An improved homopolar machine in which a solid sliding current collector, or brush, is provided for contact with a flat, annular contact surface of a rotor. The flat plane of the contact surface is orthogonal to the rotor's axis of rotation. A plurality of such contact surfaces with corresponding brushes may be employed. Also, several means for actuating the brush into contact with the rotor contact surface are provided.

BACKGROUND OF THE INVENTION 
The present invention relates to homopolar machines and more particularly 
to solid sliding current collectors or brushes for use with homopolar 
generators and motors. 
Homopolar machines have been successfully designed for providing peak 
electrical current discharges lasting several seconds and producing a peak 
current level in excess of a million amperes direct current. Such machines 
generally include a cylindrical rotor of either a drum or disc 
configuration, mounted on a frame, to be rotated about an axis through the 
center of the cylinder. A field coil encircling the rotor and connected to 
an external current supply provides an applied field excitation passing 
through the rotor. The applied field excitation is usually confined and 
directed by a ferromagnetic yoke surrounding the field coil and all, or a 
portion of, the rotor. 
A typical homopolar machine discharge is described as follows. The energy 
storing rotor or flywheel is accelerated to high speed, then the external 
magnetic field is applied via the excitation coil to create a voltage 
across the spinning rotor. When full field is reached, an external load is 
connected across the rotor terminals through sliding contacts or brushes. 
As current begins to flow through the rotor and load, Lorenz forces 
decelerate the rotor quickly, accomplishing the conversion of the stored 
kinetic energy to a single electrical current pulse. Pulse lengths depend 
upon the characteristics of the external load and are typically from 0.5 
to 3.0 seconds in duration. 
Although homopolar machines are typically operated in the pulsed mode as 
described, similar machines may also find application as continuous duty, 
low voltage, high current generators, or as low speed, high torque motors. 
When the homopolar machine rotor is spinning, the free electrons within the 
rotor experience an electromotive force resulting from their interaction 
with the applied field excitation. In prior art machines, brushes 
positioned inside the field coil, or between two halves of the field coil, 
are then lowered onto a radially outward (circumferential) surface of the 
spinning rotor to allow a current to flow under the influence of such 
electromotive force to an external circuit, and then back into the rotor 
through return conductors and additional brushes at a different location. 
To complete the electrical circuit, at least two sets of such brushes are 
required. During the discharge, the interaction of the discharge current 
and the applied field excitation creates a force which decelerates the 
rotor until its rotation stops and the discharge ends. 
It has been found that extremely high current of short duration pulses may 
be obtained after using a relatively low power conventional prime mover or 
a conventional low voltage, low amperage power source to store inertial 
energy in the rotor by gradually accelerating the rotor up to the desired 
rotational speed. 
In known homopolar generators, the brush mechanisms are subjected to 
extraordinarily difficult duty. In fact, collection of current by the 
brushes at the high peripheral speeds attained by the rotor represents the 
single most demanding task of any pulsed homopolar generator component. 
The desired pulse current magnitude and amount of stored or pulse energy 
both affect the brush collector area required to transfer the discharge 
current from the rotor. A fraction of the stored energy is necessarily 
lost at the brush/rotor interface in the form of heat, which can be 
reduced by increasing the collector area. 
Performance of homopolar machine brushes is also influenced by two external 
factors: (1) electrical load characteristics, and (2) the method by which 
the circuit is closed to initiate a discharge. Resistive loads are 
distinguished by very fast rise times, as peak current is often reached in 
less than 30 milliseconds (ms). Time available for the brush to become 
seated before carrying full current is therefore extremely limited. 
Inductive loads, on the other hand, slow the rise time considerably, but 
also extend the current pulse duration as the rotor energy is transferred 
to the inductor. This longer current pulse can significantly increase 
brush wear rates because the interface flash temperature, the peak 
temperature reached at the sliding brush/rotor interface, is maintained 
for extended periods, thus softening the brush material. 
Homopolar machines are typically switched by one of two methods. An 
external closing switch may be used, or the load may be connected directly 
to the brushes with switching performed by actuating the brushes into 
contact with the rotor. If the latter method is chosen, an arc may be 
drawn as contact is made, causing pitting of the brush surface and 
corresponding increased wear, thus reducing useful brush life. 
The foregoing is provided by way of background only. The general state of 
the art relating to pulsed power homopolar machines and generators is 
described more fully in the following publications, which are incorporated 
herein by reference: U.S. Pat. No. 4,459,504 to W. F. Weldon et al., 
issued Jul. 10, 1984; U.S. Pat. No. 4,544,874 to W. F. Weldon et al., 
issued Oct. 1, 1985; U.S. Pat. No. 4,816,709 to Weldon, issued Mar. 28, 
1989; W. F. Weldon et al., "The Design, Fabrication, and Testing of a Five 
Megajoule Homopolar Motor-Generator," presented at the International 
Conference on Energy Storage, Compression and Switching in Torino, Italy 
(November 1974); and J. H. Gully, "Compact Homopolar Generator," IEEE 
Transactions on Magnetics, vol. MAG-18, No. 1 (January 1982). In addition, 
U.S. Pat. No. 4,816,709 to W. F. Weldon, issued Mar. 18, 1989, describes 
an energy density homopolar generator as well as the general state of the 
art of homopolar generators. 
The successful development of solid sliding current collectors, or brushes, 
has advanced significantly over the past two decades (reference U.S. Pat. 
Nos. 4,459,504 and 4,562,368), driven significantly by the requirements of 
pulsed homopolar machines. The complex brush mechanisms that have resulted 
represent a significant fraction of the cost of such machines, both in 
terms of initial manufacturing cost and continuing maintenance and 
replacement cost. These costs are primarily a function of the multiplicity 
of parts that result from the following requirements. Specifically, the 
brushes must: 
(1) be retractable from the rotor surface to reduce wear and losses when 
not generating; 
(2) be capable of rapid actuation to enable use as a closing switch; 
(3) have a zero backlash mechanism to provide consistent repeatable 
alignment of the brush wear surface with the rotor or slip ring, as the 
brush wear rate has been found to be extremely sensitive to proper brush 
alignment; 
(4) have low inertia to follow rotor runout; 
(5) be current compensated to increase brush down force with increasing 
current levels to prevent arcing; and 
(6) provide sufficient brush length for adequate wear allowance. 
Meeting these diverse requirements has resulted in a multiplicity of parts 
to be manufactured and assembled, or to be disassembled and reassembled in 
the course of machine maintenance. 
The physical constraints of known brush mechanisms reduce their efficiency. 
For example, the space required for the known brush mechanisms results in 
only a fraction of the rotor periphery being contacted by brushes. Further 
the curvature of the brush surface of known brush mechanisms must 
precisely match that of the rotor under varying conditions of wear, 
temperature and alignment. Even the smallest mismatch in curvature will 
result in substantial reduction in effective contact area, increased 
contact impedance, increased brush wear, and therefore decreased machine 
performance. 
SUMMARY OF THE INVENTION 
The problems outlined above are addressed by the device and method of the 
present invention. The present invention maintains the operating 
advantages of the known current collection systems described above while 
reducing the part count, simplifying the manufacture, and reducing the 
space required by the current collection mechanism and, consequently, 
reducing the size and cost of the homopolar machine while improving its 
efficiency. 
Prior art brushes are made for contact with the radially outward rotor rim, 
which is a curved surface. In contrast, the brush of the present invention 
is designed for contact with a flat contact surface formed on an axial end 
of the rotor or a ring attached thereto or extending therefrom. Another 
improvement over the prior art is that the presently contemplated brush 
mechanism comprises a single brush or brush set per interface, as opposed 
to multiple independently mounted brushes. This results in the entire 
swept area of the rotor being contacted by the brush material. 
In a preferred embodiment, the one piece face brush mechanism of the 
present invention comprises an annular metal ring or plate with a suitable 
current collector material (such as Morganite CM-1S, electroless graphite 
or sintered copper graphite) attached to it ("brush ring"). Suitable 
attaching methods include bolting, soldering, brazing, sintering, 
clamping, or any other method effectively used to attach brush material to 
a metal ring. Alternatively, the current collector material (brush) may be 
flexibly mounted by means of a flexible or compliant shunt (such as a 
diaphragm spring) to help ensure a tight, spring loaded contact with the 
corresponding rotor surface. In this latter case, the brush material may 
be continuous or segmented, depending on the degree of flexibility and 
contact surface desired. 
The brush may be either permanently pressed against a flat annular contact 
surface on the end of the rotor or on a ring attached thereto or extending 
therefrom, or it may be pressed against such a surface repeatedly or/and 
selectively on command by actuation means. So that the electrical circuit 
is complete, at least two brushes are required, one each at opposing ends 
of a rotor or rotor face. Further, a conductor, such as a flexible shunt, 
may be provided to couple the brush ring to the load. 
A variety of actuation means are suitable for moving the brush into contact 
with the contact surface, depending on the performance required. The brush 
mechanism of the present invention may be actuated, for example, by 
hydraulic or pneumatic cylinders. In this case, some provision is 
preferably made to prevent the ring from significantly rotating with 
respect to the stator. Suitable stabilization means include splines, 
keyways (slots or grooves), radial pins or tabs on the radially outward 
surface of the brush ring, which are made to slide within a stator 
configured with complementary grooves or protrusions. Another suitable 
actuation means comprises an annular bellows-type actuator, which can be 
integrated with the brush ring and which can serve as the conducting shunt 
as well. Of course, an alternate shunt can be provided if it is desirable 
to reduce or eliminate the current flowing through the bellows material. 
An inflatable polymer diaphragm may also be used to actuate the brush 
mechanism. In this case, a separate shunt will be required, and it can be 
configured so that current flow will provide an additional force pressing 
the brush against the contact surface. This may be accomplished by shaping 
the shunt to direct the current in opposing directions. 
Yet another suitable actuation means may be incorporated into the structure 
of the stator having windings (not shown) by allowing the rotor to move 
with respect to the stator to make contact with a stationary brush, or by 
moving a portion of the stator coupled to the brush hydraulically, 
pneumatically, mechanically, magnetically, or by other suitable means. In 
the first instance, the brush mechanism may be stationarily affixed to the 
stator, and the rotor may be adapted to move into position for contact 
with the brush. In the latter instance, leakage flux from the excitation 
field of the machine or motor may be used to attract a ferromagnetic 
portion of the brush ring, thus helping to ensure that brushes are 
actuated only when adequate excitation is present. 
In some applications, it is possible that a single brush ring cannot carry 
the desired amount of current. Additionally, redundancy of brush 
collectors may be desired for other reasons. The present invention 
therefore contemplates embodiments having multiple brushes for drawing 
current from a single rotor. Any of the above-described mounting and 
actuation options may be used in multiple-brush embodiments. Furthermore, 
the planar brush and contact surface of the present invention may be 
combined with the previously known radial collection systems for 
redundancy or increased capacity. 
The advantages of the present invention will be further appreciated from 
the drawings and from the detailed description provided below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the drawings, FIG. 1 is a simplified diagram of a known 
rim-brush mechanism 12 for locating rim brush 14 into contact with the rim 
of rotor 10 by way of actuating means 13. Because it contacts the curved 
rim of rotor 10, the contact surface of rim brush 14 must also be curved 
in a complementary fashion to achieve suitable contact area. However, 
current collection brushes wear during use, often unevenly; hence, the 
proper curve may not be maintained, making it increasingly difficult for 
the brush to contact the rotor rim efficiently. 
The present invention provides an alternative brush design, as is 
illustrated in its most basic form in FIGS. 2A and 2B, for use in 
homopolar generators and other machines requiring sliding current 
transmission or collection mechanisms. The proposed design is a flat face 
brush ring 18 for contact with a flat contact surface 20 of a rotor 10. 
Annular contact surface 20 is preferably orthogonal to the rotational axis 
x (axis of rotation 23) of the rotor. Brush ring 18 may be a ring of brush 
material, centered around rotor shaft 22 and axis of rotation 23. 
Alternatively, brush ring 18 may comprise brush material (such as 
Morganite CM-1S) attached to a ring of supporting material 42, as 
illustrated in FIG. 4A, or it may comprise one or more discrete brushes, 
which are positioned to contact the contact surface 20 on end face 21 of 
rotor 10. In some installations, it may be desirable to segment the brush 
material on the brush ring into two or more segments, as shown in FIGS. 2B 
and 2C, for ease of installation and assembly. Actuation of brush ring 18 
is along axis x, as illustrated in FIGS. 2A and 2B. 
Two features of a brush system are particularly addressed by this 
invention. The first is the structure of the brush material and the brush 
ring. Because brush ring 18 contacts a flat contact surface 20, there is 
no curved contact surface, thereby reducing some of the difficulties that 
arise with rim brushes discussed earlier. The second brush-system feature 
addressed herein is the apparatus used to actuate the brush ring to bring 
it into contact with the flat contact surface. 
A variety of actuation methods are possible according the present 
invention. For example, FIG. 3 illustrates the use of discrete hydraulic 
or pneumatic cylinders 24 for actuating brush ring 18, attached to support 
ring 42, into contact with flat contact surface 28 of rotor 10. FIG. 3 
illustrates two cylinders 24 by way of example only; it will be apparent 
to those skilled in the art that a sufficient number of such cylinders 24 
may be used to ensure adequately uniform contact of brush ring 18 with 
rotor contact surface 28. 
In this case, some provision must be made to help prevent the brush ring 18 
from rotating with respect to the stator and cylinders 24. Suitable 
stabilizing means include providing splines 32, pins or tabs 33, a 
polygonal exterior 34, or keyways (slots or grooves) 35 on the brush ring 
as shown in FIGS. 3A-3D, respectively. The stator may then have a 
corresponding structure in which the brush ring travels upon actuation 
i.e., the stator is shaped in complementary fashion with brush ring 18, 
such as having grooves to admit splines 32, pins or tabs 33, or having 
protruding elements to fit within slots 35, thus preventing rotation of 
the brush ring with respect to the stator. 
A flexible conductive shunt 32, as shown in FIG. 4B, may be provided to 
conduct current from brush ring 18 to the stator (not shown), as well as 
to evenly apply force to press the brush material against the rotor face 
(not shown). Again, actuation is preferably primarily parallel to the 
x-axis. 
Alternative means of actuation are illustrated in FIGS. 5A and 5B. FIG. 5A 
shows discrete or continuous annular metal bellows 40 between support ring 
42 and brush ring 18 for actuating brush ring 18 into contact with a rotor 
(not shown). Support ring 42 may be mounted to a stator (not shown). Metal 
bellows 40 may be hydraulic, and filled with fluid through port 44 to 
expand the bellows, causing the brush to be actuated into contact with the 
contact surface on the rotor. Additionally, metal bellows 40 may form a 
single annular chamber, or a plurality of separate bellows 40 may be 
utilized in accordance with this invention. 
FIG. 5B shows inflatable polymeric diaphragms 50 connected to mounting ring 
43. When inflated with fluid (gas or liquid), diaphragms 50 actuate brush 
material 18, also mounted on mounting ring 43, into contact with a flat 
contact surface on a rotor (not shown). A separate shunt 32 is required if 
the diaphragm material is non-conductive or poorly conductive, and may be 
configured to direct the current in opposing directions, as shown in FIG. 
5B, so that the current flow will provide an additional downward force. 
In another embodiment, the actuating mechanism may be incorporated into the 
stator, as illustrated in FIGS. 6 and 7. FIG. 6 shows a partial cross 
section of a homopolar machine with brush ring 18 flexibly mounted to 
stator 26 by means of flexible conductive shunt 19. Rotor 10 is moveable 
axially along shaft 22 (as indicated by the double arrow) so that flat 
rotor contact surface 28 may be placed into contact with brush ring 18. 
FIG. 7 shows a partial cross section of a homopolar machine with brush ring 
18 flexibly mounted by shunt 19 to a moveable portion 27 of stator 26. 
Actuation of moveable stator portion 27 will cause brush ring 18 to 
contact flat rotor contact surface 28 of rotor 10. 
To ensure that the brush ring is actuated only when adequate excitation is 
present, the brush ring may have a ferromagnetic portion 36, as 
illustrated in FIG. 7A. In that case, leakage flux from the excitation 
field of the generator or motor may be used to attract the ferromagnetic 
portion of the brush ring, thus drawing the brush ring into contact with 
rotor 10 only when adequate excitation is present. 
In the event that a single ring brush cannot carry the desired amount of 
current, or if redundancy of current collectors is desired for other 
reasons, multiple ring brushes can be utilized in parallel or in 
opposition, as illustrated in FIGS. 8A and 8B. Any of the mounting and 
actuating options described herein, or other suitable means, can be used 
in either case. 
As illustrated in FIG. 8A, rotor 10 may have multiple extended rings, or 
lips 30, with flat contact surfaces 28 thereon that are orthogonal to the 
axis of rotation of the rotor. Upon actuation of the brushes, the desired 
number of brush rings 18 may be made to contact the corresponding surfaces 
28. Alternatively, as shown in FIG. 8B, multiple brush rings 18 may be 
configured to contact a single rim 29 having opposing flat contact 
surfaces. 
Another suitable type of actuating means is diaphragm spring 60, shown in 
FIG. 9. The diaphragm spring is probably best known as a component in an 
automotive "diaphragm clutch." In that application, it is useful for 
"heavy" clutches that are difficult to actuate, as although it is 
difficult to push the clutch pedal in, once in it becomes easier to hold 
in. This is due to the "over center" action of the spring, illustrated in 
FIG. 9A. 
Operation of a diaphragm spring is typically as follows. The spring may be 
biased in one of two positions. A load applied to the spring will cause 
the spring to assume a stable inverted position, as shown in FIG. 9A. In 
other words, it will turn itself inside out. This position will be 
maintained until a load is applied in the opposite direction to bring the 
spring back to its initial position. Thus, the spring has two stable 
positions, similar to an on-off switch. 
Such a diaphragm spring 60 may be used in accordance with the present 
invention to locate the brush ring(s) 18 radially, allowing the brush 
ring(s) to be actuated using any of the actuator mechanisms discussed 
herein wherein the actuator mechanism applies sufficient force to cause 
the spring to invert. The diaphragm spring can provide both the compliance 
necessary to accommodate rotor run out, and also serve as a shunt to 
conduct current from the brush ring(s) to the stator. Suitable diaphragm 
springs may be obtained from, for example, Metalcraft Manufacturing 
Corporation. 
Further modifications and alternative embodiments of this invention will be 
apparent to those skilled in the art in view of this description. 
Accordingly, this description is to be construed as illustrative only and 
is for the purpose of teaching those skilled in the art the manner of 
carrying out the invention. It is to be understood that the forms of the 
invention herein shown and described are to be taken as the presently 
preferred embodiments. Various changes may be made in the shape, size, and 
arrangement of parts. For example, equivalent elements or materials may be 
substituted for those illustrated and described herein, and certain 
features of the invention may be utilized independently of the use of 
other features, all as would be apparent to one skilled in the art after 
having the benefit of this description of the invention.