Analysis of the Mechanism of Shaft Voltage and Shaft Current in Variable Frequency Motors (2)

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This article continues the analysis of shaft voltage and bearing current mechanisms in variable frequency motors. The focus is on how these phenomena are influenced by the characteristics of PWM inverter output voltages, particularly the presence or absence of overvoltage at the motor end. The paper separates the bearing currents into two main types: dv/dt current and EDM (Electrical Discharge Machining) current, and performs a detailed simulation to understand their behavior. The simulation results reveal that bearing currents are not only affected by the inverter's carrier frequency but also by the rise time of the output pulse voltage. When the motor terminal experiences overvoltage, the bearing current increases significantly. This highlights the importance of understanding the relationship between inverter output characteristics and bearing health. Assuming zero cable length, the dv/dt current is primarily caused by the input voltage step. Thus, its magnitude depends on both the inverter’s carrier frequency and the voltage rise time. A higher carrier frequency leads to more dv/dt current pulses within a sine wave cycle, though the amplitude remains constant. The rise time of the pulse voltage is a critical factor affecting the dv/dt current amplitude. Other distributed capacitances also play a role in determining the magnitude of this current. In contrast, the EDM current is directly caused by the presence of shaft voltage. Therefore, the amplitude of the EDM current is determined by the shaft voltage level, which is influenced by the input voltage and the internal capacitance distribution within the motor. Although the inverter’s carrier frequency and pulse voltage rise time can affect the shape of the shaft voltage, they do not influence its peak value. As a result, the EDM current is less sensitive to these factors compared to the dv/dt current. To further explore the effects, simulations were conducted with different rise times (tr). The results showed that increasing tr reduces the bearing current, including both dv/dt and EDM components. The dv/dt current decreases significantly, while the effect on the EDM current is less pronounced. This is because the EDM current is mainly governed by the shaft voltage and bearing impedance. Additionally, when tr is below a certain threshold (around 200ns), the dv/dt current can be higher than the EDM current. Another important factor is the coupling parameters between the stator winding and rotor, as well as the bearing capacitance. Increasing the coupling capacitance raises the shaft voltage, leading to higher dv/dt and EDM currents. Reducing the bearing capacitance lowers the dv/dt current but has little effect on the EDM current. This is due to the parallel connection of the bearing capacitance and rotor-to-case coupling capacitance, which influences the voltage distribution. To mitigate bearing current, one effective approach is to reduce the dv/dt of the inverter output voltage. However, modifying the inverter itself to achieve this can be costly and impractical for general applications. Instead, a compromise method involves using an RC absorption network on the inverter output to suppress high-frequency harmonics. This helps lower the dv/dt of the output pulse voltage, thereby reducing the shaft voltage and subsequent bearing current. Simulation results show that the use of a filter significantly reduces both dv/dt and EDM currents. Even without grounding, the filter improves the voltage waveform and effectively minimizes bearing current. This makes it a practical solution for many industrial applications. In conclusion, under high-frequency PWM input, the distributed capacitance in the motor creates a common-mode path, leading to shaft voltage and bearing current. These currents are influenced by various factors, including coupling capacitance, pulse voltage rise time, and inverter characteristics. Understanding and managing these factors is crucial for preventing damage to motor bearings. For more information on bearings and related topics, visit our website or check out other articles such as "10 Elements of KOYO Imported Bearings" and "Injury and Discrimination of Industrial Bearings."

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