Patent Description:
It is known to produce laminated stators for electrical machines by pressing stacks of annular laminations together. The laminations are typically formed from sheets of electrical grade steel which are usually provided with insulating coatings. Each annular lamination may be formed of a single member or may itself be of segmental construction with the segments abutted against each other e.g., at generally radially extending edges.

The laminations can define axially extending teeth that define therebetween axially extending slots for receiving the conductors of a stator winding. The teeth are circumferentially spaced around the stator surface and carry the magnetic flux that links from the stator to the rotor across the airgap. The conductors are electrically insulated from the teeth.

One of the problems faced by designers of electrical machines is the heat that is created as a result of the various losses, e.g., resistive losses in the stator winding, eddy current losses in the laminations etc. The problem of heat can be particularly acute when trying to design an electrical machine with high power density (i.e., power generated per unit volume). The maximum power output of an electrical machine, for a given amount of conductive material (e.g., copper for the stator winding conductors and iron for the magnetic circuit) is limited by the efficiency of the cooling because, if heat is not removed efficiently, the temperature of the electrical machine will increase to a point that can cause the insulation material or some other part of the machine to fail.

Electrical machines can be cooled in a variety of different ways, e.g., direct liquid or air cooling, cooling by conduction to the laminations which are in turn cooled by direct cooling or an external water jacket. However, all of these known ways of cooling suffer from some disadvantages in terms of their available power density, thermal transfer efficiency, mechanical complexity, or noise. It is known to pass cooling liquid through axially extending cooling passageways in the stator core or even in the stator teeth. The axially extending cooling passageways can be defined by openings in the individual laminations that define axially extending voids when the laminations are stacked together. It is known to position a tube within each void to contain the cooling liquid and prevent any risk of the cooling liquid leaking between the laminations. But improved cooling can be obtained if the tube is omitted such that the cooling passageways are defined by the axially extending voids and the cooling liquid is in direct contact with the lamination stack. When the stacked laminations are compressed and subjected to appropriate treatment including vacuum pressure impregnation (VPI) and curing, each cooling passageway is preferably made fluid tight. However, it has not always been possible to completely eliminate the risk that the cooling liquid will leak out between adjacent laminations.

<CIT> discloses cooling of an electrical machine having a stator and a cooling passageway formed in or adjacent the stator.

It is also known to pass cooling liquid through cooling passageways in industrial transformers, and in particular through the laminated transformer core - see <CIT>, for example.

It is also known to cool electronic equipment using a negative relative pressure cooling system. For example, <CIT> uses a negative relative pressure cooling system with a pressure of <NUM>-<NUM> kPa to cool a computer system and computer components.

Similar cooling systems for cooling electrical apparatus, integrated circuit (IC) chips, computer racks etc. are described in <CIT>, <CIT>, <CIT>, and <CIT>.

The present invention provides a combination of an electrical machine (e.g., a motor or generator) comprising a stator and a cooling passageway formed in or adjacent to to the stator and adapted to cool the stator, and a negative relative pressure cooling system connected to the cooling passageway and adapted to move cooling liquid through the cooling passageway at a pressure less than the ambient atmospheric pressure (e.g., less than about 100kPa).

The cooling system can comprise a source of cooling liquid.

The cooling passageway can comprise an inlet (i.e., a first or upstream end of the cooling passage) connected to the source of cooling liquid and an outlet (i.e., a second or downstream end of the cooling passage) for discharging the cooling liquid and any entrained air that enters the cooling passageway. As will be explained in more detail below, if the cooling passageway does not contain any leaks, no air will be entrained in the cooling liquid. But in the event that there is a leak in the cooling passageway, because the cooling liquid is at a pressure that is less than ambient atmospheric pressure, air is effectively drawn into the cooling passageway. Put another way, instead of cooling liquid coming out of the hole or gap in the cooling passageway that would normally constitute the leak, air goes in to the cooling passageway and becomes entrained in the cooling liquid. It will be readily understood that this provides a significant benefit because of the serious damage that can be caused if the cooling liquid leaks out of the cooling passageway and into the electrical machine.

The cooling system can further comprise a liquid-actuated aspirator. In a typical liquid-actuated aspirator, a motive liquid flows through a tube that narrows and then expands in cross-sectional area along the flow direction. When the tube narrows, the pressure of the motive liquid decreases, and the velocity of the motive liquid must increase. Where the tube narrows, a vacuum is created as a result of the Venturi effect, which vacuum can be used to draw (or "pull") the cooling liquid from the source towards the liquid-actuated aspirator through the cooling passageway. The liquid-actuated aspirator can include a first (or motive) inlet connected to a source of motive liquid, a second (or suction) inlet connected to the outlet of the cooling passageway for receiving the cooling liquid and any entrained air, and an outlet for discharging the motive liquid, the cooling liquid and any entrained air. The first inlet and the outlet are normally aligned along an axis of the liquid-actuated aspirator. The axis of the second inlet is normally aligned substantially perpendicular to the axis of the liquid-actuated aspirator. The second inlet is also normally aligned with the part of the liquid-actuated aspirator where the tube narrows and then expands (see <FIG>). The flow of the motive liquid through the liquid-actuated aspirator, from the first inlet to the outlet, produces a vacuum that draws the cooling liquid and any entrained air towards the second inlet where it mixes with the motive liquid. The motive liquid, cooling liquid and any entrained air is then discharged from the outlet.

The present invention can be provided as a closed-loop system that comprises the cooling passageway and the components of the cooling system that are connected together by suitable tubing or pipework. The cooling system is also connected to the cooling passageway by suitable tubing or pipework. In particular, where respective inlets and outlets are described herein as being connected, it will be understood that this does not mean that they must be directly connected, but that they will normally be indirectly connected by means of suitable tubing or pipework. In such a closed-loop system, the source of the cooling liquid can be a vent tank. The vent tank can comprise a first outlet connected to the inlet of the cooling passageway, and an inlet connected to the outlet of the liquid-actuated aspirator for receiving the motive liquid, the cooling liquid, and any entrained air. The vent tank can also be the source of the motive liquid. In other words, the same liquid can be used as both the motive liquid and the cooling liquid in the closed-loop system. After being discharged from the outlet of the liquid-actuated aspirator, the mixed motive liquid and the cooling liquid can be returned to the vent tank. The vent tank is adapted to vent or remove any entrained air.

The vent tank can be an open tank which allows any entrained air to be vented naturally to the environment, or any entrained air can be actively removed by a suitable removal process or removal equipment.

The vent tank can further comprise a second outlet. The cooling system can further comprise a pump connected to the second outlet of the vent tank and to the first inlet of the liquid-actuated aspirator. Operating the pump will therefore supply motive liquid from the vent tank to the first inlet of the liquid-actuated aspirator. The strength of the vacuum created within the liquid-actuated aspirator typically depends on the flow rate of the motive liquid through the aspirator. Pump operation can therefore be used to control the vacuum that is created within the liquid-actuated aspirator and consequently the flow of cooling liquid from the vent tank to the second inlet of the liquid-actuated aspirator through the cooling passageway.

The closed-loop system can be considered in terms of:.

Although normally less desirable, the present invention can be provided as an open-loop system where the cooling liquid is drawn (or "pulled") from a source of cooling liquid and the motive liquid is provided to the liquid-actuated aspirator from the same or a separate source of motive liquid (e.g., by a pump) and where the mixed cooling liquid, motive liquid and any entrained air is simply discharged from the outlet of the liquid-actuated aspirator. Such an open-loop system might be possible, for example, if there is a reservoir, lake, river or the like from which a suitable liquid (e.g., water) can be drawn through the cooling passageway to provide cooling and from which it can be pumped to the liquid-actuated aspirator as the motive liquid. The liquid discharged from the outlet of the liquid-actuated aspirator can be disposed of in a safe manner rather than being returned to the reservoir, lake, river or the like.

The cooling system can further comprise filtration systems for filtering the cooling liquid and/or the motive liquid.

The electrical machine can comprise a plurality of cooling passageways, e.g., where the inlets and outlets of the cooling passageways are connected together by inlet and outlet manifolds, respectively. In this case, the negative relative pressure cooling system will be connected to the inlet and outlet manifolds. Other arrangements are possible. For example, where a plurality of cooling passageways are connected in series and are used to cool the same or different parts of the electrical machine.

The cooling passageway(s) can be formed in or adjacent to the stator and can be adapted to cool the stator. In one preferred arrangement, each cooling passageway can be an axially extending cooling passageway through the stator core or a stator tooth, for example. Each cooling passageway can extend along the full axial length of the stator or along just part of the stator. Preventing the leakage of cooling liquid can be particularly advantageous if the stator is a laminated stator - see above - and each cooling passageway is defined by an axially extending void in the lamination stack such that the cooling liquid is in direct contact with the lamination stack. When the stacked laminations are compressed and subjected to appropriate treatment including, for example, vacuum pressure impregnation (VPI) and curing, each cooling passageway is preferably made fluid tight. Any inlet and outlet manifolds may be also subjected to the VPI and curing process and provide a fluid tight seal around the end of each cooling passageway. But if there is a small leak between individual laminations or within a manifold, for example, air will be drawn into the axially extending void because the pressure is less than ambient atmospheric pressure and will be entrained in the cooling liquid. The cooling liquid will not leak out of the cooling passageway and into the laminated stator.

The present invention further provides a method of cooling an electrical machine (e.g., a motor or generator) comprising a stator and a cooling passageway formed in or adjacent to the stator and adapted to cool the stator, the method comprising moving cooling liquid through the cooling passageway at a pressure less than the ambient atmospheric pressure.

The step of moving cooling liquid through the cooling passageway can comprise drawing cooling liquid from a source through the cooling passageway and discharging the cooling liquid and any entrained air that enters the cooling passageway from the cooling passageway. The cooling liquid is preferably drawn from the source using a vacuum created by a liquid-actuated aspirator. The source of the cooling liquid can be a vent tank which is adapted to vent any air that is entrained in the cooling liquid.

Any suitable cooling liquid can be used, including water.

Referring to <FIG>, a cooling passageway <NUM> is adapted to cool part of an electrical machine <NUM>. A negative relative pressure cooling system <NUM> is connected to the cooling passageway <NUM> as described in more detail below.

The cooling system <NUM> includes a vent tank <NUM> that contains water. The vent tank <NUM> includes an inlet 6a, a first outlet 6b and a second outlet 6c.

The cooling passageway <NUM> includes an inlet 2a that is connected to the first outlet 6b of the vent tank <NUM> by suitable tubing or pipework <NUM>.

The cooling system <NUM> includes an aspirator <NUM>. The aspirator <NUM> includes a first inlet 10a, a second inlet 10b and an outlet 10c.

The cooling passageway <NUM> includes an outlet 2b that is connected to the second inlet 10b of the aspirator <NUM> by suitable tubing or pipework <NUM>.

The cooling system <NUM> includes a pump <NUM>. The pump <NUM> includes an inlet 14a and an outlet 14b. The inlet 14a of the pump <NUM> is connected to the second outlet 6c of the vent tank <NUM> by suitable tubing or pipework <NUM>. The outlet 14b of the pump <NUM> is connected to the first inlet 10a of the aspirator <NUM> by suitable tubing or pipework <NUM>.

The outlet 10c of the aspirator <NUM> is connected to the inlet 6a of the vent tank <NUM> by suitable tubing or pipework <NUM>.

To provide cooling, the pump <NUM> is operated to pump water from the vent tank <NUM> to the first inlet 10a of the aspirator <NUM>. The first inlet 10a and the outlet 10c of the aspirator <NUM> are aligned along an axis of the aspirator. As the water - acting as a motive liquid - flows between the first inlet 10a and the outlet 10c it passes through a tube <NUM> which narrows and then expands in cross-sectional area along the flow direction. When the tube narrows, the pressure of the water decreases, and the velocity of the water must increase. Where the tube narrows, a vacuum is created as a result of the Venturi effect, which vacuum can be used to draw (or "pull") water from the vent tank <NUM> towards the aspirator <NUM> through the cooling passageway <NUM>. The second inlet 10b of the aspirator <NUM> is aligned substantially perpendicular to the axis of the aspirator and is positioned where the tube <NUM> narrows and then expands.

In <FIG>, the water that is pumped around a closed-loop motive circuit is indicated by bold arrows. The closed-loop motive circuit includes the vent tank <NUM>, which acts as a source of motive water, the pump <NUM> and the aspirator <NUM>. The water that is drawn through a closed-loop cooling circuit by the aspirator <NUM> is indicated by non-bold arrows. The closed-loop cooling circuit includes the vent tank <NUM>, which acts as a source of cooling water, the cooling passageway <NUM> and the aspirator <NUM>. As the water is circulated around the closed-loop cooling circuit, heat is transferred to it from the electrical machine <NUM> to provide cooling.

Within the aspirator <NUM>, the motive water and the cooling water are mixed, discharged from the outlet 10c and returned to the vent tank <NUM>.

The cooling water is drawn (or "pulled") through the cooling passageway <NUM> by the vacuum created within the aspirator <NUM> by the motive water flowing through the tube <NUM>. The cooling water in the closed-loop cooling circuit is therefore at a pressure that is less than the ambient atmospheric pressure (e.g., less than about 100kPa). If the cooling passageway <NUM> does not contain any leaks, no air will be entrained in the cooling water. But in the event that there is a leak in the cooling passageway <NUM>, because the cooling water is at a pressure that is less than ambient atmospheric pressure, air is effectively drawn into the cooling passageway <NUM>. In <FIG>, the dashed arrows indicate air entering the cooling passageway <NUM> and then becoming entrained in the cooling water. The leak could be the result of a small gap between two individual stator laminations, for example. Any entrained air is subsequently vented by the vent tank <NUM> as shown.

With reference to <FIG>, an electrical machine <NUM> includes a stator <NUM> and a rotor <NUM> separated by an airgap <NUM>. The rotor <NUM> is mounted on a shaft <NUM>.

The stator <NUM> includes a stator core <NUM>, a first manifold 36A, a second manifold 36B, a first compression plate 38A and a second compression plate 38B.

The stator core <NUM> is formed from annular stacked laminations. The radially inner surface of the stator core <NUM> includes a plurality of axially extending stator slots defined between stator teeth. The conductors of a stator winding are received in the stator slots. (Note that in <FIG> the stator winding has been omitted for clarity.

Claim 1:
Combination of:
an electrical machine (<NUM>) comprising a stator (<NUM>) and a cooling passageway (<NUM>) formed in or adjacent to the stator (<NUM>) and adapted to cool the stator (<NUM>), and
a negative relative pressure cooling system (<NUM>) connected to the cooling passageway (<NUM>) and adapted to move cooling liquid through the cooling passageway (<NUM>) at a pressure less than the ambient atmospheric pressure.