Analysis of the Mechanism of Shaft Voltage and Shaft Current in Variable Frequency Motors (2)
Source: Bearing Network | Time: 2013-04-29
In this second part of the analysis, we delve deeper into the relationship between bearing currents and the voltage characteristics of PWM inverters. The paper separates the two types of bearing currents—dv/dt current and EDM current—and performs a detailed simulation to better understand their behavior. The results show that bearing currents are influenced not only by the inverter's carrier frequency but also by the rise time of the output pulse voltage. Moreover, when overvoltage occurs at the motor terminal, the bearing current increases significantly. Assuming zero cable length, it is found that the dv/dt current is primarily caused by the input voltage step. Therefore, the magnitude of the dv/dt current depends on both the inverter’s carrier frequency and the rise time of the pulse voltage. A higher carrier frequency leads to more dv/dt current pulses per sine wave cycle, although the amplitude remains unchanged. The rise time of the pulse voltage is a critical factor that affects the dv/dt current amplitude. Additionally, other distributed capacitances within the system can influence the overall dv/dt current level. The EDM current, on the other hand, is directly caused by the presence of shaft voltage. Thus, the amplitude of the EDM current is determined by the magnitude of the shaft voltage, which in turn 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 affect the shape of the shaft voltage, they do not impact its peak value. As a result, the EDM current is not significantly affected by these parameters. However, the breakdown voltage of the bearing oil layer plays an important role. A higher breakdown voltage allows for greater EDM current during discharge. 4.1 Effect of Changing the Rise Time (tr) Simulation results show that different rise times produce varying shaft voltage and bearing current waveforms. When tr increases, the bearing current decreases, including both the dv/dt and EDM components. The dv/dt current shows a significant reduction, while the effect on the EDM current is less pronounced. This is because the EDM current is mainly determined by the shaft voltage and bearing impedance. When tr is below a certain threshold (around 200ns), the dv/dt current may even exceed the EDM current. Additionally, the rise time has minimal impact on the shaft voltage itself. Notably, during voltage breakdown, the shaft voltage exhibits two oscillations, with the second one being influenced by the rise time due to the continued coupling path from the stator winding to the rotor after the bearing is shorted. 4.2 Impact of Changing Coupling and Bearing Parameters Increasing the coupling capacitance between the stator windings and the rotor raises the shaft voltage, leading to higher dv/dt and EDM currents. Conversely, reducing the bearing capacitance lowers the dv/dt current, while the EDM current remains largely unaffected. The shaft voltage rises as a result of the parallel connection of the bearing capacitance and the rotor-to-case coupling capacitance. Since these values are typically much larger than the stator-to-rotor coupling capacitance, the shaft voltage is distributed accordingly. When the bearing capacitance increases, the dv/dt current decreases, as the capacitance resistance reduces under a constant dv/dt. 5. Mitigation Strategies for Bearing Currents Based on previous theoretical research and simulations, one of the primary causes of motor bearing current is the high dv/dt of the inverter’s high-frequency pulse output. Therefore, reducing the dv/dt of the inverter output voltage is a practical solution. However, the rise time of the inverter’s pulse voltage is determined by the switching characteristics of the power devices, making it difficult to modify directly. One effective method is to add a large reactor at the inverter output, forming a "sine wave filter." This transforms the pulse voltage into a smooth sine wave, eliminating both the shaft voltage and the bearing current. While this method is effective, it has drawbacks such as high power loss, large size, and increased cost, making it unsuitable for general-purpose frequency conversion systems. To address this, a compromise approach is taken by adding an RC absorption network between the output lines to reduce the higher harmonics of the output voltage. This helps lower the dv/dt of the rising edge, effectively reducing the bearing current. Simulations show that this method significantly reduces both the dv/dt and EDM currents, even without grounding the filter. 6. Conclusion Under high-frequency PWM input, the distributed capacitance in the motor creates a common-mode path, leading to shaft voltage and subsequent bearing currents. These currents manifest in three forms: dv/dt current, EDM current, and ring current. The bearing current is influenced by the coupling capacitance of various motor components, as well as the rise time and amplitude of the pulse voltage. This paper focuses on the first two types. The dv/dt current is closely related to the PWM rise time (tr), with smaller tr values resulting in higher dv/dt current amplitudes. Higher inverter carrier frequencies lead to more dv/dt current components in the bearing current. The EDM current is more unpredictable, occurring only when the bearing oil film breaks down or there is direct contact inside the bearing. Its amplitude depends on the peak shaft voltage, which is influenced by the pulse voltage’s rise rate. Adding an inductive element and an RC absorption network at the inverter output proves effective in reducing both the shaft voltage and bearing current, as confirmed by simulation results.
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