title: The electromechanical excitation mechanism of film capacitors within highly integrated drivetrains
Creators: name: Herrnberger, Maximilian
Publisher: name: UNSPECIFIED
Year: 2024

description: DC-link capacitors are used in highly integrated power electronics for electric drives. These capacitors compensate for harmonics caused by switching operations during the conversion. The harmonics depend on the switching strategy of the transistors within the converter and cause the DC-link capacitor within the power electronics to vibrate mechanically. This can cause high-frequency acoustic noise, which propagates via structure-borne and airborne sound paths and thus can be a source of interference within electric vehicles. In the acoustic investigation of electric drives, the existing literature is focused mainly on noises that arise due to varying Maxwell forces in the air gap field between the rotor and stator. However, for a comprehensive acoustic evaluation, the excitations from the power electronics must also be considered. This is particularly important for highly integrated drives in which the power electronics, the electric machine and the gearbox are integrated into a compact drive unit. Within the power electronics, the focus is on the DC-link capacitor since it is subjected to voltage ripples. In electric drives, polypropylene-based film capacitors are widely used, which usually consist of several capacitor windings embedded in epoxy resin. In this work, a model is presented for predicting the voltage-induced acoustic excitation of film capacitors. For this purpose, a sensor model known from the literature is first transferred to a plate capacitor to analytically derive the deformation in a single capacitor layer. In addition to the electrostatic attractive forces of the electrodes, this model also considers electromechanical couplings in the dielectric, which are described by the electrostrictive parameters. The analytically calculated deformation amplitudes can then be transferred to the geometry of a capacitor winding using finite element (FE) simulations. The application of FE models makes it possible to take into account the structural-dynamic properties of different capacitor geometries. Since film capacitor windings consist of several thousand layers, the FE models are simplified in a way that modal superposition can be applied as a model reduction method. The aim is to increase the numerical efficiency of the simulation and thus to simulate larger capacitor structures with reduced resource consumption. The resulting excitation model is then validated by measurements at both the winding and the capacitor level and can be used to predict the excitation of polypropylene-based capacitors. In addition, the resulting airborne sound radiation from capacitors is estimated based on the resulting surface velocity. Acoustic measurements on capacitor windings also show that the excitation amplitude depends on the polarity of the applied DC voltage. Accordingly, polarization effects in the dielectric must also be considered (especially for small voltages), which can be caused by the manufacturing process of the metallized capacitor films. Polarization is caused by both space charges in the dielectric and a boundary layer near the metallization and leads to an intrinsic electrical field that influences the acoustic excitation of the windings. This boundary layer is detected in the course of the work using high-resolution transmission electron microscopy. Furthermore, the acoustic influence of the polarization present in the capacitor winding can be used to determine a corresponding compensation voltage that is needed to cancel the internal polarization. The compensation voltage can be influenced externally either by voltage pulses or temperature loads, which will be investigated in further experiments. The stability of the compensation voltage over time is also analyzed, and various relaxation mechanisms are discussed. The behavior of the capacitor windings can be compared to the long-term stability of electrets. The present investigations are based on polypropylene capacitors but can essentially be transferred to other dielectrics by adjusting the characteristic values. The findings presented in this work enable a prediction of the excitation amplitudes of film capacitors. The model presented combines various excitation mechanisms and takes into account the polarization caused by the manufacturing process. The results provide a basis for considering DC-link capacitors in the electric drivetrain as a source of acoustic excitation.