Electric vehicle drive train having unipolar motor

An electric vehicle drive train having a unipolar motor connected between a DC source of electrical energy and a differential or other mechanism for turning the wheels of an electric vehicle. The DC source consists of a low-voltage battery or thermoelectric generator which has first and second electrodes. One of the electrodes is the container of the DC source and the other source electrode is connected to a control member forming a part of the unipolar motor.The unipolar motor is positioned within a recess of the DC source container and is in electrical contact therewith so that current flows from the control member of the motor, through the motor's internal structure, and into the motor housing portion attached to the container. This provides a very compact electric vehicle drive train with the DC source located as close as possible to the unipolar motor. Preferably the motor is vertically positioned between the DC source located above it and the vehicle differential located beneath it.

BACKGROUND 
This invention relates to an electric vehicle drive train having a unipolar 
motor. 
A unipolar or homopolar motor is a DC dynamoelectric machine that operates 
at high current and low voltage. It is a motor which has high efficiency 
and a high power-to-weight to volume ratio. Unipolar motors are described 
in U.S. Pat. Nos. 3,185,877 to A. Sears; 3,453,467 to L. M. Harvey; 
3,916,235 to E. Massar; and 3,984,715 to D. Kullman et al. However, the 
preferred form of unipolar motor for use in connection with the present 
invention is described in commonly assigned patent application of the 
present inventor entitled, "Unipolar Dynamoelectric Machine with Variable 
Resistance Control" Ser. No. 863100, filed Dec. 1977. 
The torque-producing characteristics of the unipolar machine described in 
the inventor's patent application identified in the preceding paragraph 
makes this machine particularly suitable for use as a traction motor in 
electric vehicle applications. However, because unipolar motors operate at 
low voltage and high current, for example, 10,000 amps, the DC source of 
electrical energy used to supply such motors must be located in proximity 
to the motors to prevent undue resistive electrical power losses. 
Moreover, the DC source of electrical energy used to supply the unipolar 
motor should have a high energy density. 
Sodium-sulfur and other alkali metal batteries and DC sources developed by 
the assignee of the present invention, Ford Motor Company, are described 
in the following U.S. Pat. Nos. 3,404,035; 3,404,036; 3,413,150; 
3,446,677; 3,468,709; 3,468,719; 3,475,220; 3,475,225; 3,488,271; 
3,514,332; 3,535,163; 3,719,531; 3,811,943; 3,951,689; 3,966,492; 
3,976,503; 3,980,496; 3,985,575; 3,985,576; and 4,049,889. The drive train 
of the invention also may be used with heat engines or thermoelectric 
generators or other low-voltage, high-current DC sources of electrical 
energy. 
The sodium-sulfur batteries and heat engines mentioned above typically 
produce, for each cell in the case of the sodium-sulfur battery and sodium 
heat engine, a voltage of about 1.5 volts. Simplicity of construction of 
the heat engine or sodium-sulfur battery occurs if the various battery 
cells are connected in parallel rather than in series. If this is done, 
only a low voltage is produced but a very high current capability is 
provided. The high energy density capability of this DC source may be 
utilized to provide a practical energy source for an electric vehicle. 
The low voltage produced by a parallel-connected battery as described above 
may be used to advantage by a unipolar motor. The present invention 
provides a drive train for an electric vehicle which takes advantage both 
of the unique properties of the unipolar dynamoelectric machine and the 
unique characteristics of the DC sources mentioned above. 
SUMMARY OF THE INVENTION 
This invention relates to a drive train for an electric vehicle. The drive 
train has a unipolar or homopolar motor and includes a DC source of 
low-voltage electrical energy having a plurality of cells connected in 
parallel. The DC source includes an electrically conductive container 
forming one electrode common to each of the cells. A control member of the 
unipolar motor is electrically connected to the other electrode of each of 
the cells. The control member extends downwardly from the DC source and is 
located in a recess formed in the container. The unipolar motor has a 
stator and a rotor. The control member of the motor extends into an 
opening in the rotor and the rotor rotates about this control member. The 
housing of the unipolar motor is mounted within the recess formed in the 
DC source container and a voltage is produced between the control member 
and the housing of the motor. A liquid metal within the motor is used to 
control the current flow between the control member and the housing or 
stator of the motor, which in turn is electrically connected to the 
container. The unipolar motor preferably has an output shaft that enters a 
differential mechanism suitable for use in producing the output torque 
required to drive the wheels of an electric vehicle. 
The invention may be better understood by reference to the detailed 
description which follows and to the drawings.

DETAILED DESCRIPTION 
With reference now to the drawings, wherein like numerals refer to like 
items in the several views, there is shown in FIG. 1 a schematic diagram 
of a drive train for an electric vehicle. The drive train, generally 
designated by the numeral 10, includes a DC source of electrical energy 12 
in the form of a sodium-sulfur battery having a plurality of cells 
connected in parallel. The drive train also has a differential 14, which 
may be of conventional design, having hollow axles 16 and 18 through which 
axle shafts 20 and 22, respectively, pass to provide drive for the wheels 
(not shown) of the electrically driven vehicle with which the drive train 
is used. 
A unipolar motor 110 is positioned between the DC source 12 and the 
differential 14. A shaft 140 (FIG. 3) in the motor 110 enters the 
differential 14 to provide the drive, through the gearing of the 
differential, to the axle shafts 20 and 22. 
The sodium sulfur battery 12 includes a container or housing 24 formed from 
an upper part 26 and a lower part 28. This container preferably is formed 
from low carbon or stainless steel that may be stamped to produce the 
desired configuration. It may be seen that the lower part 28 of the 
container has a flange 118 that is in electrical contact with a 
corresponding flange on a conductive portion of the stator 112 of the 
unipolar motor 110. 
Container 26, 28 and its flange 118 actually form one electrode of the 
sodium-sulfur battery and are in electrical contact with liquid sodium, a 
liquid metal, contained in the space 30 in the battery 12. Thus, the 
container for the battery is an electrode of this DC source and is in 
direct contact with the stator of the unipolar motor 110, which is 
positioned within recessed area 32 of container part 28. The container 
parts 26 and 28 are covered with thermal insulation 34 to prevent heat 
transfer from the battery to the surrounding medium. This thermal 
insulation is necessary to limit heat loss from the sodium sulfur battery, 
as is desirable for this type of battery because it must be operated at 
elevated temperature. 
The unipolar motor 110 has a control member 116 connected to another 
electrode 35 of the battery 12. Each cell of the battery includes a rod 36 
that is electrically conductive and connected to the electrode 35. This 
rod is insulated from container part 28. The rods 36 extend into 
closed-end, tube-shaped ionically-conductive ceramic membranes 38. 
With particular reference now to FIG. 2, there is shown the detailed 
structure of two of the cells of a sodium-sulfur battery as illustrated 
more schematically in the circled portion 2 of FIG. 1. It should be noted 
in FIG. 2 that the container parts 26 and 28 are separated by a conductive 
member 62 which has the flange 118 electrically connected to the motor 
housing 112 (FIG. 1). The member 62 provides support for the various 
tubular ceramic membranes 38 forming or dividing the battery into the 
various parallel-connected cells. The members 26, 62 and 28 are fastened 
together at 64, and an aluminum foil seal is placed between the surfaces 
of member 62 and container part 28 in the flange area clamped together at 
64. This is designed to prevent leakage of liquid sodium contained in 
region 30 of the battery. Of course, the battery 12 must be maintained at 
a temperature sufficient to keep the sodium in region 30 in a liquid state 
as is well known. 
The tubular ceramic membranes 38 preferably are made from beta-alumina 
solid electrolyte material, which is a conductive ceramic material. Also, 
the rods 36 preferably are made from this conductive ceramic material or 
carbon, but may be made from other conductive materials capable of 
surviving in the presence of polysulfide materials contained in the region 
40 between the rods 36 and the tubular ceramic elements 38. The region 40 
is filled with carbon felt washer shaped elements, which are initially 
saturated with sulfur. 
In the operation of the sodium-sulfur battery, sodium ions in the sodium 
compartment area 30 migrate through the tubular conductive ceramic 
membranes 36 and react with the polysulfide and sulfur materials in region 
40 to produce additional or different polysulfide compounds. In so doing, 
the sodium ions receive an electron at the rod or cathode structures 36 
and, therefore, there is a current flow between the rods 36, connected in 
parallel by the conductive wire mesh 35 attached to the various rods 36 by 
screws 60 associated with conductive washers 58. Current flow is between 
this complete cathode structure assembly, including the rods and wire 
mesh, and the container portion 28 forms the anode of the battery. 
The polysulfide and sulfur materials in the region 40 of each of the 
tubular ceramic elements is a highly corrosive material, and it is 
necessary to seal this material within the individual cells. For this 
purpose, annular seal members 42 and 44 are provided. Member 42 is a 
washer-like element that is attached to the tubular ceramic electrolyte 
element 38 and is maintained thereon by a glass seal 48. Both of the 
annular insulating material, which is a material similar to that 
conventionally employed in the insulators of spark plugs. Member 44 is 
attached to the rod 36 by a glass seal 46. The member 42 has an annular 
ridge 43, and member 44 has an annular ridge 45. Between members 42 and 44 
and in the region of the ridge 43, a piece of annular-shaped aluminum foil 
is positioned to form a seal between members 42 and 44. A similar annular 
aluminum-foil seal is provided between members 44 and 62 at the location 
of the ridge 45. The two separate sealing members 42 and 44 facilitate 
assembly of the various cells because members 44 can be attached to 
members 36 prior to assembly of these members as a unit into the container 
holding and supporting the various tubular ceramic membranes 38. These 
membranes 38 are retained in position by annular metallic members 64 that 
are secured to member 62 by screws 66 as illustrated. It should be noted 
that the rods 36 are secured to the conductive wire mesh 35 with elements 
54 which clamp the wire mesh, by means of screws 56, to the individual 
rods 36. 
The DC source of electrical energy 12 preferably is a sodium-sulfur battery 
as described, but variations may be made in both design and type of DC 
source used in the vehicle drive train disclosed herein. For example, the 
DC source voltage may be obtained from a sodium heat engine that produces 
a voltage due to a temperature differential existing between liquid sodium 
regions separated by an ionically conductive membrane, such as 
beta-ceramic membrane 28. 
With particular reference now to FIG. 3, there is shown a sectional view of 
a unipolar dynamoelectric motor generally designated by the numeral 110. 
The motor 110, as it is described herein, includes a stator generally 
designated by the numeral 114. Also provided is a control member 116 and 
an electrode or contact plate 118. Control member 116 and contact plate 
118 are directly coupled to the opposite polarity terminals of the 
sodium-sulfur battery 12. The heat engine may be the type using liquid 
sodium at different temperatures in regions separated by a membrane to 
produce an EMF. The preferred battery is a sodium-sulfur battery, but 
others may be used to produce the low voltage required for the machine 
110. 
The stator 112 includes a first member 120 and a second member 122, both of 
these being in electrical contact with the contact plate 118 and together 
forming a housing. The stator also includes a field coil 124 that is 
surrounded by a conductive copper jacket 126, which may be formed by two 
identical annular channel-shaped elements as shown. A tube (not shown) for 
conducting a liquid coolant may be provided within the jacket 126 if 
necessary in a given machine application. Of course, other well known 
cooling techniques can be employed to improve machine efficiency and 
durability. 
The field coil 125 is helically wound and produces the magnetic field 
indicated by the dashed lines 128. Thus, the direction of this magnetic 
field may be as indicated in the drawing or opposite in direction 
depending on the direction of current flow through the helical winding. 
Preferably, the first and second members 120 and 122 of the stator are 
made from ferromagnetic and electrically conductive materials. Iron having 
the smallest amount of carbon possible is desirable for these components. 
Component or member 122 of the stator is annular in shape and is used to 
provide a flux path for the magnetic flux produced by the field coil as 
well as to fill the space in the stator left empty prior to insertion of 
the field coil and its copper jacket into the stator during machine 
assembly. The member 120 has ribs 130, which preferably are formed as an 
integral part of the member 120. 
The stator 112 has a central opening within it that is of circular 
cross-section throughout and which defines an axis 132. The rotor 114 is 
journalled for rotation about this axis by means of ball bearing 
assemblies 134 and 136. The rotor 114 includes an upper rotor portion 138 
and an output shaft 140 that is threaded into rotor portion 138. 
Preferably, the rotor portion 138 is made from iron having a minimum 
carbon content and is ferromagnetic and electrically conductive. 
The rotor portion 138 has a combined cylindrical and conical opening 142 in 
it in which the similarly shaped control member 116 is received. Rotor 
portion 138 rotates about control member 116, and these components are 
concentrically mounted with respect to axis 132. A first annular or 
cup-shaped space 144 is formed between the reduced-diameter portion of the 
control member 116 and the surface of rotor portion 138 which receives the 
control member and defines the opening 142. A second annular space 146 is 
located between the outer surface of rotor portion 138 and the surface of 
the stator opening. The stator has an insulating material 148 covering the 
surface area defining the stator opening, except on the surface of the 
copper jacket 126 located radially opposite the rotor portion 138. Current 
flow within the unipolar motor is indicated by the dot-dash lines 150 for 
the situation in which the annular spaces 144 and 146 contain a conductive 
material. It may be seen that this current flow is concentrated, due to 
the presence of the insulating material 148, in the copper jacket of the 
stator. This improves motor efficiency by tending to keep the direction of 
current flow perpendicular to the magnetic field lines 128. Of course, the 
torque produced by the motor is a function of the cross-product of these 
current and magnetic field vectors and is a maximum if they are mutually 
perpendicular. 
A conductive liquid metal 152 is located within a cavity formed between the 
rotor portion 138 and the stator housing member 120. Mercury is the 
preferred liquid metal for this application, but liquid sodium, gallium, 
or gallium-indium alloys also may be used where the machine is utilized at 
temperatures sufficient to maintain these materials in a liquid state. 
The second annular space 146 is located radially outward, with respect to 
the axis 132, of the first annular space 144, and the cavity in which the 
liquid metal 152 is located is separated from the first annular space 144 
by the portion 138 of the rotor. Passages 154 interconnect the cavity in 
which the liquid metal is located with the first annular space 144. 
A force producing means is provided in the form of an axially movable 
bellows assembly 156. The bellows assembly is of toroidal shape, is 
hollow, and has a tubular inlet 58 to permit air or other fluid pressure 
to be introduced into the assembly. The assembly walls include corrugated, 
preferably metal, walls 160 that are attached in sealing arrangement with 
upper and lower washer-sahped members 162 and 164. Member 162 is axially 
movable, is made from a metallic material, and has high-temperature 
plastic or other annular seals 166 and 168 to prevent the liquid metal 152 
from passing the bellows assembly 156. O-ring seals 170 also are provided 
for this purpose. 
When air or other fluid pressure is applied through inlet 158 to the 
bellows assembly interior, member 162 of the bellows assembly is forced to 
move with respect to the axis 132 (upwardly and against the force of 
gravity as viewed in the drawing), thereby, forcing the liquid metal 152 
into the first and second annular spaces 144 and 146, respectively. The 
liquid metal 152 enters the first annular space 144 through the passages 
154. The force applied to, and resultant movement of, the member 162 
determine the amount of liquid metal entering the first and second annular 
spaces. With the liquid metal 152 located as shown in the drawing, there 
is infinite resistance between the control member 116 and the stator 112. 
However, when the liquid metal enters the first and second annular spaces 
such that liquid metal in the first annular space contacts the control 
member 116 and the rotor portion 138 and such that liquid metal in the 
second annular space contacts the rotor portion 138 and the stator 112, 
then electrical current can flow from the control member 116 to the stator 
or vice versa. With the insulating material 148 located as shown in the 
drawing, the liquid metal in the second annular space 146 must flow 
axially upwardly as viewed in the drawing until it actually contacts the 
copper jacket 126. 
The greater the amount of liquid metal in the first and second annular 
spaces 144 and 146, the lower is the resistance between the control member 
16 and the contact plate 118. The supply voltage for motor 110 is applied 
between contron member 116 and contact plate 118 and may have a magnitude 
of about one and one-half volts. The control member 116 preferably is made 
from a substantially conductive material, that is, while it is not a 
perfect conductor, it will have a resistivity not greatly above that of 
the other conductive materials in the machine 110. Of course, the amount 
of liquid metal in the annular spaces 144 and 146 determines the motor 
current because the resistance to current flow varies as a function of the 
amount of surface area of the control member, rotor and stator contacted 
by the liquid metal. 
As the rotor 114 rotates, there is a tendency for the liquid metal in the 
first annular space 144 to decrease in volume and for the amount of liquid 
metal in the second annular space 146 to increase in volume. In other 
words, the volume of liquid metal in the annular space 144 is inversely 
proportional to rotor angular velocity and the amount in the annular space 
146 is directly proportional to rotor angular velocity. This results from 
centrifugal force acting on the liquid metal, there is a vortex-generating 
effect that occurs because the second annular space 146 is located 
radially outward from the first annular space 144. This vortex effect is 
desirable in that, with respect to the application of the motor to 
electric vehicles, it is desirable to reduce the motor current flow as a 
hyperbolic function of the motor speed. 
Since, due to the aforementioned vortex effect, the height of the liquid 
metal in the space 144 and its height in the space 146 are hyperbolic 
functions of the rotor speed and since the resistance to current flow 
thereby is a hyperbolic function of rotor speed, the motor design 
described herein is particularly suitable for vehicle traction 
applications. 
The insulating material 148 preferably is a nonconductive ceramic or 
high-temerpature polyimide material. Also, with respect to the copper 
jacket 126, nickel coating and rhodium flash are provided thereon in the 
portion thereof located in the second annular space 146 to minimize the 
resistance to current flow in this location. With respect to the 
corrugated metal portion of the bellows assembly 156, it is preferred that 
this be formed from a nickel or nickel alloy. The iron from which the bulk 
of the stator 112 is formed may be armco iron. This is a very low carbon 
iron. 
When the motor 110 is used in vehicles or other applications requiring 
speed control, the air or fluid pressure applied to the bellows assembly 
156 through the inlet 158 may be controlled by an accelerator pedal 
connected to an air bellows or the like and may be operated by a vehicle 
or machine operator.