Switched reluctance electric machine

Abstract

Switched reluctance electric machine (10) which has an axis (X) of rotation and includes: - a stator (20) provided with a plurality of salient stator poles (22), arranged circumferentially and separated by corresponding stator slots (24), - a rotor (30) contained within the stator (20) and provided with a plurality of salient rotor poles (32), arranged circumferentially and separated by corresponding rotor slots (34), - a plurality of interpolar rotor wedges (40), made of non-magnetic polymeric material, housed in corresponding rotor slots (34) and provided with a structure similar to a T-shaped beam.

Description

[0001] SWITCHED RELUCTANCE ELECTRIC MACHINE

[0002] D E S C R I PTI O N

[0003] Technical field of the invention

[0004] The present invention relates to a switched reluctance electric machine.

[0005] Background art

[0006] A switched reluctance machine (SRM) is essentially provided with a multiphase stator and a rotor with pronounced poles. The stator is made of multiple protruding (salient) ferromagnetic poles alternating with stator slots. The rotor is made of ferromagnetic material (with narrow hysteresis and high magnetic permeability, for example silicon steel), laminated, to limit the losses of the induced currents and has projections that act as salient magnetic poles to maximize the magnetic reluctance, magnetic poles alternating with corresponding rotor slots.

[0007] For switched reluctance motors, the numbers of pole pairs of the rotor and stator are different, which minimizes torque oscillation and prevents the simultaneous alignment of all the poles, in a position that could not generate torque.

[0008] When a stator pole is equidistant from two adjacent rotor poles, the rotor pole is said to be in the "fully misaligned position". This is the position of maximum magnetic reluctance for the rotor pole. In the "aligned position", two (or more) rotor poles are fully aligned with two (or more) stator poles, (which means that the rotor poles are fully facing the stator poles) and is a position of minimum reluctance.

[0009] An advantage of the switched reluctance machine is that the losses are mainly located in the stator, where the windings are located, which can therefore be easily cooled from the outside.

[0010] The main disadvantage of switched reluctance machines depends precisely on the fact that they are characterized by a doubly salient structure, that is, they have both the stator and the rotor with salient poles: during high-speed rotation, the relative motion between air and salient poles generates a parasitic (i.e. unwanted) aerodynamic force that opposes the rotation of the rotor. Air is known to oppose with its own resistance to any translational or rotary motion of a body: therefore, such resistance is also generated in electric rotating machines such as motors and generators. In particular, electric machines are influenced by the friction between the rotor and the air. Such aerodynamic friction, called windage, implies a reduction in the efficiency of the electric machine especially for predominantly high rotation speeds, since its contribution is proportional to the third power of the speed. Furthermore, this aerodynamic friction generates a sharp and unwanted acoustic noise. Efficiency losses due to parasitic aerodynamic forces occur especially when the stator slots are in correspondence with the rotor slots: the sum of the volumes of these stator and rotor slots creates a sort of "pocket" within which turbulent air motions are created and, consequently, greater aerodynamic losses.

[0011] The known solutions to this problem essentially consist of filling the rotor and / or stator slots with non-magnetic material. However, these are solutions that imply an increase in the mass of the machine, consequently, a greater inertia to rotation and, ultimately, do not allow the desired efficiency to be recovered.

[0012] There is therefore a need to define an innovative switched reluctance electric machine that is free from or at least minimizes the drawbacks inherent in the loss of efficiency and noise mentioned above.

[0013] Summary of the invention

[0014] In order to substantially solve the technical problems highlighted above, an object of the present invention is to define a switched reluctance electric machine whose rotor is equipped with a plurality of interpolar rotor wedges of innovative shape.

[0015] The invention therefore relates to a new shape and arrangement of interpolar rotor wedges for the reduction of high-speed aerodynamic losses in switched reluctance electric machines.

[0016] Therefore, according to the present invention, a switched reluctance electric machine is provided having the characteristics set forth in the independent claim, attached to this description.

[0017] Further preferred and / or particularly advantageous embodiments of the invention are described according to the characteristics set forth in the attached dependent claims.

[0018] Brief description of the drawings

[0019] The invention will now be described with reference to the attached drawings, which illustrate some non-limiting examples of its implementation, in which:

[0020] - Figure 1 is a simplified diagram of a switched reluctance electric machine, - Figure 2 illustrates a detail of a switched reluctance electric machine with an interpolar rotor wedge assembled in the rotor, according to a preferred embodiment of the present invention,

[0021] - Figure 3 illustrates the shape of the interpolar rotor wedge of Figure 2,

[0022] - Figure 4 is a perspective view illustrating the assembly of the interpolar rotor wedge of Figure 2 in the rotor of a switched reluctance electric machine, and

[0023] - Figure 5 graphically illustrates the trend of the aerodynamic losses as a function of the speed for a rotor according to the prior art (Fig. 5a) and for the rotor equipped with the interpolar rotor wedge (Fig. 5b).

[0024] Detailed description

[0025] By way of example and not limitation, the present invention will now be described with reference to the aforementioned figures.

[0026] With reference to figures 1 and 2, a switched reluctance electric machine 10 is a substantially axisymmetric structure with respect to a rotation axis X and comprises:

[0027] - a stator 20 provided with a plurality of salient stator poles 22, arranged circumferentially and which extend radially inwards. Each pole has an axial dimension equal to the axial dimension of the stator 20. Circumferentially consecutive stator poles are separated by corresponding stator slots 24, having an axial dimension equal to the axial dimension of the stator and a radial dimension equal to the radial dimension of the stator poles 22,

[0028] - a rotor 30 contained within the stator 20 and also provided with a plurality of salient rotor poles 32, arranged circumferentially. The rotor poles 32 extend radially outward and have an axial dimension equal to the axial dimension of the rotor 30. Circumferentially consecutive rotor poles are separated by corresponding rotor slots 34, having an axial dimension equal to the axial dimension of the rotor and a radial dimension equal to the radial dimension of the rotor poles 32, and

[0029] - a plurality of interpolar rotor wedges 40 made of non-magnetic polymeric material, each of them housed in a corresponding rotor slot 34.

[0030] With reference to figure 3, each rotor wedge 40 has a structure similar to that of a "T" beam and comprises:

[0031] - a wing 42 of curvilinear and convex shape, having a radius of curvature substantially equal to the radius of curvature of the rotor, an axial dimension equal to the axial dimension of the rotor 30 and a circumferential dimension slightly smaller than the maximum circumferential dimension of the rotor slot 34. At its ends in the circumferential direction, the wing 42 has respective anchoring edges 43 having a more accentuated curvature than the curvature of the wing 42,

[0032] - a shoe 45 or anchoring base of prismatic shape, "dovetail" or "dog bone", and

[0033] - a core 44 that develops in a radial direction, having a substantially parallelepiped shape, connecting the shoe 45 and the wing 42.

[0034] The interpolar rotor wedge 40 is designed with a lightweight structure designed to minimize the mass added to the rotor, but at the same time fill the space between the rotor poles and ensure the necessary mechanical resistance. This is achieved thanks to a "T" beam-shaped structure, which offers an excellent resistance / mass ratio and the chosen material, as mentioned, of a non-magnetic polymer type. In this way, the addition of the rotor wedges makes the inertia of the rotor almost unchanged and the choice of the non-magnetic material does not affect the electromagnetic performance of the electric machine.

[0035] The structure of the rotor wedge 40 aims to minimize the physical saliency of the rotor while maintaining the magnetic saliency (i.e., without negatively impacting the torque producing ability of the motor), thus obtaining a substantially cylindrical and "smooth" rotor. This is because the shape of the wing 42 of the rotor wedge 40 completely covers the rotor slot 34, filling the space between two consecutive rotor poles 32.

[0036] The rotor wedge 40 therefore reduces the physical saliency of the rotor poles, thereby mitigating the effects of aerodynamic drag at high speeds. A smooth and cylindrical rotor also leads to the reduction of tonal noise and its high tones.

[0037] Furthermore, the function of the core 44 is that of a radial strut capable of counteracting the radial deformation of the wing 42 due to centripetal forces. In other words, the core 44 provides structural integrity and minimizes radial deformation of the rotor wedge 40 towards the rotorstator air gap when the rotor rotates at high speed. The thickness of the core 44 is appropriately sized to avoid mechanical breakage phenomena due to tensile stresses.

[0038] Preferably, the material could be, for example, a polyamide polymer or it could belong to the class of perfluorocarbons (polytetrafluoroethylene or PTFE) with excellent tensile strength and resistance to high temperatures (>200 °C). The use of a non-magnetic polymer, i.e. having a relative magnetic permeability equal to unity, ensures that there is no impact on the electromagnetic performance of the motor.

[0039] Also referring to figure 4, the assembly of the rotor wedge 40 in the rotor 30 consists of inserting the rotor wedge 40 inside the rotor slot 34, or in the space between two consecutive rotor poles 32 of the rotor. The anchoring of the rotor wedge 40 to the rotor 30 is achieved:

[0040] - inserting the shoe 45 of the rotor wedge 40 inside a coupling groove 35 of the rotor. The groove extends radially inward from a radially internal surface 34' of the rotor slot 34 and reproduces the same prismatic, "dovetail" or "dog bone" shape as the shoe 45,

[0041] - locking the two anchoring edges 43 of the rotor wedge 40 with two corresponding pole pieces 33 of two consecutive rotor poles 32, pole pieces that extend circumferentially inside the rotor slot 34 between the two rotor poles 32 and are radially more external than the anchoring edges 43 on which they overlap circumferentially.

[0042] This combination of constraints acts as a self-sufficient interlock that prevents the wedge from being expelled radially outward (i.e. towards the air gap) when the rotor is rotating. Ultimately, when the rotor is rotated, the rotor / wedge structure, thanks to the constraint system created by the two circumferential pole expansions 33 and the dovetail or dogbone-shaped groove 35 of the rotor 30, is a mechanically integral system.

[0043] The advantages outlined above are analytically confirmed by numerical simulations. With reference to figure 5, two graphs are shown that calculate (according to the well-known NASA D-4849 methodology) the aerodynamic losses [W] as a function of the rotation speed [rpm] respectively for a salient pole rotor according to the known technique (Fig. 5a) and for the rotor 30 equipped with interpolar rotor wedges 40, according to the present invention (Fig. 5b).

[0044] The reduction in aerodynamic losses is considerable: for example, for a rotation speed of 20,000 rpm, the aerodynamic loss of the rotor according to the known technique is approximately 1,600 W while the aerodynamic loss of the rotor according to the present invention is approximately 470 W. This is therefore a reduction of over 1 kW which in percentage terms becomes a reduction of approximately 70%.

[0045] As regards the noise produced by aerodynamic losses, the attenuation at high speeds, due to the transition to a smooth rotor structure, is set at a value equal to or greater than 10 dB, according to literature studies. In particular, in the technical literature (IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 52, NO. 1, JANUARY / FEBRUARY 2016), the experimental data of the noise of a salient pole rotor according to the known technique are compared with those of the noise of a cylindrical "dummy" rotor, and therefore physically comparable to a rotor equipped with rotor wedges object of the present invention.

[0046] As is known, salient pole rotors, operating at high speed, are generally characterized by the emission of acoustic noise with high tones originating from the "fan-blades" effect, i.e. that of a bladed rotor of a turbomachine. On the contrary, smooth rotor electric motors are not subject to the above problem. From the above-mentioned study, it can therefore be verified, albeit by means of simulations, how the insertion of rotor wedges physically converts a salient pole rotor into a cylindrical rotor, thus eliminating the "fan-blades" effect.

[0047] In conclusion, the present invention allows to achieve the following main advantages:

[0048] - reduction of aerodynamic losses and acoustic noise, thanks to the introduction of rotor wedges in non-magnetic material that transform a rotor with salient poles into a substantially cylindrical rotor,

[0049] - inertia and mechanical resistance substantially unchanged thanks to the definition of a structure in the form of a "T" beam, very light but mechanically resistant,

[0050] - self-sufficient radial anchoring system, thanks to the fact that the rotor has two circumferential pole expansions and a central groove in the shape of a dovetail or dog bone that create a perfect fit of the rotor wedge inside the rotor slot.

[0051] In addition to the embodiment of the invention, as described above, it is to be understood that numerous other variations exist. It is also to be understood that such embodiments are exemplary only and do not limit the scope of the invention, its applications, or its possible configurations. Conversely, while the above description enables the skilled craftsman to carry out the present invention at least according to one exemplary embodiment thereof, it should be understood that many variations of the described components are possible without departing from the scope of the invention, as defined in the appended claims, which are construed literally and / or according to their legal equivalents.

Claims

C LA I M S1. Switched reluctance electric machine (10) which has an axis (X) of rotation and includes:- a stator (20) provided with a plurality of salient stator poles (22), arranged circumferentially and separated by corresponding stator slots (24),- a rotor (30) contained within the stator (20) and provided with a plurality of salient rotor poles (32), arranged circumferentially and separated by corresponding rotor slots (34), the electric machine (10) being characterized by the fact that it comprises a plurality of interpolar rotor wedges (40), made of non-magnetic polymeric material, housed in corresponding rotor slots (34) and provided with a structure similar to a T-shaped beam wherein the non-magnetic polymeric material is a polyamide polymer or belongs to the perfluorocarbon class.

2. Electric machine (10) according to claim 1, wherein each rotor wedge (40) comprises:- a wing (42) with a curvilinear and convex shape,- a shoe (45) of prismatic shape, e- a core (44) which develops in a radial direction, connecting the shoe (45) and the wing (42).

3. Electric machine (10) according to claim 2, wherein the wing (42) has a curvature substantially equal to the curvature of the rotor (30), an axial dimension equal to the axial dimension of the rotor (30) and acircumferential dimension slightly smaller than the circumferential dimension of the rotor slot (34).

4. Electric machine (10) according to claim 2 or 3, wherein at its ends in the circumferential direction, the wing (42) has respective anchoring edges (43) having a greater curvature than the curvature of the wing (42).

5. Electric machine (10) according to claim 4, wherein the rotor wedge (40) is anchored in use to the rotor (30) by means of:- a first constraint between the shoe (45) and a coupling groove (35) of the rotor (30),- a second constraint between the two anchoring edges (43) of the rotor wedge (40) and two corresponding pole protrusions (33) of two consecutive rotor poles (32), pole protrusions which extend circumferentially inside the rotor slot (34) between the two rotor poles (32).

6. Electric machine (10) according to claim 5, wherein the groove (35) extends radially inward with respect to a radially internal surface (34') of the rotor slot (34) and reproduces the same prismatic shape as the shoe (45).

7. Electric machine (10) according to claim 6, wherein the prismatic shape of the shoe (45) and the groove (35) is a "dovetail" or "dog bone" shape.

8. Electric machine (10) according to claim 5, wherein the pole protrusions (33) are radially more external than the anchoring edges (43) on which they overlap circumferentially.

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