Note: Descriptions are shown in the official language in which they were submitted.
7~;6
SERIES CONNECTED SERIES MOTORS
FOR INDEPENDENT LOADS
This invention relates to motive power systems; more particularly, it
relates to a drive system using series DC motors in a series connection for
driving independent loads.
BACKGROUND OF THE INVENTION
In certain applications, it is desirable to drive two independent load
devices with separate series wound DC motors which are connected in a
series circuit and energized from a common voltage source. A particular
example is the motive power system -for an industrial truck which gave rise
to the discovery and invention set forth herein. It has been the practice
in certain industrial trucks to use two separate series motors for driving
the two traction wheels. The reason for using two separate motors instead
of one larger motor is cost; the power train using a single large motor
costs considerably more than separate drive trains using two smaller
motors. Heretofore, the two series motors have been connected electrically
in parallel and each lnotor is provided with its own separate reversing
contractor and certain other separate circuit components.
It has been recogni~ed that additional cost savings could be realized
if the two separate series wound drive motors could be connected
electrically in series instead of parallel. In such an arrangement, a
single reversing contactor and other circuit components could be used in
common for both motors. However, the connection of DC series motors in
series with each other has heretofore required special control circuits
which add considerable cost and complexity and result in reduced
reliability of the system.
According to convention wisdom, DC series motors connected in series
with each other cannot be operated to drive independent loads without some
form of special control circuit to control the voltage across the
respective motors. The reason behind this conventional thinking is as
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follows. If one motor has no load or light load and the other motor has a
heavy load, the motor without load will have greater acceleration and will
reach a high speed beFore the heavily loaded motor starts. The motors
develop a counter electromotive force (EMF) as a direct function of speed.
The speed of the lightly loaded motor increases quickly to the point where
the counter EMF produced thereby is substantially equal to the applied
voltage while the loaded motor remains at standstill and develops no
counter EMF. The applied voltage is said to be equal to the sum of the
counter EMF and the IR drop, i.e. the voltage drop produced by the current
through the series resistances of the armatures and field windings oF the
motors. The torque produced by a series motor is proportional to the
product of the field current and the armature current. The effect of
increasing counter EMF due to the increasing speed of the unloaded motor is
to reduce the current through the fields and armatures of the two motors so
that very little torque is produced by the motors. As a result, the
heavily loaded motor remains at a standstill while the unloaded motor spins
at high speed. In the case of a vehicle drive using series connected
series motors for independent traction wheels, the vehicle would remain at
standstill if one traction wheel is on ice or is off the ground while the
other traction wheel is on the ground.
In the prior art, the problem of series motors in series connection
with independent loads has been solved~ by providing special control
circuits for controlling the voltage across the respective motors. A
typical example of such prior art is the Cronberger patent 2,930,957 which
issued March 299 1960. The system of the Cronberger patent utilizes speed
sensing or voltage sensing means for the respective motors and when one
motor runs faster than the other the field current of that motor is shunted
to allow the speeds to equalize.
A ~eneral object of this invention is to overcome certain disadvantages
of the prior art.
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1 In accordance with this invention, a traction drive
system for an industrial truck is provided which comprises
first and second series wound motors connected in series with
each other and driving first and second independent traction
wheels of the truck without any special control circuit for
controlling the voltage applied to the r~espective motors.
This is accomplished by correlating the start-up no-load
torques characteristic of the traction wheels with the
transient torque characteristic of the motors so that each
motor produces a predetermined transient driving torque.
Further, in accordance with this invention, a traction
drive system is pro~ided for a vehicle having first and second
traction wheels which are independently rotatable. Both of the
, wheels exhibit substantially the same start-up and running-
load torques when both have non-slip adhesive friction with
the roadO F'irst and second DC series motors which are
substantially identical to each other are connected
electrically in a series circui-t with each other and switching
means is provided for connecting the series circuit across a
voltage source. The first and second motors are respectively
mechanically coupled, independently of each other, in driving
relation with the first and second wheels of the vehicle.
Each traction wheel is subject to exhibiting a start-up load
ranging from the threshold-load torque to the no-load
torque. Upon start-up from a given starting condition, either
one of said wheels may exhibit the no-load torque and the
other of said wheels may exhibit the threshold-load torque.
The impedance of the series circuit is low enough to draw a
transient current, s-tarting when the switching means is
closed, which causes each motor to produce a transient
driving torque greater than the threshold-load torque
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whereby both wheels are accelerated and said driving transient torque is
reduced as a result of the counter EMF of said motors. The no-load torque
of each wheel is great enough so that the transient driving torque applied
to said other wheel is not reduced below the threshold-load torque of said
other wheel. Accordingly9 the vehicle may be moYed by the traction of said
other wheel to a location where the one wheel encounters non-slip adhesive
friction with the road. Further, speed control means is connected in the
circuit for varying the voltage applied to the motors. Also, switching
means is connected in the circuit for reversing the polarity of the voltage
applied to the field windings for reversal of said motors.
A more complete understanding of this invention may be obtained from
the detailed description that follows taken with the accompanying drawings.
FIGURE 1 is a pictorial view of an industrial truck which is provided
with a traction drive system in accordance with this invention;
FIGURE 2 is a schematic diagram of the traction drive system oF this
invention;
FIGURE 3 shows the construction of the traction drive system;
FIGURES 4a and 4b are graphical representations of motor voltage,
current and torque for use in aiding the explanation of the operation.
Referring now to the drawings, there is shown an illustrative
embodiment of this invention in the traction drive system of an industrial
truck. It will be appreciated as the description proceeds that the
invention is useful in many applications and embodiments.
FIGURE 1 shows a pictorial view of an industrial truck 10 which is
provided with a traction drive system according to this invention. The
truck 10 comprises a body 12 provided with a driver's seat 14, a steering
wheel 16 and a control panel 18. The truck has a front wheel drive which
includes a traction wheel 22 on the right side and a traction wheel 24 on
the left side. The truck is provided with a single dirigible wheel 26
which is controlled by the steering wheel 16. The traction wheel 22 is
driven by an electric traction motor 32 and the traction wheel 24 is driven
by an electric traction motor 34.
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Each of the motors 32 and 34 is a conventional DC series wound motor,
i.e. the field winding and armature winding are connected in series. The
two motors 32 and 34 are substantially identical with each other, i.e. they
are of the same construction and rating. The mechanical electrical
connection of the motors 32 and 34 will be described subsequently.
The mechanical arrangement of the traction drive system is shown in
greater detail in FIGURE 3. The motor 32 is mounted on an axle housing 36
which houses a gear reducing drive train. The armature shaft of the motor
32 is coupled through the drive train to the axle 38 which carries a wheel
hub 42. The traction wheel 22 is mounted on the wheel hub 42. The wheel
22 is provided with a disk brake 44 mounted inboard of the axle housing 36.
In a similar manner, the motor 34 is mounted on an axle housing 46 which
houses a gear reducing drive train. The armature shaft of the motor 34 is
connected through the drive train to an axle (not shown) which carrles a
wheel hub 52. The traction wheel 24 is mounted on the wheel hub 52. The
wheel 24 is provided with a disk brake 54 which is mounted in~oard of the
axle housing 46. It is noted that the axle housings 36 and 46 are
mechanically connected with each other. However, the drive train from the
motor 32 to the traction wheel 22 is entirely separate from the drive train
from the motor 34 to the traction wheel 24. The wheels 22 and 24 are
independently rotatable and constitute separate and independent loads for
the respective motors 32 and 34.
The electrical circuit of the traction drive system is shown
schematically in FIGURE 2. The motors 32 and 34 are connected in series
with each other and the motor 32 has a field winding 62 and the motor 34
has a field winding 64. The field windings 62 and 64 are connected in
series with each other and are connected in series with the armature of the
motors 32 and 34 through a reversing contactor. The contactor comprises a
set of normally closed forward contacts 66 and 68 and a set of normally
open reverse contacts 72 and 74. The series connected motors are connected
across the vehicle battery 76 through a start switch 78 and a conventional
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1 speed control device 82. The reversing contactor is
controlled by a forward-reverse con-trol device 84. A
protective Gr free-wheeling diode 86 is connected across the
armatures of the motors 32 and 34. Also, a plugging diode ~8
is connected across the series combination of the field
windings and the armatures of the motors 32 and 34.
The series circuit of the motors 32 and 3~ has an
impedance, i.e. the resi~tance and inductance of the field
windings and the armature windings and other circuit elements,
such that the transient current initiated by applying starting
voltage to the motors is sufficiently high that the driving
torque developed by each motor rises above the threshold-load
torque of the vehicle. The threshold-load torque of the
vehicle is that required to initiate motion of the vehicle
when there is a non-slip adhesive friction of the traction
wheels with the road. Further, each of the traction wheels
and its respective drive train from the motor has sufficiently
high moment of inertia and friction so that the no-load torque
exhibited by each wheel keeps the speed of the motor from
reaching a value which would produce a counter EMF that would
reduce the driving torque below the threshold-load torque.
The no-load torque is the torque required to rotate the wheel
when there is no adhesive friction with the road.
The operation of the traction drive system will be
explained with reference to the graphs of FIG~RES 4a and 4b.
For explanatory purposes, it will be assumed that the vehicle
is to be started from a standstill and brought up to a desired
speed under the control of the driver. For this purpose, the
start switch 78 is closed and the driver may advance the
control lever of the speed control device 82 as desired.
Assuming that both traction wheels 22 and 24 are resting on a
dry road surface there will be a non-slip adhesive friction
between the road and the wheels. Both the traction wheels
will exhibit substantially the same start-up and running
load torques under this starting condition. The motors
32 and 34, as is characteristic of series wound motors,
produce a high startin~ torque. Since the motors are
identical and the load torques exhibited by
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the respective traction wheels are substantially the same, the motors have
about the same acceleration. Accordingly, the wheels are brought up to
running speed together to attain the desired speed of the vehicle. As the
motors come up to speed tcgether~ the counter EMF produced by each of the
motors increases and accordingly the motor current and torque decreases.
Both motors produce the same counter EMF and since the motor current is the
same for both motors, the IR drop is the same for both motors. Thus, the
two motors share the load equally.
Now, for explanatory purposes, it will be assumed that the vehicle is
to be started from standstill and that traction wheel 22 is resting on a
patch of ice and traction wheel 24 is resting on a dry road surface. The
traction wheel 24, being on dry road, will exhibit sufficient reaction
torque when driving torque is applied thereto from the motor 34 to initiate
motion of the vehicle. This reaction torque, which is equal and opposite
to the applied torque, is herein referred to as the threshold-load torque.
On the other hand, when this torque is applied to the traction wheel 22 by
the motor 32 it will not exhibit sufficient reaction torque to
significantly assist in moving the vehicle. For discussion purposes, it
may be assumed that the reaction torque between the traction wheel 22 and
the patch of ice on which it rests is substantially zero. Accordingly, the
traction wheel 22 starts to spin as soon as power is applied to the motors
32 and 34 by closing the start switch 78.
As described previously, each of the traction wheels 22 and 24 and its
respective drive train exhibits a no-load torque which must be overcome by
the motor driving torque in order to accelerate the wheel. The no-load
torque has a value which is a function of the moment of inertia of the
traction wheel and its drive train and of the fricti~nal resistance of the
drive train. Both traction wheels 22 and 24 and the respective drive
trains are of the same design and hence have substantially the same no-load
torque. For any given operating condition, the total load torque exhibited
by either of the traction wheels at a given instant of time is the sum of
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its no~load torque and the reaction torque produced by its adhesive
friction with the road.
FIGURES 4a and 4b depict in graphical form an illustrative example of
operation of the traction drive system. It is noted that the graphs of
FIGURE 4a represent voltage as a function of time and, on the same time
scale the graphs of FIGURE 4b represent motor current and torque as a
function of time. The no-load torque T1 is substantially constant for a
given value of acceleration except for a reduction in value which occurs
with initial motion due to the difference between static and dynamic
~; 10 friction. The threshold-load torque T2 is, in a similar manner,~a~
substantially constant value but is much higher than that of the no-load
torque. The threshold-load torque is that required to initiate motion of
the vehicle whereas the no-load torque is that required to initiate
rotation of a traction wheel when there is no adhesive friction with the
road.
~e~.
Assume that the start switch 78 is closed at time-~3-ti~e and that the
driver has moved the control lever of the speed control device 82 to the
usual starting position. As soon as the switch is closed, a transient
current 106 starts to build up. Initially, the only limitation on the
current is the impedance of the series circuit, i.e. the resistance and
inductance of the field windings and armature windings of the motors 32 and
34. The current through the motors causes each motor to develop a torque
108 which varies directly with the square of the current. At a time tl the
driving torque of each motor has increased to the no-load torque T1. As a
result, motor 32 and traction wheel 22 start to rotate while motor 34 and
traction 24 remain at standstill. Rotation of motor 32 produces a counter
EMF 112 which increases as a direct function of speed. Initially, the
counter EMF 112 of motor 32 remains relatively small. The motor current
continues, after time tl, to increase toward a peak value but the rate of
increase is reduced with increasing counter EMF while the time-decreasing
value of inductive impedance tends to allow increasing current. As a
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result, the transient motor current 106, and hence the transient motor
torque 108 continue to increase toward a peak value. At a time t2, the
motor torque of each motor ~ and ~ reaches the threshold-load torque T2.
This causes the traction wheel 24 to start to rotate as a result of the
driving torque of motor 34 and hence the vehicle starts to move. At time
t2, the counter EMF of motor 22 is still small enough in relation to the
battery voltage and the instantaneous value of impedance of the motor
windings so that transient motor current 106 and motor torque 108 continue
to increase toward a peak value. Motor 24 is rotated at a much lower speed
than motor 22 because of the relatively heavy load torque which is the
threshold-load value or greater. The motor 34 produces a counter EMF 114
which when added to the counter EMF 112 of motor 32 produces a total
counter EMF 116. The total counter EMF opposes the battery voltage B+
leaving a net voltage V as the impedance drop across the motor windings.
At a time following t2, the motor current 106 and the motor torque 108
J eGRe~e
reach a peak value and begin to ~u4~e~ because of the increasing total
counter EMF. The transient motor torque 108 exceeds the threshold-load
torque T2 and the vehicle continues to move forward under the tractive
effort of wheel 24. The motor current 106 and the motor torque 108
eventually reach a stable value above threshold torque T2 and the rotation
of the traction wheel 24 continues.
Alternatively, the no-load torque characteristic of each traction wheel
and drive train and the transient torque characteristic of the motors may
be correlated so that the motor torque falls below the threshold-load
torque after a predetermined time interval. During the predetermined time
interval the vehicle will move forward by reason of the tractive effort of
the wheel 2~. If, after this travel, the traction wheel 22 remains on the
patch of ice, i.e. without adhesive friction with the road, then wheel 22
continues to spin since the motor torque remains higher than the no-load
torque. However, if the travel of the vehicle has moved the traction wheel
22 off the patch of ice to dry road surface the vehicle can be operated as
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described above with both traction wheels 22 and 24 on dry road. In the
event that traction wheel 22 remains on the ice patch and continues to
spin, as described above, the driver may turn the motors off and bring both
wheels to a standstill then turn them on again to repeat the cycle just
described. In this manner, the driver can move the vehicle off the ice
patch to thereby obtain normal operation oF the vehicle. The predetermined
kime interval may be established at a few seconds, for example in the range
of four to eight seconds, during which the vehicle 1s moved by the tractive
effort of one wheel while the other wheel spins without traction.
Although the description of this invention has been given with
reference to a particular embodiment, it is not to be construed in a
limiting sense. Many variations and modifications will now occur to those
skilled in the art. For a definition of the invention reference is made to
the appended claims.
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