ELECTRICAL COIL WITH LOW ACOUSTIC RADIATION

An electrical coil includes a wire of electrically conductive material wound in several turns around a main axis situated inside the coil, each turn making a complete revolution around the main axis, the various turns succeeding one another in an offset manner parallel to the main axis, including openings located between pairs of turns that succeed one another in the direction of the main axis.

TECHNICAL FIELD

The present invention relates to an electrical coil. It also relates to a system comprising such a coil and a method for using such a coil.

STATE OF THE PRIOR ART

The coils used to regulate the current in the electricity distribution or transmission grid are subjected to electromagnetic forces resulting from the alternating current powering them. These stresses produce vibrations and an acoustic radiation, which can create noise disturbances.

Currently, one solution is encapsulation (with a sealed structure having sound-absorbing material), which requires a subsequent treatment, which causes a problem of cooling and the efficacy of which is limited at low frequencies.

The aim of the present invention is to reduce the noise disturbance of electrical coils, preferably while decreasing the need for cooling and/or without necessarily being limited to low frequencies.

DISCLOSURE OF THE INVENTION

This objective is achieved with an electrical coil comprising a wire of electrically conductive material wound in several turns around a main axis situated inside the coil, each turn making a complete revolution around the main axis, the various turns succeeding one another in an offset manner parallel to the main axis, characterized in that it comprises openings between pairs of turns that succeed one another in the direction of the main axis.

The coil can comprise, on at least part or on all of the coil, at least one opening between each pair of turns that succeed one another in the direction of the main axis.

The coil can comprise, on at least part or on all of the coil, at least one opening between groups of turns that succeed one another in the direction of the main axis. Every group preferably comprises the same number of turns.

At least one or every opening separating two turns can comprise a hole through a material separating these two turns. As openings, the coil can comprise several perforations or slits through the material separating turns distributed over the whole circumference of these turns around the main axis, these perforations or slits preferably being spaced apart from one another along the turns by at most 2 mm.

At least one or every opening separating two turns can comprise a space running the length of these two turns over the whole circumference of these turns around the main axis.

The smallest dimension of each opening is preferably at least 0.1 mm.

The openings are preferably set up for a degree of openness of the coil:greater than or equal to 0.5% (or even 1%) and/orsmaller than or equal to 40%, preferably smaller than or equal to 30%, preferably smaller than or equal to 20%, preferably smaller than or equal to 10%.

The openings are preferably distributed uniformly over the coil.

According to yet another aspect of the invention, a system comprising a coil according to the invention and a power source set up to power the coil electrically with an electrical signal producing, continuously or as a one-off, a vibration of the coil at the frequency f is proposed.

The smallest dimension of each opening is preferably at least equal to

is the viscosity of air at 18° C. and

is the density of air at 18° C. for an atmospheric pressure of p0=101320 Pa and

is the angular frequency of the vibration.

The vibration frequency of the coil is preferably comprised between 20 Hz and 20 kHz.

The electrical signal preferably has an intensity of at least 100 A.

According to yet another aspect of the invention, a method for using a coil according to the invention is proposed, in which the coil is powered electrically, by a power source, with an electrical signal producing, continuously or as a one-off, a vibration of the coil at the frequency f.

The smallest dimension of each opening is preferably at least equal to

is the viscosity of air at 18° C. andρ0=1.213 kg·m−3
is the density of air at 18° C. for an atmospheric pressure of p0=101320 Pa and

is the angular frequency of the vibration.

The vibration frequency of the coil is preferably comprised between 20 Hz and 20 kHz.

The electrical signal preferably has an intensity of at least 100 A.

As these embodiments are in no way limitative, it is possible in particular to consider variants of the invention comprising only a selection of the characteristics described or illustrated hereinafter, in isolation from the other characteristics described or illustrated (even if this selection is isolated within a sentence comprising these other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, and/or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

A first embodiment101of a coil according to the invention will be described first of all with reference toFIG. 1.

The electrical coil101comprises a wire of electrically conductive material wound in several turns2around a main axis1situated inside the coil101and extending in one direction.

The coil101is a single-layer or multi-layer industrial coil. In the case of a multi-layer coil, the layers can be contiguous (in this case there is a single cylinder if the coil has a cylindrical shape) or separated (in this case, if the coil has a cylindrical shape, there are several concentric cylinders separated by an air gap).

The coil101preferably has a cylindrical shape, but can have different shapes depending on the variant in question, for example a coil101with a rectangular cross section.

Each turn2makes a complete revolution around the main axis1.

The various turns2succeed one another in an offset manner parallel to the main axis1. In other words, all of the turns2together progressively form a shape which surrounds and runs the length of the axis1, like a tube for example. These various turns2can optionally comprise, locally, several turns2contained in one and the same plane perpendicular to the axis1.

The coil101comprises openings3between pairs of turns2a,2bthat succeed one another in the direction of the main axis1.

The coil101comprises, on at least part or on all of the coil101, at least one opening3between each pair of turns2a,2bthat succeed one another in the direction of the main axis1.

Each opening3is set up to allow free passage of gas (typically of air) through it between the outside of the coil101and the inside (i.e. the axis1) of the coil101.

This passage of gas (typically of air) from the outside of the coil101to the axis1makes it possible to reduce the acoustic radiation of the coil101.

The inside of the coil101is defined as being the space comprised between the turns2and the axis1.

The outside of the coil101is defined as being the space comprised beyond the turns2with respect to the axis1.

If the coil is borne by a support or a frame, this support or frame is also formed openwork so as not to obstruct the openings3, such that for each opening3a straight line of free path of gas (such as air) can connect the axis1to the outside of the coil101and to the outside of the support or frame.

Each turn2preferably has a height (measured parallel to the axis1) smaller than 20 cm for the low frequencies of the human hearing range or smaller than 20 mm or smaller than 2 mm for the high frequencies of the human hearing range (20 kHz or less).

Each opening3preferably has a height (measured parallel to the axis1) smaller than the height of a turn2(measured parallel to the axis1).

Each opening2preferably has a thickness (measured perpendicular to the axis1) smaller than or equal to twice the height of each opening3(measured parallel to the axis1).

The coil101preferably has a height (measured parallel to the axis1) greater than ten times the height of a turn (measured parallel to the axis1).

The coil101preferably has an external diameter or more generally (in particular in the case of turns2making revolutions in the shape of a square, oval, etc.) at least an external width of the turns2(measured in a plane perpendicular to the axis1) greater than 40 cm.

Each opening3follows the slope of the turns2.

At least one or every opening3separating two turns2a,2bcomprises a space running the length of these two turns2a,2bover the whole circumference of these turns2a,2baround the main axis1.

The smallest dimension of each opening3is at least 0.1 mm, preferably at least 0.5 mm, preferably at least 1 mm. In this embodiment this smallest dimension is the height of each opening3measured parallel to the axis1.

The height of each opening3is smaller than 5 mm.

The openings3are set up for a degree of openness of the coil greater than or equal to 0.5% (or even greater than or equal to 1%).

The degree of openness r of the coil101is defined as being, in any profile view or image perpendicular to the axis1(cf.FIG. 1), the ratio of the surface area of the openings3to the total surface area of the coil101(this total surface area being the sum of the surface area of the turns2and the openings3). For example, τ=50% when the surface area of the openings3represents half of the surface area of the coil101.

The openings3are set up for a degree of openness of the coil101smaller than or equal to 30%, preferably smaller than or equal to 10%. This makes it possible to only very slightly modify the inductance of the coil with constant dimensions, or to only weakly modify the dimensions of the coil to preserve its inductance.

The openings3are distributed uniformly over the coil101, i.e. the openings3are arranged on the coil101with a constant spatial periodicity parallel to the axis1and/or along the turns2.

In the case of the coil101, the openings3are distributed or arranged with a constant spatial periodicity parallel to the axis1.

In an embodiment example:the turns2are made from aluminium with a rectangular-shaped cross section and the dimensions 25 mm by 20 mm composed of 36 strands with a diameter of 4 mm, and each turn2having the shape of a circle, for a total of 18 turns,the coil101forms a cylinder with a height of 394 mm measured parallel to the axis1and with an internal diameter of 550 mm measured in a plane perpendicular to the axis1and with an external diameter of 600 mm measured in a plane perpendicular to the axis1, the axis1being the axis of revolution of this cylinder,the openings or slits3have a height of 2 mm measured parallel to the axis1.

A second embodiment102of a coil according to the invention will now be described with reference toFIG. 2. The coil102will be described only with regard to its differences compared with the coil101.

The coil102comprises, on at least part or on all of the coil102, at least one opening3between groups22of turns2that succeed one another along the main axis1.

Every group22comprises the same number of turns2, typically more than or equal to 2 and/or less than or equal to 10.

The openings3are always set up for a degree of openness of the coil102greater than or equal to 0.5% (or even greater than or equal to 1%).

In an embodiment example:the turns2are made from aluminium with a rectangular-shaped cross section and the dimensions 25 mm by 20 mm composed of 36 strands with a diameter of 4 mm, and each turn2having the shape of a circle, for a total of 18 turns,the coil102forms a cylinder with a height of 384 mm measured parallel to the axis1and with an internal diameter of 550 mm measured in a plane perpendicular to the axis1and with an external diameter of 600 mm measured in a plane perpendicular to the axis1, the axis1being the axis of revolution of this cylinder,the openings or slits3have a height of 3 mm measured parallel to the axis1,each group22comprises 2 turns.

A third embodiment103of a coil according to the invention will now be described with reference toFIG. 3. The coil103will be described only with regard to its differences compared with the coil101or102.

In this variant of a coil103fromFIG. 3, for which the openings3space out individual turns2(as inFIG. 1) or groups22of turns (as inFIG. 2), at least one or every opening3separating two turns2a,2bcomprises a hole through a material separating these two turns2a,2b.

More precisely, the openings3comprise, for each pair of neighbouring turns2a,2bin question, several perforations or slits through the material (for example polymer of the ABS (acrylonitrile butadiene styrene) type) separating turns2a,2b.

These holes3are distributed over the whole circumference of these turns2a,2baround the main axis1.

These holes3are preferably spaced apart from one another by a distance4(measured along a curved line equidistant from each of the two turns2that these holes3separate) smaller than the acoustic wavelength determined at the frequency of the vibration f, preferably smaller than or equal to 20 cm (in order to cover the low acoustic frequencies), preferably smaller than or equal to 2 cm (in order to cover the intermediate frequencies), preferably smaller than or equal to 2 mm (in order to cover the high acoustic frequencies).

The smallest dimension of each opening3is at least 0.1 mm, preferably at least 0.5 mm, preferably at least 1 mm. In this embodiment this smallest dimension is the height of each hole3measured parallel to the axis1or the length of each hole (slit or perforation) measured along the two turns2a,2bthat this opening3separates.

The openings3are always set up for a degree of openness of the coil103greater than or equal to 0.5%, preferably greater than or equal to 1%.

In the case of the coil103, the openings3are distributed or arranged on the coil103with a preferably constant spatial periodicity along the turns2, i.e. along the direction of the slope of the turns2.

In the case of the coil103, the openings3are distributed or arranged on the coil103with a first constant spatial periodicity in the direction of the axis1and a second constant spatial periodicity along the turns2.

The embodiment example of a coil103differs from the embodiment example of a coil101or102in that each opening3is a hole in the form of an elongated slit having:a height of 2 mm measured parallel to the axis1,a length of 5 mm measured along the two turns2a,2bthat this opening3separatesa distance4to each of its neighbouring openings3of 3 mm measured along the two turns2a,2bthat this opening3separates.

A fourth embodiment104of a coil according to the invention will now be described with reference toFIG. 4. The coil104will be described only with regard to its differences compared with the coil103.

The embodiment example of a coil104differs from the embodiment example of a coil103in that each opening3is a hole in the form of a perforation with comparable height and length, i.e. having:a height of 2 mm measured parallel to the axis1,a length of 2 mm measured along the two turns2a,2bthat this opening3separatesa distance4to each of its neighbouring openings3of 2 mm measured along the two turns2a,2bthat this opening3separates.

The more the size of the openings3is reduced (by increasing the number of openings3), the more locally constant or uniform the degree of openness is.

Thus, the invention consists of making the coil101,102,103or104openwork (with turns2partially (FIG. 2) or completely (FIG. 1) non contiguous) so as to reduce the mechano-acoustic conversion of the vibrations to create a sound wave. The presence of spaces (of air or more generally of gas) between the turns2produces an acoustic short circuit, which greatly reduces the efficacy of acoustic radiation of the coil101,102,103or104.

The invention makes it possible to reduce the acoustic radiation of the coils present in the electricity distribution or transmission grids while substantially preserving the value of the inductance of the coil. This involves applying a vibroacoustic concept to an industrial system.

The invention makes it possible to avoid an effective acoustic radiation of a structure because of its openwork or perforated nature.

An embodiment of a system according to the invention comprising the coil101,102,103or104and a power source will now be described.

The power source is for example a current generator of an electricity distributor, set up to generate a supply electrical signal typically with a fundamental frequency higher than 10 Hz or even 40 Hz and/or lower than 1000 Hz or even 100 Hz (typically equal to 50 Hz or 60 Hz) and a current higher than 100 A or even higher than 1000 A.

The power source is set up to power the coil101,102,103or104electrically with the supply electrical signal producing, continuously or as a one-off, a vibration of the coil at the frequency f typically equal to double the fundamental frequency of the supply electrical signal. Other vibrations of the coil are possible, in particular at higher frequency harmonics or combinations of harmonics.

In other words, the coil101,102,103or104has an acoustic wavelength λ=c/f, where c is the speed of sound of 342 m/s under normal temperature and pressure conditions (18° C. and 101320 Pa) and f is the vibration frequency of the coil101,102,103,104borne by its structure.

The smallest dimension of each opening3(i.e. height measured parallel to the axis1or length measured along the two turns2a,2bthat this opening3separates) is at least equal to the viscous skin depth

is the viscosity of air at 18° C. and

is the density of air at 18° C. for an atmospheric pressure of p0=101320 Pa and

is the angular frequency of the vibration.

The smallest dimension (typically the height, measured parallel to the axis1) of the spacings3must be greater than the viscous skin depth, i.e.:greater than or equal to 15 μm at 20 kHz, i.e. greater than or equal to 0.1 mm, taking a safety margingreater than or equal to 0.5 mm at 20 Hz, i.e. greater than or equal to 1 mm, taking a safety margin

i.e. in practice at least 0.5 mm (or even 1 mm) for the range of audible frequencies (20 Hz-20 kHz).

In the case of holes or perforations3, these holes or perforations3are preferably spaced apart from one another by a distance4(measured along a curved line equidistant from each of the two turns2that these holes or perforations3separate) smaller than the acoustic wavelength at the frequency of the vibration f, i.e.:smaller than or equal to 17 m at 20 Hz, i.e. smaller than or equal to 1 m, taking a safety marginsmaller than or equal to 17 mm at 20 kHz, i.e. smaller than or equal to 10 or 2 mm, taking a safety margin

i.e. in practice smaller than or equal to 10 mm (or even 2 mm) for the range of audible frequencies (20 Hz-20 kHz).

The vibration frequency f of the coil is comprised between 20 Hz and 20 kHz.

The supply electrical signal has an intensity of at least 100 A.

An embodiment of a method according to the invention implemented in this system will now be described.

The coil101,102,103or104is powered electrically, by the power source, with the supply electrical signal producing, continuously or as a one-off, the vibration of the coil at the frequency f.

The smallest dimension of each opening3is at least equal to

is the viscosity of air at 18° C. and

is the density of air at 18° C. for an atmospheric pressure of p0=101320 Pa and

is the angular frequency of the vibration.

The vibration frequency f of the coil is comprised between 20 Hz and 20 kHz.

The supply electrical signal has an intensity of at least 100 A.

FIG. 5illustrates different sound pressure level curves generated by different variants of coils according to the invention, according to the second embodiment102according to the invention, with the following parameters:the turns2are made from copper with a rectangular-shaped cross section and the dimensions 0.5 mm by 1 mm composed of 2 strands with a diameter of 0.5 mm, and each turn2having the shape of a circle, for a total of 18 turns,the coil102forms a cylinder with a height of 90 mm measured parallel to the axis1and with an internal diameter of 138 mm measured in a plane perpendicular to the axis1and with an external diameter of 140 mm measured in a plane perpendicular to the axis1,the openings or slits3have a height, measured parallel to the axis1, of:h=1 mm for curve11, for a degree of openness of approximately 2%h=2 mm for curve12, for a degree of openness of approximately 4%h=4 mm for curve13, for a degree of openness of approximately 9%each group22comprises 2 turns, i.e. nine groups22with two turns2.

Curve10corresponds to a reference curve outside of the invention, for which h=0, i.e. without opening3.

The following is found:an inductance of 0.16 mH for curve10an inductance of 0.153 mH for curve11, i.e. a difference of 4.3% compared with curve10an inductance of 0.1505 mH for curve12, i.e. a difference of 6% compared with curve10an inductance of 0.146 mH for curve13, i.e. a difference of 8.7% compared with curve10

FIG. 5compares the sound pressure level generated at 10 m by the non-openwork coil (curve10) with that generated by the openwork coils102(curves11to13). It is noted that there is a very significant decrease in the sound level over all frequencies when the coil102is formed openwork. For a coil that is 2% open the level of the mode (2,0) decreases by approximately 20 dB, while the level of the mode (0,0) decreases by almost 29 dB. Between the two, it should be noted that some modes are more attenuated than others. Thus, the level of the mode (4,0) is attenuated by 15 dB while the level of the mode (5,0) decreases by more than 36 dB. It is also interesting to note that the progress of the decrease in the sound is quite weak between a coil that is 2% open and another coil that is 4 or 9% open.

These results show the benefit of the solution of making the coils openwork for reducing the acoustic radiation. The results obtained show overall a very significant decrease in the sound level.

Of course, the invention is not limited to the examples that have just been described, and numerous amendments can be made to these examples without departing from the scope of the invention.

Of course, the different characteristics, forms, variants and embodiments of the invention can be combined with one another in various combinations, unless they are incompatible or mutually exclusive. In particular, all of the variants and embodiments described above can be combined with one another.