undergo centrifugal forces. As a matter of fact, the resisting section of the various sectors undergoes mainly traction forces. Fixing the magnets by this essentially mechanical method results in constant seal in comparison with the continuous variation of operation temperature which can normally rise up to 150 degrees Celsius. The compact structure also permits to reduce the air gap between rotor and stator to a minimum of 0,4-0,5 mm.

Claims: A method for fixing permanent magnets to rotors of brushless motors, of the kind comprising a cylindrical rotor body (10) having its circumferential surface substantially covered with magnets (11); a sheet-like material (12) covering the outer periphery of the magnets and being secured to the rotor body (10) by means of keys (16) inserted in a plurality of longitudinal peripheral slots (15) in the circumferential surface of the rotor body (10); characterised in that it comprises the following steps: a) realizing a plurality of bars (11) made of a magnetizable material, sufficient to substantially cover the circumferential surface of the rotor body (10); b) fitting groups of adjacent bars (11) longitudinally disposed between turned up longitudinal edges (13) of a plurality of sheet metal parts (12) being as long as the rotor and as wide as together to extend around the whole circumferential surface of the rotor each between two adjacent slots (15), said edges (13) being turned up towards the bars (11); c) separately magnetizing each group of adjacent bars (11) with homogeneous polarity; d) positioning the groups around the circumferential surface of the rotor body (10); e) inserting the tips (14) of said edges (13) in said slots (15), with the bars (11) between the sheet part (12) and the surface of the rotor (10); f) locking the tips (14) of the edges (13) of said sheet parts (12) to the rotor body (10) by inserting keys (16) in said rotor slots (15). A method according to claim 1, characterised in that it also comprises the further step of: g) locking the keys (16) axially in said rotor slots (15) by fitting a keep plate (17) to their ends (19) at each end of the rotor. A method according to claim 1, characterised in that, following step c), a thin layer of glue is applied between the circumferential surface of the rotor (10) and each group of bars (11). A brushless motor rotor of the kind comprising a cylindrical rotor body (10) having its circumferential surface substantially covered with magnets (11); a sheet-like material (12) covering the outer periphery of the magnets and being secured to the rotor body (10) by means of keys (16) inserted in a plurality of longitudinal peripheral slots (15) in the circumferential surface of the rotor body (10); characterised in that the magnets are formed by groups of adjacent longitudinally disposed bars (11) magnetized with homogeneous polarity, each group being fitted between turned up longitudinal edges (13) of a plurality of sheet metal parts (12) being as long as the rotor and as wide as together to extend around the whole circumferential surface of the rotor each between two adjacent slots (15); said edges (13) being turned up towards the bars (11) and having turned tips (14) suitable to be locked inside said slots (15) by said keys (16). A rotor according to claim 4, characterised in that the tips (14) of said longitudinal edges (13) of two adjacent sheet metal parts are locked within one of said slots (15). A rotor according to claim 5, characterised in that the tips (14) of said edges (13) are curved so as to copy the surface of said slots (15), which are curved too. A rotor according to claim 5, characterised in that said tips (14) are square as well as the slots (15) and the keys (16); the ends (19) of keys (16) being locked axially and radially within axially disposed slots (18) in keep plates (17) fitted at each end of the rotor. A rotor according to claim 4, characterised in that said longitudinal peripheral slots (15) are disposed at an angular spacing of 60 degrees around the circumferential surface of the rotor body (10).

Inventors: PILATI ROMANO, PILATI, ROMANO
Applicants: LAFERT SELCA SRL, LAFERT-SELCA S.R.L.
Application year: 1989
Main classification: H02K1
IPC codes: H02K1