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Electrical discharge machining bearing currents that may occur within electric machines of variable-speed-drive motor systems have been recognized for a long time. One key influential factor, the machine's capacitive voltage divider “bearing-voltage-ratio” (BVR) strongly depends on the rotor-to-frame and the stator winding-to-rotor capacitances; these are, in turn, affected by the design of the machine's stator slot. This paper presents an approach to improve the accuracy with which these capacitances can be estimated. It is based on the well-known plate capacitance equation which is then corrected by normalization functions. The functions are defined by extensive parameter studies using electrostatic finite element method (FEM) simulations. The final expressions not only allow for the prediction of the stator-winding-to-rotor and rotor-to-frame capacitances, they are also readily applicable. Thereby, they facilitate, for example, the clear-cut study of the sensitivity of the BVR towards changes in the different stator slot parameters. [ABSTRACT FROM AUTHOR]

Inverter-induced high-frequency bearing currents are a recognized cause of bearing failure in frequency converter-fed electric machines. Mechanical bearing faults are generally identified by vibration measurements. In this work, we submit bearings to electric discharge machining bearing currents, measure the electrical stress placed on the bearings, the resulting vibration signal and operating temperature, and apply time- and frequency-domain signal processing techniques for feature extraction. Experiments are run for 940 h of operation, so as to study the incipient bearing failure behavior. After the end of the test run, the bearing surfaces are inspected using scanning electron microscopy. [ABSTRACT FROM AUTHOR]

The possibility of bearing damage caused by inverter-induced bearing currents in modern variable-speed drive systems has been well recognized. However, the breakdown and current conduction mechanisms have still not been well understood today. We present results on the HF impedance properties of rolling element bearings at 300-kHz and 1.5-MHz signals—frequencies taken from the typical spectra of HF circulating bearing currents. Different motor speeds, bearing temperatures, voltages across the bearing, and two different motor sizes are investigated. Notably, we analyze the frequency of transitions between ohmic and capacitive behavior (or vice versa)—the “mode transitions”—as a function of these operating parameters: For some operating conditions, the bearing impedance alternates between capacitive and ohmic behavior without any modification of the macroscopic conditions it is subjected to. [ABSTRACT FROM PUBLISHER]

We report on a new type of inverter-induced bearing current (BC) that can occur with large inverter-based drive systems with one journal bearing. This current is a variation of the "high-frequency circulating" BC type that has been described by different authors over the last years. As with the first, the BC mirrors the common-mode (CM) current as a result of inductive coupling. However, the voltage across the bearing also mirrors the stator winding CM voltage and is not reduced to zero by the current flow. We show experimental results of measured currents and give physical explanations from which we then derive an effective mitigation technique, which is also illustrated through measurement results. [ABSTRACT FROM AUTHOR]

Carbon brushes using electrically conductive carbon graphite have been used for shaft-grounding and current-conduction purposes for a long time. We present an alternative brush concept made from millions of conductive microfibers. If properly designed, such brushes can meet all the demands on carbon brushes but exclude the problems of excessive and hot-spotting/thermal wears commonly quoted with the first. These brushes have ultralow friction and negligible wear and are very robust toward contamination, when compared with conventional carbon-based brushes. In addition, they are particularly suitable for use with high-frequency currents and voltages, as they also allow current-conduction based on electric field emission. We develop the design criteria for such brushes, both for prevention of voltage buildup and current conduction of several tens of amperes with frequencies in the megahertz range. We support the theoretical results with measurement results taken at different rotor shaft speed. [ABSTRACT FROM AUTHOR]

Inverter-induced high-frequency bearing currents are a parasitic effect that can occur in variable-speed drive systems. While the physical cause-and-effect chains are understood, little has been known on appropriate quantitative assessment of the phenomena. This paper discusses reliable techniques for the measurement of different parameters of importance for the bearing damage assessment, including their effects on the phenomena and measured quantities. A good understanding of these correlations allows custom-design cost-effective handling of the matter. [ABSTRACT FROM AUTHOR]

The influence of different parameters of a variable-speed drive system on the phenomena of inverter-induced bearing currents has been studied under exactly the same conditions on inverter-operated alternating current motors from 1 to 500 kW. Detailed modeling may not always be applicable with practical applications in the field, where many parameters might be unknown. Therefore, the most important correlations are summarized in the form of a flowchart that is based on the physical cause-and-effect chains. This flowchart can serve as a tool for engineers to estimate the endangerment of a drive system due to inverter-induced bearing currents and select an appropriate mitigation technique if necessary, where detailed knowledge of the different design parameters is not available. [ABSTRACT FROM AUTHOR]

The high-frequency (HF) circulating bearing current that may occur in the machines of the inverter-based drive systems can be reduced or even completely suppressed by the use of the electrically insulated bearings or the insulated inner bearing seats. This paper proposes a method to calculate the influence of the thickness of the insulating coat on the obtained current reduction as a function of the several machine-design parameters. The parameters of the equivalent circuit are derived from an eddy-current model. The calculation considers the transient nature of the current flow. For the standard fan-cooled motors with up to 500-kW rated power, a minimum thickness of the insulating coat of 250 μm is required to reduce the circulating bearing currents to values where the apparent bearing-current density is less than 0.1 A/mm², which is commonly taken as the threshold to the bearing failure. [ABSTRACT FROM AUTHOR]