Philip Reichner

Metallic Brushes for Extreme High-Current Applications

Recent studies of homopolar machines and electric vehicles have generated a renewed interest in sliding contacts which can carry a very high-current density. Simple metallic brushes were shown to be capable of the efficient transfer of very high currents under proper conditions of mechanical load and with control of the surrounding atmosphere. In stable operation under a carbon dioxide cover gas, copper fibers have transferred 12.4 MA/m2(8000 A/in2) with contact drops less than 50 mV. During a 240-h test, the dimensionless wear rate was found to be 5 x 10-11. Current densities as high as 23.2 MA/m2were demonstrated in short-term tests. To evaluate the feasibility of practical application a larger copper fiber brush was assembled and tested in a standard holder with a conventional spring loading system. The observed contact voltage drop was quite similar to that found for the smaller brushes. The test results conformed to elastic contact theory and indicated that the controlling resistive component was due to film rather than constriction. The ability to use metallic contacts under a controlled atmosphere provides an opportunity for the investigation of sliding electrical contacts without the complexity introduced by excessive oxide formation and without the added parameters that would be introduced by graphite or other lubricant films.

High Current Tests of Metal Fiber Brushes

Simultaneous measurements Were made of the friction force and electrical contact drop for small metal fiber brushes at high current densities (12.4 MA/m2). Comparative testing was performed with both polarities of copper, silver-plated copper, and silver fiber brushes on copper and On silver-plated copper slip rings in humidified carbon dioxide. Friction increased from 0.3 to 1.4, while the contact drop decreased as more silver was added. At a contact force of 0.23 N and a sliding speed of 12 m/s, minimum total power loss was found for the bare and the silver-plated copper fibers on the copper sllp ring. However, the optimum material selection will depend on current density, sliding speed, and contact force Used for a specific application. Maximum test duration has been extended to almost 500 h, with indications of slight improvement of both contact drop and wear rate as the operating time is increased. Dimensionless wear rate was only 1.9 x 10-11. With added cooling capability, successful Shortterm operation has been achieved with current density in the individual fibers as high as 100 MA/m2 (65 000 A/in2).

Mechanical Load Aspects of High-Current Brush System Design

Selection of proper mechanical loading is necessary to assure satisfactory brush operation in rotating electrical machines. Irregularities in the moving surface produce forces such as brush inertia and holder friction which alter the contact force. The range of force variation depends upon controllable factors in the brush system design. For high metal-content brushes operating in an inert atmosphere, the sliding contact is modeled using theory applicable to metallic contacts with high conductivity films. Based on this model and under conditions of fluctuating contact force, the brush power loss is higher than that which would occur for a steady force equal to the average value. The results may be used to determine the sensitivity of a brush system design to irregularities in the dimensions of the slip ring or commutator and in the selection of proper brush loading. In addition, the results may explain differences in performance between laboratory brush tests and machine operation, which are caused by differences in acceleration forces and current redistribution.

Pressure-Wear Theory for Sliding Electrical Contacts

The concept of wear-dependent contact phenomena is developed for sliding electrical contacts, where geometric constraints on wear directly influence contact pressure distribution under the face of a brush. In terms of general contact theory, this pressure distribution is related to the number of contacting asperities, the true area of each contact, or the frequency of asperity encounters. The concept is proposed as a potential new tool to be used in the formulation of a descriptive analytical model of the brush interface. Analytical relationships are developed for friction, wear, and electrical contact resistance for composite and other multicomponent brush configurations.

Shunts for High-Current Density Brushes

The conventional shunt size cannot be maintained with brush body current densities in the range of 1000A/in2(1.55 MA/m2) and higher. The large shunts which are necessary to preserve low-voltage drops and acceptable temperatures ate found to be heavy and stiff. Brush body voltage drop may also become a significant contributor to electrical power loss. With bulky shunt leads, it is difficult to avoid adverse mechanical restraint of the brush by the shunt, and this in turn prevents achievement of the contact load control necessary for good brush performance. In some machines, the contact load is also modified by electromagnetic forces on the brush body associated with conventional shunting techniques. A sliding contact shunt is being developed to replace the conventional shunt. The new shunt greatly reduces the electrical power loss and the effect of magnetic forces. Friction interference with brush movement is made acceptable through the application of multiple-point contact configurations. Increasing the number of contact points reduces the total contact force required to achieve a given value of resistance. This force reduction associated with well-defined multiple contacts presents itself as a useful design tool in electrical machinery development. The concept of the multiple-point contact shunt has been demonstrated experimentally, and test results are found to be consistent with the theoretical predictions.

Stress concentration in the multiple-notched rim of a disk

A photoelastic study shows the effect of notch radius and spacing on the stress-concentration factor in a multiple-notched, parallel-sided disk, for thermal and rotational stress distributions.