Screw it in and drive

Among the companies pushing the boundaries of packaging efficiency is the German engineering colossus Siemens. The company has developed an integrated motor and inverter contained in a single housing, the Siemens Sivetec MSA 3300. The new drive unit reduces weight, frees up six to seven liters of additional space, lowers costs, and requires fewer assembly steps during vehicle production.

They key goals in any automotive project are to achieve the right performance, high quality and low prices in something that can be easily integrated into new cars. “We wanted a traction drive that was easy to integrate, especially on the rear axle of the car,” Dr. Karsten Michels, Head of Development at Siemens eCar Powertrain Systems, told Charged.

The idea is to target an existing platform, such as a passenger car with a combustion engine, that an OEM is interested in making into a hybrid. A compact motor and inverter assembly could easily be added to the rear axle without many additional parts. “We have to make it easy for the OEM,” said Dr. Michels. “That’s the first big advantage of combining the motor and the inverter. To put it simply, screw it in and drive. Today, the cabling is a nightmare for OEMs, especially the thick cables needed for a traction motor.”

Combining the motor and inverter completely eliminates the cables between the two, which is an advantage for both space and price. “Copper is expensive stuff,” said Dr. Michels. “Also, transferring high currents is not that easy and can bring up some issues in the field. If you simply do not have these cables between the inverter and motor, it completely eliminates the concern for an OEM. The space you need is smaller and the cost is lower. These are key advantages.”

The savings

At this point of the development process, precisely defining the savings in weight and cost of such a system is difficult, because it depends on a lot of factors. There are weight savings gained from the reduction in housing and die-cast material, which Dr. Michels says are “a few kilograms, but not the major part of the weight efficiency.”  Again, he highlights elimination of “the cables that are very heavy because it’s thick copper, many kilograms, and you get completely rid of it.” Overall, he estimates weight savings to be around 10-15%.

“It’s hard to talk specifically about costs, as you can imagine,” said Dr. Michels. “Cost savings come from the reduction in materials and parts, but the main cost advantage lies in the car integration and the cables.”

Creative cooling

One of the biggest challenges that Siemens engineers faced was the heat generated by the electric motor. At high temperatures, the output of the IGBT modules has to be limited. For this reason, inverters in automotive applications often have their own dedicated liquid cooling systems. So, a key feature of the integrated drive unit was the creation of a special cooling system and, according to Dr. Michels, it’s the part of the project that contains the company’s most innovative IP.

“The motor can be more than 160° C, and in an inverter you have electronic parts which will be damaged at 120° C,” explained Dr. Michels. “So, you have to isolate the inverter from the motor in some way. You can do this with a very sophisticated design of the cooling system.”

Siemens designed the cooling channels so that the coolest liquid first flows around 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 liquid flow system is designed so that a kind of water screen is created between the inverter electronics and the motor, thermally isolating the two units from one another.

“This is where the real intellectual property is,” said Dr. Michels. “We did a lot of design work in this area, a lot of simulations and calculations of temperatures. In the end, we have more homogeneous cooling in this inverter than we’ve seen in distributed parts. Very good results.”

SKiN in the game

In 2013, Siemens signed a cooperation agreement with Semikron to get access to power electronic systems specifically for the hybrid and electric vehicle market, based on the Semikron SKiN technology, which does not require conventional soldered joints or wire bonds.

Semikron claims that by replacing wire bonding with SKiN flex layers, some major increases in performance can be achieved, including an increase of about 25% in phase current in the power module, and excellent thermal and electrical properties that increase the module lifetime up to tenfold. Also, if the Direct Copper Bonded substrate is sintered directly onto the heat sink, Semikron says that the thermal resistance to the heat sink is reduced drastically, which decreases the thermal resistance between the semiconductor chip and coolant by up to 30%, enabling even more power increases, or a reduction in volume of up to 35%.

The advantages of SKiN technology can also be realized in a traditional system without an integrated motor and inverter, and Dr. Michels says that by the middle of this year we will begin to see the first cars on the street with SKiN-powered modules. But the advantages are particularly well-suited for an integrated approach where volume and heat generation are tightly constrained.

Commercialization

Siemens’ announcement of the Sivetec MSA 3300 integrated motor-inverter marks the end of the technology development and the beginning of application development for specific OEM projects.

“We’ve completed testing with internal prototypes that had very good results. And feedback from the OEMs has been great,” said Dr. Michels. “This is this mostly geared towards passenger cars, because of their space limitations. We are now in a good position to design applications specifically for the spaces where OEMs need an integrated solution, otherwise the space requirements would not be fulfilled.”

This article originally appeared in Charged Issue 17 – January/February 2015