Photovoltaic inverter extended section hollow cantilever beam wall bracket assembly

The commonly used wall brackets for photovoltaic inverters are shown in Figures 1 and 2. The bearing structure is a small folded edge of sheet metal, which is equivalent to a cantilever beam in mechanics. The width of the cantilever beam section in Figure 1 is the sheet metal wall thickness, and the height of the cantilever beam section in Figure 2 is the sheet metal wall thickness. In order to obtain sufficient strength, either a small folded edge with high and low repetition is used as shown in Figure 1 (causing the outline size to increase, and the radiator or sheet metal parts that match it need to be slotted at all heights), or the sheet metal thickness is increased as shown in Figure 2. Moreover, in order to facilitate the marking of wall holes, the outline of Figure 1 must cover all the upper and lower wall holes, which will inevitably increase the outline size. After I proposed the "split secondary load wall bracket assembly" in January 2018 (based on the lightweight research of photovoltaic inverter wall brackets in ANSYS WORKBENCH), the industry has already seen inverter products using "wall bracket + wall bracket", but because the wall bracket and the wall bracket are not directly connected, a "wall hole marking special" cardboard is added, as shown in Figure 3.

A sheet metal part for a wall bracket that has appeared is shown in Figure 4. Welding technology must be used. The normal direction of the sheet metal fold that fills the gap between the inverter and the wall is parallel to the direction of gravity. In order not to block the cooling airflow from bottom to top, enough holes are processed, and the processing cost is high.

In view of the above shortcomings, this paper proposes four design goals that the new wall bracket assembly must meet:

1) Without repeated small folds, it still meets the "3 times additional deadweight" load test standard specified in IEC 62109;

2) The bracket outline on the wall only covers the upper wall-mounting hole, and no additional parts provided by the inverter seller are required, and it can also be used as a marking ruler for the lower wall-mounting hole;

3) The bracket under the wall does not need welding, does not require porous structure and does not affect heat dissipation;

4) In order to reduce costs to the greatest extent, the upper and lower wall brackets are made of the same material as the box body, and can be processed with the same sheet metal as the box body (or front cover).

Using the design concept of "split secondary load", this paper first completed the structural design of the new wall-mounted bracket assembly, then used the finite element method to check the strength and stiffness, and finally passed the physical prototype test. A utility model patent has been applied for, application number 201920806157.1, the name of the invention: a wall-mounted bracket assembly and a photovoltaic inverter having the same.

Design and simulation of a wide and low 17kg inverter wall bracket assembly

As shown in Figure 5, the new wall-mounted bracket assembly carries a certain type of wide and short inverter. After the known heat generation is optimized (using response surface optimization to evaluate the heat dissipation solution of 150KW inverter), the radiator cross-section width is 403, the bottom plate thickness is 10, the rib height is 40, the stretched length is 222, and the mass is 5.2kg. The box body is 403 wide, 402 high, 94 deep, and has a mass of 11.8kg. In order to not block the airflow, the porous flat plate structure in Figure 4 is removed from the wall bracket. In order to avoid the instability of the rectangle, the isosceles trapezoidal profile shown in Figure 6 is adopted after comprehensive consideration. No welding is required. The folded edge marked B fills the gap between the inverter and the wall. The normal direction is perpendicular to gravity and does not block the cooling airflow. The outline size after unfolding is 30 x224. It needs to be fixed to the box before hanging on the wall. After hanging on the wall, it is fixed to the wall with screws at A. The padlock cover is shown in Figure 7. The shape matches the wall bracket, and the screw head is covered by the sheet metal folded edge in the space marked A.

The bracket on the wall is shown in Figure 8, and the bottom plate (outline size 410 x45) is fixedly connected to the load-bearing sheet metal parts on both sides. The load-bearing sheet metal parts are shown in Figure 9. The sheet metal stamping process is used to obtain an extended cross-section hollow cantilever beam structure. The width and thickness in the cross section are both greater than 1 sheet metal wall thickness, which greatly increases the strength. The outermost fins of the radiator need to be processed with grooves that match the cantilever beam.

The marking and marking operation of the wall-mounted holes is shown in Figure 11. Using the bracket on the wall as a ruler, mark the positions of the upper wall-mounted holes A, C, F and the directional hole D. Then, the bracket on the wall is rotated 90 degrees so that the marks of A and C coincide, and the marks of the directional holes B and D coincide (the hole distance between C and D is equal to the hole distance between A and B in design). The position of the lower bracket wall-mounted hole G can be marked using the spacing hole E (the hole distance between C and G is equal to the hole distance between A and E in design). In Figure 11, the highest point of the radiator rib is 8mm away from the bottom plate of the bracket on the wall, which is enough to accommodate the head of the 3-in-1 M6 screw. If the bracket on the wall is fixed with an expansion bolt, the internal expansion bolt (M6) as shown in Figure 10 should be selected. In Figure 11, the lower edge of the lower bracket on the wall is flush with the lower edge of the box body, and the gap is 58mm, which is enough to accommodate the ratchet torque wrench and the M6 ​​screw socket. If the lower edge of the wall bracket must be lowered due to other reasons, the form shown in Figure 12 can be used (correspondingly, to maintain the spacing between holes C and G, the wall bracket must also be moved down), the padlock cover is universal, and the wall fasteners are the same as the wall bracket. The wall thickness of all sheet metal parts is 2mm.

The boundary conditions of mechanical simulation are: hole C is not fixed, only holes A, F and G are fixed, bolt preload is considered, "secondary load" at G is considered, 4 load steps are set, the inverter deadweight is applied in the second step, and 3 times the weight is applied to the top surface of the box in the fourth step. The box and radiator are set as rigid bodies, and the material of the upper and lower brackets on the wall is nonlinear aluminum alloy 5052H32. The maximum overall deformation is 0.44mm as shown in Figure 13, the maximum deformation of the cantilever beam is 0.24mm as shown in Figure 14, and the maximum equivalent stress of the cantilever beam is 191.23MPa as shown in Figure 15. The maximum deformation of the lower bracket of the wall is 0.44mm as shown in Figure 16, and the maximum equivalent stress of the lower bracket of the wall is 193.31MPa as shown in Figure 17. When the lower bracket of the wall is shown in Figure 12, the calculation results change very little.

Design and simulation of narrow and high 40kg inverter wall bracket assembly

As shown in Figure 18, the new wall-mounted bracket assembly carries a narrow and high inverter. The cross-section width of the radiator is 450, the height of the bottom plate rib is 80, the stretched length is 350, and the mass is 12.4kg. The box is 470 wide, 600 high, 175 deep, and has a mass of 27.6kg. The upper and lower bracket structures on the wall are the same as before. The outline size after unfolding is 30 x286, the wall-mounted hole marking method is the same as before, and the cantilever beam size remains unchanged. Due to structural limitations, the radiator is not slotted, and two box brackets are added as shown in Figure 19. The sheet metal material is the same as before. The boundary conditions of the mechanical simulation are the same as before, and the hole C is not fixed.

The maximum overall deformation is 1.0mm as shown in Figure 20, the maximum deformation of the cantilever beam is 0.64mm as shown in Figure 21, and the maximum equivalent stress of the cantilever beam is 198.45MPa as shown in Figure 22. The maximum deformation of the wall bracket is 0.91mm as shown in Figure 23, and the maximum equivalent stress of the wall bracket is 196.89MPa as shown in Figure 24. The maximum deformation of the box bracket is 1.0mm as shown in Figure 25, and the maximum equivalent stress of the box bracket is 214.14MPa as shown in Figure 26.

If the bracket on the wall must be as close to the upper edge of the box as possible due to structural restrictions, as shown in Figure 27, the distance between the upper and lower wall-mounted holes is larger than the outline of the upper bracket, a certain orientation pin can be set on the upper bracket, and then the plumb line method can be used to mark the position of the lower wall-mounted hole as shown in Figure 28. After mechanical simulation, it was found that the calculation results changed very little, with the maximum overall deformation of 0.91mm as shown in Figure 29, and the rest is omitted.

17kg inverter wall bracket assembly prototype verification

The 17kg inverter prototype is shown in Figure 30. B is the equivalent counterweight of the radiator, A and C are the two-side aluminum plate connecting beams, and A also plays the role of contact between the top of the equivalent rib and the folded edge of the bottom plate of the upper bracket. The bracket is installed on the wall as shown in Figure 31. It can be seen that the middle wall-mounted hole is not fixed. With the counterweight, the total weight of the inverter is 17kg as shown in Figure 32. The counterweight equivalent to 3 times the deadweight is shown in Figure 33. The 3 times weight bearing test is shown in Figure 34. After one minute, the wall-mounted bracket assembly has no obvious deformation and no collapse. For the experimental video, please search "inverter cantilever beam" on Youku.

Cost reduction ideas based on sheet metal processing

As shown in Figure 35, taking the aforementioned 17kg inverter as an example, since the same sheet metal material and wall thickness are used, the raw materials for the wall bracket, cantilever beam, and padlock cover can be obtained from the blanking at the radiator notch during the box processing, and then processed by stamping die. The bottom plate of the wall bracket is relatively long, but the shape is regular after unfolding, so it can be considered to be obtained from the scraps of the box or front cover blanking process.