Motor cooling

Abstract: An electric motor assembly (10) e.g. for an air cycle air conditioning apparatus (11) comprises stator windings (14) and a rotating member (15) mounted for rotation about a motor axis (A) and wherein, the stator windings (14) comprise therethrough at least one axially extending passage (18), and there being means to connect the passage (18) to a source (50) of cooling gas, whereby as the cooling gas flows along the axially extending passage (18) the motor assembly (10) is cooled.

Description: This invention relates to an electric motor assembly and more particularly but not exclusively to such an assembly for use in an air conditioning apparatus e.g. of the air cycle kind in which load air is cooled by system air which is cooled by an expansion turbine or other air expansion member of the apparatus. In such an apparatus the motor will generate heat in operation which needs to be removed from the motor. It is known to establish a flow of cooled system air over the motor but particularly where the motor comprises e.g. a switched reluctance motor or a permanent magnet DC motor or a high speed induction motor, in which a rotating member of the motor rotates at high speed, the cooling effect of such system air is inadequate. It has been proposed in US-5703421 to provide radially extending passages through stator windings to allow for the venting of spent cooling fluid from a space between the rotor and stator. Although such passages may permit of some winding cooling, the effect is somewhat limited. In EP-A-0644334 there is disclosed a vacuum cleaner in which a motor is cooled by air drawn through the motor, by the action of an impeller rotated by the rotor. The air is then exhausted together with the remainder of the drawn-in air. According to a first aspect of the invention we provide an electric motor assembly for an air conditioning apparatus, according to claim 1. Thus the primary cooling gas will primarily cool the motor and additional cooling of the motor assembly may be achieved by the secondary cooling gas which will remove heat from a position immediately close to where the heat primarily is generated i.e. in the stator windings. In one construction conveniently the axially extending passage communicates at the one end with the inlet through a manifold from which the secondary cooling gas passes into the axially extending passage. For example, the manifold may be provided towards one end of the casing and the axially extending passage communicating with the exhaust passage towards an opposite end of the casing. If desired the primary cooling gas will be ambient air at a reduced pressure compared with the source of the secondary cooling gas and so to establish the secondary cooling gas flow, the exhaust passage may open into the lower pressure primary cooling air which flows or has flowed over the casing. Where the source of cooling gas is ambient air, and a manifold is provided, the ambient air may be cooled prior to flowing into the manifold. In another arrangement, which does not fall within the scope of the invention, the primary cooling gas may be at a higher pressure than ambient air in which case the source of cooling gas may comprise primary cooling air which flows or has flowed over the casing and the exhaust passage communicates with the lower pressure ambient air. Although the passage or passages through the stator windings may be of any desired configuration, the or each axially extending passage through the stator windings preferably comprises a generally axially extending gap between a pair of adjacent winding sections. The rotating member may comprise a shaft which is journalled in bearings, and if desired the secondary cooling gas prior to and /or subsequent to flowing along the axially extending passage or passages, may cool the bearings. This is particularly beneficial where the rotating member rotates at high speed when much heat can be generated in the bearings. The rotating member of the motor assembly may comprise a shaft which carries an air compressor member and/or an air expansion member of the air conditioning apparatus. Although the invention may be applied to any suitable motor assembly type, the invention has particularly been developed for use where the motor assembly is of the switched reluctance or other type in which the rotating member in use is rotated at high speed by which we mean typically at least 10,000 revolutions per minute, and more typically up to 40,000 revolutions per minute or more. According to a second aspect of the invention we provide an air conditioning apparatus comprising a motor assembly according to the first aspect of the invention. The invention will now be described with reference to the accompanying drawings in which:- FIGURE 1 is a illustrative cross sectional view along the line YY of Figure 2, of an electric motor assembly in accordance with the invention. FIGURE 2 is an illustrative cross section along the line XX of Figure 1. Referring to the drawings, an electric motor assembly 10 comprises a motive means of an air cycle air conditioning system 11. The motor assembly 10 is of the switched reluctance kind having no permanent magnet, but stator windings indicated at 14 including stator poles P arranged generally cylindrically, and a rotating member 15 comprising a shaft 16 carrying laminations 17, which shaft 16 rotates about a rotor axis A. The operation of switched reluctance motors is not described more fully herein but the invention is concerned with a means for cooling the electric motor assembly 10 be it of the switched reluctance kind as illustrated, or of an alternative kind. The stator windings 14 are provided in circumferentially arranged sections, with there being axially extending gaps 18 between the sections which, as will hereafter be described, provide passages for cooling gas. Although the gaps 18 open to the rotating member 15 and the laminations 17 of the rotating member 15 may have cut-outs C as shown, due to the speed at which the rotating member 18 rotates, there is negligible transfer of cooling gas from the gaps 18 into the cut outs C. In any event, the ends of the laminations 17 are closed by closure discs D. The motor assembly 10 comprises a casing 19 which contains the windings 14 and rotating member 15 axially within the casing 19. At one end of the shaft 16 of the rotating member 15 there is provided an expansion turbine wheel 22 which rotates with the shaft 16. Ambient air is drawn into the expansion turbine 22 via an ambient air inlet 23, is expanded and thus cooled by the turbine wheel 22, and then passes along a passage 24 of the air conditioning apparatus 11 to a heat exchanger 25 to which hot load air is delivered at an inlet 26. As the expanded cooled air passes through the heat exchanger 25, heat is thus removed from the load air, and the cooled load air is returned to an air conditioned environment (not shown) such as a train carriage compartment via an outlet 27 from the heat exchanger 25. The warmed system air leaves the heat exchanger 25 and passes along a conduit 30 an inlet 32 of the motor assembly 10. It can be seen particularly from Figure 2 that the system air is then constrained to flow around the casing 19 of the motor assembly 10 before passing to a further conduit 31. The system air 30 from conduit 30 will be of a relatively low pressure still, but will be warmer than the system air entering the heat exchanger 25. However, the system air will still be cooler than the casing 19 of the motor assembly 10 and thus as it flows round the casing 19, the system air will remove heat from the motor assembly 10. Fins F are provided to enhance heat exchange between the casing 19 and system air. The thus further warmed but still low pressure system air in conduit 31 is fed to a compressor wheel 33 which is also carried on shaft 16 which compressor wheel 33 returns the system air to ambient pressure at least, before exhausting the system air through an exhaust outlet 34 to ambient. At one axial end of the casing 19, which in this example is the end closest to the compressor wheel 33, there is provided a manifold 35 in which bearings 36 in which the shaft 16 is journalled at its end towards the compressor wheel 33, are positioned. At an opposite end of the casing 19, adjacent to the turbine wheel 22, further bearings 37 for shaft 15 are provided. Thus the bearings 36 are located adjacent to a space 38 within the manifold 35, the space 38 communicating with the axially extending gaps 18 between the sections of stator windings 14 of the motor assembly 10. A conduit 40 is provided which communicates at one end with the space 38 of the manifold 35, and at an opposite end with an ambient air inlet 50. The conduit 40 extends through system air inlet 32 to the motor assembly 10. At an opposite end of the casing 19, there are provided a plurality of openings 51 into the system air flow around the casing 19. As the system air flowing around the casing 19 is a relatively low pressure, it will be appreciated that in use ambient air will be drawn into inlet 50, pass along the conduit 40 into the space 38 of the manifold 35, and be drawn along the axially extending gaps 18 before passing via the openings 51 into the lower pressure system air flow. Thus the ambient air acts as a cooling gas. This flow of air has two effects. First, the air will cool the windings 14 of the motor assembly 10 as it flows along the gaps 18, and secondly, because the bearings 36 in which shaft 16 is journalled are located in the space 38 into which the air flows from conduit 40, the bearings 32 too will be cooled by the flowing air. The bearings 37 may also be subjected to a cooling effect as the air flows past the bearings 37 into the openings 51. Furthermore as the system air flowing in the inlet 32 to the motor assembly 10 may be of lower temperature than ambient, the ambient air whilst flowing in conduit 40 will be cooled by the system air prior to entering space 38 of the motor assembly 10. Thus the motor assembly 10 is cooled by a flow of primary cooling system air passing over the casing 19 (which may be finned or ribbed to promote heat transfer), which air will at least be of lower temperature than the casing 19, and by a secondary flow of cooling air/gas from the ambient air inlet 50. It will be appreciated that the introduction of the ambient air 50 into the system air flow will have two further effects. First this may increase the pressure of the system air flowing around casing 19, and secondly may increase the temperature of the system air flowing. This will impose an increased load on the motor assembly 10 and the compressor wheel 33 will thus need to be slightly larger than otherwise would be required, in order to accommodate that extra load. The extra load will of course also impose a greater heating effect within the motor assembly 10 and hence a balance needs to be obtained between the decrease in efficiency arising out of the increase in pressure and temperature of the system air, flowing over casing 19 and the increase in operating efficiency and bearing life etc. of the motor assembly 10 by virtue by it being cooled. Many modifications may be made without departing from the scope of the invention. Most particularly, although the invention has been described with reference to a switched reluctance type motor, the invention may be applied to any other motor in which there are stator windings through which passages for the flow of cooling air or an other cooling gas may be established. If desired, to control the amount of cooling gas drawn into the ambient air inlet 50, a valve may be provided, the flow through which valve may be adjusted in response to the amount of cooling required within the motor assembly 10. Although as described, the motor assembly 10 carries on the shaft 16 of the rotating member 15, a compressor wheel 33 and a turbine wheel 22, other arrangements by which the motor assembly 10 provides a motive force to at least expand air and/or compress air as desired, may be provided. Although it is preferred for the motor assembly 10 to be cooled by a primary flow of system air from the load heat exchanger 25 over a casing 19 thereof, any other primary motor cooling means could be provided. The arrangement described has the advantage though that no auxiliary pump or the like is required for providing cooling air gas or other fluid to the motor assembly 10, air flow occurring solely as a result of the operating conditions of the motor assembly 10. Instead of the passages for cooling gas through the stator winding 14 of the motor assembly 10 comprising axially extending gaps 18 or slits, any other means for allowing an axial flow of cooling gas through at least one axially extending passage of the windings 14, may be provided. The invention may be applied to an electric motor assembly for another kind of air conditioning assembly but preferably to an air cycle air conditioning assembly. In another arrangement, outside the scope of claim 1, the system air may be of high pressure within the inlet 32 to the assembly 10 in which case the exhaust outlet openings 51 from the casing 19 may be vented to ambient rather than to the system air flow, such that the higher pressure system air is drawn through the passage or passages through the stator windings 14 and exhausted to ambient, although again this will result in a slight loss of operating efficiency of the motor assembly 10 to be balanced against the increase in efficiency of the motor 10 being cooled. The invention may even be applied to a system in which the system air is heated air which has previously delivered heat to load air for use in the air conditioned environment. The hot system air will of course need to be cooler than the motor assembly 10 to provide any cooling effect in such circumstances.

Claims: An electric motor assembly (10) for an air conditioning apparatus (11), the motor assembly (10) comprising: stator windings (14) provided within a casing (19), the stator windings having therethrough at least one axially extending passage (18) through which, in use, secondary cooling gas flows to cool the stator windings (14), a rotating member (15) mounted for rotation about a motor axis (A), characterised by means for constraining, in use, primary cooling air from the air conditioning apparatus, which primary cooling air is at a pressure lower than ambient air pressure, to flow around the casing (19), and wherein the axially extending passage (18) through the windings (14) at one end communicates with an inlet for ambient air, and at an opposite end communicates with an exhaust passage (51) into the lower pressure primary cooling air flowing in said means. An assembly according to claim 1 characterised in that the axially extending passage (18) communicates at the one end with the inlet through a manifold (35) from which the secondary cooling gas passes into the axially extending passage (18). An assembly according to claim 2 characterised in that the manifold (35) is provided towards one end of the casing (19) and the exhaust passage (51) towards an opposite end of the casing (19). An assembly according to claim 2 or claim 3 characterised in that the ambient air is cooled prior to flowing into the manifold (35). An assembly according to any one of the preceding claims characterised in that the axially extending passage (18) through the stator windings (14) comprises a generally axially extending gap between a pair of adjacent winding sections. An assembly according to any one of the preceding claims characterised in that the rotating member (15) comprises a shaft (16) which is journalled in bearings (36, 37), the secondary cooling gas prior to and /or subsequent to flowing along the axially extending passage (18), cooling the bearings (36, 37). An assembly according to any one of the preceding claims characterised in that the rotating member (15) has a rotatable shaft (16) which carries an air compressor member (33) and/or an air expansion member (22). An assembly according to claim 7 characterised in that the motor assembly (10) is of the switched reluctance type in which the rotating member (15) in use is rotated at high speed. An air conditioning apparatus (11) characterised in that the apparatus has a motor assembly (10) according to any one of the preceding claims.

Inventors: BRINSDEN JAMES DUDLEY, HUNT RICHARD GRANT, WATSON STEPHEN RICHARD, WILSON MALCOLM PHILIP, BRINSDEN, JAMES DUDLEY, HUNT, RICHARD GRANT, WATSON, STEPHEN RICHARD, WILSON, MALCOLM PHILIP
Applicants: NORMALAIR GARRETT LTD, NORMALAIR-GARRETT (HOLDINGS) LIMITED
Application year: 1998
Main classification: H02K9
IPC codes: H02K3, H02K5, H02K9