Laboratory Results

A One-Piece Faraday Generator With Liquid Metal Brush

With three preliminary models, a fourth working prototype of the OPFG was fabricated using 8" ceramic magnets (four on each side of disk) and a 1/2" thick copper disk (Fig. 3). A General Electric current shunt (50 mV @ 2500 A) was used for accurate current measurement and a DC motor with 12 V battery power. Between Trombly, DePalma, Wil-helm and the author, however, this OPFG had the highest internal resistance, (which alone limits the theoretical maximum power output). Using an Electronics Limited Milliohmeter (Model 47A), it was measured at 230 microhms (+/- 10) with the current shunt disconnected. The GE current shunt contributed 20.0 microhms to the circuit while the contact resistance of the milliohmeter leads was about 30 microhms. Noting the contact resistance of copper brushes, the danger of mercury, and the wetting problem of mercury, it was decided to fabricate a circular trough with AC heaters for a low temperature solder (Wood's Metal) brush. Through one year of trial and error, it was discovered that the brush of an HPG has to be located as close to the disk as possible to obtain the maximum emf and current. Therefore, a

Proc. Intersoc. Energy Conver. Eng. Confer., 1991

special circular flange was designed (see Fig. 4) to pass through the circular trough filled with hot, melted solder.

Without the internal resistance of the liquid metal brush, the internal resistance of the copper disk and 1" brass shaft was measured to be about 0.1 microhm. Woodson and Melcher show that the maximum current can be calculated using the open circuit voltage, which yields about 500,000 Amps, at 1000 RPM. The maximum power that could therefore theoretically be delivered at that speed is about 30 kW [28]. While impressive, this shows the vital importance of a very low resistance brush system for high power output HPGs. For example, with the help of superconducting magnets, Northern Engineering Industries in England has designed an HPG capable of 1300 megawatts of continuous output [29].

Testing for Output and Back Torque

The 8" ring magnets used were the Ferrimag 5 #MF-51239 which have a 4" hole in the center, where the flux actually reverses. Though the center flux is of much lower intensity than the rest of the magnet area, the effect forced the the average magnetic flux density (B) to become a calculated quantity. Using the standard V=wBR/2, where R is the difference of the squares of the inner and outer radii, we solved for the average flux density experienced by the OPFG during open circuit emf production. For six trials, the average B = 0.163 Tesla.

With B determined, the torque delivered to the generator by the DC motor could be calculated for each trial, using P=wT. The GE motor was a 24 V, 20 A, 3400 RPM, 1/2 Hp motor. However, since the efficiency curve could not be obtained from the dealer nor from GE, we were forced to assume 100% efficiency for all power calculations. Since the load to the motor varied less than 10% between open and

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