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The motor controller will be screwed (M4) onto a an aluminum heat sink which will be enclosed by a multi print 3-D printed (cracks sealed with JB weld) ASA enclosure with splash proof connectors and lid. The heat sink will need to be cut and drilled. An intake duct may need to be attached to provide air from the wheel well. This enclosure will be mounted behind the back right wheel (wheel with motor) and onto tabs from the frame.
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https://www.digikey.com/en/products/detail/advanced-thermal-solutions-inc/ATS-EXL6-254-R0/5848412
Fans Selected:
Enclosure Connector:
https://amphenol-industrial.com/products/epower-lite-and-epower-lite-mini/
Calculations: (no ducts)
Ploss = ReqIo2 + (αIo + β)Vbus + C𝑓eqVbus2 (from datasheet)
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We can assume that all of the power loss will be converted to heat, therefore the heat generated will be considered 43.78W. For the sake of simplicity, we are also assuming all of this heat is being transferred to the heat sink attached to the motor controller . Also, and that the airflow will not be is distributed evenly distributed across each fin with the two fans, but for the sake of simplicity we are assuming it isthe heat sink.
Q = hAtotal(Tobject−Tambient) (Newton’s Law of Cooling)
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Pr = 0.71 (Prandtl number for air at ambient temp 100F)
Re = ρvL/μ (Reynold’s number)
v = 4.16m/s (Air velocity caused by fans, I used a an online calculator)
ρ is the air density (typically 1200 g1.2 kg/m³ at room temperature).
v is the airflow velocity (m/s), which can be estimated from the fan's specifications.
L is a characteristic length (0.250m)
μ is the dynamic viscosity of air (0.01918 g1.918E-5 kg/ms, at ambient temp 100F)
We can calculate Nusselt’s number (Nu) with the Dittus-Boelter Equation which is used for turbulent flow through a smooth pipe. Although our enclosed fins are more akin to a rectangular prism.
Nu = 0.023 ⋅ Re0.8 ⋅ Pr0.33 ⋅ (L/D)0.5 (Nusselt’s number for airflow parallel to fins)
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L = Length of the fin (in the direction of the flow, 0.250m)
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Nu = hL/k (Nusselt’s number equation relating to h)
h: is the heat transfer coefficient
L: is the characteristic length (0.250m)
k: is the thermal conductivity air (0.026 W/mk, at ambient temp 100F)
h = (0.023 ⋅ Re0.8 ⋅ Pr0.33 ⋅ (L/D)0.5 ⋅ k)/L
Q = ((0.023 ⋅ Re0.8 ⋅ Pr0.33⋅ (L/D)0.5 ⋅ k)/L)Atotal(Tobject−Tambient)
Q = ((0.023 ⋅ ρvL/μ0.8 ⋅ Pr0.33⋅ (L/D)0.5 ⋅ k)/L)Atotal(Tobject−Tambient) (heat transferred)
Q = 27.2885 ⋅ ΔT
Q = 8.9679 ⋅ ΔT
We can conclude that with the given cooling system, the temperature difference between the surface of the motor controller and the ambient temperature will need to be >5C to transfer the heat generated at peak power usage. Basically, it only cools the heat plate if we’re not just blowing hot air over hot metal.
However, given our ideal assumptions that heat and airflow is evenly distributed throughout the heat sink, velocity is constant and not accounting for humidity and that the motor controller is inside an enclosure, we can assume that the temperature difference needed for effective heat transfer is considerably greater than 5C.
Assuming the highest temperature conditions of 100F, we can estimate that the ambient temperature within the aeroshell will be able to reach at least 120-130F given that it’s outside in the sun all day and the battery, motor controller, MPPT’s, and other electronics are producing heat that’s not entirely expelled outside the aeroshell. The MPPT shutoff temperature is 70C (158F). With our effective ΔT value, the motor controller will hover dangerously close to it’s max temperature given bad weather conditions and continuous high current draw, which is dangerous and affects performance.
Conclusion: The cooling efficiency without ducts is questionable, but should be adequate if we add ducts leading from the wheel well to provide cooler air for convection.
Sources:
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