Riassunto analitico
The demand for more reliable and efficient electric machines and drives is constantly growing in the realm of transport electrification. Semiconductor switch-based inverters usually feed such drive systems and, unlike balanced pure sine-wave AC sources, generate large-amplitude, high-frequency common-mode voltage (CMV) waveforms, stemming from pulse-width modulation (PWM). These CMV waveforms give rise to high-frequency parasitic currents that can traverse the machine’s bearings, causing lubrication film deterioration and surface damage of bearing elements. This results in increased maintenance, extra costs, reduced mechanical efficiency and NVH performance, ultimately compromising reliability. Various parasitic currents exist, but only electric discharge machining (EDM) currents are present in electric vehicles (EVs) and hybrid electric vehicles (HEVs) motors. After exploring the origins and implications of EDM currents in electric machine bearings and briefly introducing common mitigation strategies, this work estimates the shaft voltage, recognized as the primary trigger of EDM currents, in order to predict and prevent their occurrence for enhanced reliability and efficiency in EVs. It also strikes a balance between accuracy and computational efficiency for the models used to calculate such voltage. To accomplish these purposes, adjustments are made to equivalent circuits from existing literature for both the induction machine (IM) and the surface mounted permanent magnets (SMPM) synchronous motor. These modifications account for various machine components and enhance accuracy. The associated stray capacitances are computed using established and custom analytical models, and validated through finite element analysis (FEA). These stray capacitances’ analytical and FE models facilitate a sensitivity analysis on an existing automotive motor to identify the necessary accuracy and critical parameters for shaft voltage assessment. The tailored equivalent circuit for the motor under investigation is integrated in Simulink/SIMSCAPE to extract the bearing voltage waveform and assess its response to changing switching frequencies. This circuit is subsequently employed to establish a correlation with experimental data. In this respect, to ensure relevance to the application, the stray capacitance values of hybrid bearings are computed and incorporated in the circuit, together with those of a parallel path through the test bench components. Ultimately, the evaluation of key parameters in building shaft voltage leads to the identification of various design solutions and recommendations aimed at minimizing shaft voltages and mitigating EDM currents.
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