Several electronic elements in a train or metro need cooling.

Introduction

Railways mobility is a strategic area for business development and growth: all Countries look to infrastructures for moving people and goods in a rapid, safe, and economic way as a key asset, and for this reason are investing many resources.

Trains and metros have several electronic components and elements to guarantee their normal functionality.

Therefore, electronic cooling is fundamental and Priatherm, being one of the major heat sink manufacturers, is readily suggesting heatsink design and technology capability to support manufacturers in the traction applications field properly.

Application field

The complexity of railways vehicles is pretty high, having they:
  • motors
  • batteries
  • compressors
  • fans
  • circuit breakers

Electrical energy, coming from the main overhead line, “enters” the system and needs rectifiers and inverters to be properly managed and transformed to let devices work.

Priatherm’s customer designs the inverter for the auxiliary system onboard the train and metro.
The specifications are demanding because on the one hand there is much power to be dissipated (e.g. 6 kW) in order to guarantee a long working life of the electronic devices; but, on the other hand, boundary conditions are challenging (T ambient from -30 °C to 60°C, weight constraints, space limitations for heatsink, shock & vibration specs., salt fog).

The heatsink is used in cooling electronic devices, to create a mechanical structure for many other smaller elements and to secure the electronic “heart” of the machine to the complete system in movement.

In the design stage, many parameters have to be considered in parallel to optimize the solution and satisfy all safety and reliability requirements.

The challenge

In this specific case, the customer needed a big heatsink to sustain a metal box to be installed on top of the roof of the vehicle: base plate dimensions were approx. 500 x 900 mm. The space available for the cooling fins was max. 120 mm.

Since the requested Rth was lower than 6,5 °C/kW, with forced convection cooling given by axial fans, the fins pack had to be studied, reaching the following optimum: fins thickness 2 mm, height 90 mm, pitch 6 mm, for a total of 74 fins.

Another important input data was that the customer needed a free frame of 50 mm width all around the fins in order to implement a sealing element during the assembly of the heatsink to the main box, able to protect the inside electronics from external agents.

Last point: the surface of the fins had to be resistant to the aggression and corrosion of salty air coming from the external environment.

Solution

After receiving the technical specifications from the customer and having deeply clarified them clause by clause, our engineers started the first steps of thermal simulation.
The goal was to select the proper heat sink manufacturing process to satisfy thermal requirements.
Extruded aluminum heat sinks, the first solution analyzed, were not able to solve the problem, not only for dimensional limitations (partially resolvable with double or triple welding of different “pieces”) but for the impossibility to create fins 90 mm high with a pitch of 4-8 mm.
Brazing technology was identified as the best one! Fins of pure aluminum (best thermal conductivity available) were brazed onto a base, made by two extrusion 250 mm wide, joined to create a unique base 500 mm wide.
By one:
  1. assembly operation step and
  2. a single cycle in the brazing oven,
it was possible to make a metal joint (top thermal and mechanical performance!) between the two bases and amongst them to all the fins.
Levering on the fact that fins were brazed, we designed the width and length of the fins pack in order to leave the needed 50 mm wide free frame for sealing material.
With this solution, we have reduced the material content and eliminated any further milling to remove unnecessary fins. Our competitors, that offer mechanical pressed fins, meet the difficulty to obtain “3D” design of the fins and must use more material which needs to be later removed with CNC, increasing costs and quality issues.
After sandblasting and CNC phases, the heatsink was moved to the Surtec 650 surface treatment, which added the required protection to the aluminum against corrosion.

Top challenges

  • The dimensions of the heatsink: we had to find a solution that would allow us to meet the request, approx. 500x900x120 mm.
  • The requested Thermal Resistance, a consequence of high power to be dissipated (6 kW) with a low delta T (40 °C).
  • The request to achieve a cost-effective solution to have free space around the fins for applying sealing material and the aggressive environment where the application had to work.

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