Offset liquid metal storage method and means

An offset or asymmetrical self-contained liquid metal current collector for cyclic or homopolar electrical generators or motors is provided which includes a narrow annular volume on one side and a wide annular volume on the other side of the collector disc. Selectively positioned circular troughs effectively communicate with each other so that the liquid metal is trapped and retained when the rotating speed of the disc is too low to centrifugally maintain the liquid metal at the outer periphery of the disc.

The present invention concerns machines using liquid metal for electrical 
contact between a collector disc and its mating ring and, more 
particularly, providing such contact to assure retention of the liquid 
metal at the collector site by sufficient centrifugal forces in the 
rotating liquid to overcome the electrical expulsion forces generated in 
the machine. 
In order to obtain superconducting electric motors and generators having 
high efficiency, small volume, and low weight, it is necessary to utilize 
liquid metal current collection in an unflooded construction 
configuration. In machines of this type, the current is transferred 
between the rotor and the stator by means of a brush consisting of a disc 
mounted on the rotor revolving in a channel in the stator which contains 
liquid, typically a sodium-potassium eutectic NaK-78 or NaK. 
In any practical machine, it is necessary to utilize multiple current 
carrying members on the rotor in order to obtain reasonable terminal 
voltages and good flux utilization which use necessitates the employment 
of multiple brushes which are necessarily adjacent to each other. To avoid 
shorting out the machine, the NaK must be contained within each individual 
brush site. Electromagnetic Lorentz forces acting on the liquid metal tend 
to expel the NaK from the brush site under certain conditions and 
hydrodynamic forces may cause the liquid metal to migrate between brush 
sites. The present invention is directed to a method and means for 
overcoming these and other problems entailed in the use of liquid metal 
for current collection. 
Accordingly, it is an object of the present invention to provide improved 
stability of the current carrying capability of liquid metal current 
collectors in acyclic drum and other machines in their low speed ranges. 
Another object of this invention is to provide a method and means for 
substantially reducing the differential head across the rotor in 
superconducting electric motors and generators utilizing liquid metal for 
current collection. 
A further object of this invention is to provide an improved current 
carrying capability in liquid metal current collectors by means of which a 
maximum useful centrifugal force is achieved using a minimum quantity of 
liquid metal in machine ranges in which the rotational speeds are 
sufficient to develop adequate centrifugal forces.

The present invention, in general, provides an asymmetrical configuration 
of a liquid metal current collector ring rotating with respect to a 
stationary collector ring which assures retention of the liquid metal at 
the electrical collector site by incorporating circular troughs in both 
the rotor and stator which overlap and cooperate with one another. The 
troughs provide annular volumes at the sides of the collector discs so 
that the local supply of liquid metal is trapped and retained within each 
individual collector volume when the rotating speed of the disc is too low 
to centrifugally maintain the liquid metal at the outer periphery of the 
rotating collector disc. 
The advantages of liquid metal collector rings lie primarily in the much 
higher current density that can be carried, e.g. 3,000 to 10,000 amperes 
per square inch, by the liquid metal as compared to 50 to 100 amperes per 
square inch in conventional carbon brushes, and that as compared to the 
approximately one (1) volt voltage drop across conventional carbon 
brushes, the voltage drop across the brush site is reduced to a few 
microvolts by the use of properly made liquid metal collectors. Electrical 
resistance losses are correspondingly reduced. 
Present collector configurations in commercial use are directed to 
continuously removing liquid metal from each individual collector site to 
cool and purify the liquid metal, and to continual replenishment through 
electrically insulated, external sets of pipes, heat exchanges, etc. The 
present invention provides a current collector in which the needed 
quantity of liquid metal for the external collector ring is placed and 
retained in the collector region which thus becomes self-contained and 
does not require continual extraction and withdrawal from the retainer to 
the collector. 
Referring to the drawings, FIG. 1 shows the portion of armature current I 
through the collector gap, indicated at 11, and the direction of the 
resultant expulsion force P, indicated by arrow 12, together with the 
centrifugal hydraulic head, indicated at h, which successfully retains 
liquid metal in the collector gap with higher machine speeds. The armature 
current running in the circuit through the collector gap and the liquid 
metal induces a magnetic field which acts upon the liquid metal electrical 
conductor 15 in gap 16 between a rotor disc 18 and a stator 19. The liquid 
metal level during operation at low speeds without the features of the 
invention is indicated at 22 while the liquid at operating speeds is 
substantially within the lines indicated at 23 and 24. Depending upon the 
speed of disc 18, there is a specific height, H=h-d, of liquid metal in 
the liquid retention area indicated at 26 at which the centrifugal forces 
on the liquid metal in this area will be sufficient to counterbalance the 
expulsion force and maintain the liquid metal in the area indicated at 
23-24. 
FIG. 2 illustrates a preferred embodiment of the invention wherein a rotor 
disc 30 is specifically formed to retain liquid metal in a collector gap 
31 at such low operating speeds where otherwise liquid metal would 
normally leave this area. A salient collector head 32 of disc 30 is offset 
from an encompassing stator collector 33 a small amount. A lateral gap, on 
the order of 0.040 inches in this embodiment, is provided in the liquid 
metal retention area indicated at 35 to provide the counterbalancing 
centrifugal force. When rotor 30 is stationary, the liquid metal will drop 
to the bottom of a pair of cooperating opposed channels 36 and 37 in the 
lower disc and stator levels, respectively. A larger volume of liquid 
metal will collect at 40 which volume must be sufficient to store the 
principal portion of the total amount of liquid metal required by each 
individual collector without spilling over into adjacent collectors. Such 
spilling over, of course, would electrically connect consecutive 
conductors and short out the machine. 
All of the surfaces of disc 30 along the liquid metal channels at either 
side thereof in the area indicated at 35 and 40 are preferably coated with 
an electrical insulating material, not shown. This material confines the 
Lorentz expulsion forces to the collector side region 31. The narrow area 
at 35 provides the necessary height for the development of sufficient 
centrifugal forces while requiring a minimum quantity of liquid metal to 
do so, and the side area at 40 provides adequate storage volume to retain 
this minimum quantity of liquid metal even when the rotor is stationary or 
when there is a pitch to the center line of the motor shaft. 
FIG. 3 shows the liquid metal storage concept of the invention installed in 
an acyclic machine. Armature current enters a cylindrical rotor 42 at a 
point not shown and moves axially therethrough in the direction indicated 
by arrow 43 to a cylindrical stator 45 through liquid metal in a liquid 
metal collector region 46 in a direction indicated by arrow 47 and 
thereafter through stator 45 in the direction indicated by arrow 48 and 
back to rotor 42 at the opposite end of the motor, not shown. Both rotor 
42 and stator 45 are typically made of copper while the liquid metal may 
typically be NaK-78 or NaK. The main flux field extends radially outward 
from the center of the machine and thus is in a direction perpendicular to 
that of the current travel indicated by arrows 43 and 48. A second 
magnetic field is generated by the armature current flowing in the circuit 
through successive liquid metal collectors 46, 50 and 51, this field being 
that which acts upon the liquid metal conductor as shown in FIG. 1. 
There is thus provided a new and novel means for liquid metal retention in 
a self-contained collector for an acyclic drum type machine. The novel 
offset means provides for a radial, gravitational and centrifugal force 
activated column or head height, H, of liquid metal which furnishes an 
opposing pressure to overcome unbalanced axial pressure in the liquid 
metal at the collector sites. This pressure retains the liquid metal in 
its proper location, using a minimum practical amount of liquid metal, 
while still providing an adequate storage volume for the liquid metal in 
each collector when the motor is stationary, i.e. not rotating. The 
electrical circuit of the motor armature, as seen in FIG. 3, is from rotor 
conductor 42 through regions 46, 50 and 51 thence along the return path 
indicated at 55 for as many turns as are formed in the machine.