BLDC motor for automotive cooling fan assembly: heat sink optimization

Davide Parodi
Johnson Electric

One of the main product segment of Johnson Electric is represented by cooling fan modules (CFM) for automotive applications. Such cooling fan modules can be summarized as assemblies of: fan (creating air flow), electric motor (maintaining fan rotation) and shroud (generally, with a supporting function). The complete system is mounted on a series of heat exchangers (car engine radiator, intercooler, condenser, etc.) where hot fluids to be cooled flow. For this reason, temperature becomes one of the most important parameters to take into account already at the concept phase. The continuous growing of overall vehicle performance requirements have caused, in the last years, the increase of automotive products characteristics, not only in terms of efficiency, but also from more general concepts such as: robustness, reliability, lightness, compactness, etc. In order to take all these necessities into account, sub-components suppliers have to design their products always addressing to innovative solutions to achieve Customers targets. All these technical needs are often in conflict with other industrial important topics, such as: timing, costs, production efficiency (fastness, easiness, etc.), etc.
The temporal contraction occurred between Concept Phase and Time To Market has pushed more and more cars manufacturers and their suppliers to increase Virtual Prototyping usage, nowadays possible thanks to more powerful “desk sized” hardware, which has allowed to extend numerical simulations application. In Johnson Electric Asti, complex Thermal-Fluid Dynamic Simulations were traditionally performed by external suppliers; thanks to CPU improvements, from some years, a big amount of them can be performed internally with contained timings, accurate models and important advantages:

• Timing and costs reduction
  
• No necessity of confidential agreements with external suppliers (generally time constraining)
  
• The whole process is completely managed by the competence center of product
  
• Fast optimizations, models modification, data exchange and analysis
  
• Possibility to directly and completely verify simulation results accuracy
  
• Company know-how improvement

Purpose of the present work is the optimization of a Brushless Electric Motor (BLDC) heat sink for automotive application. Main task of the aforementioned heat sink is the cooling of the integrated electronic board, whose BLDC is equipped, allowing a safe operating condition for its sub-components, so improving its robustness and reliability. During their normal working, in fact, the electric motor “active parts” (copper and magnets) and electronics apparatuses self-heat because of electric currents passage. Internal electronic board components especially are characterized by maximum allowed temperatures. Considering that the typical installation for this kind of product is a hot environment (road vehicle engine compartment), electric motor cooling aspects become the key to prevent its failure. This problem may become even more complex when electric motor is directly faced to the thermal engine or some of its hot parts (exhaust collector, etc.), as it is for the most common installations.
The optimization has been focused on several aspects not only related to traditional technical issues, but also commercial and industrial. Starting from an already optimized solution, previously defined by means of a similar design approach, the improvement of the new heat sink has been focused on motor weight and costs reduction by means of the investigation on new geometry and new materials, always considering some peculiarities required by these kind of products, such as: interchangeability with existing products, building easiness, etc., but always warranting proper internal temperature conditions. The whole study has been driven by an accurate internal board components modeling, based on preliminary experimental data analyses and dedicated simulations performed with the purpose to define the most correct resultant thermo-physical properties and geometries to model each electronics part and its thermal behavior. Simulations results have been continuously compared with real experiments, both to verify design achievements and validate the numerical model.
With the electronics system defined in this way, the complete simulation model consists of all CFM parts, so that all heat production and rejection mechanisms can be taken into account.
ANSYS-CFX has been used for thermal-fluid-dynamic analyses, performing stationary analyses, which are the closest condition to the real test set-up (thermal stabilization in dedicated oven).