Both the Tesla Model 3 and BMW i3 have novel approaches to cooling the motor. This week’s segment compares these approaches. Let’s get to it.
The BMW i3 uses a two-piece motor housing to allow for large coolant channels in the housing walls to cool the housing, which in turn cools the motor and stator laminate stack. The Tesla Model 3 has a coolant to oil heat exchanger on the exterior of the housing and uses the oil to flow through grooves and channels in the stator laminate stack to cool the motor.
The advantage of the BMW i3 design is that it eliminates the need for a heat exchanger, along with the thermal transfer loss between the two fluids. However, the two-part housing requires more processing costs related to casting and machining a second housing component.
The advantage of the Tesla Model 3 design is that it only requires one housing and allows for direct cooling of the stator through contact with the cooling fluid (versus through a housing wall). However, this design requires a heat exchanger assembly and an oil pump to circulate the fluid. These two commodities add significant cost to the motor assembly, but they also have a dual purpose of providing heat to the battery as well as cooling the motor. In this design, the systems pays for some additional functionality.
The general conclusion is that Tesla is increasing their cooling performance of the motor by running oil directly through the laminates, but accommodating this system requires a cost increase for added commodities of a heat exchanger and pump. This drives approximately a $31 cost increase on the Tesla Model 3 versus the BMW i3 design, even though the Tesla was able to use a single piece housing.
Each of the motor housings and companion cooling components were analyzed in Design Profit to understand the full cost of cooling the motor. Specifically, the housing full fabrication process was captured in the software, including the casting and all machining operations.