Field of the invention The present invention generally finds application in the field of energy conversion devices and particularly relates to a rotating electric machine.

Background art Rotating electric machines are known to be often use to provide a mechanical torque to operating machines or, when operating as generators, to convert an external torque into electrical energy.

Nevertheless, during operation electric machines have non-negligible energy losses, caused by electrical and magnetic dissipation, mainly concentrated in the stator windings and the rotor core.

These losses turn part of the electrical energy that is used or generated by the electric machine into heat, which is developed by Joule effect in the stator and rotor.

The heat so generated must be appropriately dissipated out of the case of the electric machine to maintain the temperatures in the cavity of the stator below predetermined maximum values, which are non-destructive for the electric and mechanical connections contained therein.

A simple, cost-effective method for dissipating such heat consists in fixing one or more cooling fans to the driven shaft of the rotor, which fans are adapted to cause forced circulation of an air flow in the gap between the rotor and the stator. Nevertheless, this method does not allow optimal cooling of all the internal parts of the electric machine and allows a relatively low maximum heat dissipation, particularly in medium-to-high power electric machines. In an attempt to at least partially obviate the above drawbacks, electric machines have been developed in which the heat generated in the stator can be exchanged with a cooling fluid that circulates in a conduit formed in the case. The conduit in the case is in fluid connection with a closed-loop cooling circuit external to the electric machine, which has heat exchange means for cooling the cooling fluid.

Furthermore, at least one pump is associated with the cooling circuit, to ensure continuous circulation of the cooling fluid in the conduit formed on the case.

A first drawback of this prior art solution is that an electric machine comprising an external cooling circuit has a very large size, demanding space requirements and a heavy weight, which may hinder its use in relatively small spaces or in areas having a limited load-carrying capacity.

An additional drawback of this solution is that the case of the electric machine needs a particularly accurate design, to prevent any intentional or accidental contact between the cooling fluid and the windings in the stator.

Furthermore, the outer cooling circuit and the conduit of the case must be periodically checked, to remove any obstructions or sediments that might impede or block circulation of the cooling fluid.

Also, during operation of the pump and the cooling means, a relatively loud noise is produced, which adds to the noise produced during operation of the electric machine.

The overall sound emission of the electric machine during operation is particularly high, especially for high-power electric machines and sound- absorbing devices are required to meet the relevant standards.

Disclosure of the invention The object of the present invention is to overcome the above drawbacks, by providing a rotating electric machine that is highly efficient and relatively cost- effective.

A particular object of the invention is to provide a rotating electric machine that has a high ability to dissipate the heat resulting from its operation, while maintaining relatively small size, low space requirements and light weight.

A further particular object of the invention is to provide a rotating electric machine that has a particularly simple and easy-to-fabricate construction of the case and the other parts.

A further particular object of the invention is to provide a rotating electric machine that has a relatively high durability, and reduced periodic check requirements and maintenance costs as compared with electric machines having the same power, which have external cooling systems.

Yet another particular object of the invention is to provide a rotating electric machine that is relatively noiseless and reduces the use of sound-absorbing devices.

These and other objects, as better explained hereinafter, are fulfilled by an electric machine which is adapted to be connected to an electric network as defined in claim 1 , comprising an outer casing, a substantially tubular stator housed inside the casing and having a longitudinal axis, a rotor inserted in the stator with a predetermined interspace and having at least one substantially longitudinal passage extending lengthwise thereof, and air cooling means associated with the rotor and adapted to promote forced circulation of an air flow into the stator and the rotor.

The electric machine is characterized in that said cooling means comprise an annular peripheral cavity defined between the stator and the case, the cavity being in fluid connection with the longitudinal passage and the air gap to define a closed-loop path therewith for circulation of the cooling air flow for cooling the stator and the rotor. With this particular configuration, an electric machine may be provided that has particularly high electrical performances, allows particularly effective dissipation of the generated heat, and ensures a particularly high durability, while having lower maintenance requirements. Advantageous embodiments of the invention are defined in accordance with the dependent claims.

Brief description of the drawings Further characteristics and advantages of the invention will become more apparent upon reading of the detailed description of a few preferred, non exclusive embodiments of a generator having a reluctance synchronous electric machine, which are described as non limiting examples with the help of the accompanying drawings in which :

FIG. 1 is a schematized perspective view of an electric machine of the invention, when used in a wind generator; FIG. 2 is a front broken away view of the electric machine of Fig. 1 ; FIG. 3 is an enlarged view of a detail of Fig. 2;

FIG. 4 is a perspective view of a detail of the electric machine of Fig.

1 ;

FIG. 5 is a sectional side view of an electric machine of the invention in a first configuration ;

FIG. 6 is a sectional side view of an electric machine of the invention in a second configuration ;

FIG. 7 is a sectional front view of an electric machine of the invention in a third configuration ;

FIG. 8 is a sectional side view of Fig. 7;

FIG. 9 is a schematized side view of a detail of a generator using an electric machine of the invention. Detailed description of a preferred embodiment

Referring to the above figures, the electric machine of the invention, generally referenced 7, may be used in a system for generating electric energy, generally referenced 1 , from fossil or chemical energy sources, or renewable energy sources, such as wind or water energy sources.

The figures show, as a non-limiting example, a generator 1 that utilizes a renewable energy source, particularly an aeroturbine 2. It will be appreciated that the aeroturbine 2 may be replaced by a water or steam turbine, i.e. a fossil fueled internal combustion engine.

The wind generator 1 may be used both for small-to-medium power systems, such as in civil and industrial installations, and for high-power systems in terrestrial or on-shore or off-shore installations.

Furthermore, the generator 1 may have a horizontal-axis or vertical-axis design and be wholly contained in a nacelle, not shown, located at the end of the tower, also not shown.

The generator 1 comprises a driving shaft 3 rotating about a longitudinal axis L and adapted to be coupled to the external energy source.

In a wind generator 1 , the energy source may comprise a turbine 2 having one or more vanes 4, directly fitted to the driving shaft 3, like in the schematized configuration of Fig. 1 , or to a different shaft operably connected to the drive shaft 3 for imparting the motion of the turbine 2 thereto.

As is known, the turbine 2 comprises a propeller 5 with vanes 4, whose angle of inclination may be fixed or be designed to change according to wind force, e.g. using appropriate actuator means, not shown, associated with each vane 4.

Furthermore, the generator 1 may be rotatably supported by a support structure, not shown, to rotate about a vertical axis defined by the support tower, not shown, thereby allowing the wind turbine 2 to be always oriented downwind even when the wind changes its direction.

Therefore, the driving shaft 3 of the generator 1 is coupled to the driven shaft 6 of an electric rotating machine 7 comprising a stator 8 and a rotor 9 equipped with the driven shaft 6.

Thus, the electric machine 7 may generate electric energy with predetermined torque ripple during its operation.

For example, the driving shaft 3 may be coupled to the driven shaft 6 of the electric machine 7 by means of a joint 1 0. In this configuration, the rotation speed of the driving shaft 3 is substantially coincident with the rotation speed of the driven shaft 6 of the electric machine 7. In an alternative configuration of the invention, a multiplier device, not shown, may be interposed between one end 1 1 of the driving shaft 3 and one end 12 of the driven shaft 6, for increasing the rotation speed of the driven shaft 6 as compared with the speed of the driving shaft 3. The multiplier device may be selected from currently available devices and may include kinematic transmission means, not shown, such as gear wheels, epicyclic gears, or drive belts.

This configuration may be used in generators 1 that include an electric machine 7 operating at a rotation speed substantially higher than that of the drive shaft 3.

The generator 1 further comprises electric connection means 1 3 for injecting the electric energy generated by the electric machine 7 into the network R and control means 14 for adjusting electrical and dynamic parameters of the electric machine 7.

In accordance with a peculiar feature of the invention, the electric machine 7 is of synchronous magnetic reluctance type and the rotor 9 is of the transverse lamination type.

In a particularly advantageous configuration, the stator 8 has an even number n _s of grooves, generally referenced 15 for each pair of poles and the rotor 9 has a plurality of adjacent slots, generally referenced 16, defining an even number n _r of magnetically equivalent slots. The grooves 1 5 may be substantially longitudinal, and their length may substantially coincide with the length I of the stator 8.

In the illustrated configuration of the invention, the stator 8 has two pairs of poles and twenty-four grooves 1 5.

However, it shall be understood that the pairs of poles and hence the grooves 1 5 may be also provided in different numbers, without limitation to the scope of the present invention.

The rotor 9 is composed of a plurality of disc-line sheet elements, generally referenced 17, which have a predetermined thickness s, are integral with the driven shaft 6, and are arranged in longitudinally side-by-side relation. The inside diameter d of the disc-line sheet elements 17 may substantially coincide with the diameter Φ of the driven shaft 6 of the rotor 9.

Advantageously, the slots 1 6 may have an elongate curved shape, symmetrical to a radius of the relevant disc-line sheet element 1 7, with end portions 1 8, 1 8' close to the edge 1 9 of the disc-line sheet element 1 7, so that each slot 1 6 can delimit at least one pair of ribs 20, 20' at such edge 1 9.

The slots 1 6 of the rotor 9 are configured so that, during operation of the electric machine 7, the magnetic field in the stator 8 will saturate the ribs 20, 20' which, in essentially magnetic terms, will act as if the disc-line sheet element 1 7 had an additional slot thereat.

Further saturation areas are located at the central portions 21 of the slots 16 of the rotor 9, that are placed near the peripheral edge 1 9 of the disc-line sheet element 17. Particularly, each of these slots 21 near the edge 1 9 has an additional pair of ribs 22, 22' defining magnetically equivalent slots.

Therefore, the total number of magnetically equivalent slots n _r will be equal to the number of ribs so defined.

In a particularly advantageous aspect of the invention, the number n _r of magnetically equivalent slots and the number n _s of grooves 1 5 are selected according to a predetermined relation, to minimize torque ripple in the electric machine 7 during operation.

Particularly, according to a first relation, the number n _r of equivalent slots is greater than 6. Conveniently, the number n _s of grooves 1 5 is other than the product of equivalent slots by an integer m, and the difference between the number n _s of grooves 1 5 and the number n _r of equivalent slots is other than 0, +2 and - 2. Concerning the above relations, a particular configuration of the invention may be provided, as shown, in which the rotor 9 comprises a plurality of discline sheet elements 1 7, in which the ratio between their inside diameter d and the outside diameter D is equal to or greater than 0.45, and which have a number n _r of equivalent slots equal to the number n _s of grooves 1 5 decreased or increased by four units.

Nevertheless, it will be understood that this configuration is only provided by way of example and without limitation to the invention and that the rotor 9 may be also formed in different configurations.

In a further configuration, as shown in Figs. 6 and 7, the generator 1 may include a plurality of permanent magnets, generally referenced 23, associated with the rotor 9.

The magnets 23 may be provided in such number as to have a smaller magnetic mass than the permanent magnets that are used in permanent magnet electric machines having approximately the same power.

Conveniently, the permanent magnets 23 may be housed in one or more predetermined portions 24, 24', 24", 24"' of the disc-line sheet element 1 7 of the rotor 9.

Each disc-line sheet element 1 7 will accommodate a permanent magnet 23 with one or more pairs of magnetic poles. Preferably, but without limitation, the permanent magnets 23 will be held in a central portion 24, 24', 24", 24"' of the disk-shaped element and accommodated in at least one adjacent slot 1 6, as shown, to form a rotor 9 with embedded magnets. Furthermore, the permanent magnets 23 may be evenly arranged along the axis L of the rotor 9.

Also, the number of permanent magnets 23 will be selected in view of reducing the electrical and/or power specifications of the control means 14 as compared with the control means 14 of the electric machines 7 with permanent magnets 23 having approximately the same power.

For example, the permanent magnets 23 may be designed to generate a magnetic flux ranging from 1 0% to 20% of the value of the magnetic flux generated by a prior art electric machine 7 with permanent magnets 23, of equal rated power. This particular configuration of the electric machine 7 can reduce the electrical and power specifications of the control means 14 by a value ranging from 20% to 30% as compared with the control means 14 that are used in electric machines 7 with permanent magnets 23.

The control means 14 of the electric machine 7 include a circuit 25 which is electrically coupled to the stator 8 for adjusting the electrical and dynamic parameters of the electric machine 7.

For example, the control means 14 include an inverter 26, which is designed to maintain the rotation speed of the driven shaft 6 of the electric machine 7 at a predetermined value. Preferably, the inverter 26 is designed to control the instantaneous rotation speed imparted to the driven shaft 6 by the external source.

The rotation speed of the driven shaft 6 may change in a range from predetermined minimum and maximum values.

Furthermore, the inverter 26 may be designed to control the rotation speed of the driven shaft 6 at periodic and predetermined times.

Any deviation of such speed value from the predetermined value will trigger the action of the inverter 26 and the change of certain electrical parameters of the control signals transmitted thereby to the electric machine 7.

Such changed control signals will allow the electric machine 7 to change its operation and maintain the generated power substantially constant.

Furthermore, in a configuration of the invention, not shown, a programmable logic unit may be provided, which is connected to the control means 14 and can be also connected to the external source, e.g. a wind turbine 2.

For example, in the case of a wind generator 1 , the programmable logic unit can keep the power delivered by the electric machine 7 constant as wind conditions change, by operating in a simultaneous and synchronized manner on the inverter 26 and the turbine 2.

The programmable logic unit may change the operating point of the electric machine 7 through the action of the inverter 26 and may also change the action of the wind on the drive shaft 3 by operating on the vanes 4 of the turbine 2, e.g. by changing their angle of inclination.

Furthermore, the control means 14 also include a converter element 27, which is designed to change the electric parameters associated with the energy generated by the electric machine 7, thereby allowing injection thereof into the external transmission network R.

Conveniently, the operating point of the electric machine 7 may be adjusted by changing the electric quantities associated with the signals provided by the control means 14 to the stator 8.

According to a peculiar aspect of the invention, as shown in Figs. 6 to 9, the electric machine 7 may comprise an outer casing 28, which stably housed a substantially tubular stator 8 having a longitudinal axis L and a rotor 9 inserted in the stator 8 with a predetermined interspace 31 and has at least one substantially longitudinal passage 1 6 extending along its entire length.

Air cooling means 33 are also provided, which are associated with the rotor 9 and are adapted to promote forced circulation of an air flow into both the stator 8 and the rotor 9. The cooling means 33 comprise an annular peripheral gap 29 defined between the stator 8 and the casing 28, which is in fluid connection with both the longitudinal passage 16 and the interspace 31 to define a looped path 30 therewith for circulation of the cooling air flow for cooling the stator 8 and the rotor 9.

The longitudinal passages 16 of the rotor 9 may substantially coincide with the plurality of adjacent slots 16 formed in the sheet elements 17 thereof.

The gap 29 may be formed in the proximity of the peripheral outer edge 32 of the stator 8 to promote heat exchange by the cooling air from within it with the environment. It shall be further understood that the cooling means 33 operated by the rotor 9 may be also provided in synchronous reluctance electric machines 7 that are not used as electric energy generators 1 and may not comply with the mathematical relations associated with the number n _s of grooves and the number n _r of equivalent slots as mentioned above.

According to a particular characteristic of the invention, the cooling means 33 may comprise one or more impeller blades, generally referenced 34, coaxial with the rotor 9, at least at one, preferably at both of the longitudinal ends 35, 35' of the rotor 9.

In a preferred, non-limiting configuration of the invention, as shown in the figures, the cooling means 33 may comprise a pair of impeller blades 34, each at one end 35, 35' of the rotor 9, to increase the air flow fed into adjacent slots 16.

Each impeller blade 34 may be located at a predetermined distance from the corresponding end 35, 35' of the rotor 9. Such distance may be identical or different for the two impeller blades 34.

In a further configuration of the invention, not shown, the individual propeller blades 34 may be accommodated at the ends 35, 35' of the rotor 9 by ring- shaped fastening members, not shown, which are connected to the driven shaft 6 of the electric machine 7.

Furthermore, each impeller blade 34 may include a plurality of helical surfaces, generally referenced 36, which are specially shaped according to the end of the rotor 9 towards which they face.

The helical surfaces 36 may be variously shaped to generate an air flow which flows into one end 35, 35' of the rotor and flows out of the opposite end 35', 35.

The two impeller blades 34 may have the same number of helical surfaces 36 or different numbers thereof, so that different air flows may be fed at each end 35, 35' of the rotor 9.

Conveniently, the cooling means 33 may comprise a cooling air conduit 37 in fluid connection with the outer casing 28 at one or both ends 35, 35' to convey at least part of the cooling air flow in the path 30 out of the stator 8. The conduit 37 may be closed and comprise the adjacent slots 1 6 of the rotor 9, the cavity 29 and the ends 35, 35' of the rotor 9.

The conduit 37 may also have a cooling air flow inlet 39, 39' at one end 35, 35' of the rotor 9 and a cooling air flow outlet 39', 39 at the opposite end 35', 35 of the rotor 9. Advantageously, as shown in Fig. 8, the cooling means 33 may comprise an air cooling heat exchanger 38 located outside the stator 8, in fluid connection with the cavity 29 and the conduit 37. The exchanger 38 may be of the air-air or air-fluid type and the cooling air may be fed therethrough via the inlets and the outlets 39', 39 located at the corresponding longitudinal ends 40, 40' of the stator 8.

Advantageously, the cooling means 33 may include one or more pumps, generally referenced 41 , which are driven by the driven shaft 6 of the rotor 9 for feeding corresponding cooling loops, generally referenced 42, associated with the closed loop 30.

Conveniently, the pumps 41 may be designed to also feed cooling loops 42 associated with the heat exchanger 38 and the control means 14.

In this configuration, the pumps 41 are only actuated during the movement of the driven shaft 6 and will not operate when the latter does not rotate or has a rotation speed lower than a predetermined value, i.e. not sufficient to actuate them.

Furthermore, each pump 41 may feed the same fluid into all the cooling loops 42 or feed different fluids into one or more loops 42. Conveniently, the flow rate of each pump 41 may be selected according to the amount of fluid requested by each loops 42.

The configuration so obtained will be particularly efficient and will avoid undesired energy wastes in the cooling circuit. Also, it will allow the generator 1 to be compact, while decreasing its overall weight. Furthermore, the stator 8 may be of the toothed coil type 43, with stator windings, generally referenced 44, of smaller axial length.

The reduced length of stator coils 44 will allow the conduit 37 to have a greater width and the dimension I of the stator 8 to be smaller than in a conventional stator, which will involve a reduction of the longitudinal dimension of the generator 1 .

In this configuration, the stator 8 of the electric machine 7 will require a smaller amount of conductive filament, e.g. made of copper, than in conventional stators 8.

The stator 8 of the toothed coil type 43 has a lighter weight and is more cost- effective than a conventional stator 8.

The above disclosure clearly shows that the invention fulfills the intended objects and particularly meets the requirement of providing a generator that has high electrical performances while being cost-effective and having reduced dimensions and weight.

The rotating electric machine of the invention is susceptible to a number of changes and variants, within the inventive concept disclosed in the appended claims. All the details thereof may be replaced by other technically equivalent parts, and the materials may vary depending on different needs, without departure from the scope of the invention.

While the rotating electric machine has been described with particular reference to the accompanying figures, the numerals referred to in the disclosure and claims are only used for the sake of a better intelligibility of the invention and shall not be intended to limit the claimed scope in any manner.