Bosch Designs Electric Vehicle Components with Simulation

As the automotive trade races towards an electrified future, Bosch is accelerating its R&D into the important constructing blocks of electrical drivetrains. One of those elements is the inverter, which modifications direct present (DC) from the automotive’s batteries into alternating present (AC) to energy its drive motor (Figure 1). The inverter’s potential to offer a easy move of present will depend on its integral DC hyperlink capacitor (Figure 2). “The capacitor is one of the most expensive components of the inverter. Its performance has a direct impact on the performance and reliability of the inverter, which is fundamental to the operation of the drivetrain,” explains Martin Kessler, Bosch senior skilled for automotive electronics.

For the worldwide automotive sector to fulfill its formidable electrification objectives, inverters and their capacitors should bear steady enchancment and optimization. Martin Kessler and his workforce depend on multiphysics simulation to check and refine Bosch’s DC hyperlink capacitors. Their simulation-enabled predictive evaluation enhances and optimizes the stay prototyping of latest designs. “It is simply not possible to predict potential problems with testing alone; we need both simulation and prototyping working hand in hand,” says Kessler.

The Emerging Era of the Electric Automobile

“Drivers, start your engines!” As if heeding the decision to start a worldwide race, folks all over the place start their days by firing up a rumbling IC engine. But this acquainted sound can appear ominous, particularly because the environmental influence of auto emissions grows extra obvious. To reduce these emissions and their contribution to world local weather change, the car trade is ramping up the manufacturing of electric-powered vehicles and vans. Many of the electrical automobiles obtainable right now have acquainted model names, however beneath the hood, these vehicles typically depend on the expertise and experience of outdoor suppliers.

It is value noting simply how important a shift that is for a significant world trade. Leading automakers are among the world’s largest employers, and an enormous share of their employees, R&D, and manufacturing capability is devoted to producing IC engines. The centrality of inside combustion to those firms could be discovered of their names, from General Motors to Bayerische Motoren Werke (higher often called BMW). Why would firms identified for his or her engines flip to outsiders to make their vehicles go? Perhaps it’s as a result of, in a way, electrification is forcing the trade to learn to produce a wholly totally different kind of machine.

Anatomy of an Electric Drivetrain

To make a completely electrical automotive, it’s not sufficient to exchange the engine with an electrical motor and the gasoline tank with a battery. Such acquainted gadgets are solely components of a bigger system, which helps ship easy, dependable efficiency by adjusting to the continually various circumstances beneath which each and every automobile should function (Figure 3).

Indispensable Inverter, Crucial Capacitor

The function of the inverter in an automotive drivetrain is straightforward in idea, however advanced in follow. The inverter should fulfill the AC calls for of the motor with the DC supplied by the battery, but it surely should additionally alter to ongoing fluctuations in load, cost, temperature, and different components that may have an effect on the conduct of every a part of the system. All of this should happen inside tight value and spatial constraints, and the part should maintain this efficiency for years to return.

To perceive the inverter’s operate, take into account what a three-phase AC motor wants with a view to function. If related to DC, the motor merely is not going to rotate. Instead, it should be supplied with alternating present with three distinct however complementary waveforms, enabling the motor’s three-part subject coil to magnetically appeal to the segments of its rotor in a sequential sample. “To control the activity of the motor, we must control the amplitude and frequency of the inverter’s current output,” explains Kessler. “The speed of the motor is proportional to frequency, while amplitude helps determine its torque.”

“The desired current waveform through the transistors has a relatively steep gradient. The only way to achieve switch-mode current with this high gradient is to have very low inductance in the source path,” Kessler says. Inductance is the actual pressure opposing modifications in present move. Every slight change in present shall be restricted by an induced counteracting voltage, which is able to disrupt the specified waveform — and the sleek rotation of the motor.

To cut back the inductance within the supply path of the transistors, a capacitor is positioned in parallel throughout the enter lead from the battery, which known as the DC hyperlink. The DC hyperlink capacitor (Figure 5) is positioned in direct proximity to the transistors and offers the specified present waveforms by means of the transistors. The low impedance of the capacitor minimizes any remaining ripple voltage on the battery facet.

A typical capacitor consists of two electrodes separated by an insulating hole, which can merely be airspace or some sort of materials. In this software, Bosch makes use of capacitors made with metallized polypropylene movie. A skinny coating of steel (forming the electrodes) is sprayed on either side of the movie, which offers the mandatory dielectric hole. The metallized movie is then wound tightly right into a canister form. As with the inverter itself, the capacitor’s conceptual simplicity conceals a multifaceted engineering design drawback.

Challenges with DC Link Capacitor Design for Vehicle Inverters

Capacitors are extensively obtainable elements which are put in in numerous digital gadgets. For the previous seven years, Martin Kessler has been accountable for DC hyperlink capacitor design at Bosch. He has been with the corporate since 1989 and has labored on electrical automotive expertise since 2010. That such an skilled engineer is devoted to this one part reveals its significance — and its complexity.

“Why can we not just pick up a capacitor from the marketplace?” asks Kessler, rhetorically. “There are multiple interdependent factors at work. First, we have high demands for performance and reliability. Second, there are very tight spatial requirements. Third, we face difficult thermal constraints, as the polypropylene film in a capacitor can only withstand temperatures up to around 105°C. This issue is compounded by the interaction of electromagnetic and thermal activity throughout the inverter. And finally, the capacitor is relatively expensive,” Kessler explains.

Simulation (Not Luck) Helps Solve the Black Box Problem

To meet the design challenges of a DC hyperlink capacitor, Kessler developed a course of that mixes experimental testing with multiphysics simulation. As an instance of why simulation-based evaluation is a essential a part of his work, he cites the problem of discovering and measuring potential sizzling spots, the place excessive warmth and matched results may cause failures. “We try to locate hot spots by placing a lot of thermocouples inside prototypes and measuring temperatures at various load points,” Kessler says. “But my mantra is that you will never find a hot spot like this without a lot of luck! You will need to be lucky to place the thermocouple in the right position,” he laughs.

“A simple 2D model of a capacitor is also insufficient,” Kessler continues. “The inverter is a distributed system with internal resonances and a complex loss distribution. Our coupled EM and thermal analysis must account for skin effects and proximity effects. We cannot calculate an absolute value for peak temperatures without a 3D finite element approach, which also enables us to model the spatial distribution of coupled EM and thermal effects. This is an ideal task for the COMSOL Multiphysics software,” Kessler says. (Figures 6–7)

Kessler’s design course of validates simulation fashions in opposition to measured outcomes, the place potential, after which makes use of the validated fashions to pinpoint potential issues (Figure 8). “By helping us locate hot spots in the model, the simulation helps us avoid issues that would have appeared late in the development process, or even after production had started,” says Kessler. “Instead, we can get specific results and make adjustments early in the process.”

“We perform EM modeling and validation of every new design. We compare the calculated equivalent series resistance (ESR) curve with the ESR curve as measured from a prototype (Figure 9). If these curves are aligned, we can set up boundary conditions for stationary and transient heat calculations,” says Kessler. “We can compare the temperature curves from our thermocouples with the results of probes in the COMSOL Multiphysics model. If they match, we can then simulate all the critical points where we must keep temperatures within limits.” The curve knowledge is put into the COMSOL Multiphysics software program by way of the ResideLink for MATLAB interfacing product.

“Before we can do this, we have to think about which factors should be incorporated into the model,” says Kessler. “Some of the variables we receive from the OEM, such as maximum DC link voltage, are not very relevant to our simulation,” he continues. “But the current, switching frequency, e-machine values, and modulation schemes all help define a current spectrum. We need to calculate the current spectrum for all three phases of our output in order to establish power losses. Once we have this, we can do the harmonic analysis with COMSOL Multiphysics for the frequencies of the current spectrum. Then we sum up our losses for every harmonic,” Kessler explains.

Other vital values embody the boundary circumstances, which assist Kessler and his workforce decide coupled results. “We calculate parasitic inductance of the capacitor with the AC/DC Module,” Kessler says. “We also find the complete AC loss distribution through the capacitor windings or internal busbar. Then we can couple the results and determine a temperature-dependent resistivity of the cover parts with the Heat Transfer Module,” he says. “This enables us to establish the maximum element hot spot temperature resulting from the EM activity.”

Findings from their analyses can then result in design modifications. Kessler explains that every new capacitor design sometimes undergoes three rounds of testing. “With simulation, the improvement curve gradient is much steeper from one phase to the next. Our knowledge grows quickly, and this is reflected in the final product.” The newest era of Bosch inverters guarantees 6 p.c larger vary and a 200 p.c soar in energy density in comparison with earlier designs.

Electrification Shifts into High Gear

As automakers convert extra of their product strains to electrical propulsion, Martin Kessler believes that the necessity for speedy, cost-conscious R&D can even enhance. “Electric mobility is growing up now,” he says. “We expect that the OEMs will come to us with more varied needs, for inverters in different power classes and that meet tighter spatial constraints,” says Kessler. “I do think that the number of products that require new capacitor designs will keep expanding. With our simulation-driven development methods, we are confident that we can keep up with this growth.”

In the years to return, maybe guests to Stuttgart’s automotive museums will cease to admire the historic motors and inverters that powered the trade into a brand new electrical age.

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