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Claims  |
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What is claimed is:
1. In an aircraft the combination of
a fuselage,
wings on said fuselage,
a rudder and a stabilizer on said fuselage,
movable control surfaces on said rudder and on said stabilizer for
controlling the flight path of said aircraft,
an electric motor in said aircraft,
a propeller coupled to said motor and driven thereby, said propeller being
disposed in the atmosphere and effective to propel said aircraft through
the atmosphere,
a battery pack in said aircraft coupled to said motor for supplying
electric power to said motor,
a radio receiver on said aircraft for receiving command signals from a
remotely located radio transmitter,
control means interconnected with said radio receiver and responsive to the
command signals from said radio transmitter, said control means being
coupled to said movable control surfaces for positioning said control
surfaces according to said command signals,
a switch coupled to said motor for turning said motor "ON" and "OFF",
means interconnecting said switch to said control means, said last means
being responsive to the position of one of said control surfaces for
turning said switch "OFF" and stopping said motor when said control
surface is moved into a preselected position,
a voltage dividing and regulating circuit coupled to said battery pack,
a first portion in said circuit,
a second portion in said circuit,
at least one Zenner diode in said first portion of said circuit for
providing at least one regulated voltage,
means connecting said receiver to the Zenner diode in said first portion
for supplying the regulated voltage thereto,
means connecting said motor to said second portion whereby both said
receiver and said motor receive their power from said battery pack,
a ventilation passage, and
an entrance to said passage, said entrance being open to the atmosphere and
positioned in the direct blast from the propeller, said ventilation
passage extending through said casing and into and around the armature
whereby the blast from the propeller is directed into direct and intimate
heat exchanging relation with the rotating armature.
2. The combination of
an aircraft,
an electric motor mounted on said aircraft,
an outer casing on said motor secured to said aircraft,
an armature in said motor rotatably disposed inside of said casing,
a propeller connected to said armature and driven thereby for propelling
said aircraft through the atmosphere, said propeller being adapted to
free-wheel when said motor is turned "OFF" and said aircraft is gliding,
a ventilation passage extending through said casing and into and around the
armature,
an entrance to said passage open to the atmosphere,
an enlarged scoop on the entrance to said passage positioned behind the
propeller and in the direct blast of air therefrom whereby the blast from
the propeller directs a flow of air through said passage and into direct
and intimate heat exchanging relation with the rotating armature, and
a battery pack on said aircraft operatively connected to said motor for
supplying electrical power to drive the motor, said battery pack being
disposed in said ventilation passage whereby the flow of air through the
passage passes into direct and intimate heat exchanging relation with the
battery pack after it has passed through the motor.
3. The combination of
an aircraft,
an electric motor mounted on said aircraft,
an outer casing on said motor secured to said aircraft,
an armature in said motor rotatably disposed inside of the casing,
a battery pack mounted on said aircraft and operatively connected to the
motor for supplying electric power to drive the motor,
a propeller connected to said armature and driven thereby for propelling
said aircraft through the atmosphere,
intake scoop means mounted on said aircraft and positioned behind the
propeller in the blast of air from the propeller for diverting a flow of
air into the scoop,
ventilation passage means having the entrance thereof connected to said
intake scoop means and the outlet thereof opening into the atmosphere
whereby the blast of air from the propeller causes a forced draft of air
to flow through the passage and discharge into the atmosphere,
a portion of said ventilation passage means extending through said casing
and into and around the armature whereby the flow of air from the blast
from the propeller directs a flow of air through said passage and into
direct and intimate heat exchanging relation with the rotating armature,
and
a portion of said ventilation passage means extending around said battery
pack whereby the flow of air from the blast from the propeller directs a
flow of air through said passage and into direct heat exchanging relation
with the battery pack.
4. The combination of claim 3 including
a radio receiver on said aircraft for receiving command signals from a
remotely located radio transmitter, and
control means on said aircraft interconnected with said radio receiver and
responsive to the command signals for controlling the operation of said
aircraft in response to the command signals.
5. The combination of claim 4 including
a voltage dividing and regulating circuit coupled to said battery pack,
a first portion in said circuit,
a second portion in said circuit,
at least one Zenner diode in said first portion of said circuit for
providing at least one regulated voltage,
means connecting said radio receiver to the Zenner diode in said first
portion for supplying the regulated voltage thereto, and
means connecting said motor to said second portion whereby both said radio
receiver and said motor receives their power from said battery pack.
6. The combination of claim 4 including
an operative payload carried on said aircraft, and
means coupling said payload to said battery pack whereby the payload
receives its power from said battery pack, said last means coupling said
payload to said radio receiver so that the payload is operated in response
to said command signals.
7. The combination of claim 4 including
a fixed, rigid wing on said aircraft,
a rudder and a stabilizer on said aircraft,
a movable control surfaces on said rudder and stabilizer for controlling
the flight path of said aircraft, said control means being interconnected
with said movable control surfaces for positioning said control surfaces
in response to said command signals.
8. The combination of claim 3 including a throttle control on said aircraft
connected to said receiver and to said motor, said throttle control being
effective to regulate the speed of the motor in response to a throttle
command signal from the transmitter.
9. The combination of claim 8 wherein said electric motor has ball bearings
whereby the friction torque on said motor is less than the torque produced
by the propeller wind-milling when the aircraft is gliding.
10. The combination of claim 8 including means for electrically
"shorting-out" said electric motor when said motor is not being driven by
said battery pack to electrically increase the torque required to turn the
motor.
11. The combination of
an aircraft,
an electric motor mounted on said aircraft,
an outer casing on said motor secured to said aircraft,
an armature in said motor rotatably disposed inside of the casing,
a battery pack mounted on said aircraft and operatively connected to the
motor for supplying electric power to drive the motor,
a propeller connected to said armature and driven thereby for propelling
said aircraft through the atmosphere,
intake scoop means mounted on said aircraft and positioned behind the
propeller in the blast of air from the propeller for diverting a flow of
air into the scoop,
ventilation passage means having the entrance thereof connected to said
intake scoop means and the outlet thereof opening into the atmosphere
whereby the blast of air from the propeller causes a forced draft of air
to flow through the passage and discharge into the atmosphere,
a portion of said ventilation passage means extending through said casing
and into and around the armature whereby the flow of air from the blast
from the propeller directs a flow of air through said passage and into
direct and intimate heat exchanging relation with the rotating armature,
a portion of said ventilation passage means extending around said battery
pack whereby the flow of air from the blast from the propeller directs a
flow of air through said passage and into direct heat exchanging relation
with the battery pack,
a radio receiver on said aircraft for receiving command signals from a
remotely located radio transmitter,
control means on said aircraft interconnected with said radio receiver and
responsive to the command signals for controlling the operation of said
aircraft in response to the command signals,
a fixed, rigid wing on said aircraft,
a rudder and a stabilizer on said aircraft,
a movable control surfaces on said rudder and stabilizer for controlling
the flight path of said aircraft, said control means being interconnected
with said movable control surfaces for positioning said control surfaces
in response to said command signals,
an "ON/OFF" switch coupled to the electric motor for turning the motor "ON"
and "OFF", and
said switch being operatively interconnected to be responsive to the
position of at least one of said control surfaces for turning said switch
"OFF" and stop said motor when said control surface is moved into a
preselected position.
12. The combination of
an aircraft,
an electric motor mounted on said aircraft,
an outer casing on said motor secured to said aircraft,
an armature in said motor rotatably disposed inside of the casing,
a battery pack mounted on said aircraft and operatively connected to the
motor for supplying electric power to drive the motor,
a propeller connected to said armature and driven thereby for propelling
said aircraft through the atmosphere,
intake scoop means mounted on said aircraft and positioned behind the
propeller in the blast of air from the propeller for diverting a flow of
air into the scoop,
ventilation passage means having the entrance thereof connected to said
intake scoop means and the outlet thereof opening into the atmosphere
whereby the blast of air from the propeller causes a forced draft of air
to flow through the passage and discharge into the atmosphere,
a portion of said ventilation passage means extending through said casing
and into and around the armature whereby the flow of air from the blast
from the propeller directs a flow of air through said passage and into
direct and intimate heat exchanging relation with the rotating armature.
a portion of said ventilation passage means extending around said battery
pack whereby the flow of air from the blast from the propeller directs a
flow of air through said passage and into direct heat exchanging relation
with the battery pack,
a throttle control on said aircraft connected to said receiver and to said
motor, said throttle control being effective to regulate the speed of the
motor in response to a throttle command signal from the transmitter,
said throttle command signal is a pulse having a magnitude which is a
function of the desired speed of said motor, and
pulse responsive means in said throttle control, said pulse responsive
means being responsive to the magnitude of said pulse for modulating the
amount of battery power coupled to said motor in response to the magnitude
of the pulse.
13. The combination of
an aircraft,
an electric motor mounted on said aircraft,
an outer casing on said motor secured to said aircraft,
an armature in said motor rotatably disposed inside of the casing,
a battery pack mounted on said aircraft and operatively connected to the
motor for supplying electric power to drive the motor,
a propeller connected to said armature and driven thereby for propelling
said aircraft through the atmosphere,
intake scoop means mounted on said aircraft and positioned behind the
propeller in the blast of air from the propeller for diverting a flow of
air into the scoop,
ventilation passage means having the entrance thereof connected to said
intake scoop means and the outlet thereof opening into the atmosphere
whereby the blast of air from the propeller causes a forced draft of air
to flow through the passage and discharge into the atmosphere,
a portion of said ventilation passage means extending through said casing
and into and around the armature whereby the flow of air from the blast
from the propeller directs a flow of air through said passage and into
direct and intimate heat exchanging relation with the rotating armature,
a portion of said ventilation passage means extending around said battery
pack whereby the flow of air from the blast from the propeller directs a
flow of air through said passage and into direct heat exchanging relation
with the battery pack,
a throttle control on said aircraft connected to said receiver and to said
motor, said throttle control being effective to regulate the speed of the
motor in response to a throttle command signal from the transmitter, and
said electric motor is reversible and may be run in either direction, said
control means being effective to cause said motor to run in the forward
direction whereby said propeller may be used for propelling said aircraft
through the atmosphere and to cause said motor to run in the reverse
direction whereby said propeller may be used for braking the aircraft. |
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Claims |
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Description |
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BACKGROUND
There are many uses for small or even intermediate size unmanned aircraft
for performing various types services. Some of these aircraft are
preprogrammed to fly a predetermined course and/or they are remotely
controlled by radio, etc. An example of this type of aircraft are drones,
remotely piloted vehicles for taking photographs, etc. Also it is common
now adays to employ model aircraft for training and/or amusement purposes.
Heretofore, it has been customary to use a gasoline engine for driving such
airplane. Although such engines are highly developed and reasonably
reliable, they do require a considerable amount of servicing and
maintenance. This is particularly true with engines having relatively low
power ratings for example on the order of 5 or 10 horsepower or less and
especially so called fractional horsepower engines. In addition, they also
require a considerable amount of special equipment and supplies such as
messy gasolines, oils, batteries, etc. Moreover, the gasoline engines are
extremely noisy whereby their operation has been banned in many areas.
SUMMARY
The present inventor provides means for overcoming the foregoing
difficulties. More particularly, the present invention provides an
airplane which uses an electric motor as its driving source. The motor
includes means for maintaining its temperature at a low level whereby the
motor can produce large amounts of power for its size and weight. In
addition, means are provided for obtaining a large amount of power from a
small size, light weight battery pack and for recharging the battery pack
at a high rate in a very short period of time.
DRAWINGS
FIG. 1 is a side view of an airplane, a position thereof being broken away,
utilizing a battery driven electric motor and emboding one form of the
present motor.
FIG. 2 is a fragmentary cross sectional view on an enlarged scale of the
motor in the airplane of FIG. 1.
FIG. 3 is a block diagram of the control systems embodied in the airplane
of FIG. 1.
FIG. 4 is a set of waveform present in the throttle control portion of the
control system in FIG. 2.
FIG. 5 is a wiring diagram of a battery charging circuit for use in
recharging the battery power supply in the airplane.
FIG. 6 is a graph illustrating the operating characteristics of the battery
charger of FIG. 4.
FIG. 7 is diagram of a multi-engine system for use in an airplane.
DESCRIPTION
Referring to the drawings in more detail and particular to FIG. 1, the
present inventor is particularly adapted to be embodied in an airplane 10.
The airplane 10 may be of any desired variety and used for any desired
purpose. By way of example, the airplane 10 may be a small scale model
which is flown for amusement purposes or it may be a pilotless aircraft or
drone which is used for any desired variety of purposes.
In the present instance the airplane 10 is of the so called remotely
piloted vehicle variety suitable for flying various types of reconiscense
or survielance missions. It is equipped with a suitable pay load such as a
television camera 12 for taking pictures of the ground and relaying them
back to the operator. Of course it should be understood that other types
of equipment such as scientific instruments, etc. may be carried on the
aircraft 10. Alternatively, the aircraft 10 may merely be a small model
aircraft used for amusement and/or training purposes.
The present aircraft 10 is of a generally conventional design and includes
a fuselage or body 14, a pair of wings 16, a vertical tail fin 18 and
horizontal stabalizers 20. Depending upon the intended use of the aircraft
10, one or more of the foregoing may be provided with movable control
surfaces. More particularly in the present instance, the wings 16 include
ailerons 22, the tail fin 18 includes a rudder 24 and the stabilizers 20
include elevators 26. These control surfaces 22, 24 and 26 are effective
to respectively cause the aircraft 10 to roll, to turn and to climb or
dive.
The airplane 10 is propelled through the air by means of an electric motor
28. Although a more or less conventional electric motor may be used, it
has been found that because of the normally critical weight limitations,
the motor 28 should preferably be of a light weight, high performance
variety.
In the present instance, the motor 28 is of the so called permanent magnet
variety and may be made to run in either direction. It includes an outer
casing 30 which is of a generally cylindrical shape. This casing 30 is
adapted to be rigidly secured to the aircraft 10 in a fixed position by
any suitable means. By way of example in this embodiment it is mounted on
a bulkhead or firewall. The rear end 34 of the motor 28 projects slightly
through the bulkhead 32. The casing 30 of the motor 28 should be fastened
by means sufficiently strong to withstand the torque from the motor 28 and
the usual shocks, impacts, etc. encountered during normal operation. At
the same time, it is highly desirable for the motor 28 to be easily
removable for servicing replacement.
A permanent magnet assembly 36 is mounted on the inside of the casing 30.
The magnet assembly 36 forms one or more pairs of opposed pole faces.
These faces are of a cylindrical shape so as to form a cylindrical passage
axially through the motor 28.
An armeture 38 is mounted on a drive shaft 40. The drive shaft 40 extends
beyond the ends of the armeture 38 whereby it may be supported on ball
bearings which are attached to the casing 30. In addition, one end of the
drive shaft 40 extends a substantial distance from the motor 28. It
projects from the front of the airplane 10 whereby a propeller 42 may be
attached thereto.
The armeture 38 includes a core with one or more pairs of arms which
projects radially from the shaft 40 so as to form accurate pole faces.
These pole faces are disposed adjacent to the pole faces of the permanent
magnet 36. The faces on the armeture and the permanent magnet are
separated from each other by a small clearance space or air gap. As a
result the armeture 38 can freely rotate within the casing 30 at a very
high speed.
An electric coil 44 is wound upon the armeture 38 for producing a magnetic
field. This field reacts which the field from the magnet 36 to produce the
torque for driving the armeture.
The coil 44 is usually wound upon the radial arms of the armeture core.
This leaves a substantial amount of open space in and around the armeture
38. More particularly there are large openings between the coils. These
openings extend axially off the armeture 38 and are filled with air.
The opposite ends of the casing 30 are preferrably open to the atmosphere
whereby the air can flow from the atmosphere through the open end of the
casing, in and around the armeture 38 and out the other end of the casing
30.
More particularly, it may be seen that the front of the motor 28 includes a
"spider" 46 for supporting the front ball bearing. This in turn leaves
several large openings 48 which extend into the interior of the motor 28.
In addition, the rear end of the motor 28 also includes several large
openings through which air may flow. It may thus be seen air may easily
flow through the openings 48 on the front of the motor 28, axially through
the motor 28 and around the armeture 38 and be discharged from the rear of
the motor 28 and into the space behind the fire wall 32.
A propeller 42 is mounted on the exposed end of the driveshaft 40 whereby
it is driven directly by the motor 28. The motor 28 is preferrably mounted
on the airplane 10 so as to position the propeller 42 in the atmosphere
whereby the propeller 42 is free to propel the airplane 10 through the
atmosphere. In the present instance, by way of example, the motor 28 is
mounted on the front of the airplane 10. However, it should be understood
it can be mounted anywhere that is convenient. For example, if a pair of
motors are being used, they could be mounted on the wings, etc. In
addition the motor or motors may be arranged in a so called "pusher type"
of configuration.
It may seem that the open end of the casing 30 is disposed at the front of
the aircraft 10 and directly aligned with the direction of travel. Thus,
the airplane 10 travels through the atmosphere, the air will flow through
the interior of the motor 28.
In addition, the open end of the casing 30 is directly aligned with at
least a portion of the blast of air from the propeller 42. As a
consequence, the propeller 42 will force a large volume of air axially
through the motor 28. If desired a somewhat enlarged cowling or scoop 50
may be provided on the front of the fuselage 14. This is effective to
increase the volume of air forced through the motor 28.
It can be appreciated that the cool atmospheric air is flowing axially
through the motor 28. It is also flowing through the space wherein the
armeture 38 is rotating. Because of the high velocity of the blast from
the propeller 42 and the turbulence produced by the high speed rotation of
the armeture 38, there is an intimate heat exchanging relationship between
the armeture 38 and the cooling air. As a consequence, a large amount of
heat is rapidly absorbed out of the armeture 38.
It should be noted that virtually all of the heat generated in a permet
magnet motor is generated by the current flow in the coil 44 on the
armeture 38. Since a large volume of cool air is forced directly against
this coil 44 and other parts of the armeture 38, the waste heat is most
effectively dissipated directly from the coil 44 without its having to
flow outwardly through the stator, outer casing 30, etc. As a result,
there is not a localized concentration of heat which can cause a large
build-up in temperature.
Because the heat is effectively dissipated at a high rate from its original
source, the motor 28 can be driven at a much higher power level than is
otherwise possible. In fact, it has been found that designing the electric
motor 28 to force the air to flow through and around the armeture 38 and
by directing a portion of the prop wash through the motor 28 and around
the armeture 38 the motor 28 may be operated at a load level considerable
in excess of ten times its original rating. In other words, the weight of
the motor 28 for a given power rating may be reduced by roughly 90%. Such
a dramatic reduction in weight is, of course, extremely important in an
airplane.
The motor 28 is driven by a suitable source of electric power. In the
present instance, this is accomplished by means of a battery pack 52 which
is mounted in and carried by the airplane 10. Although any suitable type
of batteries may be employed, it is desirable for the battery pack 52 to
be of a high performance variety. That is, the battery pack 52 should be
capable of producing large amounts of power for reasonably extended
periods of time, while having as little weight as possible. The batteries
should also be suitable for use in the type of environment normally
encountered in an aircraft, i.e., inverted flight, vibration, shock, etc.
It is also highly desirable for the battery pack to be rechargable.
One type of battery pack 52 which satisfies all of these requirements and
has been found satisfactory, is the variety which employs one or more
nickel cadmium cells. Nickel cadmium cells can store a large amount of
energy in a relatively small size and weight. Also, it is possible to draw
a very large current from them and discharge them at a high rate without
causing any serious damage to the cells.
However, as the individual nickel cadmium cells are discharged, each one
generates waste heat. The rate at which the heat is generated is a
function of the discharge rate, i.e., the amount of current flow. If the
discharge rate is very high and the heat is generated faster than it is
being dissipated, the temperature of the cell will rise far about the
ambient temperature. Fairly substantial rises in temperatures can be
tolerated. However, if the temperature rises too much, a substantial
pressure builds up inside the cell. This build up in temperature and
pressure reduces the performance of the battery pack 52 i.e., the voltage
drops etc. Moreover, if the pressure build-up is uncontrolled, it can
cause the individual cells to explode and destroy themselves. Such a
catostrophic failure at the very least will damage the aircraft 10 and may
even injure any persons in the vicinity.
In order to increase the rate at which the battery pack 52 can be safely
discharged, the individual cells are disposed in a case having ventilating
openings for the flow of air.
In addition the battery pack 52 is disposed immediately behind the firewall
32 so as to be in line with the blast of air discharged from the rear 34
of the motor 28. A second bulkhead 54 is disposed immediately behind the
battery 52. This is effective to prevent the flow of air into the rear of
the airplane 10.
An enlarged opening 56 or series of smaller openings may be provided in the
bottom of the fuselage 14. As a consequence, the air flowing through the
motor 28 is trapped in the sealed space above the battery pack 52. The air
is thereby forced to flow downwardly through the battery pack 52 and into
intimate heat exchanging relation with each of the individual cells in the
pack. The air is then discharged through the opening 56 in the bottom of
the aircraft 10 and returned to the atmosphere.
It will be seen that this arrangement insures a forced draft of air through
the battery pack 52 in intimate heat exchanging relations with the
individual cells. Thus whenever a heavy discharge current is being drawn
from the battery pack 52 (i.e. the motor 28 is running), a flow of cooling
air is always circulated across the individual cells.
The charging of the battery pack 52 presents a somewhat similar type of
problem. The nickel cadmium cells can be rapidly charged. However, as they
are charged, they produce heat with a resultant build-up in pressure. If
the charge rate is so high the heat builds up faster than it is being
discharged, the temperature will rise.
If this heat is not dissipated from the battery pack 52 at an adequate
rate, the temperature of one or more of the cells may rise to an excessive
level. This, in turn, can cause the individual cells to explode.
In addition, if the cells are overcharged, they can be damaged. As a cell
becomes overcharged, its voltage and/or its internal resistance reduces.
The greater the amount of overcharge, the greater the rate at which the
voltage and resistance decrease. Under these circumstances, if the cell is
being charged from a constant voltage source once the cell if fully
charged and its voltage and resistance begin to decrease, the charging
rate begins to increase. This, of course, results in a snowballing effect
that will destroy the cell.
This type of a problem is particularly acute where the airplane 10 is
operating out of a remote location where complicated charging equipment,
accurately regulated voltage sources, etc. are not available. For example,
if the airplane 10 is a small model operating from a vacant field, etc.,
there is not a regular power source available. Accordingly, if the battery
pack 52 is going to be recharged, it is customary to use a portable power
source. Heretofore, it has been customary to employ a "trickle" charger
which charges the battery at a very slow rate. This, of course, required a
long time to charge the battery pack.
Alternatively, it has been possible to completely discharge the battery
down to ground level. This insures the charging process starting from a
known reference level. Thus if a predetermined charge is fed to the
battery pack, it should be fully charged. This avoids the problem of
overcharging an already partially charged battery pack. Theoretically,
this then permits the known charge being fed into the battery pack at a
relatively high rate whereby the charging time is relatively short.
However, if the battery pack was completely discharged just prior to the
charging operation, the battery will be hot and the rapid charge will
cause it to overheat and fail.
The present battery charger 58 which is best seen in FIGS. 4 and 5, may be
used. This charger 58 includes a connector 60 having a pair of contacts
62-64 which are adopted to mate directly with a pair of complimentary
contacts 66-68 on the side of the aircraft 10. If the battery switch in
the aircraft is moved from the operating or running position to the charge
position, the charger 58 will be connected directly to the battery pack 52
and the motor 28 will be disconnected.
The charger 58 includes an electric energy source such as a rechargable
lead acid wet cell battery 70. By way of example, this may be a battery
out of an automotive vehicle, motorcycle, etc.
A timer 72 and a regulating or limiting resistor 74 are connected in series
with the storage battery 70 and the contacts 62-64. The value of the
resistor 74 may vary over a fairly wide range. As will be explained in
more detail subsequently, it has been found preferable for the resistor 74
to be approximately equal to the internal resistance of the battery pack
52. This allows a very rapid charging rate with little or no danger of
damaging any of the nickel cadmium cells in the battery pack 52.
The voltage of the charging source (i.e., the battery 70), may also vary
over a considerable range. However, it is preferable equal to or just a
little greater than the fully charged voltage on the battery pack 52.
Normally, the back voltage on a nickel cadmium cell as it is being charged
is about 1.5 volts or a little less when it reaches the fully charged
level. The voltage on a lead acid, wet cell is about 2.0 volts or a little
more. Therefore, if there are four nickel cadmium cells in series with
each other, they will have a voltage equal to or slightly less than the
voltage from three lead acid cells.
It has been found the foreging ratio of cells is ideally suited for this
purpose. More particularly, if a 6-volt system is being used to run the
motor 28, the battery pack 52 will include four cells and the energy
source battery will include three cells. Similarly in a 12, 18, or 24 volt
system, the battery pack 52 will have eight, 12 or 16 cells while the
energy source will have six, nine or 12 cells, respectively. It should
also be understood that the cells may be arranged in various series and
parallel configurations to preserve these ratios for any set of operating
voltages.
The charging characteristics of the battery pack 52 and charger 58 are best
seen in FIG. 5. If the battery pack 52 is completely discharged down to
ground level, it will correspond to the left-hand vertical line 76 and the
voltage will be zero or ground. As a consequence, when a fully discharged
battery is connected to the charger 58, there is initially a "very high"
current surge. The amount of this current is limited by the resistor 74
and the internal resistances of the battery pack 52 and the energy source
battery 70.
As the battery pack 52 begins to charge, the voltage on the pack 52 very
rapidly builds up to some predetermined intermediate level. Depending on
the characteristics of the individual nickel cadmium cells 73 and the
parameters of the system, this may take about 15 to 45 seconds.
Thereafter, the voltage rises more slowly and linearly as the charge
increases.
At the same time, the current follows a somewhat inverse function. More
particularly, the current gradually decreases. This results in the
"charging rate" decreasing correspondingly. "Charging rate" as used
herein, means the amount of time that would be required to fully charge a
discharged battery if the current remained constant at that level.
When the battery pack 52 is fully charged (as indicated by the broken
vertical line 78 in FIG. 5) the voltage on the battery pack 52 will reach
a maximum level. This is the rated voltage of the battery pack 52. At this
point, the charging rate is at a minimum level. By way of example, this
rate may be at a level of about 3 hours or more. In other words, if the
current remained constant at this amount, it would require about 3 hours
or more for the battery pack 52 to go from the fully discharged condition
to the fully charged condition.
If the charger 58 continues to charge the battery pack 52 beyond its fully
charged condition, the voltage on the battery pack 52 will gradually
decrease from its rated level. At the same time, the current and therefore
the charging rate will increase. As this overcharging continues, the
voltage continues to drop at an ever-accelerating rate whereby the
charging rate begins to increase at an ever-accelerating rate. As a
consequence, if this overcharging is allowed to persist for an excessive
time, it will reach a rate where the battery pack 52 will be destroyed.
However, since the limiting resistor 74 reduces the current flow to a very
low level at the fully charged condition, any overcharging is initially
very limited. It will take a considerable amount of time before the
charging rate builds up to a dangerous level. More particularly, the
current limiting and voltage dropping resistor 74 will allow the charging
rate to initially be very high. For example, initially the charging rate
may be on t | | |
