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

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