ROTATING ELECTRIC MACHINE WITH EXTERNAL ROTOR

Abstract: A rotating electric machine with external rotor constituted for example by an asynchronous motor with cage rotor, in which the rotor structure (6′), instead of being mounted protrudingly on a stationary axial supporting element is supported thereon by means of two rotoric shields (8′c, 14) mounted by means of respective bearings (18, 19) on the axial supporting element (1′), which is conveniently monolithic with the stationary supporting shield (3′).

Description: The present invention relates to a rotating electric machine with external rotor, and more particularly to an electric motor with external rotor cage, with an improved mechanical structure. Rotating electric machines with external rotor, and in particular asynchronous electric motors with external cage rotor, are known. The purposes of this constructive solution are well known and dictated by the requirements of the particular applications (such as,in particular, axial and centrifugal fans mounted directly on the rotor frame, devices which require a high inertia of the rotating masses and an enhancement of the self-ventilating effect, etc.). A typical known form of construction of such a motor with external rotor, given by way of comparison in figure 1, which will be commented in greater detail hereinafter, comprises a supporting axial element constituted by a flanged bush which is protrudingly mounted on a stationary supporting shield, and externally supports the active stator parts and in its interior a coaxial shaft, mounted rotatable on bearings also internal to the bush, and with a shaft end protruding from said bush. On the shaft end which protrudes from the bush there is keyed the opposite shield, protrudingly and monolithically supporting the rotor frame and the related active parts contained therein ("rotor cage" and lamellar core). Such a construction of the prior art has several disadvantages among which: difficult centering and critical plays of the rotating masses, due to the protruding mounting of the external rotor part on the shaft end protruding from the bush; high stress and wear of the bearings, consequent to the protruding mounting of the rotating structure and to the concentricity errors deriving therefrom, with the aggravation that the need to contain the bearings inside the bush limits their size, and forces their operation in a hot environment and in an axially close mutual position; noise, due to the above reasons; need for forced mounting of the stator pack on the bush, with consequent deformation of the bearing seats; oversizing of the diameter of the bush, which must contain the shaft at the expense of the active stator material; plurality of parts, and of mechanical machinings of the related coupling surfaces. Other known motors are disclosed by US patent No. 3.385.510 and German patent No. 2536094. Both these prior art documents disclose a state which extends out of the rotor on both sides. The aim of the present invention is to eliminate the above described disadvantages of the prior art. This aim is achieved, according to the present invention, in a rotating electric machine with external rotor comprising a stationary axial supporting element with substantially cylindrical outer surface, having an end rigidly associated with a stationary supporting shield, active stator parts mounted externally to the axial supporting element, and a rotor structure adapted to rotate externally and coaxially to said active stator parts and rotatably supported by said stationary axial supporting element; said rotor structure comprises at least two rotating shields for the support of the active rotor part, said shields being rotatably supported by means of respective bearings at the two ends of the outer surface of the stationary axial supporting element, and in that the stationary axial supporting element has an axial hole communicating with the outside of the machine through an opening of the stationary supporting shield and a radial hole which connects said axial hole with the connections side of the field winding, characterized in that the stationary axial supporting element is a tubular sleeve obtained by casting monolithically with the stationary supporting shield, and in that the active rotor part supporting shield furthest from the stationary supporting shield is provided monolithically with the rotor cage, and comprises a solid wall in correspondence with the axis of said stationary axial supporting element. Advantageously, the stationary axial supporting element is a tubular sleeve obtained by casting monolithically with the stationary supporting shield. The structure according to the invention allows the following advantages: 1. Solid construction, which ensures a greater centering and squaring of the machine, allowing to eliminate the noise due to the protruding rotor construction of the prior art; 2. Possibility of adopting greatly oversized bearings with normal play which allow a longer life, positioned in non-hot regions and axially well spaced; 3. Reduction of dimensions of the structural parts in favour of the active stator parts; 4. Significant reduction of the concentricity errors and therefore further reduction in noise, by virtue of the possibility of performing a single-step mechanical machining of the bearing seats, of the stator pack seat and of the coupling abutment; 5. Possibility of easily reaching,the IP 55 protection degree according to the IEC norms, with the addition of a small gasket ring. Appropriately, the stator pack is blocked on the stationary axial supporting element by a spline or key type coupling. This is facilitated by the greater radial dimensions of the crown of the stator active iron with respect to the structure according to the prior art, and allows to eliminate the deformation of the bearing seats due, in the embodiment according to the prior art, to the forced keying of the pack on the bush constituting the axial supporting element. Other advantages and peculiarities of the present invention will become apparent from the following detailed description of a preferred embodiment thereof, with reference to the accompanying drawings, wherein: figure 1 is an axial sectional view of the typical structure of an external-rotor motor according to the prior art; figure 2 is an axial sectional view of an example of structure of rotating electric machine with external rotor constituted by an asynchronous motor with external cage rotor, according to the present invention; figure 3 is an axial sectional view of a structure similar to that of figure 2, in fully enclosed version corresponding to the IP 55 protection degree according to IEC norms. Figure 1 is an axial sectional view of an example of typical constructive form of an asynchronous motor with external cage rotor according to the prior art. It mainly comprises a stationary axial supporting element or bush 1, substantially cylindrical, having an end insertingly coupled and fixed with screws 2 to a stationary supporting shield 3; active stator parts, in turn comprising a stator pack of lamellar magnetic material 4 coupled by forcing on the outer surface of the bush 1 and a field winding, of which only the heads 5 protruding from the recesses of the pack 4 are visible; and a rotor structure, generally indicated at 6 and constituted by a lamellar pack of magnetic material 7 included in an aluminum die-casting which embraces like an external frame 8 the pack, fills its recesses forming the rotor bars therein (not visible in the sectional plane shown) and also forms the heads 8a, 8b rigidly associated with the bars and monolithic therewith, with function both of short-circuiting rings of the cage and of rotor pack containment flanges. In the example shown, the flange 8a has a perforated radial extension 13 to which it is possible to fix directly, for example, the blades of a fan, not shown. On the side of the field winding opposite to the connections 9 the same aluminum die-casting forms a shield 8c, monolithically with said parts 8, 8a, 8b. The shield 8c is rigidly coupled to an end of a pivot 10 rotatably supported by means of ball bearings 11, 12 inside the stationary bush 1 and coaxially thereto. The above described rotor structure 6 , exemplifying the prior art, is therefore protrudingly supported by means of the shield 8c on the pivot 10, rotatable within the bush 1 with the interposition of the bearings 11, 12 internal to said bush. With reference to figure 2, a preferred embodiment of the rotating electric machine with external rotor according to the present invention is now described, by way of non-limitative example, constituted by an asynchronous motor with external cage rotor. The parts having a function equal or similar to those of the structure according to the prior art shown in figure 1 are indicated in figure 2 with a same numeral accompanied by an index ('). In the following description, only the differences of the structure according to the present invention from that according to the prior art of figure 1 will be pointed out. The rotor structure 6 ' (figure 2) comprises two shields (14, 8'c) for supporting the active rotor parts, and supported by means of bearings, respectively 11' and 12', at the two ends of the outer surface of a stationary axial supporting element or sleeve 1. The sleeve 1 has an axial hole 15 communicating with the outer terminal board 18 through an opening of the supporting shield 3' and a radial hole 16 for the exit of the electric connections 17 of the field winding 5'. In this structure according to the present invention the rotor part 6', differently from the solution according to the prior art, rests at both ends on the axial supporting element, or sleeve 1, instead of protrudingly, with the already mentioned advantages of greater solidity and centering. Furthermore the bearings 11', 12', being external to the axial supporting element 1, may be oversized and have normal play, with the also already mentioned advantages of greater solidity and durability. For the same reason, the sleeve 1' can have a diameter smaller than that of the prior art supporting element 1 (figure 1), leaving greater room for the active material of the stator pack 4', with consequent improvement of the electric performance of the motor. Ample room is also left to interleave coupling elements between the sleeve 1 and the stator pack 4', for example of the type with key or tab and related seat, so as to avoid a keying obtained exclusively by forcing, as used in the prior art and source of deformation of the bearing seats. In the embodiment shown, the sleeve 1' is made by monolithic casting with the stationary supporting shield 3'. This allows a single-step mechanical machining of the bearing seats 18, 19, of the seat of the stator pack 20 and of the coupling abutment 21. This, besides reducing machining times, allows more strict tolerances, with enhancement of the above described advantages of centering etc. Furthermore, a single part 1' replaces four ones of the prior art construction of figure 1 (supporting shield 3, axial supporting element 1, pivot 10 and centering bush 22). In the example of embodiment of figure 2, the connections-side rotor shield 14 is separable from the rotor structure 6 ' and connected thereto by means of an insertion coupling and of screws 23, for ease in assembly. A further advantage of the construction according to the present invention is to easily allow the obtainment, in a rotating electric machine with external rotor of the type described, of an IP 55 protection degree according to IEC norms, corresponding to a fully enclosed construction. With reference to figure 2, this is obtainable by means of the simple addition of an elastomer gasket ring 24 in front of the connections-side bearing, between the rotor shield 14 and the stationary supporting shield 3'. The same figure also shows a variated aspect of the locking system with key and key-seat between the stator pack and the sleeve 1. The rotor with squirrel-cage shorted winding represented herein may be replaced with a wound rotor, the winding whereof leads to a slip ring arranged externally on the rotor part, for example to a frontal slip ring applied to the outer rotating shield. Furthermore, the construction according to the present invention is applicable to a synchronous machine, for example with rotating external inductor with permanent magnets, or with a field winding connected to a slip ring as described above, or even to a direct-current machine, with obvious modifications.

Claims: Rotating electric machine with external rotor comprising a stationary axial supporting element with substantially cylindrical outer surface, having an end rigidly associated with a stationary supporting shield, active stator parts mounted externally to the axial supporting element, and a rotor structure adapted to rotate externally and coaxially to said active stator parts and rotatably supported by said stationary axial supporting element; said rotor structure (6') comprises at least two rotating shields (8'c, 14) for the support of the active rotor part (7, 8'a, 8'b), said shields being rotatably supported by means of respective bearings (18, 19) at the two ends of the outer surface of the stationary axial supporting element (1'), and in that the stationary axial supporting element (1' ) has an axial hole (15) communicating with the outside of the machine through an opening (15a) of the stationary supporting shield (3') and a radial hole (16) which connects said axial hole (15) with the connections side (17) of the field winding (5'), characterized in that the stationary axial supporting element is a tubular sleeve (1') obtained by casting monolithically with the stationary supporting shield (3'), and in that the active rotor part supporting shield (8') furthest from the stationary supporting shield (3') is provided monolithically with the rotor cage (8'a, 8'b), and comprises a solid wall in correspondence with the axis of said stationary axial supporting element. Rotating electric machine with external rotor according to claim 1, characterized in that the stator pack (4') is fixed on the stationary axial supporting element (1') at least predominantly by means of a tab (25, 25') and groove coupling. Rotating electric machine according to one or more of the preceding claims, characterized in that an elastomer gasket ring (24) is mounted between the stationary supporting shield (3') and the rotor shield contiguous thereto (14) proximate to the related bearing (11') and in that the rotor shield completely encloses said bearing (12') on the opposite side with respect to the outer environment, so as to provide a fully enclosed construction with IP 55 protection degree according to IEC norms.

Inventors: MASSERINI, NATALINO, MAZZUCCHELLI, ATTILIO, VIVALDI, SILVANO
Applicants: GRUPPO INDUSTRIALE ERCOLE MARELLI S.P.A.
Application year: 1988
Main classification: H02K5
IPC codes: H02K1, H02K5, H02K15, H02K17