Electric machine with a displacement body

Abstract

The invention relates to an electric machine (1) with a displacement body (6), wherein the electric machine (1) is designed in the form of a separately excited, directly cooled synchronous machine, comprising a housing in which a rotor (2), comprising winding carriers (3) arranged star-shaped to one another, each having a winding (4), is rotatably mounted, and which is encompassed by a stator, and wherein a winding gap (5) is formed between adjacent windings (4), in which the displacement body (6) with a cooling channel (7) is mounted. According to the invention, the displacer body (6) is designed in multiple parts, consisting of at least two individual parts (12, 21), wherein the individual parts (12, 21) comprise a first module (14) in the form of an intermediate element (11) of the displacer body (6), and / or a second module (15) in the form of a first end region (9) of the displacer body (6) and / or a third module (22) in the form of a second end region (13) of the displacer body (6).

Description

[0001] The invention relates to an electrical machine with a displacement body according to the preamble of patent claim 1.

[0002] It is known to equip electrical machines in the form of separately excited, directly cooled synchronous machines with so-called displacement bodies that usually do not contribute to the power of the synchronous machine, whereby the displacement bodies are used to guide the coolant. In order to achieve the desired power of the synchronous machine, the displacement bodies are usually designed with a delicate design, so that the introduction and / or manufacture and / or sealing of cooling channels can usually be achieved with great effort, or their design is only possible to a limited extent. Furthermore, an installation space formed between the winding overhangs offers the opportunity to integrate and optimize additional functions. However, this installation space is not usable, for example, with the radial insertion of one-piece displacement bodies.

[0003] German Patent Application DE 19 62 591 A1 discloses a directly cooled, separately excited synchronous machine with a metal displacement body arranged between winding heads. Cooling channels are formed in the displacement body, and the displacement body is axially defined by an end cap. The end caps allow a cooling medium to be collected and discharged. The multi-part displacement body comprises several identical components and is designed to be longitudinally splayed.

[0004] The published patent application DE 10 2012 217 711 A1 describes a directly cooled, separately excited synchronous machine that has a displacement body between its winding ends, which also has cooling channels. Cooling areas allow a cooling medium to be collected and / or distributed.

[0005] From the published patent application DE 10 2021 210 498 A1, a separately excited synchronous machine is known, with a displacement body arranged between winding heads, which has cooling channels.

[0006] Published patent applications EP 2 985 885 A1 and EP 3 916 963 A1 each disclose a directly cooled, separately excited synchronous machine with a displacement body arranged between winding heads. The displacement body has cooling channels and is axially delimited by an end cap. A cooling medium can be radially conducted via the end caps.

[0007] Published patent application WO 2010 / 099974 A2 discloses a directly cooled, separately excited synchronous machine that has a displacement body comprising cooling channels between its winding heads. The displacement body is axially limited by an end cap. The displacement body is formed as an injection-molded component injected directly between the winding heads.

[0008] The published patent application DE 10 2019 212 391 A1 describes a directly cooled, separately excited synchronous machine that has a displacement body with cooling channels between its winding ends. The displacement body is designed to be longitudinally spread.

[0009] From published patent application DE 10 2021 111 321 A1, an electrical machine in the form of a separately excited, directly cooled synchronous machine with a displacement body is known. The electrical machine has a housing in which a rotor is rotatably accommodated. The rotor has winding supports arranged in a star configuration, each with a winding. A winding gap is formed between adjacent windings, in which the displacement body is accommodated with a cooling channel. The displacement body is formed in several parts, consisting of at least two individual components.

[0010] Laid-open publication KR 10 2018 115 911 A also discloses an electric machine with a displacement body having a two-part cooling channel, wherein the cooling channel is divided into two parts by a web. The displacement body is divided along a longitudinal axis of the electric machine, transverse to the latter, into several identical components that are manufactured separately.

[0011] The object of the present invention is to provide an improved electrical machine with a displacement body.

[0012] This object is achieved according to the invention with an electric machine having a displacement body with the features of patent claim 1. Advantageous embodiments with expedient and non-trivial further developments of the invention are specified in the respective subclaims.

[0013] An electrical machine according to the invention with a displacement body, wherein the electrical machine is designed in the form of a separately excited, directly cooled synchronous machine, comprises a housing in which a rotor, comprising winding supports arranged in a star shape relative to one another, each having a winding, is rotatably received, and which is surrounded by a stator, and wherein a winding gap is formed between adjacent windings, in which winding gap the displacement body is received with a cooling channel.According to the invention, the displacement body is designed in several parts, consisting of at least two individual parts, wherein the individual parts have a first module in the form of an intermediate element of the displacement body, and a second module in the form of a first end region of the displacement body and / or a third module in the form of a second end region of the displacement body, and wherein the displacement body has the cooling channel in several cooling sections, wherein at least the first cooling channel section and the second cooling channel section are formed separately from one another by means of a web, and the first module is designed in two parts along a longitudinal axis of the electrical machine, and has a first sub-module and a second sub-module which are produced in the form of an identical part.The advantage of the displacement body, which is formed separately in individual parts, is that complex geometries can be realized, particularly cost-effectively, such as a cooling channel with multiple cooling channel sections or a collecting volume for the supply and / or removal of coolant. This not only allows the cooling channel to be technically optimized, but also allows for improved sealing surfaces at the coolant transfer points in the end sections. Another significant advantage is that windings adjacent to the end sections can also be securely fixed.

[0014] To improve cooling of stressed areas of the electrical machine, the cooling channel has separate cooling channel sections, whose geometries can be adapted to the cooling requirements. These cooling channel sections are stabilized by the web against possible deformation caused by winding forces acting on the displacement body. In other words, the overall stiffness of the displacement body can be locally adjusted using the webs.

[0015] Because the first module having the cooling channel has a first sub-module and a second sub-module, the cooling channel or the cooling channel sections can be manufactured cost-effectively, in particular by an injection molding or similar process.

[0016] According to the invention, the first sub-module and the second sub-module are manufactured in the form of a single identical part. To create the first module, one of the two identical parts must be turned over so that it is arranged opposite the other identical part in a mirror image. This allows, in particular, manufacturing costs to be reduced, since only one mold needs to be made to produce the identical part, and manufacturing costs are further reduced due to the doubled number of identical parts.

[0017] In one embodiment of the electrical machine according to the invention, the individual parts are connected to one another by means of a material-locking and / or form-locking and / or force-locking connection.

[0018] In a further embodiment of the electrical machine according to the invention, the displacement body is designed to be spread along its longitudinal extent along the longitudinal axis of the electrical machine. This allows for secure fixation of the windings even in the end regions. If the displacement body were designed as a one-piece component, it could not be inserted or mounted axially into the winding gap due to the spread at its end regions. Radial mounting would not allow radial support of the displacement body on a sheet metal section of the electrical machine and would not achieve the necessary displacement effect for fixing the windings.

[0019] Advantageously, the second module and / or the third module are designed to be spread along the longitudinal extent of the displacement body. With the help of the displacement body, which is locally spread, particularly at its end regions, an enlargement of a sealing surface is achieved for improved sealing at a coolant transfer point formed between the end regions and the cooling channel or its cooling channel sections. The second module and the third module are designed to seal against the rotor.

[0020] In a further embodiment of the electrical machine according to the invention, the first module has a shoulder at its end facing the second module or the third module. With the help of the shoulder, a stop for axially joining the second module or the third module can be easily formed. Furthermore, the shoulder defines a joining contour for a sealing surface formed between the first module and the second module or the third module.

[0021] In a further embodiment of the electric machine according to the invention, the second module and / or the third module have a collecting basin for conveying the coolant. This allows the cooling channel formed in the first module, or the cooling channel sections, to be supplied with coolant, preferably continuously.

[0022] In a further embodiment of the machine according to the invention, a reinforcement ring of the electric machine is designed to accommodate the second module, and / or a cover ring of the electric machine is designed to accommodate the third module. Thus, a cost-effective reinforcement ring and / or a cost-effective cover ring can be produced, since complex machining of the reinforcement ring and / or the cover ring is eliminated due to the provision of the correspondingly necessary geometries in the second module and / or third module.

[0023] The reinforcement ring is generally designed for the coolant inlet from the rotor. It can be implemented as a second component, in which case it has the necessary channel structure to direct the coolant into the cooling channel. However, this is generally cost-intensive, as different manufacturing processes are required. If it is designed to accommodate the second module, the collecting basin, for example, can be manufactured simply and thus cost-effectively.

[0024] In a further embodiment of the electric machine according to the invention, the third module is designed for coolant outlet, with an element surface facing the stator having at least one outlet opening. Thus, the third module can be easily and cost-effectively manufactured in the form of the second end region by incorporating the outlet opening and connecting it to the collecting basin formed in the third module.

[0025] Further advantages, features, and details of the invention will become apparent from the following description of preferred embodiments and from the drawings. The features and feature combinations mentioned above in the description, as well as the features and feature combinations mentioned below in the description of the figures and / or shown alone in the figures, can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention. Identical or functionally equivalent elements are assigned identical reference numerals. They show: Fig. 1 in a radial plan view an electrical machine 1 according to the prior art, Fig. 2 shows an axial plan view of a section of the electrical machine 1 according to the prior art, Fig. 3 in a radial plan view of a section of an electrical machine according to the invention on a second end face designed for coolant removal, Fig. 4 in an axial plan view of the electrical machine according to the invention. Fig. 3, Fig. 5 in a radial plan view a section of the electrical machine according to the invention. Fig. 3, Fig. 6 in a radial plan view a section of the electrical machine according to the invention with a displacement body formed from two individual parts, Fig. 7 in a plan view a third module of the displacement body, Fig. 8 in a perspective view the third module, Fig. 9 in another perspective view the third module, Fig. 10 in an axial plan view a section of the electrical machine according to. Fig. 6, Fig. 11 shows a plan view of a section of a reinforcement ring of the electrical machine according to the invention, Fig. 12 shows a section through the reinforcement ring of the electrical machine according to the invention, Fig. 13 in a section the reinforcement ring in a variant, Fig. 14 in one section the reinforcement ring acc. Fig. 13 in connection with the rotor shaft, Fig. 15 in one section the reinforcement ring acc. Fig. 13 in connection with the rotor shaft and a second module of the displacement body, Fig. 16 in one section the reinforcement ring acc. Fig. 13 in connection with the rotor shaft, a first module and the second module of the displacement body, Fig. 17 in a perspective view the second module of the displacement body, Fig. 18 shows a perspective view of a section of the first module of the displacement body of an electrical machine not according to the invention, Fig. 19 in an axial plan view a section of the electrical machine according to the invention with the displacement body, and Fig. 20 in a front view the displacement body of the electrical machine according to the invention.

[0026] An electrical machine 1 according to the prior art, which is designed in the form of a separately excited, directly cooled synchronous machine, has a cylindrical rotor 2, which has windings 4 on its star-shaped winding supports 3. The winding supports 3 are enclosed by separately formed lamination sections 10. The electrical machine 1 is according to Fig. 1. The rotor 2 is accommodated in a housing (not shown in detail) of the electric machine 1 and is surrounded by a stator (also not shown in detail).

[0027] A winding gap 5 is formed between adjacent windings 4, which is filled with the aid of a displacement body 6 in order to fix the windings 4 and to prevent deformation of the windings 4 due to, for example, centrifugal forces.

[0028] Between the windings 4, the winding gap 5 has a gap cross-section SQ which is larger in the region of end faces 19, 19' of the electrical machine 1, a first end face 19 and a second end face 19' arranged opposite the first end face 19, than in the remaining winding gap 5. In other words, this means that the displacement body 6 should have a variable cross-section Q for the preferred fixing of the windings 4.

[0029] In Fig. Figure 2 illustrates the prior art electric machine 1 in a section of an axial plan view. The displacement body 6 of the electric machine 1 has a constant cross-section Q over its entire length. It can be seen that the winding gap 5 between the windings 4 and the displacement body 6 is not completely closed, so that the fixation of the windings 4 is not ensured.

[0030] The displacement body 6, which is exemplified in Fig. 2 in an axial plan view of the electrical machine 1, has a first end region 9 and a second end region 13 facing away from the first end region 9, as is shown for example in Fig. 15 is shown.

[0031] The first end region 9, which faces the first end face 19, is designed for the inlet of cooling water flowing through the displacement body 6, which is guided in a cooling channel 7 located in the displacement body 6. The second end region 13, which faces the second end face 19', is designed to drain the cooling water toward the stator.

[0032] The displacement body 6 is designed to extend axially along a longitudinal axis 8 of the electric machine 1. An intermediate element 11 of the displacement body 6 is arranged between the first end region 9 and the second end region 13, which has a cross-section Q that is constant over its length.

[0033] In Fig. 3 shows the electrical machine 1 according to the invention according to a first exemplary embodiment in a section of a radial plan view of the second end face 19'. The displacement body 6 has a variable cross-section Q, wherein the cross-section Q is larger in its first end region 9 than the cross-section Q of the intermediate element 11. Preferably, the cross-section Q of the displacement body 6 is larger in its two end regions 9, 13 than the cross-section Q of the intermediate element 11. Or in other words, the displacement body 6 is designed to be locally spread along the longitudinal axis 8. The winding gap 5 is completely filled so that a secure fixing of the windings 4 is realized.

[0034] The displacement body 6 of the electric machine 1 according to the invention is constructed in multiple parts, using a modular design. In other words, the displacement body 6 is not designed as a single component, but rather consists of at least two individual parts, a first individual part 12 and a second individual part 21, which are preferably manufactured separately from one another. The displacement body 6 could also be manufactured from more than two individual parts 12, 21.

[0035] At this point, the displacement body 6 is to be explained in more detail with regard to the possible modular construction and the resulting individual parts 12, 21.

[0036] The intermediate piece 11 of the displacement body 6 can be designed as a first module 14. The first end region 9, which is designed for coolant supply, can be designed as a second module 15. The second end region 13, which is remote from the first end region 9 and is designed for coolant removal, can be designed as a third module 22. In other words, this means that the displacement body 6 consists of at least three modules 14, 15, 22, which can each be manufactured independently of one another or partially in a composite in the form of one of the individual parts 12, 21.

[0037] In Fig. 3, the electric machine 1 according to the invention according to the first embodiment has the displacement body 6 designed in such a way that the first module 14 and the third module 22 are designed in one piece with each other, and the second module 15 with the aid of a Fig. 11 of the reinforcement ring 16 of the electrical machine 1. Thus, the displacement body 6 also has at least two individual parts 12, 21. Or, in other words, the displacement body 6 is realized in two parts, with the aid of the first individual part 12 and the reinforcement ring 16 designed in the form of the second individual part 21.

[0038] In Fig. 4 illustrates the electric machine 1 according to the first exemplary embodiment in a section of a radial plan view of the second end face 19'. Coolant is preferably removed by means of a cover ring (not shown in detail) of the electric machine 1. The cover ring can be designed to accommodate the third module 22.

[0039] In Fig. 5, the electrical machine 1 is illustrated in a radial plan view of the second end face 19', wherein the first module 14 is accommodated between the windings 4, wherein the Fig. 5 second module 15, not shown in detail, is formed integrally with the first module 14.

[0040] In Fig. Figure 6 illustrates a radial plan view of a section of the electric machine 1 according to the invention with the displacement body 6 formed from the two individual parts 12, 21. The first module 14, which together with the second module 15 is designed as the first individual part 12, has the third module 22, which is designed as the second individual part 21.

[0041] The third module 22 is designed for the removal of coolant from the cooling channel 7 and has two outlet openings 23 on an element surface 28 facing the stator, which are connected to a collecting basin 18' formed in the third module 22 in a flow-through manner.

[0042] The third module 22 is in the Fig. 7 to 9 in different views. It has the collecting basin 18', which is arranged opposite the cooling channel 7 formed in the first module 14.

[0043] In Fig. 10 is the electrical machine 1 according to the invention. Fig. 6 in an axial plan view. The third module 22 is designed to seal the cooling channel 7 in the axial direction. In other words, this means that the third module 22 is closed on its outer surface 24 facing away from the collecting basin 18'.

[0044] The Fig. The reinforcement ring 16 shown in Figure 11 has a radially extending annular projection 17 arranged opposite the first module 14. This projection has a collecting basin 18 for receiving and conveying the cooling water into the cooling channel 7, as well as a supply channel 25, which are created by means of a milling process. The supply channel 25 is connected to the collecting basin 18 in a flow-through manner.

[0045] In Fig. Figure 12 illustrates a section of the reinforcement ring 16 in connection with a rotor shaft 20 of the electric machine 1. The coolant, which is preferably in the form of a lubricant, is conveyed by means of the rotor shaft 20 via the collecting basin 18 into the cooling channel 7.

[0046] Likewise, in particular to achieve a cost-effective reinforcement ring 16' as shown in Fig. As illustrated in Figure 13, the second module 15, which is manufactured independently of the first module 14 and the third module 22, can be configured similarly to the third module 22, wherein it is configured to supply coolant into the cooling channel 7. Thus, it would be possible to manufacture the first module 14 and the second module 15 integrally as the first individual part 12 and to configure the third module 22 as the second individual part 21.

[0047] It would also be possible for all three modules 14, 15, 22 to be manufactured independently of one another, so that in addition to the first individual part 12 and the second individual part 21, a third individual part forms the displacement body 6.

[0048] The Fig. 14 shows a section through a reinforcement ring 16', which is a variant of the Fig. 11 formed reinforcement ring 16, also has the annular projection 17, which, however, only has the supply channel 25. The production of the supply channel 25 of the according Fig. 14 can be carried out in a simple manner, since, starting from an outer ring surface 26 of the reinforcing ring 16', a tool for producing the supply channel 25 formed in the ring projection 17 can be attached, which tool can produce the supply channel 25 in a single axial line L. Thus, the supply channel 25 can be produced in the form of a bore in a cost-effective process.

[0049] In an outer ring 27 of the reinforcement ring 16' having the outer ring surface 26, a passage opening 29 is to be provided, which is, however, closed with the aid of the second module 15, as shown in the Fig. 15 and Fig. 16 is illustrated.

[0050] In Fig. 14 shows the reinforcement ring 16' in a section in connection with the rotor shaft 20. The passage opening 29 is open. In Fig. 15 illustrates a section of the reinforcement ring 16' in connection with the rotor shaft 20 and the second module 15, wherein the second module 15 has a closed element surface 28 facing the outer ring 27, which closes the passage opening 29 against an outflow of the coolant.

[0051] Especially in Fig. 16, in which the reinforcement ring 16' is illustrated in section in connection with the rotor shaft 20, the second module 15 and the first module 14, it can be seen that, starting from the rotor 20, the coolant is supplied via the supply channel 25 into the second module 15 and further from the second module 15 into the coolant channel 7. The second module 15 has an element opening 31 on its further element surface 30 facing the supply channel 25 to receive the coolant, through which opening the coolant can enter the second module 15. According to the invention, the coolant channel 7 has three cooling channel sections: a first cooling channel section 36, a second cooling channel section 37 and a third cooling channel section 38, which are formed separately from one another in the first module 14.

[0052] In Fig. Figure 17 illustrates the second module 15 in a perspective view. It has an element base 32 containing the collection basin 18', which is completely enclosed by an element frame 33. In addition to creating the collection basin 18', the element frame serves to form a positive connection with the first module 14.

[0053] It can be seen that the second module 15 and the third module 22 have the same base body, and thus this base body can be produced in large quantities, wherein the openings 23, 31 necessary for the functions of the modules 15, 22 are to be formed in the base body in a further manufacturing step.

[0054] In Fig. 18 is a perspective view of a section of the first module 14 comprising the three cooling channel sections 36, 37, 38 of an electrical machine 1 not according to the invention. The first module 14 has, at its end 34 facing the second module 15, a shoulder 35 which serves to receive the second module 15 in the form of a plug connection. Or, in other words, the second module 15 can be securely arranged on the first module 14 with the aid of the shoulder 35, wherein the first module 14 is at least partially received by the second module 15. Likewise, the first module 14 could also have the shoulder 35 for corresponding reception at its end facing the third module 22, which is not shown in detail.

[0055] In Fig. Figure 19 illustrates a section of the electric machine 1 according to the invention in an axial plan view. The displacement body 6 has the cooling channel 7 with its cooling channel sections 36, 37, 38.

[0056] For cost-effective production of the displacement body 6, having the cooling channel 7 in several cooling channel sections 36, 37, 38, the first module 14 is divided along the longitudinal axis 8, in particular divided into two parts, as in Fig. 20, a front view of the first module 14 according to the variant.

[0057] Between the cooling channel sections 36, 37, 38, webs 41 are formed, which form a support structure for absorbing forces exerted on the displacement body 6 by the windings 4. Inserting the displacement body 6, in particular in the form of the first individual part 12, comprising the first module 14 and either the second module 15 or the third module 22, into the winding gap 5 along the longitudinal axis 8 exerts forces on the first individual part 12 by displacing the windings 4.

[0058] The effect of the displacement body 6 depends on its overall rigidity. If, for example, it is too soft, thus having a low overall rigidity, its effect is less than if it has a high overall rigidity. With the help of the webs 41, the overall rigidity can be increased, since the integrated cooling channel sections 36, 37, 38 can no longer be deformed, or can no longer be deformed as severely. Likewise, it could also happen that the cooling channels are compressed to such an extent that the coolant can no longer flow through the cooling channel sections 36, 37, 38, thus blocking the channel, preventing coolant flow, and thus preventing cooling. This can be avoided with the help of the webs 41.

[0059] It goes without saying that the connection of the individual parts 12, 21, regardless of whether it is a material-locking, a form-locking or a force-locking connection, is designed to bring about a sealed cooling channel 7 or sealed cooling channel sections 36, 37, 38. List of reference symbols 1 Electric machine 2 rotors 3 winding carriers 4 windings 5 Winding gap 6 displacement bodies 7 Cooling channel 8 Longitudinal axis 9 First end area 10 Sheet metal cutting 11 Intermediate element 12 First individual part 13 Second end area 14 First Module 15 Second Module 16, 16' reinforcement ring 17 ring projection 18, 18' collection basin 19, 19' first front side, second front side 20 Rotor shaft 21 Second individual part 22 Third Module 23 Exit opening 24 exterior area 25 supply channel 26 Outer ring surface 27 Outer ring 28 Element area 29 Passage opening 30 Additional element area 31 Element opening 32 Element floor 33 element frames 34 End 35 paragraph 36 First cooling channel section 37 Second cooling channel section 38 Third cooling channel section 39 First sub-module 40 Second sub-module 41 jetty L Line SQ gap cross-section Q cross-section

Claims

[1] Electrical machine (1) with a displacement body (6), wherein the electrical machine (1) is designed in the form of a separately excited, directly cooled synchronous machine, comprising a housing in which a rotor (2), comprising winding supports (3) arranged in a star shape relative to one another, each having a winding (4), is rotatably received, and which is surrounded by a stator, and wherein a winding gap (5) is formed between adjacent windings (4), in which the displacement body (6) is received with a cooling channel (7), characterized bythat the displacement body (6) is designed in several parts, consisting of at least two individual parts (12, 21), wherein the individual parts (12, 21) have a first module (14) in the form of an intermediate element (11) of the displacement body (6), and a second module (15) in the form of a first end region (9) of the displacement body (6) and / or a third module (22) in the form of a second end region (13) of the displacement body (6), and wherein the displacement body (6) has the cooling channel (7) in several cooling sections (36, 37, 38), wherein at least the first cooling channel section (36) and the second cooling channel section (37) are designed separately from one another by means of a web (41), and the first module (14) is designed in two parts along a longitudinal axis (8) of the electrical machine (1), and a first sub-module (39) and a second sub-module (40) which are manufactured in the form of an identical part. [2] Electrical machine (1) according to claim 1, characterized by that the individual parts (12, 21) are connected to one another by means of a material-locking and / or form-locking and / or force-locking connection. [3] Electrical machine (1) according to one of the preceding claims, characterized by that the displacement body (6) is designed to be spread along its longitudinal extent along the longitudinal axis (8) of the electrical machine (1). [4] Electrical machine (1) according to claim 3, characterized by that the second module (15) and / or the third module (22) are spread out along the longitudinal extent of the displacement body (6). [5] Electrical machine (1) according to one of the preceding claims, characterized by that the first module (14) has a shoulder (35) at its end (34) facing the second module (15) or the third module (22). [6] Electrical machine (1) according to one of the preceding claims, characterized bythat the second module (15) and / or the third module (22) each have a collecting basin (18') for forwarding the coolant. [7] Electrical machine (1) according to one of the preceding claims, characterized by that a reinforcement ring (16') of the electrical machine (1) is designed to receive the second module (15), and / or that a cover ring of the electrical machine (1) is designed to receive the third module (22). [8] Electrical machine (1) according to one of the preceding claims, characterized by that the third module (22) is designed for coolant outlet, wherein an element surface (28) facing the stator has at least one outlet opening (23).

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