In machines such as homopolar motors utilizing high armature current at low voltage, high current-carrying capacity is required for making electrical connections between the rotor and the stator. It is well known in the art to utilize liquid metal current collectors for such machines due to the high current-density capacity of such collectors, for example, 3,000 to 10,000 amperes per square inch, by the liquid metal as compared to approximately one-tenth of this current density in the case of conventional solid carbon brushes. Further, solid brushes exhibit an order of magnitude higher voltage drop than that of liquid metal brushes, even at the lower current density. As a result of this higher voltage drop, solid brushes generate more heat than liquid metal brushes and wear both the brushes and the slip rings down rapidly. To provide the high current carrying capacity required by homopolar motors, a large number of solid brushes are necessary. The current is not necessarily shared equally by all of the brushes resulting in unequal heating and wear of the brushes. Moreover, a failure of one brush allowing debris to be transported around the slip ring may result in catastrophic failure of all the brushes.
In high current, high speed operation of homopolar machines, it has been common practice to provide a constant supply of liquid metal by pumping it to the stator and rotor current collector sites. This ensures that the current collectors are continuously wetted.
U.S. Pat. No. 4,628,221 to Young discloses a homopolar motor with pressurized liquid metal contact. The invention of Young uses a rotor having a circular cylindrical shell utilized as a conductor ring, a stator current collector ring of one polarity encircling one edge of the rotor conductor ring and another stator current collector ring of the opposite polarity encircling the other edge of the rotor conductor ring. Liquid metal is utilized within the cylindrical enclosure to provide continuous electrical contact between the stator current collector and the rotor ring.
U.S. Pat. No. 2,588,466 to Barnes discloses a unipolar or homopolar generator using a sodium/potassium alloy as a liquid brushes. Structure adapted to employ liquid brushes is also disclosed.
Great Britain Patent Application No. 2,203,293 to Parson discloses a homopolar generator comprising a rotor having an armature of conductive, ferromagnetic material. The rotor being surrounded by a ferromagnetic stator, field coils being provided on the stator, the field coils being connected to an alternating current supply to provide an alternative magnetic field.
European Patent Application No. 0,347,089 discloses a homopolar device along the line of the above GB '293 application with the exception that it utilizes first and second annular elements having conductive and non-conductive sectors. Formation of annular electrical currents is thus avoided.