Annette Muetze

Calculation of Motor Capacitances for Prediction of the Voltage Across the Bearings in Machines of Inverter-Based Drive Systems

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

Electric bicycles - A performance evaluation

In this article, the term electric bicycle is used to describe electric-motor-powered bicycles, including both fully and partially motor-powered bicycles. Here, the electric bicycle market would benefit from further research both on the battery and on the drive technology and their use with electric bicycles. In the United States, electric bicycles are currently used most commonly for short trips to grocery stores or for leisurely rides.This article provides a systematic, comprehensive classification of electric bicycles that includes an overview of the state of the art of todays commercially available electric bicycles. The power requirements in different typical riding situations are also identified. The results are confirmed by experiments. From the results, the key parameters, needs, and challenges involved in improving the performance of electric bicycle are identified.The article gives the summary of the different results that can serve as a roadmap for such improvements. This summary includes both market trends and regulations and technical-science-related aspects.

Simplified Design of Common-Mode Chokes for Reduction of Motor Ground Currents in Inverter Drives

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.

Techniques for Measurement of Parameters Related to Inverter-Induced Bearing Currents

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 the 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.

Economic Factors and Incentives for Ocean Wave Energy Conversion

Ocean energy conversion systems have recently seen renewed interest, stimulated mostly by todays increasing energy discussions. In addition to ocean current energy converters, several commercial ocean wave energy projects have already been undertaken; however, no clear economic model exists for developers to follow. Therefore, this paper focuses on U.S. economic factors that affect the use of wave power for generation of electric energy. These are addressed with regard to economic and business incentives for renewables and wave energy. This paper also investigates incentive programs needed to promote wave energy conversion technology.

Scaling Issues for Common-Mode Chokes to Mitigate Ground Currents in Inverter-Based Drive Systems

The additional high frequency common mode current of inverter-based drive systems can cause parasitic effects such as high frequency bearing currents. This high frequency common mode current can be significantly reduced by purpose-designed common mode chokes that are inserted in the drive at the inverter-output. The paper discusses scaling issues related to the design of such common mode chokes, both of wound and feed-through toroidal types. The requirements for the design of chokes for these applications can differ from those for common mode chokes designed for use in the inverter-input, such as those used for the purpose of EMI filtering. This difference is notably due to the comparatively large magnitude of the currents involved. The developed correlations allow taking full advantage of the possibilities offered by such chokes to reduce the high-frequency common mode current and mitigate the resultant parasitic effects. The discussed relationships are derived analytically, and the results are supported by measurements.

Simulation Model of Common-Mode Chokes for High-Power Applications

Linear and nonlinear Cauer-network models for nanocrystalline inductor cores are developed and used for circuit simulations of common-mode chokes for inverter-based drive systems of several hundred kW. Experimental measurements of the chokes performance in a high-power test bed are compared to the model results. When the model is based on measured characteristics of the material used, the match to measured behavior is good. Damping inherent in the material characteristic is adequate, such that additional damping is not useful. The primary limitation to accurate prediction of loss in practice is the tolerance on specifications of core characteristics, particularly at high frequency where limits are not specified.

Influence of motor operating parameters on discharge bearing current activity

While the cause-and-effect chains of inverter-induced bearing currents have been well understood today, not much is known about the damage mechanism inside the bearings. Research has suggested that the bearing degradation is approximately proportional to the frequency of occurrence of the bearing currents. For bearing currents that occur statistically distributed-such as discharge bearing currents-this parameter has not been thoroughly researched yet. In this paper, we present an analysis of the influence of different motor operating parameters on the frequency of occurrence (“discharge activity”) of discharge bearing currents, using a non-intrusive detection method. When analyzed individually, the motor speed, motor shaft temperature, inverter switching frequency, and inverter dc-link voltage influence the discharge activity. However, the overall running time of the motor, the state-steady-state versus instantaneous-and the “operating history” on the time-scale of several days have the strongest effect on the frequency of the discharge bearing current occurrence.

Design Aspects of Conductive Microfiber Rings for Shaft Grounding Purposes

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 wear and hot-spotting/thermal wear commonly quoted with the first. These brushes have negligible frictional wear, are free of maintenance, and robust towards contamination. In addition, they are especially 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 build-up and current-conduction of several 10s of Amperes with frequencies in the MHz range. We support the theoretical results with measurement results taken at different rotor shaft speed.

An Alternative Approach to Analytic Force Computation in Permanent-Magnet Machines

We present an alternative approach to analytic force computation in permanent-magnet machines, focusing on the tangential forces that generate the armature and the cogging torque. We extend previously presented methods using conformal mapping to overcome the limits imposed by the singularity of the magnetic flux density at the tooth tip that occurs during the transformation. Using our approach, the cogging and armature torques developed in lightly loaded electric machines can be computed without these limits and without introducing auxiliary parameters. We also revisit Maxwells stress theory to compute the force on the interface of two materials with different permeabilities, in contrast to the common application to compute the force on a rigid body placed in an electromagnetic field. Using this technique, we then calculate the forces at the slot sides so that the influence of the machine design parameters on the result is directly available.