GB2025706A

GB2025706A - Cooling superconducting rotor windings of electricmachines

Info

Publication number: GB2025706A
Application number: GB7923600A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1970-02-18
Filing date: 1979-07-06
Publication date: 1980-01-23
Also published as: DE2107535B2; GB2025706B; DE2107535A1; DE2107535C3

Description

SPECIFICATION An electrical machine rotor with a superconductive winding and a cooling arrangement therefor This invention relates to rotors for electrical machines, which rotors having superconductive windings and cooling arrangements therefor.

In the dissertion by A. Bejan :"Improved thermal Design of the Cryogenic Cooling System for a Superconducting Synchronous Generator", PH. D.Thesis, Massachusetts Institute of Technology (USA), December 1974, page 151, there is disclosed a cooling arrangement for the rotor of an electric machine comprising a superconducting exciter winding and a co-rotating mixing chamber which is situated close to the rotor axis and contains in the operating condition a phase mixture of a coolant supplied from externally, and from which liquid coolant is extracted for cooling the exciter winding and into which coolant passed through the exciter winding is returned, and from which gaseous coolant is discharged externally. In this cooling arrangement, a coolant taken from an external coolant supply unit and expanded in a Joulethomson valve is fed through a coolant feed duct to the co-rotating mixing chamber. Owing to the JouleThomson expansion, there is then situated in the mixing chamber a two-phase mixture consisting of liquid and gaseous coolant. In the operating state during rotation, the phases of this two-phase mixture are separated by the centrifugal forces acting on them, and the coolant vapour accumulates in regions of the mixing chamber which are close to the rotor axis and the coolant liquid in regions which are remote from the axis. A stream of coolant comprising liquid coolant is fed from the mixing chamber through radially disposed supply ducts, for example at one end face of the rotor, to the exciter winding.

The coolant then flows through the exciter winding in a direction parallel to the axis of rotation and is thereafter returned into the central mixing chamber through a further, radially directed discharge duct.

The quantity of heat thus absorbed brings about a temperature increase and a partial evaporation of the coolant. The necessary pumping action for developing the flow of coolant through the exciter winding is produced by a self-pumping effect due to density differences, the isentropically compressed coolant carried radially outwards in the supply ducts being accelerated owing to the centrifugal forces, so that it can enter the exciter winding. Since it is heated owing to the power losses occurring therein, or is heated by heat transmission from the outside, its density decreases. Hence, hydrostatic pressure difference is set up between the supply and return ducts. There is thereby developed along the winding between the points at which the coolant is fed in and escapes a pressure gradient which results in a convection flow and causes the coolant to return into the mixing chamber close to the axis by way of the discharge ducts (see"Cryogenics", July 1977, pages 429 to 433, and German Offenlegungsschrift 25 30 100).

However, difficulties arise in such a cooling arrangement during the cooling-down phase through which the exciter winding passes, because during this phase the rotor is not rotating or is rotating only at low speed, so that substantially no centrifugal forces are present which might force the coolant into the still-warm exciter winding. When the coolant enters the exciter winding, it becomes heated and therefore evaporates. However, the gaseous coolant then forms a buffer and prevents further flow of the still relatively cool coolant.

Therefor, additional means are necessary for ensuring uniform cooling of the exciter winding during the cooling-down phase. for this purpose, there is usually provided a forced cooling, but in this kind of cooling, difficulties are encountered in subsequently utilising the aforesaid self-pumping effect after the cooling-down has taken place.

The present invention aims to provide a generally improved arrangement for the superconducting winding of a rotor. More particularly, in preferred embodiments of the invention, it is intended to utilise the aforesaid self-pumping effect during the state of operation in the cooling arrangement and nevertheless to be able to effect during the coolingdown phase a cooling of the exciter winding which is simple from the viewpoint of flow technology and fully effective in respect of heat exchange.

According to the present invention, there is provided a rotor for an electrical machine, the rotor having a superconductive winding and a cooling arrangement therefor which comprises: a mixing chamber which rotates with the rotor in use, radially inwardly of the winding, and is arranged to contain a phase mixture of a coolant ; a coolant distribution system which rotates with the rotor in use, radially outwardly of the winding; at least one coolant supply duct for supplying coolant to said distribution system from externally of the rotor; coolant channels extending through the winding for the passage of coolant in thermal contact with the winding, from said distribution system to the mixing chamber; coolant communication ducts for supplying liquid coolant, out of contact with the winding, from said mixing chamber to said distribution system; and at least one coolant discharge duct for the discharge of gaseous coolant from the mixing chamber to externally of the rotor.

We have found embodiments of the invention to have the advantage that the coolant always flows from the outside towards the inside through the parts of the winding during a cooling-down phase, while absorbing heat, independently of the speed.

Therefore, flow instabilities cannot occur, because the direction of flow imparted to the coolant is always identical with the direction of decreasing density from outside towards the inside. The coolant heated in the parts of the winding then flows radially inwards into the mixing chamber by way of the cooling channels provided. Consequently, there is not formed a buffer of warmer coolant gas which prevents further flow of cooler coolant in the winding during the cooling-down phase. The discharge of the gaseous coolant from the mixing chamber takes

place by pumping in the operating state. Consequently, the temperature rise set up in a radial direction due to the compression of the coolant resulting from centrifugal forces can be balanced out. In this way, temperatures of, for example, less than 4.2 K can be reached overall in the winding if the coolant is fed into the rotor at a corresponding low pressure and at a corresponding temperature. In this case, a Joule-Thomson expansion in the supply duct is generally provided for.

If, on the other hand, normally boiling coolant is to be fed in a pressures around 105 Pa, the coolant supply duct may include a portion which is in the form of a heat exchanger, and in which there is preferably effected a heat exchange with coolant taken from the mixing chamber. More particularly, the duct portion in the form of a heat exchanger may be disposed at least approximately radially in relation to the axis of the rotor. By these means, coolant introduced into the rotor at a point close to the axis and flowing radially from the inside towards the outside into coolant distribution system is prevented from being adiabatically compressed and thus correspondingly heated. Therefore, a Joule-thomson expansion in this part of the duct for compensating for such heating can generally be dispensed with, and boiling helium can be introduced into the coolant feed duct under normal pressure or only slightly elevated super-atmospheric pressure.

To assist in understanding the invention and to show how it may be carried out, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic longitudinal sectional view through a part of a rotor with a superconducting winding and a cooling arrangement therefor; and Figure 2 is a diagrammatic transverse sectional view of part of the rotor.

Figure 1 shows the guiding of a coolant through a part of a rotor of the electric machine, more especially a turbo-generator, at which the rotor adjoins an overhang. The rotor parts not particularly shown in the figure may correspond, for example, to parts of a machine disclosed in German Offenelgungsschrift 24 39 719. The figure shows only the upper half of the rotor. The rotating parts of the machine which are to be cooled are disposed in a vacuum in order thus to limit the introduction of heat from the outside to these parts. The rotor body 3, mounted on an axis 2, of the machine is therefore surrounded by evacuated spaces 4,5 and 6 which are situated within a hollow cylindrical housing part 8 and within a disc-form end housing part 9 of a co-rotating vacuum housing which is at or above room temperature. The disc-form end portion 9 of the vacuum housing at the same time forms part of a connecting head (not particularly shown) of the rotor. The rotor body 3 has formed in its outside surface slots 11 into which turns of an exciter winding 12 are introduced.

These portions of the exciter winding are held inside the slots 11 by wedges 13. The conductors of the winding 12 contain superconducting material, and therefore helium is provided as the coolant. The directions of flow of the individual streams of coolant set up when the machine is in the operating condition are indicated by arrow.

A coolant denoted by Ao, which is preferably normally boiling helium under a pressure of 1. 2 x 105 Pa at a temperature of 4.4 K, is taken from a coolant supply arrangement (not shown in the figure) and introduced into the rotor by way of a helium coupling at the connecting head (not particularly shown). With the said of such a helium couPLING (which is disclosed, for example, in the publication"Siemens Forschungs-und Entwicklungsberichte", volume 5 (1976), No. 1, page 13, the coolant Ao is transferred from fixedly located machine parts to rotating parts. The coolant Ao is then introduced by way of a fixed supply duct 15 extending into the rotor in the neighbourhood of its axis, into a co-rotating fore-chamber 18 situated on the end face 17 of the rotor body 3. The supply duct may be a double tube concentric with the axis 2, with a coolant guiding space of annular cross-section, or it may consist of a number of individual tubes which are commonly mounted on the surrounding surface of a cylinder. Connected to the fore-chamber is at least one coolant supply duct 20 which extends radially in relation to the rotor axis 2 and through which the coolant is introduced into a coolant distribution system 21 provided on the outer periphery of the exciter winding 12. From this coolant distribution system, which consists of a network of interconnected coolant ducts extending with their axes parallel to one another and in the peripheral direction of the rotor, the coolant passes by way of inlet apertures 23 in the outer slot edge into cooling channels 24 which extend substantially radially in relation to the rotor axis 2 through. those parts of the exciter winding 12 which are disposed in the slots 11. The coolant which reaches the slot base is then introduced by way of radial coolant tubes 25 into a mixing chamber 27 situated in the interior of the rotor.

In the operating condition, there is situated in the chamber 27 a two-phase mixture of liquid coolant A, abnd gaseous coolant 8,. On rotation, a phase separation takes place under the influence of centrifugal forces, so that the heavier liquid coolant A, accumulates concentrically around the gaseous coolant B, held along the rotor axis 2.

For cooling the superconducting exciter winding 12, there is provided a circuit in which a selfpumping effect is utilised. For this purpose, there are provided, between the coolant distribution system 21 on the external periphery of the exciter winding 12 and the mixing chamber 27, radially disposed coolant communication ducts 29 which extend in tooth-like intermediate portions 30 of the rotorDody 3, between which portions are defined the slots 11.

By way of these communication ducts 29, cold coolant A, is conveyed from the mixing chamber 27 into the coolant distribution system 21, from which it passes into the coolant channels 24 in the superconducting winding. Owing to the introduction of heat from the outside, the coolant in the coolant ducts of the coolant distribution system 21 is heated. In addition, it is heated by the power losses set up in

the winding parts of the exciter winding 12. The resultant reduction of the density of the coolant brings about a reduction of the hydrstatic pressure in relation to the hydrostatic pressure of the cold coolant in the communication ducts 29. Owing to this pressure reduction, the coolant flows back radially inwardly into the mixing chamber 27 by way of the channels 24 and the tubes 25.

The quantity of evaporated coolant flowing into the mixing chamber 27 in the operating condition is discharged by way of a waste gas tube 32. Here again, a self-pumping effect is utilised to suck away the gaseous coolant denoted by Bz. The coolant extracted by way of the tube 32 at a point close to the axis becomes heated while being utilised for cooling a connecting piece 34 between the co-rotating, warm vacuum housing 8 and 9 and colder parts of the rotor. Such a colder part of the rotor may be, for example, a radiation shield 36 acting as an electromagnetic damping means. Therefore, there is provided at the connecting piece 34 remote from the axis a heat exchanger 37 in which the coolant 82 is brought approximately to the temperature of the vacuum housing 8 and 9. The coolant waste gas thus heated, which is denoted by Bg, can then be led centrally out of the rotor at the connecting head (not particularly shown in the figure) and fed to a refrigerator. Owing to the self-pumping effect for exhausting the coolant gas 82, Bg, a negative pressure is then advantageously set up in the waste gas tube 32, which is under 105 Pa.

In order to prevent heating of the coolant introduced into the coolant distribution system 21 by way of the coolant supply duct 20 due to an adiabatic compression, the radially extending part of this duct is made in the form of a heat exchanger 39 through which liquid coolant A2 taken from the mixing chamber 27 is fed by way of a duct 40 to the part thereof remote from the axis and reintroduced into the mixing chamber at the part close to the axis. The flow through the heat exchanger 39 is also effected on the basis of a self-pumping effect.

During the cooling-down phase, during which the rotor is not rotating or is rotating only at low speed, and consequently no high centrifugal forces are acting on the coolant and the exciter winding 12 is initially still in the warm condition, the coolant Ao is first introduced by way of the supply duct 20 into the coolant distribution system 21 situated on the external edge of the winding. There is thus formed between those parts of the rotor body 3which are ctose to the axis and the distribution system 21 remote from the axis a high temperature gradient.

Due to this temperature gradient, the coolant flows from the distribution system 21 through the channels 24,25 and also through the communication ducts 29 into the mixing chamber 27, in a direction conforming with the pressure difference impressed from externally.

When the mixing chamber 27 has been partially filled with liquid coolant A, and the rotor is brought to its required speed, the operating condition of the machine is automatically established, wherein the cooling of the exciter winding 12 is effected by the two described self-pumping effects. Only the evapo- rated parts odf the cooiant Bz, 63 which are discharged externally are then regenerated by liquid coolant Ao supplied from externally.

There are also indicated in the figure, at the end face 9 of the vacuum housing or connecting head of the rotor, externally guided, waste-gas-cooled current supply conductors 42 for the exciter winding 12.

In the figure, it has been assumed that a single coolant supply duct 20 is situated at one winding head. However, it is equally possible to provide a further supply duct also on the opposite end face of the rotor body, in which case there is provided an axis-parallel communication duct extending from the fore-chamber 18 through the mixing chamber 27.

In addition, there may be provided, instead of the coolant supply duct situated on at least one of the end faces of the rotor, or in addition to one or more of such ducts, corresponding supply ducts which are situated at points between the end faces of the rotor body.

In the embodiment illustrated in the figure, the cooling channels 24,25 extend at least approximately radially in relation to the rotor axis 2 through the exciter winding 12, along the boundary faces between the turns of the exciter winding 12 and the slot teeth 30. Alternatively, there may be provided radial cooling channels which are not situated on the edges of the individual winding assemblies, but extend centrally through these assemblies.

Figure 2 is a cross-section through the rotor according to Figure 1, taken ontheiine ! i-ii of Figure 1. There are situated in this plane a number of radial coolant communication ducts 29 through which liquid coolant A, is passed from the central mixing chamber 27 into the coolant distribution system 21 situated on the external periphery of the exciter winding. A number of distribution ducts 44 extend in an axis-parallel direction from the coolant distribution system 21 and the coolant communication dusts 29 have portions 46 which project somewhat beyond the base portions 47 of the ducts 44. The coolant ducts 29 are sealed at their outer ends 48 and have lateral outlet apertures 49 through which the coolant can pass into the ducts 44. By this means, it is ensured that only the cold coolant which has already issued from the apertures 49 is heated by the heat emanating from warmer parts of the rotor and consequently flows radially inwards in the direction of the inlet apertures 23 at the slots 11 and not within the communication ducts 29. In this way, a definitive direction of flow from the outside towards the inside through the exciter winding can be ensured.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

1. A rotor for an electrical machine, the rotor having a superconductive winding and a cooling arrangement therefor which comprises: a mixing chamber which rotates with the rotor in use, radially inwardly of the winding, and is arranged to contain a phase mixture of a coolant ; a coolant distribution system which rotates with the rotor in use, radially outwardly of the winding; at least one coolant supply duct for supplying coolant to said distribution system from externally of

the rotor; coolant channels extending through the winding forthe passage of coolant, in thermal contact with the winding, from said distribution system to the mixing chamber; coolant communication ducts for supplying liquid coolant, out of contact with the winding, from said mixing chamber to said distribution system; and at least one coolant discharge duct forthe discharge of gaseous coolant from the mixing chamber to externally of the rotor.

2. A rotor according to claim 1, wherein said coolant channels extend generally radially of the rotor.

3. A rotor according to claim 1 or 2, wherein at least part of said coolant supply duct is arranged as a heat exchanger.

4. A rotor according to claim 3, wherein said heat exchanger is arranged to receive coolant from the mixing chamber.

5. A rotor according to claim 3 or 4, wherein said part of said coolant supply duct extends generally radially of the rotor.

6. A rotor according to any preceding claim, wherein at least one coolant supply duct extends over or adjacent an end face of the rotor.

7. A rotor according to any preceding claim, wherein at least one coolant supply duct extends through a portion of the rotor intermediate the end faces thereof.

8. A rotor according to any preceding claim, wherein the coolant distribution system comprises interconnected coolant distribution ducts which extend along the periphery of the rotor substantially parallel to the rotor axis.

9. A rotor according to any preceding claim, wherein the rotor body is formed with winding slots defined between tooth-like intermediate body portions, and said coolant channels are disposed within the slots on the boundary faces between the winding portions and said intermediate body portions.

10. A rotor according to any preceding claim, wherein the rotor body is formed with winding slots defined between tooth-like intermediate body portions, and said coolant communication ducts extend through said intermediate body portions.

11. A rotor for an electrical machine, the rotor being substantially as hereinbefore described with reference to the accompanying drawings.

12. An electrical machine having a rotor according to any preceding claim.

13. An electrical machine according to claim 12, wherein coolant is fed through said supply duct into the rotor, at a pressure of at least 105 Pa.

14. An electrical machine according to claim 13, wherein normally boiling helium is fed through said supply duct, into the rotor, at a pressure of substantially 1.2 xy 105 Pa.

15. An electrical machine according to claim 12, 13 or 14, wherein the extraction of liquid coolant from the mixing chamber to said distribution system and the return of the coolant from said distribution system, via the winding, to the mixing chamber take place by a self-pumping effect.

16. An electrical machine according to claim 12, 13,14 or 15, wherein the discharge of gaseous coolant from the mixing chamber to externally of the rotor takes place by a self-pumping effect.

17. An electrical machine according to any one of claims 12 to 16 as appendant to claim 4 or 5, wherein the flow of coolant through said heat exchanger takes place by a self-pumping effect.

18. An electrical machine according to any one of claims 12 to 17, wherein the gaseous coolant is discharged from the rotor at a pressure below 105 Pa.