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Simulations of Brush Contacts of Homopolar Motor/Generators

Time-dependent distributions of electromagnetic force and heating were studied.

A computational- simulation study of distributions of electric current and temperature in brushes and slip rings in two model homopolar- motor/generator configurations was performed in support of the development, by the U.S. Navy, of a superconducting homopolar motor/generator (SCHPMG) machine for ship propulsion. Electrical-contact performance (more specifically, brush/slip-ring contact performance) is a limiting factor in the performance of an SCHPMG machine. The present study and similar studies are needed to gain understanding of brush/slip-ring contact performance in order to enable optimal design of brushes for homopolar machines.

Half Symmetric Finite-Element Models of eight-brush stators with electromagnet coils and slip rings were used in the computational simulations.

At the interface between a brush and a slip ring, where transfer of electric current occurs, factors that affect wear include the spatial distributions of electric current, magnetic field, temperature, and electromagnetic forces as the brush slides at a given velocity on an imperfect rotor (slip-ring) surface. There is experimental evidence that wear is not only highly asymmetric between anode and cathode brushes but is also nonuniform within the contact area of an individual brush. The asymmetry and nonuniformity of wear limits the life and efficiency of a brush assembly, inasmuch as the full volume of brush fiber material is not fully utilized. Moreover, debris from worn brushes can give rise to internal short circuits and/or other adverse effects on the operation of a machine containing a brush assembly.

Each of the two models in the computational- simulation study was a half-symmetric finite-element model representing eight stator brushes, an electromagnet coil, a slip ring, and buses (see figure). The two models differed in the aspect ratios of the brushes (1:2 versus 1:3). The brush contact area was kept constant at 2 cm2, a transport current of 325 A was considered to be applied to each brush, and the current applied to the electromagnet coil was chosen to produce a magnetic flux density of 0.4 T at the brush locations. Coupled electromagnetic and thermal behaviors (including bulk ohmic heating) were included in the simulations. Local adiabaticity and a specific form of the temperature dependence of electrical conductivity were assumed. Time-dependent spatial distributions of electric current, temperature, and electromagnetic force were calculated. The conclusions drawn from the numerical results include the following:

  • Circumferential components of electric current at leading and trailing edges of brushes could interact with the magnetic field to produce forces tending to destabilize the brushes, but these forces could be reduced through appropriate design.
  • A brush having a higher aspect ratio is subject to smaller destabilizing forces.
  • Power loss per brush due to bulk ohmic heating is about 0.74 W for an aspect ratio of 1:2 vs. 0.72 watts for an aspect ratio of 1:3.
  • Assuming a contact pressure of 2 psi (≈14 kPa) and a tip speed of 12 m/s, frictional heating is about 16.5 W per brush.
  • The power loss per slip ring due to bulk ohmic heating is about 0.98 W for an aspect ratio of 1:2 vs. 0.9 W for an aspect ratio of 1:3.

This work was done by Kuo-Ta Hsieh of the University of Texas at Austin for the Naval Research Laboratory.

NRL-0018

Topics:
Conductivity Electric motors Engines Fibers Machinery Magnetic materials Marine vehicles and equipment Materials properties Mechanical Components Powertrains Research and development Simulation and modeling Vehicles Wear
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Simulations of Brush Contacts of Homopolar Motor/Generators

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    Defense Tech Briefs Magazine

    This article first appeared in the December, 2007 issue of Defense Tech Briefs Magazine (Vol. 1 No. 6).

    Read more articles from the archives here.

    Overview

    The document is a Quarterly Progress Report prepared by Dr. Kuo-Ta Hsieh from the Institute for Advanced Technology at The University of Texas at Austin, covering the period from January 1 to March 31, 2006. It details the ongoing research and development efforts related to superconducting homopolar motor/generator (SCHPMG) technology, which is being explored for naval ship propulsion.

    The report emphasizes the critical role of electrical contact systems, specifically the performance of brushes and slip rings, in the efficiency and longevity of SCHPMG machines. It highlights that brush wear is a significant challenge, exhibiting asymmetric and non-uniform patterns between brushes of opposite polarity (anode and cathode) and within the contact area of individual brushes. This wear not only limits the operational life of the brush assembly but also generates debris that can negatively impact the machine's operating environment.

    To address these issues, the report underscores the importance of accurate electromagnetic (EM) characterization of the brush/rotor and brush/slip ring interfaces. Understanding the wear behavior of brushes is essential for optimizing brush design and improving overall system performance.

    The report also outlines the key personnel involved in the project, including Dr. Hsieh, who is an expert in electromechanical codes, and Dr. Sikhanda Satapathy, who focuses on mechanical simulations and brush design improvements. Their combined expertise is crucial for advancing the research objectives.

    Figures included in the report illustrate temperature distributions on brushes and slip rings at various time intervals, providing insights into the thermal behavior of these components under operational conditions. The data presented in these figures is vital for understanding the thermal dynamics that contribute to brush wear and overall system efficiency.

    Overall, the report serves as a progress update on the SCHPMG project, detailing the challenges faced, the methodologies employed, and the ongoing efforts to enhance the performance and reliability of superconducting motor technology. The findings and developments reported are expected to contribute significantly to the advancement of naval propulsion systems, aligning with the U.S. Navy's goals for innovative and efficient ship propulsion solutions.

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