GB2326983A

GB2326983A - Cooling systems for electric machines

Info

Publication number: GB2326983A
Application number: GB9707986A
Authority: GB
Inventors: Paul Greenhough
Original assignee: BRITISH JEFFREY DIAMOND 100 LI
Current assignee: BRITISH JEFFREY DIAMOND 100 LI
Priority date: 1997-04-21
Filing date: 1997-04-21
Publication date: 1999-01-06
Anticipated expiration: 2017-04-21
Also published as: GB9707986D0; GB2326983B

Description

Cooling Systems for Electric Machines This invention relates to cooling systems for electric machines, for example electric motors.

Electric motors are widely used in driving mechanical devices ranging from miniature devices up to extremely large devices such as fans, conveyors, compressors and mineral crushers.

More particularly in electric motors which are large in construction and which have drive applications in industrial situations, large drive currents are involved which result in a substantial amount of iron loss and heat generation within the body of the motor.

Thus when high current is passed through for example stator coils wound onto a laminated stator core, heat generated in the core laminations leads to inefficiency owing to high temperatures produced in the stator core and in the stator windings.

Many attempts have been made to cool electric machine stators and their windings. Conventionally, the stator core in encased in a substantially cylindrical iron frame. The stator core is in thermal contact with the frame, and the outer cylindrical wall of the frame contains a hollow region providing a water jacket through which cooling water can be circulated.

Heat in the stator core and stator windings is thus transferred away by the cooling water via the frame.

Another method of providing stator core and stator winding cooling is for a fan to be mounted on an end of the rotor of the machine, in order to blow or suck air in an axial direction of the machine, through the gap between the rotor and stator. This air can, in some cases, be recirculated back to the input end of the stator/rotor gap through closed channels formed in the frame of the machine, the air itself being subjected to a cooling effect whilst returning through those stator channels.

Yet another method of achieving stator cooling is to provide external cooling fins on the machine frame, in order to dissipate, to the ambient atmosphere, heat generated in the stator core and stator windings and then transferred to the machine frame.

Although these prior cooling systems have all been partially successful at least to some extent, there is nevertheless still a need to improve the dissipation of heat away from stator cores and stator windings in electric machines, leading to the greater efficiency of electric machines (particularly but not exclusively motors) and to get a greater power output from a given size of machine.

It is accordingly an object of the present invention to provide an improved and more efficient means of cooling an electric machine.

According to the present invention there is provided an electric machine stator, comprising a stator core with stator windings and providing a magnetic path for magnetic flux produced when the stator windings are energized, the stator core having an extent outwardly beyond the magnetic path and being formed, in a region outwardly of that magnetic path, so as to provide a cooling structure operable when the stator is in use to provide a cooling effect for the stator core.

It is to be appreciated that the shape of the external surface of the stator core is not to be restricted to any particular form, and could be circular or, for example, square or hexagonal or any other suitable cross-sectional shape.

In one version the cooling structure may provide fins on the outer periphery of the stator core, whilst in another version the cooling structure may provide a plurality of ducts along the periphery of the stator core.

In some embodiments it may be possible to combine cooling ducts with cooling fins, for example by providing cooling fins alongside or radially outwardly of the cooling ducts.

A cooling structure according to the present invention may simply be in contact with stationary air, or the electric machine may be adapted to blow or suck air through the cooling ducts or along the cooling fins. This could be achieved by a fan fixed to the end of the rotor of the machine.

Alternatively, water cooling may be employed, for example by pumping cooling water through cooling ducts.

In addition to employing a cooling system according to the present invention, an electric machine may combine such a cooling system with one of the already-known systems. For example, if a fan is employed to blow or suck cooling air through the cooling ducts, this fan may also blow or suck cooling air through the gap between the stator and rotor.

Usually, but not essentially, the invention will be applied to a laminated stator core, with the cooling structure being stamped into each one of the core laminations. When stacked together, the thus-stamped laminations may provide the entire cooling structure, for example the cooling ducts and/or the cooling fins.

A preferred embodiment of the present invention may be employed in an electric machine, such as an electric motor, wherein the conventional statorcontaining frame of the machine is omitted. Thus the outer periphery of the cooling structure provided according to the present invention, may provide the cylindrical outer periphery of the machine itself, without any further frame being necessary. It is, however, to be noted that a stator frame may be provided in some embodiments if required, and in particular where the cooling structure comprises external cooling fins, it may be advantageous to sheath the outer periphery of the fins within a smooth cylindrical envelope made, for example, from rolled steel. This envelope could be welded to the outer peripheral tips of the fins, and could provide protection against any sharp tips of the external cooling fins injuring a person handling the machine.

Another advantage with such an envelope would be that a cooling medium (e.g. air or water) could then be passed in the axial direction of the machine between the external cooling fins and within the protective envelope, thus leading to additional cooling efficiency.

Although we envisage the present invention being applicable to any type of electric machine, and particularly motor, we feel that the invention can be particularly usefully applied to the case of a switched reluctance motor. Such a motor can provide a variable speed brushless motor ranging from lower than 50kW to 500kW in output or even higher, having a salient pole rotor with no windings, and a stator core upon which salient pole pieces are wound with individual stator windings.

In such a motor the supply current to the stator windings is switched so that different combinations of the stator windings are energised in turn, creating magnetic flux which flows through a magnetic flux path in the stator core, through the stator salient poles, and through the rotor salient poles also. The rotor is in effect pulled around in rotation in order to shorten these lines of magnetic flux.

Because the stator poles and their windings are highly localised at specific angular positions around the stator core, heat is not generated evenly around the stator core, unlike say the situation for an AC induction motor wherein the windings are evenly distributed around the stator core.

Accordingly, a switched reluctance motor requires an extremely efficient cooling of the stator core so that the localised heating at hot-spots of the stator core can be effectively dissipated, therefore allowing the stator poles and their associated windings to be cooled efficiently and accept higher energization current thereby to provide a more efficient and effective drive.

Since the present invention enables direct cooling of the stator core laminations to be provided, with the stator core laminations being directly contacted at or close to their outer periphery by a cooling medium such as air or water, heat can be dissipated extremely effectively and efficiently. This advantage has particular (although not exclusive) applicability to the case of a switched reluctance motor, enabling the heat which is generated at localised discrete positions around the stator core, to be effectively dissipated.

This is particularly the case when no stator frame is provided, enabling a direct thermal contact between the stator core laminations and the cooling medium.

For a better understanding of the present invention and to show how it may be put into effect, reference will now be made by way of example to the accompanying drawings in which: Figures 1 and 2 show radial cross-sections of the stator and rotor cores of a conventional switched reluctance motor; Figure 3 shows an axial section of a conventional switched reluctance motor incorporating the stator and rotor cores of Figures 1 and 2; Figure 4 shows a radial sectional view of a stator core according to a first embodiment of the present invention; and Figure 5 shows a radial sectional view of a second embodiment of stator core according to the present invention.

Figure 1 shows a laminated stator core 1 of a conventional switched reluctance motor, with a laminated rotor core 2 being rotatably mounted within the stator core 1 on a rotary shaft 3.

The stator core 1 comprises a continuous outer annular region 4, from which extend salient stator poles 5 directed radially inwardly. In each lamination of the stator core 1, the outer annular region 4 and the stator poles 5 are stamped-out together and form a single continuous sheet of metal.

Each stator pole 5 carries its own stator winding 6, with all the windings 6 being coupled to a mains source via a controlled switching circuit. The particular details of such a switching circuit will be apparent to the man skilled in the art, and so are not shown here in detail.

The rotor core 2 has salient poles 7 formed thereon as shown, but not having windings thereon. It is a feature of a switched reluctance motor that the rotor is unwound, and has a simple robust construction with no significant heat being generated in the rotor.

Figure 1 shows the motor in a first switched state, showing four closed circuits 8 of magnetic flux, each extending in the outer annular region 4 of the rotor core 1, through a pair of stator salient poles 5 and then, via the air gap between the stator and rotor, through a pair of salient poles 7 on the rotor core 2.

Figure 2 shows a subsequent switched state of the stator coils 6, without the reference numerals of Figure 1 being repeated. It can be seen from Figure 2 that the energization of the stator windings 6 has been switched to the next set of stator windings, the resultant change in the positions of the magnetic flux circuits 8 resulting in the rotor 2 being pulled-round in a clockwise direction as is indicated.

Continual rapid switching of the stator windings 6 results in rapid rotation of the magnetic flux circuits 8 around the motor, and thus rapid rotation of the rotor core 2. Such a motor has advantages such as high efficiency throughout the speed range and high torque generated at low speeds, together with a better power to volume ratio than an equivalent DC motor, with the ability to remain in a stall condition for long periods.

Figure 3 shows how the rotor core 2 and conventional stator core 1 of Figures 1 and 2 have previously been housed and cooled. The laminated stator core 1 can be seen in axial cross-section, indicating stator windings 6 located on upper and lower salient stator poles 5.

The rotor shaft 3 and salient rotor core poles 7 are shown also, whilst the stator core 1 can be seen to be housed in a cylindrical motor frame 9 which extends around the outer periphery of the stator core 1, and in close contact with it.

The rotor shaft 3 is mounted in bearings 10 and 11 at opposite end regions of the frame 9, with a coupling 12 being provided at one end of the shaft 3 so that the motor can be coupled to drive an external apparatus.

Mounted at the opposite end of the shaft 3 is a rotor position sensor 13 which supplies to the stator switching control system feedback signals indicative of the rotor position.

The frame 9 has a cylindrical peripheral wall of substantial thickness, which is hollow around its entire extent in order to provide a cooling water jacket 14 for the stator core 1. Heat generated in the stator windings 6 is transferred to the salient stator poles 5, and from there to the outer annular region 4 of the stator core 1, then to the stator frame 9 where it is carried away by water circulating through the water jacket 14.

It can clearly be seen that the efficiency with which heat is taken away from the stator windings 6 depends to a great extent on the efficiency of the thermal connection between the stator core 1 and the frame 9, whilst at the same time the frame 9 must be made of iron or other material of good thermal conductivity and is an expensive component of the motor as a whole.

First and second embodiments of stator cores of the present invention are shown in Figures 4 and 5.

According to Figure 4, the stator core is shown in similar cross-sectional view to that of Figures 1 and 2. The rotor core 2 is not shown however and nor are there shown constructional details of the stator poles 5 and their associated windings 6. Instead a shaded annular region 15 is shown, denoting the region at which the stator poles 5 and windings 6 are located.

It is to be understood that the stator poles and windings may be of any desired form, depending upon the type of machine to which the present invention is to be applied. Thus, although we find the invention to be particularly applicable to the case of a switched reluctance motor as shown in Figures 1 to 3, the invention is not restricted to this particular type of motor, and the shaded annular region 15 could be occupied by the windings of, say, an AC induction motor, DC motor or any other type of winding requiring cooling. The winding could also, for example, be a winding of an electrical generator.

Radially outwardly of the shaded area 15 is the annular region 4 of Figure 1, which carries the magnetic flux path within the stator core when the stator windings are energised.

The radial width of this annular region 4 may vary depending upon the type of machine for which the stator core is intended, whilst it is particularly important in the case of a switched reluctance motor that the radial extent of this annular region 4 be sufficient to allow the required magnetic flux to have an easy passage.

The stator core in Figure 4 is extended radially outwardly beyond this annular region 4, and is shaped in a region radially outwardly of the magnetic flux path so as to provide a cooling formation or structure operable when the machine is in use to provide a cooling effect for the stator.

Thus in Figure 4 it can be seen that the stator core is extended outwardly from the outer edge of the annular region 4 to a cylindrical outer periphery 17, and with there being formed therebetween a plurality of ducts 16 which are arranged entirely around the periphery of the stator core, and each of which has a longitudinal extent along the entire axial length of the stator core.

Adjacent ones of the ducts 16 are separated by a relatively thin solid wall 18, with the ducts 16 each having the same cross-sectional shape with a radial extent greater than its peripheral extent, substantially flat side and bottom walls, and with a curved outer wall. This particular shape of duct cross-section is entirely optional, and other shapes could be used also, for example square, circular, triangular or more complex shapes.

A motor stator can comprise the stator core shown in Figure 4, sandwiched between end plates carrying bearings for the rotor shaft. No frame outside the outer peripheral surface 17 of the stator core need be provided, and the ducts 16 provide direct cooling for the stator laminations. In this way the stator poles and windings can be efficiently cooled, particularly if the machine is adapted to provide a cooling flow of air or water through the ducts 16.

Figure 5 shows an alternative form of stator core according to the invention, wherein instead of ducts 16 as in Figure 4, external cooling fins 18 are formed on the outer periphery of the annular region 4 of the stator core. The fins are approximately triangular in cross-section and extend along the whole length of the stator core in an axial direction, and may in themselves provide sufficient cooling for the stator core without it being necessary to provide a flow of cooling air or water. It would of course, however, be possible to mount a fan on one end of the rotor and provide an axial cooling flow of air along the fins 18.

In an optional development of the stator core of Figure 5, the external periphery of the cooling fins 18 could be sheathed in a cylindrical envelope 19 of, for example, rolled steel welded to the tips of the cooling fins 18. This is shown partially in Figure 5 merely for the purpose of illustration. In practise the envelope 19 would extend all around the stator core.

Such an envelope 19 could protect operators handling the motor from being injured on the sharp edges of the cooling fins 18 when no outer frame is provided, and could have the additional purpose of providing axially-extending ducts between itself and the cooling fins 18, through which cooling air or water could be pumped.

Although in each of Figures 4 and 5 the cooling formations (i.e. ducts and fins) can be seen equally distributed around the periphery of the stator core, when applied to a motor such as a switched reluctance motor it may be possible to concentrate the cooling formations at the locations of the salient stator poles. In this way it may be possible to achieve an efficient cooling of the localised hot spots occurring at the locations of the individual stator windings, whilst nevertheless retaining a more solid construction inbetween these locations.

It will be appreciated that several advantages can be achieved in a machine, e.g. motor, having a stator core constructed in accordance with the present invention. For example, the conventional motor frame can be omitted, yet an efficient cooling system, possibly involving a flow of cooling medium at the periphery of the motor, can be achieved.

Further, with an improved cooling effect it may be possible to achieve more mechanical power from a given volume or size of motor, or alternatively it may be possible to reduce expense on the stator core laminations. For example, the laminations could be made thicker and/or could be provided from a cheaper grade of steel, thereby reducing expense, for the same output.

Utilising the same thickness and quality of laminations as previously, a more efficient cooling effect enables the temperature of the stator coils to be reduced more efficiently, enabling the stator windings to conduct more current and thus more power to be achieved. This advantage can be particularly noted when employing a switched reluctance motor at high speed, involving the magnetic flux changing at high frequency and substantial iron loss being generated, meaning higher temperatures in conventional motors.

Claims

CLAIMS:

1. A switched reluctance motor stator, comprising a stator core with switchable stator windings and providing a magnetic path for magnetic flux produced when the stator windings are energised, the stator core having an extent outwardly beyond the magnetic path and being formed, in a region outwardly of that magnetic path, so as to provide a cooling structure operable when the stator is in use to provide a cooling effect for the stator core.

2. A switched reluctance motor stator according to claim 1, wherein the cooling structure formed by the stator core is equally distributed around the stator core.

3. A switched reluctance motor stator according to claim 1, wherein the cooling structure formed by the stator core is concentrated at predetermined locations around the stator core.

4. A switched reluctance motor stator according to claim 3, wherein the cooling structure is concentrated at the locations of the stator windings.

5. A switched reluctance motor stator according to any one of the preceding claims, wherein the cooling structure provides a plurality of axially-extending ducts around the stator core.

6. A switched reluctance motor stator according to claim 5, wherein adjacent ones of the ducts are separated by a relatively thin solid wall, with the ducts each having the same cross-sectional shape with a radial extent greater than its circumferential extent.

7. A switched reluctance motor stator according to any one of the preceding claims, wherein the cooling structure provides axially-extending fins at the outer periphery of the stator core.

8. A switched reluctance motor stator according to claim 7, wherein the fins are substantially triangular in cross-sectional shape.

9. A switched reluctance motor stator according to claim 7 or 8 when combined with claim 5 or 6, wherein the cooling fins are provided radially outwardly of the cooling ducts.

10. A switched reluctance motor stator according to claim 7, 8 or 9, wherein the external periphery of the fins are sheathed in a cylindrical envelope.

11. A switched reluctance motor stator according to any one of the preceding claims, wherein the stator core is a laminated stator core, with the shape of the cooling structure being stamped into each one of the core laminations.

12. A switched reluctance motor stator according to any one of the preceding claims, wherein the stator is a frameless stator so that the outer periphery of the stator core provides the outer periphery of the stator itself.

13. A switched reluctance motor stator substantially as hereinbefore described with reference to Figure 4 or 5 of the accompanying drawings.

14. A switched reluctance motor comprising a stator according to any one of the preceding claims.

15. A switched reluctance motor according to claim 14, which is adapted to provide a flow of cooling fluid (e.g. air or water) through the cooling structure formed by the stator core.