GB2289576A

GB2289576A - Stator cooling channels

Info

Publication number: GB2289576A
Application number: GB9512176A
Authority: GB
Inventors: Arnold Fenocci; Mohammad Reza Etemad; Mihailo Ristic; Colin Bowden Beasant; Keith Robert Pullen
Original assignee: Turbo Genset Co Ltd
Current assignee: Turbo Genset Co Ltd
Priority date: 1991-11-06
Filing date: 1991-11-06
Publication date: 1995-11-22
Anticipated expiration: 2011-11-06
Also published as: GB2289576A8; GB2261327B8; GB2289576B; GB2261327A; GB2261327A8; GB2261327B; GB2289802B8; GB2289802A8; GB9512176D0; GB9511976D0; GB9123576D0; GB2289802A; GB2289802B

Abstract

A stator 40 - 46 of an axial field electrical machine comprises at least one radial channel 106 for ducting cooling air. The channel has an entrance 108 at or substantially near the rim 110 of the stator and an exit 114 at or substantially near its centre 112. The electrical machine may be part of a turbine engine 20. The stator may comprise a plurality of windings 114 for various phases of electric current wound in substantially the same plane. <IMAGE>

Description

ROTARY ELECTRICAL MACHINES The present invention relates to rotary electrical machines such as electrical generators and electric motors and to particularly advantageous components for use in such machines.

British Patent Specification GB 2 222 031 A describes an axial field electrical generator capable of operation at very high speeds. However, certain features of this known design have drawbacks and it is these in particular which the present invention seeks to overcome.

As is normal with such machines, the known design employs a pair of keeper discs at either end, abutting the outermost ends of the permanent magnet rotors (the coils are present as stators). The discs serve to provide closed flux paths between the magnets in the end rotors.

As illustrated and described, these keeper discs have a 'hairpin profile. That is to say, when viewed in axial cross section, they have the shape generally as shown in Figure 1 of the accompanying drawings in which numeral 3 denotes part of the shaft and 5 the keeper disc. As shown this is a left-end disc so that the rotor would be immediately to the right of the disc in the drawing.

The two hairpin 'arms' as seen in axial section are denoted 7 and 9 and are separated by an annular channel 11.

The hairpin profile is necessary to enable the discs to withstand the strong centripetal forces in high speed rotation. However, this form of construction is difficult to manufacture because of the bore detail which must be very accurately machined.

We have now devised a new form of keeper disc which overcomes this drawback.

Thus a first aspect of the present invention provides a keeper disc for an axial field electrical machine, the disc for an axial field electrical machine, the disc comprising a pair of concentric parts made from magnetically permeable material.

The concentric parts are held together using the interference technique, the amount of the interference being determined by the operating speed of the machine.

The first aspect of the present invention also extends to an axial field electrical machine comprising a pair of such discs as hereinbefore defined in the preceding paragraph and configured to be located at respective ends of the machine.

Preferably, the outer concentric part and the inner concentric part are both made of high strength steel.

However, the inner part may for example be made of titanium.

In the context of the present invention, the term electrical machine' means any electrical machine having at least one stator and at least one rotor, for example an electrical generator or an electric motor.

The generator described in the aforementioned patent specification employs air cooling for the stators. The air is directed by means of radial channels entering at the rim. The channels conduct air towards the centre of the stators and back again to the rim. As a result, on the return path the air is already hot and so performs little cooling. At the same time, the multiplicity of channels detracts from the strength of the stators.

We have also now devised a new means of stator cooling which does not suffer from the disadvantages of the aforementioned known equipment.

Therefore, a second aspect of the present invention provides a stator for an electrical machine, the stator comprising at least one radial channel for ducting of cooling air, the channel having an entrance at or substantially near the rim of the stator and an exit at or substantially near the centre of the stator.

We may also claim an axial field electrical machine comprising at least one stator according to the second aspect of the present invention.

Although it is preferred that the at least one radial channel should take a substantially straight path from the rim of the stator to its centre, that is not absolutely necessary. It could equally take a curved or meandering path, provided that the general direction is radial, from the rim to the centre.

The second aspect of the present invention is particularly suited to stators provided with electrical windings, for example as described hereinbelow.

Preferably, the at least one radial channel is arranged such that leaving the channel exit, cooling air provided at the rim of the electrical machine is conveyed away via a channel provided through the centre of the electrical machine. The cooling air flow is produced via the pressure differential from inlet to outlet.

This cooling air flow may be provided by a variety of means such as: a) external compressor driven by an external drive; or b) air compressor for cooling air mounted on the electrical machine's shaft.

Additionally, the following two means are particularly attractive when the electrical machine is connected to a turbine engine: c)air flow induced by a pressure drop at engine/compressor intake; or d)air flow induced by introducing a venturi on compressor exit.

Another method is to run the electrical machine' s rotating components (rotors, keeper discs, etc) in vacuum, cooling the stators internally. In this case the cooling medium may be oil and is not restricted to air. This method has the advantage of reducing windage (air friction) losses.

The three phase stator windings in the generator according to the aforementioned patent specification are standard wave windings as conventionally used in electrical machines. That is to say, the respective windings for each phase are in separate overlapping planes. However, we have also devised a means of reducing the thickness of stators by forming the windings in a different way, thereby aiding miniaturisation of such electrical machines.

Thus, a third aspect of the present invention provides a stator for an electrical machine, the stator comprising a plurality of windings for various phases of electric current, the windings being substantially in the same plane.

We may also claim an axial field electrical machine comprising at least one stator as hereinbefore defined in the preceding paragraph.

The generator according to the aforementioned patent specification also utilises a carbon fibre-reinforced hoop for retaining magnets on a rotor thereof.

According to a fourth aspect of the present invention there is provided a retaining hoop for magnets of a rotor, the hoop being formed from reinforced carbon fibres, wherein the fibres extend around the hoop at a non-normal angle relative to the axis of symmetry of the hoop.

We may also claim a rotor comprising such a hoop as hereinbefore defined in the preceding paragraph and also we may claim an axial field electrical machine comprising at least one such rotor.

The present invention also encompasses any two or more aspects thereof in combination, or an axial field electrical machine comprising such a combination, as appropriate.

The present invention will now be explained in more detail by the following description of a preferred embodiment and with reference to the accompanying drawings, in which: - Figure 1 shows a known form of keeper disc; Figure 2 shows an alternator coupled to a gas turbine; Figure 3A and 3B respectively show left and right-hand keeper discs according to the first aspect of the invention; Figure 4 shows a stator according to the second aspect of the invention; Figure 5 shows the alternator of Figure 2 in more detail, illustrating how cooling air is obtained for the stator shown in Figure 4; Figure 6 shows a method of manufacturing a retention hoop according to the fourth aspect of the invention; and Figure 7 shows sleeving of the windings used in the stator shown in Figure 4.

As shown in Figure 2, a gas turbine 20 is arranged to drive an electrical generator in the form of an alternator 22 via a drive shaft 24. A first air compressor 26 is arranged to provide cooling air to the alternator whilst a second compressor 28 is provided for the turbine.

A plurality of rotors 30, 32, 34, 36, 38 are alternately interleaved with a plurality of stators 40, 42, 44, 46.

The rotors each comprise a plurality of radially spaced magnets 48, 50 etc each having a substantially right angled apex facing adjacent the drive shaft, as described in British Patent Specification GB 2 222 031 A.

The magnets are retained by respective hoops 52 etc.

The stators each comprise encapsulated 3-phase windings, connected to output coupling 54 to provide an electric current in known manner.

A closed path for the magnetic field resulting from the magnets is maintained by keeper discs 56, 58 at either end of the electrical machine.

As shown in more detail in Figures 3A and 3B, and in accordance with the first aspect of the present invention, the left hand keeper disk 56 and the right hand keeper disk 58 respectively comprise an inner collar 60, 62 and concentric therewith, an outer collar 64, 66. The inner and outer collars are all made of high strength steel.

The inner collars are provided with recesses 68, 70, 72, 74 enabling them to be secured to the central shaft.

The outer collars have respective cylindrical portions 76, 78 and are provided with respective annular flanges 80, 82 on their ends which are for facing the rotors.

Referring now to Figure 4, there is shown one of the stators 42 between respective rotors 32, 34. The rotors are provided with segmented magnets 82, 84 retained on respective steel bosses 86, 88 by carbon fibre containment hoops 92, 94. Respective duralumin rings 96, 98 are sandwiched between the hoops and the magnets. Air gaps 102, 104 are defined between the stator and the rotors, the latter being freely rotatable about the stator as the central drive shaft 24.

The stator is provided with radial channels, one of these 106 being shown in Figure 4. The radial channel 106 has an inlet port 108 at its rim 110. The stator is substantially circular and is provided with a central hole 112 into which emerges an exit port 114 of the radial channel, remote from the inlet port 108.

A stator holder 116 holds the stator firmly in position. The stator holder is provided with a conduit 118 allowing cooling air 120 to enter the radial channel of the stator via the inlet port. After travelling through the stator, the air is drawn through a perforated distance piece 122 into the hollow core 124 of the drive shaft by virtue of an air current 126.

Cooling of the electrical machine is provided by the air pressure differential from inlet to outlet (regardless of the source). The passage in the stator, 114, and the passages between the rotor/stator components, 102 and 104, carry the cooling air as shown in Fig. 4. The exit port is denoted by reference numeral 126. This provides a further advantage over the cooling system of the operator described in GB 2 222 031 A. Figure 5 shows two of the means mentioned hereinbefore for providing the required pressure differential.

One such means comprises an air compressor 26 mounted on the shaft of the electrical machine. The second means is indicated in Figure 5 by reference numeral 130, which is a tapping off the engine/compressor air inlet duct 132. The air tapping is assisted by a venturi arrangement in the compressor inlet ducting.

Referring again to Figure 4, the stator comprises three-phase coil winding 134 encapsulated in a potting compound which may be an epoxy resin. All three windings are in substantially the same plane, relative to the axis of symmetry of the electrical machine.

The windings are manufactured on a purpose-built jig consisting of pins, around which the coil wire strands are wound. Bundles 135 of coil wire strands are sleeved in glass fibre tubing 136 (see Fig. 7) to facilitate the winding process and gives protection during compaction.

When winding commences, the second 'turn' of the wave winding fits inside the first 'turn' rather than overlapping it, as with the conventional wave winding.

The result is a hybrid wave/concentric winding which is more compact than the conventional wave winding, especially at the inner periphery where space is critical. The use of a winding jig avoids the need for formers which would have to be left in the winding, increasing its thickness. Moreover, it means the winding process is two-dimensional.

The retention hoops 52, 92, 94 etc are made by filament winding of carbon fibres wound in the hoop as well as cross-hoop direction. The proportion of cross-hoop windings to the linear hoop windings depends on the strength characteristics required.

The method of hoop manufacture is shown in Figure 6. A drum 138 of dry tow (carbon fibre bundles) 140 delivers a fibre strant 142 to an epoxy resin bath 144 and hence, via a variable tension loader 145 to a heated rotating drum 146. The strands are angled by virtue of internal movement of the bath and loader assembly, as indicated by the arrows A, B. The resin is cured by means of a heat lamp 148.

In the resulting hoop 150, the majority of windings are in a substantially circumferential direction with additional windings nearly axial. The optimum number of windings is dependent upon the strength required.

Claims

1. A stator for an electrical machine, the stator comprising at least one radial channel for ducting of cooling air, the channel having an entrance at or substantially near the rim of the stator and an exit at or substantially near the centre of the stator.

2. A stator according to claim 1, wherein the at least one radial channel -takes a substantially straight path from the rim of the stator to its centre.

3. A stator according to claim 1, wherein the at least one radial channel takes a curved or meandering path from the rim of the stator to its centre, the general direction of the channel being radial from the rim to the centre.

4. An axial field electrical machine comprising at least one stator according to any preceding claim.

5. An axial field electrical machine according to claim 4, wherein the at least one radial channel is arranged such that on leaving the channel exit, cooling air provided at the rim of the electrical machine is conveyed away via a channel provided through the centre of the machine.

6. An axial field electrical machine according to claim 5, wherein the cooling air flow is arranged to be produced via a pressure differential from inlet to outlet of the channel through the machine.

7. An axial field electrical machine according to claim 6, wherein the pressure differential is arranged to be produced by an external compressor driven by an external drive.

8. An axial field electrical machine according to claim 6, wherein the pressure differential is arranged to be produced by an air compressor for cooling air, the compressor being mounted on a central motor shaft of the electrical machine.

9. An axial field electrical machine according to claim 6, wherein the pressure differential is arranged to be produced by an air flow induced by a pressure drop at an engine or compressor air intake.

10. An axial field electrical machine according to claim 6, wherein the pressure differential is arranged to be produced by an air flow induced by introducing a venturi on a compressor exit.

11. An axial field electrical machine according to any of claims 4-10, wherein the stator is provided with a plurality of windings for various phases of electric current, the windings being substantially in the same plane.

12. An axial field electrical machine according to any of claims 4-11, further comprising at least one rotor having a carbon fibre-reinforced hoop for retaining magnets on said at least one rotor.

13. An axial field electrical machine according to claim 12, wherein the hoop or hoops of the at least one rotor is/are formed from reinforced carbon fibres, wherein the fibres extend around the hoop or hoops at a non-normal angle relative to the axis of symmetry of the hoop or hoops.

14. A stator for an axial field electrical machine, the stator being substantially as hereinbefore described with reference to any one of Figures 2, 4, 5, 6 and 7 of the accompanying drawings.

15. A turbine engine comprising an axial field electrical machine according to any of claims 4-14.