Comparison of Several Liquid Metal Sliding Electric Contacts

Abstract

Liquid metals have been used as sliding electric contacts since the time of Michael Faraday, who used Mercury in his early experimental work on electromotive force. In contrast to solid sliding electric contacts, liquid metals provide uniform coverage to a slip ring and therefore have very low electrical contact losses and are essentially wear free. However, the linear velocity dependent frictional losses in solid brushes are replaced by a cubic fluid frictional loss for liquid metals with a density dependent coefficient. Therefore, early attempts at minimizing total losses favored the use of the liquid metal eutectic alloy of Sodium and Potassium abbreviated NaK, whose mass density is less than water, 850 kg/m3 rejecting other liquid metals such as Gallium and its alloys which have mass densities around 6500 kg/m3. Furthermore, the chemical reactivity of NaK requires a carefully controlled atmosphere to prevent oxidation by water or oxygen. However, if the electrical contacts are in strong magnetic fields, the current-carrying liquid metals are accelerated inversely proportional to their density and the higher velocities lead to higher losses. This paper presents a comparison of the power losses of sliding electric contacts of both NaK and Gallium Indium Tin in magnetic fields. It is shown that, indeed at low magnetic fields, the GalnSn contacts are a factor of 6-8 more dissipative than NaK contacts as their density ratios would predict. However at higher magnetic fields, where Lorentz forces modify velocity distributions and circulating currents become stronger, the losses are similar. Furthermore, the present study shows that at high currents and high power levels, where electromagnetic expulsion forces become significant, the GalnSn collectors are as stabile as NaK collectors under similar conditions.