OEM&Lieferant Ausgabe 2/2021

100 Developers need to understand the system For the design of an electric drivetrain sev- eral parameters such as acceleration, top speed and maximum gradient as well as wheel diameter and vehicle mass have to be considered in order to determine required wheel torque and highest rotational speed. It’s important to regard the single compo- nents of the powertrain as well as the entire vehicle system. Ultimately, the propulsion solution also has to meet specific require- ments, e.g. the Functional Safety Standard ISO26262. Electric vehicles powered by two different motor types For the propulsion of their electric drive- trains car manufacturers primarily use two types of engines: a synchronous elec- tric motor with Interior Permanent Mag- nets (IPM) or an asynchronous Induction Machine (IM) (Figure 1). The IPM, on the one hand, provides higher torque density, which is advantageous while driving in the city at lower speeds. The IM configura- tion, on the other hand, is best for driving at higher speeds and with low torque on the highway, since the induction machine provides enhanced engine performance in these situations. There are significant differences in system behavior, which is why the choice of the mo- tor type influences Functional Safety as well. Using an IPM could cause critical driving conditions with braking torque acting on the wheels. The reason being: Flux is generat- ed permanently by the magnets. If the stator voltage is higher than the battery voltage the motor turns into an uncontrolled generator in case the inverter’s power stage switches are turned off at high speed. When connect- ed to a rotating IM all inverter power stage switches can be switched off if an inverter error occurs. There is no magnetic flux, no torque production and the voltage induced in the stator windings is low. This means the inverter must be designed to operate differently with the two motors, to meet functional safety requirements. For IPM machines, it is common to use an Active Short Circuit (ASC) option. With ASC enabled, the inverter power stage is used to short circuit the motor terminals, thus mitigating voltage-related hazards and preventing regeneration into the battery. It is important to ensure that the peak short circuit current in ASC does not damage the motor magnets or inverter, and the braking torque applied to the wheels is acceptably small. Wire windings also play an important role Round wire windings are the cheaper option, but they do not use the available stator slot area to the full extent, and they display poor thermal properties. Rectangular wire, how- ever, has a better thermal performance and utilizes the slot area properly. Hence, rectan- gular wire windings are the best choice for a high-powered electric motor. BorgWarner has been using this type of winding in their motors for over 15 years, in their patented High Voltage Hairpin (HVH) and continuous S-wind structures. eMobility Electric Vehicle Development By Steven Stover, Senior Manager Product Strategy, BorgWarner in Noblesville, IN (USA) and David Fulton, Director of Electric Drive Innovation, BorgWarner in Noblesville, IN (USA) For their design of high-performance electric vehicles OEMs require innovative powertrain solutions and highly efficient components. Backed by vast knowledge and experience in various propulsion technologies as well as strategic acquisitions especially in the field of electric propulsion, BorgWarner can supply automakers with overall solutions, customized modules and numerous components. Graphics: © BorgWarner Figure 1: Sectional views of an IPM and IM