Outer rotor wheel hub motor with u-shaped oil cooling circuit

Abstract

The utility model discloses a kind of outer rotor wheel hub motor with U-shaped oil cooling circuit, including outer rotor, inner stator, inside and outside oil seal, wheel hub bearing, brake caliper, brake disc etc., the outer rotor includes shell body, magnet steel etc., outer rotor drives wheel rotation, the brake caliper and brake disc are integrated in the inside of inner stator, the inner stator, including structural electrical part and U-shaped oil cooling circuit sealing part, two parts together formed the U-shaped oil cooling circuit of groove bottom parallel connection in groove of inner stator then again series connection, the utility model wheel hub motor integrates brake and wheel hub bearing, structure height is compact, realize the function of driving and brake integration. Cooling oil in groove of inner stator and winding directly contact in large area, cooling effect is greatly improved compared to existing outer rotor water-cooled wheel hub motor, stator support inside does not need to lay water channel, adopt ultra-thin structure, provide greater space for brake.

Description

Technical Field

[0001] This utility model relates to an external rotor hub motor for vehicle drive used in pure electric or hybrid passenger vehicles and light commercial vehicles, belonging to the field of hub motor technology. Background Technology

[0002] In the drive motors of new energy vehicles, the main type is the external stator and internal rotor motor. Among them, oil cooling technology has developed rapidly, but it is basically a mode of cooling by slotting the outer circle of the iron core or spraying at the winding ends, or very rarely, there are cases of oil cooling by slotting the iron core teeth or yoke where they contact the windings. The above methods all have some drawbacks. First, if slots are made on the iron core, the magnetic flux density of the stator yoke or teeth will increase, which requires increasing the thickness of the teeth or yoke, thus increasing the size of the motor. Second, even if slots are made on the teeth, the contact area between the cooling oil and the windings is very limited, so the heat dissipation effect is still not good enough. Third, if cooling is only done on the outer circle of the iron core or the winding ends, the motor windings generate a lot of heat in the slots, and the inability to directly cool the hottest part in the slots will further limit the heat dissipation effect.

[0003] In the application of external rotor hub motors in new energy vehicles, the external rotor permanent magnet motor is directly installed in the wheel, which brings a series of benefits to the whole vehicle. The motor directly drives the wheel, the transmission chain is greatly simplified compared to the central motor drive, the transmission efficiency is greatly improved, the overall vehicle layout is very simple, and more space is provided inside the vehicle. However, while bringing these advantages, the external rotor hub motor has high cooling requirements due to the limited space inside the wheel and the harsh environment.

[0004] Most external rotor hub motors use a direct-drive mode. With an internal stator structure, and the stator inside the rotor, if an oil-cooling system is implemented, the cooling oil will flow onto the rotor housing. As the rotor housing rotates at high speed, the oil impacting the housing causes unacceptable churning noise and mechanical wear. The oil on the rotor housing also worsens the dynamic balance, and collecting the cooling oil is difficult. Due to these issues, current external rotor hub motors typically use a water channel system inside the stator support. The coolant circulates in the water channel to cool the support, which then cools the inner stator core, which in turn cools the windings. This method has poor cooling efficiency. Furthermore, because of the water channel, the inner stator support becomes thicker in the radial direction, compressing the space for the internal brake and often resulting in insufficient mechanical braking capability of the hub motor.

[0005] To address these issues, we provide an external rotor hub motor with a U-shaped oil-cooling circuit. Summary of the Invention

[0006] 1) Technical problems to be solved

[0007] This utility model proposes an external rotor hub motor with a U-shaped oil cooling circuit. The hub motor adopts a permanent magnet direct drive design, integrates hub bearings and brakes, and sets up a U-shaped oil cooling circuit system on the inner stator, thus solving the problems mentioned in the background art.

[0008] Technical solution

[0009] To achieve the above objectives, this utility model provides the following technical solution: an external rotor hub motor with a U-shaped oil-cooling circuit, comprising an external rotor, an inner stator, inner and outer oil seals, a hub bearing, a rim nut, a rim bolt, a brake caliper, a brake disc, a suspension bracket, and suspension bracket fixing bolts. The inner stator includes structural and electrical components as well as a U-shaped oil-cooling sealing system. The structural and electrical components include a stator bracket, a stator core, stator external teeth, slot insulation, a concentrated winding coil, and a three-phase cable, providing support and electrical circuitry for the inner stator.

[0010] The U-shaped cooling circuit sealing system consists of a non-magnetic ultra-thin sheath, a cable outlet end plate, a non-cable outlet end plate, an epoxy sealant strip, a rubber sealant strip, an oil cooling pipe joint, a cable sealing joint, and an O-ring. Together with the structural electrical parts, it forms a U-shaped cooling circuit that is connected in parallel and then in series within the slot bottom of the inner stator.

[0011] Furthermore, the outer rotor includes an outer casing, magnets, and an oil seal seat. The outer casing is fixed to the wheel hub bearing with bolts to drive the wheel to rotate. The magnets are bonded to the outer casing using Loctite adhesive. The oil seal seat is installed on the outer casing with connecting bolts. The inner and outer oil seals include an inner oil seal and an outer oil seal. The inner oil seal is installed on the inner stator, and the outer oil seal is installed on the oil seal seat. The inner and outer oil seals seal the inside of the motor. The wheel hub bearing is fixed to the wheel rim with wheel rim bolts and wheel rim nuts, and can bear the rotational and radial loads of the entire vehicle.

[0012] Furthermore, the brake caliper is mounted on the stator bracket, the brake disc is connected to the housing and wheel rim by bolts, and can provide braking force to the whole vehicle. The suspension bracket is fixed to the stator bracket by mounting bolts, and the suspension of the whole vehicle is mounted on the suspension bracket.

[0013] Furthermore, the motor is designed with concentrated windings, in which the coils are all mounted on the corresponding stator external teeth. There is an assembly gap between the two coils in each slot, which forms the cooling circuit in the slot.

[0014] Furthermore, the stator support is heat-sheltered onto the stator core, and each stator support has a cooling groove on its outer surface facing each core slot. The cooling groove is connected in parallel with the cooling circuit inside the groove to achieve a better cooling effect.

[0015] Furthermore, the stator core is thermally fitted with a non-magnetic ultra-thin sheath on its outer circumference. The non-magnetic ultra-thin sheath is made of non-magnetic material and is very thin, thus isolating the cooling oil from the external space.

[0016] Furthermore, the flow channel partition strips are distributed at both ends of the coil. The flow channel partition strips are composed of epoxy sealant strips and rubber sealant strips fitted on the outside. The epoxy sealant strips are pressed into a shaped form at a set position. The flow channel partition strips determine the direction of the U-shaped cooling circuit.

[0017] Furthermore, the non-outgoing end plate is provided with a sealing strip holder, which is directly opposite the flow channel separator strip at the end of the non-outgoing coil. The outgoing end plate is provided with a sealing strip holder, which corresponds one-to-one with the flow channel separator strip at the end of the outgoing coil.

[0018] Furthermore, the cooling circuit is designed to be routed as follows: based on the gap between the coils in the slots and the dimensions of the cooling slots on the stator support, the cross-sectional area of ​​the oil passage is calculated. Combined with the heat loss of the external rotor hub motor, it is determined that N slots are connected in parallel to form a group. The total number of slots is divided by N to obtain the total number of groups. Each adjacent group is connected in series to form a total U-shaped cooling circuit, with the oil inlet and oil outlet adjacent to each other.

[0019] (iii) Beneficial effects:

[0020] Compared with existing technologies, this external rotor hub motor with a U-shaped oil cooling circuit has the following advantages:

[0021] This utility model designs an external rotor direct drive motor scheme, which adopts a concentrated winding, does not have a reducer, and has the brake built into the motor. It also integrates wheel hub bearings and suspension mounting devices. The motor is compact in size, has high power density, and achieves high-efficiency and high-reliability transmission within the wheel.

[0022] This invention employs a parallel oil circuit method connecting the stator slots and the stator support cooling slots, which can cool the windings and the core together. At the same time, the oil inlet and outlet of the cooling circuit are adjacent, with the oil inlet having a lower temperature and the oil outlet having a higher temperature. The two can conduct and balance, further improving the cooling effect.

[0023] 3. There is a gap between the two coils in each slot of the inner stator. This utility model uses this gap for cooling. The cooling oil in the slot has direct and large-area contact with the winding, which greatly improves the cooling effect compared with the existing water-cooled hub motor with external rotor, and is also significantly better than the existing oil-cooled motor with external stator and internal rotor.

[0024] 4. This utility model uses a non-magnetic ultra-thin sheath to achieve external sealing of the stator. The cooling oil circulates only inside the stator, which solves the problem caused by cooling oil leakage into the rotor housing. The U-shaped cooling circuit does not require slotting in the teeth and yoke of the iron core, thus avoiding increasing the local magnetic flux density of the teeth and yoke, and will not weaken the performance of the hub motor.

[0025] 5. The stator support of this utility model does not require the liquid cooling circuit commonly used in existing external rotor hub motors. Therefore, the stator support can be designed to be thinner in the radial direction, which allows for a larger internal cavity size and the design of a larger brake disc. This improves the mechanical braking capability of the hub motor and solves the problem of insufficient mechanical braking in hub motors.

[0026] 6. The inner stator coil of this utility model can be kept at a low temperature and is basically unaffected by the ambient temperature. As a result, the stator resistance will not increase due to the thermal effect. The copper loss is small during operation, the motor has higher efficiency, and the motor can output greater torque, further improving the torque density of the hub motor. Attached Figure Description

[0027] To more clearly illustrate the specific embodiments of this utility model or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0028] Figure 1 This is a cross-sectional view of an external rotor hub motor with a U-shaped oil cooling circuit according to this utility model;

[0029] Figure 2 This utility model relates to a main view of an external rotor hub motor with a U-shaped oil cooling circuit. Figure 1 ;

[0030] Figure 3 This utility model relates to a main view of an external rotor hub motor with a U-shaped oil cooling circuit. Figure 2 ;

[0031] Figure 4 This is a diagram of the U-shaped oil cooling circuit of the inner stator connected in parallel inside and at the bottom of the tank according to this utility model.

[0032] Figure 5 This is a front view of the inner stator of an external rotor hub motor with a U-shaped oil cooling circuit according to this utility model;

[0033] Figure 6 This is a cross-sectional view of the inner stator of an external rotor hub motor with a U-shaped oil cooling circuit according to this utility model;

[0034] Figure 7 This is a diagram of the stator support of this utility model;

[0035] Figure 8 This is a diagram of the stator core of this utility model;

[0036] Figure 9 This is a schematic diagram of the stator external teeth of this utility model;

[0037] Figure 10 This is a diagram of a single concentrated winding coil of this utility model;

[0038] Figure 11 This is a diagram of the coil core assembly of this utility model;

[0039] Figure 12 This is a schematic diagram of the molded epoxy sealant strip (outgoing and non-outgoing ends) of this utility model;

[0040] Figure 13 This is a schematic diagram of the rubber sealing strip (outlet end and non-outlet end) of this utility model;

[0041] Figure 14 This is a schematic diagram of the flow channel separator (a combination of rubber sealing strip and epoxy sealing putty strip) of this utility model;

[0042] Figure 15 This is a diagram of a coil core assembly with a non-magnetic ultra-thin sheath and flow channel separator strips according to this utility model.

[0043] Figure 16 This is a schematic diagram showing the distribution of the flow channel separator strips at the end of the coil at the output end of this utility model;

[0044] Figure 17 This is a schematic diagram showing the distribution of the flow channel separator strips at the non-outgoing coil end of this utility model;

[0045] Figure 18 Figure 2-1 shows the output end plate of this utility model;

[0046] Figure 19 Figure 2-2 shows the output end plate of this utility model;

[0047] Figure 20 Figure 2-1 shows the non-outgoing end plate of this utility model;

[0048] Figure 21 Figure 2-2 shows the non-outgoing end plate of this utility model.

[0049] In the diagram: 1. Oil cooling pipe joint; 2. First O-ring seal; 3. Outlet end rubber sealing strip; 4. Outlet end epoxy sealing putty strip; 5. Stator external teeth; 6. Non-magnetic ultra-thin sheath; 7. Non-outlet end epoxy sealing putty strip; 8. Non-outlet end rubber sealing strip; 9. Second O-ring seal; 10. Concentrated winding coil; 11. Non-outlet end plate; 12. Third O-ring seal; 13. Stator core; 14. Stator bracket; 15. Outlet end plate; 16. Fourth O-ring seal. 17. Connecting bolt; 18. Three-phase cable; 19. Cable sealing joint; 20. Stator bracket cooling tank; 21. Flow channel separator; 22. Sealing strip holder; 23. Rim nut; 24. Rim bolt; 25. Hub bearing; 26. Inner oil seal; 27. Outer shell; 28. Magnet; 29. ​​Oil seal seat; 30. Outer oil seal; 31. Brake caliper; 32. Brake disc; 33. Suspension bracket; 34. Suspension bracket fixing bolt; 35. Inner stator; 36. Wheel rim. Detailed Implementation

[0050] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.

[0051] like Figure 1 - Figure 21 As shown, this utility model provides a technical solution: an external rotor hub motor with a U-shaped oil cooling circuit, including an external rotor, an inner stator 35, an inner oil seal 26, an outer oil seal 30, a hub bearing 25, a rim nut 23, a rim bolt 24, a brake caliper 31, a brake disc 32, a suspension bracket 33, and a suspension bracket fixing bolt 34.

[0052] like Figures 1-3 As shown, the outer rotor consists of a housing 27, magnets 28, and an oil seal seat 29. The housing 27 is bolted to the hub bearing 25 to drive the wheel rotation. The magnets 28 are bonded to the housing 27 using Loctite adhesive. The oil seal seat 29 is mounted to the housing 27 using connecting bolts. The inner oil seal 26 is mounted on the inner stator 35, and the outer oil seal 30 is mounted on the oil seal seat 29. The inner and outer oil seals provide a sealed environment inside the motor. The hub bearing 25 is fixed to the rim using rim bolts 24 and rim nuts 23, bearing the rotational and radial loads of the entire vehicle.

[0053] The brake caliper 31 is mounted on the stator bracket 14, and the brake disc 32 is connected to the housing 27 and the wheel rim 36 by bolts, providing braking force for the entire vehicle. The suspension bracket 33 is fixed to the stator bracket 14 by suspension bracket fixing bolts 34, and the suspension of the entire vehicle is mounted on the suspension bracket 33.

[0054] like Figure 5 , 6 As shown, the inner stator 35 of the external rotor hub motor with U-shaped oil cooling circuit mainly includes structural electrical parts and a U-shaped cooling circuit sealing system.

[0055] The structural electrical components include stator support 14, stator core 13, stator external teeth 5, slot insulation, concentrated winding coil 10, and three-phase cable 18, providing support and electrical circuits for the entire inner stator 35. The U-shaped cooling circuit sealing system consists of a non-magnetic ultra-thin sheath 6, a terminal plate 15, a non-terminal plate 11, a flow channel separator 21, an oil cooling pipe joint 1, a cable sealing joint 19, a first O-ring 2, a second O-ring 9, a third O-ring 12, and a fourth O-ring 16. Together with the structural electrical components, they form a U-shaped cooling circuit in which the slot bottom of the inner stator 35 is connected in parallel and then in series.

[0056] Each inner stator 35 has 24 concentrated winding coils 10. The concentrated winding coils 10 are assembled on the teeth of the stator core 13. There is a certain gap between the two coils in each slot. This utility model utilizes the space between the two coils in the slot for large-area direct cooling of the winding.

[0057] like Figure 7 , Figure 8 , Figure 9 , Figure 10 and Figure 11 As shown, the stator core 13 is heat-fitted onto the outer circumference of the stator support 14. The stator support 14 has a stator support cooling groove 20 on the outer surface of each core slot, through which cooling oil can flow. Slot insulation is laid in the slots of the stator core 13. After the 24 concentrated winding coils 10 are wound, they are installed onto the teeth of the stator core 13. Then, the connecting wires between the coils are soldered at the output end. Three three-phase cables 18 are soldered and insulated at the three-phase output end. The 24 stator external teeth 5 are punched separately. After the coils are installed, the stator external teeth 5 are installed onto the teeth of the stator core 13, thus forming the coil core assembly.

[0058] The third O-ring 12 and the fourth O-ring 16 are installed in the slots at both ends of the stator bracket 14. The non-magnetic ultra-thin sheath 6 is placed on the outer circle heat sleeve of the coil core assembly. The non-magnetic ultra-thin sheath 6 is made of 0.5 mm thick non-magnetic stainless steel material. The non-magnetic ultra-thin sheath 6 isolates the cooling oil from the external space.

[0059] like Figures 12-21 As shown, epoxy sealant is pressed into a molded shape at designated positions at both ends of the coil. After curing, epoxy sealant strip 4 at the lead end and epoxy sealant strip 7 at the non-lead end are formed. Rubber sealant strip 3 at the lead end and rubber sealant strip 8 at the non-lead end are respectively installed on the outside. Each assembly of epoxy sealant strip and rubber sealant strip forms a flow channel separator 21. There are four flow channel separators 21 at the non-lead end and five flow channel separators 21 at the lead end.

[0060] The second O-ring 9 is fitted onto the outer circle of the non-outgoing end plate 11. Then, the four sealing strips 22 of the end plate are aligned with the flow channel separator 21 at the end of the coil. The non-outgoing end plate 11 is pressed into the coil core assembly and the connecting bolts 17 are tightened.

[0061] The first O-ring 2 is fitted onto the outer circle of the end plate 15 at the output end. The five sealing strip positions 22 of the end plate are respectively aligned with the flow channel separator 21 at the end of the coil. The three-phase cable 18 is led out through the hole on the end plate. Then the end plate 15 at the output end is pressed into the coil core assembly and the connecting bolt 17 is tightened.

[0062] Install three cable sealing connectors 19 on the end plate 15 at the outgoing end and tighten them to prevent leakage, then install two oil cooling pipe connectors 1.

[0063] In this embodiment, the stator core 13 has 24 slots, arranged in groups of three. The three slots are cooled in parallel, totaling eight groups. The groups are cooled in series, forming a cooling system as shown below. Figure 4 The U-shaped oil cooling circuit shown is connected in parallel inside and at the bottom of the tank.

[0064] There are five flow channel separators 21 at the outlet end, and their positions must be strictly in accordance with... Figure 16 There are four non-outgoing line terminals, and their locations must be strictly in accordance with... Figure 17 The wiring is installed with five sealing strip slots 22 on the end plate 15 at the outgoing end. Figure 18 The distribution includes four sealing strip slots 22 on the non-outgoing end plate 11, according to... Figure 20 The distribution, flow channel partition strip 21, and sealing strip positioning 22 are key to forming a U-shaped oil cooling circuit.

[0065] The group of slots 1, 2 and 3 are directly opposite the oil inlet, and the group of slots 22-24 are the oil outlet. Of course, since they are completely symmetrical, the oil inlet and outlet can be interchanged, and the cooling effect is the same.

[0066] In this embodiment, a non-magnetic ultra-thin sheath of sequence 6 is used to establish the inner stator sealing environment. Alternatively, the inner stator sealing environment can be established by bonding stator laminations and then injection molding. The effect achieved by both methods is the same.

[0067] In the description of this utility model, it should be understood that the terms "left", "right", "up", "down", "top", "bottom", "front", "back", "inner", "outer", "back", "middle", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this utility model.

[0068] However, the above description is only a specific embodiment of this utility model and should not be construed as limiting the scope of implementation of this utility model. Therefore, any substitution of equivalent components or equivalent changes and modifications made in accordance with the scope of protection of this utility model should still fall within the scope of the claims of this utility model.

Claims

1. An external rotor hub motor with a U-shaped oil-cooling circuit, comprising an external rotor, an inner stator (35), inner and outer oil seals, a hub bearing (25), a rim nut (23), a rim bolt (24), a brake caliper (31), a brake disc (32), a suspension bracket (33), and suspension bracket fixing bolts (34), characterized in that: The inner stator (35) includes a structural electrical component and a U-shaped oil-cooled sealing system. The structural electrical component includes a stator bracket (14), a stator core (13), stator external teeth (5), slot insulation, a concentrated winding coil (10), and a three-phase cable (18), which provide support and electrical circuits for the inner stator. The U-shaped oil-cooled sealing system consists of a non-magnetic ultra-thin sheath (6), a cable outlet end plate (15), a non-cable outlet end plate (11), a flow channel separator (21), an oil-cooled pipe joint (1), a cable sealing joint (19), and an O-ring seal. Together with the structural electrical parts, it forms a U-shaped cooling circuit that is connected in parallel and then in series with the bottom of the slot in the inner stator.

2. The external rotor hub motor with a U-shaped oil-cooling circuit according to claim 1, characterized in that: The outer rotor includes an outer shell (27), a magnet (28), and an oil seal seat (29). The outer shell (27) is fixed to the wheel hub bearing (25) by bolts, directly driving the wheel to rotate. The magnet (28) is bonded to the outer shell (27) using Loctite adhesive. The oil seal seat (29) is installed on the outer shell (27) by connecting bolts. The inner and outer oil seals include an inner oil seal (26) and an outer oil seal (30). The inner oil seal (26) is installed on the inner stator (35), and the outer oil seal (30) is installed on the oil seal seat (29). The inner and outer oil seals seal the inside of the motor. The wheel hub bearing (25) is fixed to the wheel rim by wheel rim bolts (24) and wheel rim nuts (23), and can bear the rotation and radial load of the whole vehicle.

3. The external rotor hub motor with a U-shaped oil cooling circuit according to claim 1, characterized in that: The brake caliper (31) is mounted on the stator bracket (14), and the brake disc (32) is connected to the outer shell (27) and wheel rim (36) by bolts, which can provide braking force to the whole vehicle. The suspension bracket (33) is fixed to the stator bracket (14) by the suspension bracket fixing bolt (34), and the suspension of the whole vehicle is mounted on the suspension bracket (33).

4. The external rotor hub motor with a U-shaped oil cooling circuit according to claim 1, characterized in that: The motor is designed with a concentrated winding. The concentrated winding coils (10) are all mounted on the corresponding stator external teeth (5). There is an assembly gap between the two coils in each slot. The assembly gap constitutes the cooling circuit in the slot.

5. The external rotor hub motor with a U-shaped oil cooling circuit according to claim 1, characterized in that: The stator support (14) is heat-sheltered onto the stator core (13). The stator support (14) has cooling grooves on the outer surface of each core slot. The cooling grooves are connected in parallel with the cooling circuit inside the grooves to achieve better cooling effect.

6. The external rotor hub motor with a U-shaped oil cooling circuit according to claim 1, characterized in that: The stator core (13) is covered with a non-magnetic ultra-thin sheath (6) on its outer circumference. The non-magnetic ultra-thin sheath (6) is made of non-magnetic material and is very thin. The non-magnetic ultra-thin sheath (6) isolates the cooling oil from the external space.

7. The external rotor hub motor with a U-shaped oil-cooling circuit according to claim 1, characterized in that: The flow channel partition strip (21) is distributed at both ends of the coil. The flow channel partition strip (21) is composed of an epoxy sealant strip and a rubber sealant strip sleeved on the outside. The epoxy sealant strip is pressed into a shaped form at a set position. The flow channel partition strip (21) determines the direction of the U-shaped cooling circuit.

8. The external rotor hub motor with a U-shaped oil cooling circuit according to claim 1, characterized in that: The non-outgoing end plate (11) is provided with a sealing strip slot (22), which is directly opposite to the flow channel separator (21) at the end of the non-outgoing end coil. The outgoing end plate (15) is provided with a sealing strip slot (22), which corresponds to the flow channel separator (21) at the end of the outgoing end coil.

9. An external rotor hub motor with a U-shaped oil-cooling circuit according to claim 1, characterized in that: The cooling circuit is designed to be designed by calculating the cross-sectional area of ​​the oil passage based on the gap between the coils in the slot and the size of the cooling slot on the stator support (14). The heat loss of the outer rotor hub motor is combined with the calculation to determine that N slots are connected in parallel to form a group. The total number of slots is divided by N to obtain the total number of groups. Each adjacent group is connected in series to form a total U-shaped cooling circuit with the oil inlet and outlet adjacent to each other.

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