Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 20-TR-1216
This invention relates to apparatus for excitiny and
regulating rotating alternating current generators and, more
particularly, to a static control system for a self-excited
variable speed generator.
Alternating current ta-c) genexators or alternators are
electrical machines having relatively rotatable magnetic core
members forming a magnetic cirucit. A field winding is commonly
provided on one of the core members and a main or armature
winding which developed the alternating current produced by the
alternator is formed on another core member. Direct current
(d-c) is supplied to the field winding to establish magnetic
poles within the alternator. Within its normal operating range
the magnitude of alternating current produced by the alternator
ls directly related to the relative rotational velocity between
-the armature and field winding and the magnitude of currentin
the field windings. Once the alternator is in operation a
portion of the alternating current from the main winding can
be rectified and used to provide excitation to the field winding.
Alternators employing such techniques are well known and are
referred to as self-excited machines.
The present inven-tion is of particular utility in
an electric propoulsion system for traction vehicles such as,
for example, off-highway ~ehicles of the earthmoving or mining
type. Such a vehicle may include an articulated frame and a
four-wheel drive. Both front and rear axles may be driven by
an electrical system comprising a pair of variable speed
reversible d-c motors which are energi~ed by an alternator
coupled to a diesel engine or other suitable prime mover. sy
appropriate manipulation of a speed control pedal, an operator
can control the electric drive system so as to determine the
vehicle speed. The speed control pedal acts as a throttle
control to vaxy the speed of the diesel engine driving the
generator and thus varies the electrical power output of the
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a-c generator.
A typical a-c generatox for a traction vehicle may
provide 750 amperes a-c RMS current at 1600 volts RMS and
may require 300 amperes of d-c current excitation for its
field winding. In such an application it is possible to
shiphon off a portion of the alterna-ting current developed
by the main winding in order to provide a relatively low
voltage excitati~on to the field windings. However, if the
alternator is started under substantially short circuit load
conditions, such as in electric traction motor applications,
th~ short circuit condition will prevent the main winding voltaye
from attaining a magnitude sufEicient to allow the alternator
to operate in a self-excited mode. A more practical method
for implementing self-excitakion i5 to provide an auxiliary
or tertiary winding on the armature which pxoduces an alter-
nating current suitable for exciting the field winding after
appropriate rectification. Such a system is shown in Canadian
Patent No. 812,936 issued May 13, 1969. In this patent the
alternator is stationary and adapted to operate with a sub-
stantially constant relative rotational velocity and the alter-
nating current output is regulated by control of the direct
current excitation of the field winding. During initial start-
up of the alternator, the field winding is e~cited by a battery
which provides sufficient excitation under no-load conditions
to cause current to be generated in the auxiliary winding
without appreciably discharging the battery.
In a system in which the generator is operated over
a range of rotational Yelocities and the alternating current
produced by the generator is a function of the relative ro-
tational velocity between the core members, it will be appreci-
ated that at low speeds the energy produced by the auxiliary
winding may be insufficient to provide field curren-t at the
desired level. Accordingly, at such low speeds the battery
~:3LQ~6~ 20~TR-1216
necessaril~ supplies at least part of the excitation for the
field winding. If the generator is operated any appreciable
percentage of time at low speeds, it will. be apparent that the
battery may become weakened and unable to supply adequate
Eield excitation.
~ lthough discharge of the battery may be avoided by
drawing power at low armature velocities from a battery
charger circuit such as a rotating exciter, this alternative
does not provide a ready means of regulating the average
magnitude of alternator field current. If the field current is
allowed to stabilize at its own level, i.e., if the current is
determined solely by the magnitude of avai].able battery voltage,
the power output of the alternator may exceed a desired level.
Furthermore, if the battery circuit is continuously required
to supply field excitation, the physical capability and size of
the battery circuit becomes economically impracticaly particularly `
in traction vehicle applications where equipment space is
limited. Thus, it wi.ll be appreciated that the level of ex-
citation supplied by the battery or other means should be
regulated to prevent overexciting the alternator field winding and
to blend the excitation energy withdrawn from the battery cir-
cuit with the excitation from the auxiliary winding iri a manner
to ~inimize the use of battery supplied excitation current.
It is an object of the present invention to provide
an improyed method and apparatus for augmenting self-excitation
of an alternator field winding.
It is a ~urther object of the present invention to
provide a method and apparatus for controlling a secondary
excitation source in a self-excited alternator circuit to
minimize power drain of the secondary source.
In accordance with the present invention there is
provided a control system for a sel~-excited alternator which
blends secondary excitation with self-excitation in a manner
~ 20-TR-1216
to minimize secondary source power. The alternator is
preferably of the type having an armature mounted auxiliary
or tertiary winding for producing excitation current for a
field winding of the alternator. A secondary source is
connected in circuit with the auxiliary winding in order to
supply field winding excitation when the excitation current
available from the auxiliary winding is less than the desired
magnitude. The inventive system includes a first control
means, such as a phase controleld rectifier, connected in
circuit with the auxiliary winding and the field winding for
controlling the average magnitude of excitation current supplied
from the auxiliary winding to the field winding. A secondary
excitation source and a second control means, such as a chopper,
is also connected in circuit with the auxiliary winding. A
regulating circuit is responsive to selected parameters of
the generator to provide signals to control both the first and
second control means. The regulating circuit utilizes a first
feed forward loop for supplying control signals to the first
control means and a second feed forward loop for supplying
control signals to the second control means. The amplification
factor of the first loop is chosen to be greater than that of
the second loop whereby the excitation current is primarily
supplied by the auxiliary winding and only the current deficit
is provided by the secondary source.
These and other objects, features and advantages
of the present invention will be better understood by ref-
erence to the detailed description taken in conjunction with
the accompanying drawings in which:
Fig. 1 is a simplified schematic of an ele-c-tric
vehicle propulsion system incorporating a self-excited
alternator control system according to the present invention;
Fig. 2 is a block diagram of a regulating system
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~ 20-TR-1216
according to the present invention for controlliny the
alternator of Fig. l; and
Fig. 3 i.s an alternate embodiment of the system of
Fig. l.
Referring now to Fig. 1, there is shown a simplified
schematic diagram of a power control system for an electric
traction vehicle according to the present invention. A prime
mover lO which may be, for example, an internal combustion
engine such as a diesel engine, is adapted to drive an alter-
nating current generator or alternator 12 through a connecting
shaft 14. The rotational velocity or revolutions per minute
(RPM) of shaft 14 is determ.ined by the relative displacement of
accelerator pedal 16 which is adapted to control the prime
mover lO.' Alternator 12 includes relatively rotatable first and
second magnetic core members forming a magnetic circuit. A
Field winding 18 wound on one of the core members proYides
magnetizing flux for the magnetic circuit. A main winding
(not shown) wound on the other of the core members provides
alternating curren-t output power. In the illustxated embodiment
alternator 12 is a three phase machine providing three phase
output on lines A,B, and C. For illustration purposes the
output power developed on li.nes A,B, and C is shown as being
supplied to a direct current motor 20 after being converted
to direct current power by means of a bridge rectifier circuit
22. The motor 20 may be connected to drive a wheel-axle
assembly (not shown) for propelling the traction vehicle.
The magnitude of power developed on lines A,B, and
C by alternator 12 is determined by the relative rotational
velocity of the :Eirst and second magnetic core members and by
the level of excitation applied to the field winding 18. As
is well known, the magnitude of the electrical power output
de~eloped by alternator 12 is determinative of the magnitude
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LÇ~ 20-TR-1216
of the mechanical load reflected to the prime mover 10. For
any given RPM of prime mover 10 there is a maximum level of
loading which can be imposed without causing the prime mover 10
to bog or lose speed. Accordingly~ the level of excita-tion
applied to Eield winding 18 is preferably controlled as a
function of the RPM of alternator 12. To accomplish this
function, speed sensor 24 is operakively connected to alter-
nator 12 for providing an output signal representative of the
RPM of alternator 12. This output signal is supplied to a
function generator 2~ of a type well known in the art. The
function generator 26 may be, for example J the type illustrated
in section 11.23 of the Philbrick/Nexus Application Manual for
_perational Amplifiers, second edition, 3rd printlng in August,
1969 by Philbrick/Nexus Research of Dedham, Mass. As indicated
by the graph illustrated in block 26, the function generator 26
produces an output signal or load reference signal as a pre-
determined function of the RPM signal from speed detector 24.
In a preferred embodiment the output signal developed by function
generator 26 is substantially zero at a minimum operating or
idle RPM, for example, 700 RPM, and increases in a piecewise
linear fashion to a maximum value at a maximum RPM reference
level, for example, 1900 RPM. The signal developed by function
generator 26 is applied to one input terminal of a summing
junction 28.
Depending on the load characteristics, in this instance
the load being represented by motor 20, the alternator 12
may be controlled as a function of its power output, its vol-
tage output or its c~rrent output. For example, with very low
or zero velocity of the motor 20, the current produced by
alternator 12 may be relatively large while the vol-tage output
is near zero. Under this condition~ current should be the
con trolling or limiting param~ter. In order to control alternator
12 as a function o$ voltage, current or power, there is provided
a selection circuit 30 having a firs t set of input terminals
~ 6~ 20-T.R-1216
connected to lines 32 and 34 for monitoxing the voltage out-
put of alternator 12. A second set of input terminals is
connected to lines 38 and 40 for monitoring the current out-
put of alternator 12. The current outpu-t signal is preferably
developed by a current shunt 36 serially connected in the
motor current path between motor 20 and rectifier circuit 22,
although other well known means may be utilized.
The selection circuit 30 is shown as comprising
a first differential amplifier 30A having input terminals con-
nected to lines 32 and 34 for developing an output signal
representative of the voltage produced by alternator 12. A
second differential amplifier 30B has input terminals connected
to lines 38 and 40 for developing an output signal represen-
tative of the current ~duced by alternator 12. The voltage
representative signal and the current representative signal
are supplied to an analog multiplier 30C which multiplies
the two signals together to develop a signal representative :~
: of the electrical power bei.ng produced by alternator 12. The
power sginal is then applied to one input terminal of an analog
OR circuit 30D. Two additional in~ut terminals or OR circuit
30D are connected to receive the voltage signal and the current
signal from amplifiers 30A and 30B, respectively. OR circuit
- 30D thus provides at its output terminal a signal representative
of the magnitude of the larges of the power, voltage or
current input signals. The signal developed by OR circuit 30D
then becomes the actual measurement signal produced by
selection circuit 30 for comparison with the reference signal
from function generator 26. Although the circuit 30 is
illustrated only in functional form, the construction of such
a selection circuit including the necessary biasing resistors
and gain or scaling adjustments are well known to those
skilled in the art. Such circuits are shown, for example, in,
~ ~0~6~ 20-TR-1216
and in the references cited in, U.S. patent No. 3l970,858
issued July 30, 1976 and assigned to the General Electric Co.
The signal from selection circuit 30 is representative
of the mechanical load being reflected from alternator 12
onto the pr.ime mover 10. This signal is applied via line 42
to a second input terminal of summiny junction 28 where i.t is
algebraically summed with the signal from function generator 26.
Summing junction 28 thus pro~ides an error signal representative
of the difference between the actual power output being developed
by alternator 12 and the power level represented by the load
reference signal from function generator 26.
The error signal developed by summing junction 28 is
supplied via line 44 to an input terminal of a thyristor or
SCR phase control circuit 46. Phase control circuit 46 is
connected to supply gating signals to a phase controlled recti-
fier circuit 48 which is connected in series circuit with the
alternator field winding 18. Power for the phase controlled
rectifier circuit 48 is supplied by an auxiliary or tertiary
winding 50 which is wound on one of the magnetic members of the
alternator 12. Preferably the auxiliary winding 50 is wound in
conjunction with the main winding supplying the three phase
output of the alternator in a manner as illustrated in the
pre~iously identified Canadian Patent No. 812,936. The phase
controlled rectifier circuit 48 has one output terminal con-
nected to field winding 18 via a line 50 and a second output
terminal connected to field winding 18 through a diode 52 and a
current shunt 54. When the output current developed by auxiliary
winding 50 is available to supply the excitation requirements
of field winding 18, control of the thyristors 48a and 48b in
phase control rectifier circuit 48 will allow the current from
field winding 50 to be controllably supplied as exci.tation to
field winding 18. As can be seen the current path between
~ 20-TP~-1216
winding 18 and winding S0 is through the rectifier circuit
48, field winding 18, current shunt 54, and diode 52.
When the alternator is running at a very low ~PM,
the output current developed by auxiliary winding 50 may be
insufficient to supply the desired amount of excitation for
field winding 18. When this occurs, additional excitation may
be supplied by a battery circuit 56 thorugh a chopper circuit
58 of a type well known in the art such as that described in
the SCR Manual, fifth edition r published in 1972 by the
General Electric Co., Semiconductor Products Dept., Syracuse J
NY. In the embodiment of Fig. 1 the battery circuit 56 and
chopper circuit 58 are serially connected in parallel with the
diode 52. If the excitation current available from field winding
50 is insufficient to satisfy the desired excitation level,
chopper circuit 58 may be time ratio controlled to provide
additional excitation current. Control of chopper circuit 5
is implemented by a chopper control circuit 59 in response to
the error signal on line 44 and a current feedback signal from
current shunt 54. In general the chopper circuit 58 will be
gated into conduction for very large exror signals on line 44;
however, for currents in field winding 18 above a predetermined
level the signal developed by current shunt 54 may be utilized
to completely gate off the chopper circuit 58 since in that
instance the power available from winding 50 will be sufficient
to supply the requirements of field winding 18. A more
detailed description of the operation of phase control circuit
46 and chopper control circuit 59 is illustrated in the
accompanying Fig. 2. The battery circuit 56 of Fig. 1 may
comprise a battery alone or, as is more common, may comprise
a battery and associated battery charger. A typical battery
charger will include a rotating machine such as a d-c generator
or an alternator/rectifier unit and a voltage regulator.
_9_
"
~ 20-TR-1216
The charger is generally a low current unit connected to be
directly driven by the prime mover 10. Since it is economically
impractical to provide a battery circuit capable of supplying
a relatively large amo~mt of current for extended time periods,
the phase control circui-t 46 and chopper control circuit 59
are adapted to minimize the current drain on battery circuit
56.
Referring now to Fig. 2 there is shown a preferred
embodiment of the phase control circuit 46 and chopper control
circuit 59. The current signal developed by current shunt ~4
is representative of the current in field winding 18 and is
coupled to input terminals of a differential amplifier 61. The
signal then developed by amplifi0r 61 is supplied to an ampli~
fier 60 which is biased such that for a current in shunt 54
which is less than a predetermined levell for example, 40
amperes, the output signal developed by amplifier 60 remains
at substantially zero potential. When the curren-t through shunt
54 attains the predetermined le~el, the signal level at the
output terminal of amplifier 60 rises very rapidly to a maxi-
mum or saturation level which is thereafter maintainedO The
signal developed by amplifier 60 is applied to a first input
terminal of a summing junction 62. A second input terminal
of summing junction 62 is connected to receive an amplified
version of the error signal on line 44. This amplified error
signal is provided by an amplifier 64, which has one input
terminal connected to line 44 and an output terminal connected
via line 66 to the second input terminal of summing junction
62. It will be appreciated that the output signal from sum-
ming junction 62 on line 68 is substantially equivalent to
the output from amplifier 64 during the time period in which
amplifier 60 produces a signal of substantially zero potential.
However, as the signal developed by amplifier 60 increases,
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20-TR-1216
the signal on line 68 is very rapidly driven to a negative
value. The output signal developed by amplifier 61 is applied
via a line 88 to an input terminal of a switching amplifier
90 which may bel or example, a Schmitt trigger which changes
state when t~ie signal on line 88 exceeds a pre-determined value.
The output signal de~eloped by amplifier 90 is applied to
control an oscillator 92. Oscillator 92 is preferably a ~Eixed
frequency free running oscillator of a type well known in the
art. The signal from amplifier 90 is -used to turn off the
oscillator when it goes to a high level, i.e., when the amplifier
90 changes state upon the current through shunt 54 reaching
a predetermined value, oscillator 92 is turned of.
Oscillator 92 provides clock signals which are
applied through an amplifier 94 to gate a main current carrying
thyristor (not shown~ in the chopper circuit 58 into con-
duction. Accordingly, at each clock pulse signal from oscillator
92, the main thyristor in chopper circuit 58 is tricJgered.
The clock signals from oscillator 92 are also applied to a ramp
function generator 96 causing the ramp function generator 96
to be reset in synchronism with the clock pulses from oscillator
92.
The ramp function signals from ramp function generator
96 are applied to a first input terminal of a comparator 98, a
second input -terminal of the comparator 98 being connected to
line 68. When the ramp flinction signal from generator 96
reaches a magnitude greater than the magnitude of the error
signal on line 68, comparator 98 changes state producing a
positive going output signalr This signal is app~ied to trigger
a one shot multivibrator 100 which then produces a pulse o~ a
predetermined pulse width. This pulse is amplified by ampli-
fier 102 and applied to chopper circuit 58 to commutate the
main current carrying thyristor.
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~ 20-TR-1216
The error signal on line 44 is also applied to an
input terminal of a second amplifier 70 which preferably has a
higher gain than amplifier 64. ~ccordingly, the signal
developed at the output of amplifier 70 on a line 72 will be a
larger signal than that on line 66 at the same time. As will
become apparent, the larger gain of amplifier 70 allows the
circuit 46 to maximize the current supplied by the auxiliary
winding 50 ana minimize the current supplied by battery circuit
56.
Alternating current is generated by alternator 12
on auxiliary winding 50. Since the frequency of this alter-
nating current is proportional to the RPM of the prime mover 10,
in order to properly control the rectifier circuit ~8, it is
necessary to provide control signals which are synchronous with
the alternating voltage from winding 50. Synchronism of the
control function is provided by tracking the voltage waveform
in winding 50 by means of a voltage sensing transformer 7~
having a primary winding connected in parallel with auxiliary
winding 50 and a secondary winding connected to input terminals
of a zero crossing detector 76. The zero crossing detector 76
provides an output pulse each time that the alternating current
waveform crosses the zero voltage reference axis, i.e., at the
beginning of each half-cycle of the alternating voltage wave-
form. The pulse signals developed by zero crossing detector
76 are applied yia line 78 to a frequency-to-current converter
83 which converts the pulses to a d-c current proportional to
the frequency of the pulses. The output signal from the fre-
quency-to-current converter 80 is applied to a ramp generator
circuit 82. The ramp generator circuit 82 may co~prise, for
example, a resettable integrating circuit. The ramp is
reset at each zero crossing by pulses supplied by zero crossing
detector 76 via line 83. Since the rate of rise of the ramp
. .
~ -12-
~ 20-TR-1216
function is proportional to the d-c input current and since
-the ramp is reset at each zero crossing, the ramp rises to
substantially the same amplitude during each half-cycle of
the a-c voltaye waveform and thus provides a synchronous
control function. A detailed illustrat:Lon and description of a
zero-crossing detector, frequency~to-current converter and
ramp generator circuit for developing a ramp function synchron-
ized to a variable frequency a-c voltage waveform is given
in U.S. Patent issued July, 1977 and assigned to the General
Electric Co.
The synchronized ramp function signal from ramp
function generator 82 is applied to one input terminal of a
comparator circuit 84, a second input terminal of comparator
circuit 84 being connected to line 72 for receiving the
amplified error siynal. When the amplitude of the error signal
on line 72 exceeds the amplitude of the ramp function signal,
the comparator 84 will change state and provide an output
signal to an SCR firing circuit 86. The SCR firing circuit 86
may be of a type well known in the art for providing firing
pulses to phase controlled rectifier circuits. The design
of such phase control firing circuits is described in Chapter
9 of the aforementioned GE SCR Manual. The SCR firing circuit
86 provides firing pulses to phase control the rectifiers
48a and 48b in a manner well known in the art.
To better understand the operation of the circuit
in Fig. 2~ consider the situation in which the alternator
12 is operating at relatively low RPM and is supplying power
to the motor 20. The signal developed by selection circuit
30 on line 42 will be of lesser magnitude than the signal
developed by function generator 26. Thus the error signal on
line 44 supplied by summing junction 23will be greater than
zero. Since the alternator RPM is relatively low, the alter-
nating current produced by auxiliary winding 50 will be in-
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~ 20-TR-1216
sufficient to supply the required level of field curren-t
excitation. Accordingly, the battery circuit 56 will be called
upon to supply current to field winding 18. Note, however,
that the error signal on line 44 is amplified by amplifier
70 and provides on line 72 a relatively large signal. This
large error signal 72 will cause comparator 84 to change
state early in the period of each alternation of the current
supplied by winding 50. The comparator 84 will thus supply
signals to cause firing circuit 86 to phase the phase controlled
rectifier circuit 48 into a full "on" condition to thereby
take maximum a~ailable energy from field winding 50. Since
this energy from field winding 50 is less than the desired
level of excitation current for field winding 18, the error
signal developed by amplifier 64 on line 66 will cause the
comparator 98 to change state relatively late in the cycle of
the ramp signals from ramp function generator 96 thus maintaining
the chopper circuit 58 in a highly conductive mode, it being
noted that the main thyristor in the chopper circuit is fired
on each clock pulse from oscillator 92 and is commutated off
by a change of state of comparator 98.
As alternator RPM is increased the current output
of alternator 12 will reach a magnitude such that the excitation
available from auxiliary winding 50 is sufficient to excite
field winding 18 at the desired level. When the field excitation
reaches this desired level, the error signal on line 44 becomes
relatively small. The gain of amplifier 64 is set such that
for a small error signal the amplifier output voltage is also
relatively small. Thus, the signal on line 66 is of such a
low magnitude that the chopper circuit 58 is commutated off
~; 30 almost as soon as it is gated into conduction. Of course, the
transition to this state is relatively smooth and follows the
rate of reduction of the magnitude of the error signal on line
-14-
20-TR-1216
44. Because oE the higher gain of amplifier 70, however,
the signal on line 72 remains of sufficient magnitude to
control the operation of phase control rectifier circuit 48 in
a manner to minimize the error signal on line 44.
Since the illustrated feedback control system is
a non-integrating (type O) regulating system, the error signal
on line 44 always has a finite magnitude. Accordingly, the chop-
per circuit 58 is not completely inactivated by a minimum
magnitude error signal. In order to assure complete inactivation
of the chopper circuit 58, the switching amplifier 90 is set
to change state when the current in shunt 54 reaches a predeter-
mined magnitude, ine., when the signal developed by amplifier
61 reaches a predetermined magnitude. A change of state by
amplifier 90 inhibits the operation of oscillator 92 so that
gating signals are no longer supplied to chopper circuit 58.
Thus, once the field current excitation exceeds a predetermined
magnitude, all the excitation is supplied by auxiliary
winding 50 and regulated via phase control rectifier circuit
48.
Referring now to Fig. 3 there is shown an alternate
embodiment of the present invention in which the chopper circuit
58 and battery circuit 56 are connected in parallel circuit
arrangement with the phase controlled rectifier circuit 48.
A free-wheeling diode 106 is also connected in parallel with the
rectifier circuit 48 and, as a consequence of the parallel
connection of rectifier circuit 48 and field winding 18, is
also connected in parallel with field winding 18. Diode 106
shunts the battery circuit 56 to prevent large inductive cur-
- rents from damaging this circuit. The operation of the system
of Fig. 3 is essentially identical to that of Fig. 1. However,
the variable frequency firing of the phase control rectifier
.~
circuit 48 sometimes interacts with the chopper circuit 58
-15-
~ Q 20-TR-1216
causing the latter to be gated at undesirable times. An
additional diode 108 serially connected hetween chopper c:ircuit
58 and rectifier circuit 48 aids in reducing this interaction.
As will be apparent the present invention provides
an effective method and apparatus for operating a self-excited
alternator under conditions in which the self-excitation
current is insufficient for sustaining the operation of the
alterantor and for blending the required excitation current in
a manner to maximize the contribution from the self exciting
source while minimizing the excitation from a secondary source.
Although the invention has been described in a
preferred embodiment as being implemented using analog
circuitry, there will be apparent to those skilled in the art
other modifications and arrangements including implementation
using digital techniques which come within the true spirit
of the invention and thus it is contemplated that the scope of
the invention be determined by the appended claims.
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