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As for mass-produced sub-fractional horsepower drives, non-optimal motor behavior (e.g., high cogging torque, output torque ripple, and noise) is often accepted when cost can be reduced, provided reliability is not compromised. This paper proposes an innovative claw-pole motor design for a low-cost single-phase brushless direct current fan drive, improving motor behavior with no increase to the manufacturing cost: 1) reducing cogging torque by proposing unconventional claw-pole skewing and 2) ensuring self-starting capability by implementing air-gap asymmetry. Both measures help reduce the total output torque ripple. The goal is to reduce fan drive noise, especially at low-speed operation, where cogging torque is often the dominating noise source. The design of stator claw skewing and air-gap asymmetry is presented; their effects on motor quantities are studied via simulations and experiments. With the exception of a small reduction of about 1.8% in the back electromotive force fundamental, skewing the stator claws by 30 $^{\circ }$ can reduce the cogging torque by 23% in the simulations and by 28% in the experiments. [ABSTRACT FROM AUTHOR]

Three-phase passive harmonic filters (e.g., LCL filters) are often utilized to attenuate harmonic currents caused by pulsewidth modulation of voltage-sourced converters (VSC) to achieve high power quality. However, LCL filters may jeopardize the system stability due to their inherent susceptibility to resonance. This paper proposes an active damping current controller based on the internal model principle for a grid-connected VSC equipped with an LCL filter. This controller can be considered a high-order multi-input multi-output transfer function, implemented in the $dq$ reference frame, including decoupling terms for improved transient behavior. Simulation case studies are performed in the PSCAD/EMTDC environment to evaluate the transient behavior of the controller and test its robustness against parameter variations and unbalanced and distorted grid conditions. Experimental results verify the performance of the proposed controller and confirm its ability to operate at low switching frequencies. The results are compared with two existing active damping strategies (virtual RC and notch filter methods), confirming that the proposed controller shows significantly improved robustness with no degradation of transient behavior, has the ability to operate at low switching frequencies, and does not need additional sensors. [ABSTRACT FROM AUTHOR]

This paper introduces a detailed nonlinearity characterization technique for insulated-gate bipolar transistor inverters in a drive system which is able to measure device nonlinearities. Methods are derived from measurements of the voltage on the inverter phases. These measurements are obtained while applying a set of direct currents to an electrical machine. These methods are then applied to tests using alternating current. The study of inverter nonlinearities is useful in order to conduct condition monitoring. These methods fulfill the need to characterize each device in an inverter. In addition to characterizing every device using either ac or dc current, the method is resistant to noise; for these reasons, the method is applicable in the field. Simulations and experimental work demonstrate the benefits and applicability of the methods. [ABSTRACT FROM AUTHOR]

Common-mode voltage (CMV) in voltage-source inverters (VSIs) has been well defined and is frequently referred to in the literature. While often referred to, there is no real consistency with respect to how the CMV properties of an individual modulation scheme are evaluated, lacking tools to assess the relative merits or disadvantages of a certain topology or modification of a control scheme. The way that it is analyzed is still mostly a matter of the different authors' foci. In this paper, we propose a metric for the analysis of the CMV, aiming to fill the aforementioned gap. Thereby, the CMV in VSIs is quantified by the number of possible levels and the step sizes between different levels, as well as the respective frequencies of occurrence. The first two are given by the number of levels and the number of phases of the inverter. Depending on the modulation (control) schemes, all possibly available levels or a few of these levels are present in the CMV. We illustrate the use of the tool by applying it to thoroughly study the CMV properties intrinsic to the topologies of multilevel multiphase single- as well as double-ended diode-clamped VSI drive systems. As part of this analysis, we uncover new relationships concerning the existence of zero CMV level. Furthermore, we exemplarily apply the developed tool to the assessment of the CMV properties of selected pulse width modulation (PWM) control schemes and translate the findings into an interpretation of the endangerment of a given drive to suffer damage from inverter-induced bearing currents. The theoretical findings are supported by measurements. This paper also reviews the literature on CMV in multilevel multiphase VSI systems. With the proposed metric in mind, the CMV being mostly discussed as an element of different foci becomes even clearer, confirming again the need for the development of such a tool. [ABSTRACT FROM AUTHOR]

A simplified design calculation for common-mode chokes for reducing motor ground currents provides insight on the effects of various parameters and allows easy design. Key simplifying assumptions are that the voltage rise time is short compared to the ring period and that the damping is small. Example designs for machines up to 450-mm frame size show how effective ground-current reduction is possible with small inexpensive single-turn chokes. [ABSTRACT FROM AUTHOR]

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]

In this survey paper, we show by straightforward mathematical scaling considerations that small inverter-fed ac motors of up to typical 20 kW at 1500/mm are likely to suffer from discharge (electric discharge machining) bearing currents, whereas larger motors are likely to be subjected to high-frequency (HF) circulating bearing currents. When comparing motors that operate at the same voltage level, the resulting bearing-current density is high for very small and very large motors. Smaller values occur in between these two extremes with medium size motors in the range of 10-100 kW. Therefore, electric bearing insulation is useful for larger motors to interrupt the HF circulating bearing-current path, whereas small motors need rotor shielding, common-mode voltage filters, or hybrid bearings. [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 occurrence of high-frequency discharge bearing currents (also called "electric discharge machining (EDM) currents") that can occur in machines of inverter-based drive systems depends strongly on the value of the capacitive voltage divider "bearing voltage ratio" (BVR) of the machine. This paper presents a straightforward approach to calculate the involved motor capacitances in order to predict the voltage across the bearing and, thereby, the likeliness of such EDM currents that are caused by this voltage to occur. The approach is based on the design parameters of the machine and does not involve additional measurements. It combines ease of application and satisfying accurateness, and allows prediction of the risk of endangerment of an inverter-based drive system due to discharge bearing currents. [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]