Motor-integrated inverter saves space, weight

October 17, 2014 // By Christoph Hammerschmidt
Siemens has developed a traction motor for electric vehicles with mechanics and power electronics in one casing. The integrated powertrain saves space, weight and costs for electric vehicles.

Hitherto motor and inverter were built and delivered as two separate components. The new solution is based on a new cooling concept which ensures that the power electronic components do not overheat despite its close proximity to the engine, thus ruling out negative effects on power and life expectancy.

A decisive criterion for the purchase of an electric vehicle is driving range. For this reason, car makers continuously are putting efforts on optimising the vehicle's weight. The Siemens approach takes this into account: An inverter integrated into the motor requires only one casing and saves space and weight. Plus, the space formerly occupied by the inverter is now freed up and can used otherwise, for example for a charger. Also, the cost for the cabling between motor and inverter can be omitted and the production is simpler because it lakes less process steps.

The design of Siemens' integrated powertrain unit Sivetec MSA 3300 is based on a serial model; the company modified the casing to accommodate the inverter. The core issue was the heat generated by the motor: At high temperatures, the IGBTs in the inverter cannot be fully utilised; their power has to be limited. Automotive inverters typically are equipped with water cooling for this very reason. The power modules in SKiN technology contributed to the overall solution of the heat problem. SKiN is a semiconductor joining technology developed by Siemens cooperation partner Semikron. This technology connects the semiconductor element extensively, doing away with bonding wires. Under operational conditions with varying thermal stress, the contact between semiconductor and bonding wire is one of the most frequent points of failure in semiconductor components.

Essential for the integrated was the design of the cooling water flow within motor and inverter. The liquid with the lowest temperature first flows around thermally critical components such as the IGBTs and the DC link capacitor and then is directed into the motor's cooling jacket. The