Siemens integrates EV motor and inverter in single housing; common cooling and SKiN

17 October 2014

Siemens has developed a solution for integrating an electric car’s motor and inverter in a single housing. Click to enlarge.

Siemens has developed a solution for integrating an electric car’s motor and inverter in a single housing. The motor and the inverter, part of the power electronics which converts the battery’s direct current into alternating current for the motor, have up to now been two separate components. The new integrated drive unit saves space, reduces weight, and cuts costs.

The solution’s key feature is the use of a common cooling system for both components. This ensures that the inverter’s power electronics don’t get too hot despite their proximity to the electric motor, and so prevents any reduction in output or service life.

Because range is a decisive criterion for purchasing an electric car, automakers are always striving to reduce vehicle weight. This was also the aim of the Siemens engineers. Their idea was to integrate the inverter into the motor, as this would reduce weight because only a single housing would be needed.

In addition, it would create six to seven liters of additional installation space, which could be used for a charging unit, for example. Integration would also eliminate the costs of wiring the motor to the inverter and fewer assembly steps would be needed to produce the vehicle.

Siemens developed the integrated drive unit—Sivetec MSA 3300—on the basis of a series electric motor. The engineers adapted the housing in such a way that the inverter could be integrated into the motor.

One problem they faced was the heat generated by the electric motor. At high temperatures, the output of the IGBT modules—the high-performance semiconductors that convert the battery’s current into alternating current—has to be limited. For this reason, inverters in electric cars always have their own cooling system.

A key feature of the integrated drive unit was therefore the creation of a special cooling water system around the motor and inverter. The coolest water first flows around very thermally sensitive components such as the IGBT modules and the intermediate circuit capacitor, after which it is led into the motor’s cooling jacket.

The water flow system is designed in such a way that a kind of water screen is created between the inverter electronics and the motor. As a result, it thermally isolates the two units from one another.

Another component of the overall solution is the very robust power modules featuring SkiN technology. Introduced in 2011 by power electronics leader Semikron, SKiN technology comprises the consistent application of sintering technology on all material combinations significant to load-cycling in a power module—i.e., sintered compositions replace all soldering and bond connections. When thermal load fluctuates, the electrical contact between the chip and the bonding wire is a weak point of semiconductor components.

(Siemens began collaborating more closely with Semikron on automotive power electronics in 2013, and took over Semikron subsidiary VePOINT. VePOINT developed power electronic components and systems, based on Semikron SKiN technology, specifically for the hybrid and electric vehicle market. (Earlier post.)

SKiN flex layers allow an increase of about 25% surge current in the power module due to the sintered layer on the chip tops. Compared to conventional power modules, the additional performance allows an approximate doubling of the current density. Excellent thermal and electrical properties of the sintered layers increase the module lifetime up to tenfold, Semikron says.

If, in addition, the DCB substrate is sintered directly onto the heatsink, the thermal resistance to the heatsink is reduced drastically over traditional interface materials—such as thermal pastes or foils. This decreases the thermal resistance R_th[j-a] between the semiconductor chip and coolant by up 30%, which enables a power increase or a reduction in volume by up to 35%.

Top: Comparison of standard connection technology (solder/bonding) and SKiN technology. SkiN is a bonding technology that connects the surface of the semiconductor chip without requiring bonding wire. When the thermal load fluctuates, the electrical contact between the chip and the bonding wire is a weak point of semiconductor components.

Bottom: Comparison of the partial thermal transfer resistances in a standard module and a SKiN power module with sintered heatsink. Click to enlarge.

The integrated motor/inverter concept’s feasibility has already been demonstrated in a lab under the typical load curves and operating conditions of an electric motor in an automobile. Siemens said that the industry has expressed interest in Sivetec MSA 3300, and the system was recently nominated for the eCarTec Award 2014, which is the Bavarian State Award for Electric and Hybrid Mobility.