U.S. patent application number 09/771543 was filed with the patent office on 2001-08-30 for power unit.
This patent application is currently assigned to Honda Giken Kogyo Kabushiki Kaisha. Invention is credited to Asai, Koichi, Eguchi, Hiroyuki, Shimizu, Motohiro.
Application Number | 20010017784 09/771543 |
Document ID | / |
Family ID | 18550405 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017784 |
Kind Code |
A1 |
Asai, Koichi ; et
al. |
August 30, 2001 |
Power unit
Abstract
A power unit is provided which can perform stable feedback
control by reducing a feedback gain, and sufficiently cope with
fluctuations in the output voltage occurring according to
fluctuations in the input voltage caused by fluctuations in the
rotational speed of an engine. A pair of variable control bridge
circuits are connected to three-phase output windings of a
three-phase generator driven by the engine, and connected in an
antiparallel manner to each other to form a cycloconverter for
generating a single-phase alternating current to be supplied to a
load. An effective voltage value-detecting circuits detects an
effective value of a voltage of the alternating current. A
reference effective voltage value-generating circuit generates a
reference effective voltage for controlling the single-phase
alternating current. A target wave-forming circuit forms a target
wave for making the effective value of the voltage closer to a
value of the reference effective voltage. A firing angle control
device performs switching control of the variable control bridge
circuits, based on the target wave, such that the variable control
bridge circuits are alternately switched to operate every half a
repetition period of the alternating current. A rotational
fluctuation-detecting circuit detects fluctuations in a rotational
speed of the engine in a rotation cycle thereof. A gain-adjusting
circuit generates a reverse characteristic signal having a
characteristic reverse to a characteristic of the fluctuations in
the rotational speed of the engine. A comparator corrects amplitude
of the target wave based on the reverse characteristic signal.
Inventors: |
Asai, Koichi; (Tokyo,
JP) ; Shimizu, Motohiro; (Saitama-ken, JP) ;
Eguchi, Hiroyuki; (Tokyo, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN
LANGER & CHICK, P.C.
25th Floor
767 Third Avenue
New York
NY
10017
US
|
Assignee: |
Honda Giken Kogyo Kabushiki
Kaisha
Tokyo
JP
|
Family ID: |
18550405 |
Appl. No.: |
09/771543 |
Filed: |
January 29, 2001 |
Current U.S.
Class: |
363/148 |
Current CPC
Class: |
H02M 5/4505
20130101 |
Class at
Publication: |
363/148 |
International
Class: |
H02M 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 2000 |
JP |
2000-024413 |
Claims
What is claimed is:
1. A power unit comprising: an engine; a three-phase generator
having three-phase output winding, and driven by said engine; a
pair of variable control bridge circuits connected to said
three-phase output windings of said three-phase generator and
connected in an antiparallel manner to each other to form a
cycloconverter for generating a single-phase alternating current to
be supplied to a load; effective voltage value-detecting means for
detecting an effective value of a voltage of said single-phase
alternating current generated by said pair of variable control
bridge circuits; reference effective voltage-generating means for
generating a reference effective voltage for controlling said
single-phase alternating current; target wave-forming means for
forming a target wave for making said effective value of said
voltage of said single-phase alternating current detected by said
effective voltage value-detecting means closer to a value of said
reference effective voltage; control means for performing switching
control of said pair of variable control bridge circuits, based on
said target wave formed by said target wave-forming means, such
that said pair of variable control bridge circuits are alternately
switched to operate every half a repetition period of said
single-phase alternating current; rotational fluctuation-detecting
means for detecting fluctuations in a rotational speed of said
engine in a rotation cycle thereof; reverse characteristic
signal-generating means for generating a reverse characteristic
signal having a characteristic reverse to a characteristic of said
fluctuations in said rotational speed of said engine detected by
said rotational fluctuation-detecting means; and correction means
for correcting amplitude of said target wave based on said reverse
characteristic signal generated by said reverse characteristic
signal-generating means.
2. A power unit according to claim 1, wherein said effective
voltage value-detecting means detects said effective value over a
predetermined number of repetition periods of said voltage of said
single-phase alternating current.
3. A power unit according to claim 2, wherein said predetermined
number of repetition periods of said voltage of said single-phase
alternating current is one repetition period.
4. A power unit according to claim 1, including a synchronizing
signal-forming circuit for forming a synchronizing signal in
synchronism with an output frequency of said generator, and wherein
said rotational fluctuation-detecting circuit detects said
fluctuations in said rotational speed of said engine based on said
synchronizing signal delivered from said synchronizing
signal-forming circuit.
5. A power unit according to claim 1, wherein said three-phase
generator is a magneto generator having a permanent magnet
rotor.
6. A power unit according to claim 2, wherein said three-phase
generator is a magneto generator having a permanent magnet rotor.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a power unit which generates a
single-phase AC power having a commercial frequency or a like
frequency.
[0003] 2. Prior Art
[0004] Conventionally, a power unit which is a combination of a
small-sized engine and a synchronous generator, for instance, is
widely used for emergency purposes, outdoor works, leisure time
amusement, etc.
[0005] In this type of conventional power unit, however, the output
frequency depends on the rotational speed of the engine. Therefore,
in the case of a bipolar generator, to obtain an AC output of 50 Hz
(or 60 Hz), the rotational speed of the engine is required to be
held at 3000 rpm (or 3600 rpm), i.e. a relatively low rotational
speed, which degrades the operating efficiency of the power unit,
and further, necessitates designing the generator to be large in
size, resulting in an increased total weight of the power unit.
[0006] To overcome this inconvenience, a so-called inverter
generator has been proposed by the present assignee, e.g. in
Japanese Patent Publication (Kokoku) No. 7-67229 and Japanese
Laid-Open Patent Publication (Kokai) No. 4-355672, which is
constructed such that the engine is operated at a relatively high
rotational speed to obtain a high AC power from the generator, the
AC power is once converted to direct current, and then the direct
current is converted to alternating current having a commercial
frequency by an inverter.
[0007] The conventional inverter generator, however, requires
provision of two power conversion blocks, i.e. an AC-to-DC
conversion block for once converting the AC power to DC power, and
a DC-to-AC conversion block for converting the DC power to AC power
having a predetermined frequency, as well as a circuit for
temporarily storing the DC power. Thus, the use of a lot of
expensive power circuit components is necessitated. This makes it
difficult to reduce the size of the generator and leads to an
increased manufacturing cost.
[0008] As a solution to this problem, a so-called cycloconverter
generator has become commercially available, in which a
cycloconverter is employed for use with the generator to directly
convert the high AC power generated by the generator (the generator
is operated at a relatively high engine rotational speed, and hence
the frequency of the alternating current generating the AC power is
higher than a commercial frequency) to AC power having a
predetermined commercial frequency, without carrying out AC-to-DC
conversion.
[0009] In the conventional cycloconverter generator, however, since
the AC power generated by the generator is directly converted to
the AC power having the predetermined frequency (commercial
frequency), without being once converted to direct current, as
described above, to promptly reduce an undue fluctuation in the
output voltage caused by a rapid change in the input voltage which
inevitably occurs when the cycloconverter generator is implemented
in the form of a generator having a relatively small capacity, or
more specifically, an undue fluctuation in the output voltage
occurring when the power unit is switched between a no-loaded
condition thereof and a loaded condition thereof, in short, to
reduce an output voltage regulation, a very large feedback gain is
required of the power unit.
[0010] Therefore, if a conventional control method, or more
specifically, a control method of reducing the output voltage
regulation simply by feedback of the waveform of the output voltage
is applied to the above conventional cycloconverter generator, a
very large feedback gain is required, as mentioned above, which
makes it difficult to achieve stable control of the generator.
[0011] A possible solution to this problem is to modify the
cycloconverter generator such that the effective value of the
output voltage is detected over a predetermined number of
repetition periods of the output voltage and feedback control is
carried out based on the detected effective voltage value, thereby
reducing the feedback gain to enable the generator to perform more
stable feedback control.
[0012] The cycloconverter generator modified as above can
sufficiently cope with a fluctuation in the output voltage caused
by a rapid change in the input voltage occurring when the power
unit is switched between a no-loaded condition thereof and a loaded
condition thereof. However, the AC power is generated from the
generator driven by an engine operating with a relatively high
rotational speed which is changing, and such AC power is directly
converted to AC power having a predetermined frequency (commercial
frequency), without being converted to direct current. Therefore,
it is impossible for the generator to sufficiently cope with
fluctuations in the output voltage occurring according to
fluctuations in the input voltage ascribable to fluctuations in the
engine rotational speed. This inconvenience cannot be eliminated
even if the effective value of the output voltage is detected by
limiting the number of repetition periods of the output voltage to
one, because a time period taken for detection of an effective
value of the output voltage per repetition period thereof is
considerably longer than a time period over which a change in the
input voltage takes place according to a change in the engine
rotational speed. More specifically, assuming that a
single-cylinder four-cycle engine is rotated at 3600 rpm to drive
the generator, and a rated load is connected to the power unit, the
engine rotational speed varies to an extent of approximately
.+-.150 rpm from 3600 rpm. Within this range of variation of the
engine rotational speed, particularly an explosion stroke of the
engine during which a change in the engine rotational speed takes
plase with a sharp gradient of rise continues over a time period of
approximately 5 msec. On the other hand, assuming that the
frequency of the AC output of the cycloconverter generator is a
commercial frequency, i.e. 50 Hz, a time period taken for the
detection of an effective value of the output voltage per
repetition period thereof is 20 msec. This means that when a factor
has been detected to carry out effective feedback control based
thereon, a change in the engine rotational speed responsible for
the factor, which is to be controlled, has already ended, and hence
even the method based on detection of the effective value of the
output voltage per repetition period cannot enable the power unit
to sufficiently cope with fluctuations in the output voltage
occurring according to fluctuations in the input voltage caused by
fluctuations in the engine rotational speed.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide a power
unit which is capable of performing stable feedback control by
reducing the feedback gain, and at the same time, sufficiently
coping with fluctuations in the output voltage occurring according
to fluctuations in the input voltage caused by fluctuations in the
rotational speed of an engine as a drive source of the power
unit.
[0014] To attain the above object, the present invention provides a
power unit comprising:
[0015] an engine;
[0016] a three-phase generator having three-phase output windings,
and driven by the engine;
[0017] a pair of variable control bridge circuits connected to the
three-phase output windings of the three-phase generator and
connected in an antiparallel manner to each other to form a
cycloconverter for generating a single-phase alternating current to
be supplied to a load;
[0018] effective voltage value-detecting means for detecting an
effective value of a voltage of the single-phase alternating
current generated by the pair of variable control bridge
circuits;
[0019] reference effective voltage-generating means for generating
a reference effective voltage for controlling the single-phase
alternating current;
[0020] target wave-forming means for forming a target wave for
making the effective value of the voltage of the single-phase
alternating current detected by the effective voltage
value-detecting means closer to a value of the reference effective
voltage generated by the reference effective voltage-generating
means;
[0021] control means for performing switching control of the pair
of variable control bridge circuits, based on the target wave
formed by the target wave-forming means, such that the pair of
variable control bridge circuits are alternately switched to
operate every half a repetition period of the single-phase
alternating current;
[0022] rotational fluctuation-detecting means for detecting
fluctuations in a rotational speed of the engine in a rotation
cycle thereof;
[0023] reverse characteristic signal-generating means for
generating a reverse characteristic signal having a characteristic
reverse to a characteristic of the fluctuations in the rotational
speed of the engine detected by the rotational
fluctuation-detecting means; and
[0024] correction means for correcting amplitude of the target wave
based on the reverse characteristic signal generated by the reverse
characteristic signal-generating means.
[0025] Preferably, the effective voltage value-detecting means
detects the effective value over a predetermined number of
repetition periods of the voltage of the single-phase alternating
current.
[0026] More preferably, the predetermined number of repetition
periods of the voltage of the single-phase alternating current is
one repetition period.
[0027] Preferably, the power unit includes a synchronizing
signal-forming circuit for forming a synchronizing signal in
synchronism with an output frequency of the generator, and the
rotational fluctuation-detecting circuit detects the fluctuations
in the rotational speed of the engine based on the synchronizing
signal delivered from the synchronizing signal-forming circuit.
[0028] Preferably, the three-phase generator is a magneto generator
having a permanent magnet rotor.
[0029] The above and other objects, features, and advantages of the
invention will become more apparent from the following detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a block diagram schematically showing the whole
arrangement of a power unit according to an embodiment of the
present invention;
[0031] FIG. 2A is a traverse cross-sectional view of an AC
generator appearing in FIG. 1;
[0032] FIG. 2B is a longitudinal cross-sectional view of the AC
generator;
[0033] FIG. 3 is a circuit diagram showing the construction of a
cycloconverter appearing in FIG. 1;
[0034] FIG. 4 is a circuit diagram showing an example of a
synchronizing signal-forming circuit 18;
[0035] FIG. 5 is a timing chart showing changes in voltages applied
between a U phase, a V phase, and a W phase, each appearing in
FIGS. 6A to 6D or 7, timing of turn-on of photocouplers, and timing
of turn-on of gates of thyristors;
[0036] FIG. 6A is a diagram showing an output waveform of a
positive converter exhibited when each thyristor thereof is fired
at a firing angle of 120 degrees;
[0037] FIG. 6B is a diagram showing an output waveform of a
negative converter exhibited when each thyristor thereof is fired
at a firing angle of 120 degrees;
[0038] FIG. 6C is a diagram showing an output waveform of the
positive converter exhibited when each thyristor thereof is fired
at a firing angle of 60 degrees;
[0039] FIG. 6D is a diagram showing an output waveform of the
negative converter exhibited when each thyristor thereof is fired
at a firing angle of 60 degrees;
[0040] FIG. 7 is a diagram showing reference sawtooth waves
generated for controlling the firing angles of the thyristors;
[0041] FIG. 8 is a diagram which is useful in explaining a problem
to be solved when the firing angle is controlled to a range of 120
degrees to -60 degrees; and
[0042] FIGS. 9A to 9D are diagrams useful in explaining grounds of
the method employed by the present embodiment for removing
fluctuations (pulsation) in the output voltage caused by
fluctuations in the engine rotational speed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0043] Next, the present invention will be described with reference
to drawings showing an embodiment thereof.
[0044] FIG. 1 shows the whole arrangement of a power unit according
to an embodiment of the present invention.
[0045] In the figure, reference numerals 1 and 2 designate output
windings independently wound around a stator of an AC generator,
i.e. reference numeral 1 designates a three-phase main output
winding (hereinafter referred to as the "three-phase main coil"),
and reference numeral 2 designates a three-phase auxiliary output
winding (hereinafter referred to as the "three-phase sub coil"),
respectively.
[0046] FIGS. 2A and 2B show the construction of the AC generator in
longitudinal cross-section and transverse cross-section,
respectively. The three-phase main coil 1 is formed of coils
forming twenty-one poles within an area A1, and the three-phase sub
coil 2 is formed of coils forming three poles within an area A2. A
rotor R is formed with eight pairs of magnetic poles of permanent
magnets, and driven for rotation by an internal combustion engine,
not shown. The rotor R also serves as a flywheel of the engine.
[0047] Referring again to FIG. 1, the three-phase main coil 1 has
three output terminals U, V, and W which are connected to
respective input terminals U, V and W of each of positive and
negative converters BC1 and BC2 of a cycloconverter CC.
[0048] FIG. 3 shows the construction of the cycloconverter CC
appearing in FIG. 1. As shown in the figure, the cycloconverter CC
is comprised of twelve thyristors SCRk.+-. (k=1, 2, . . . , 6),
with six thyristors SCRk+ thereof forming a bridge circuit
(hereinafter referred to as "the positive converter") BC1 for
delivering positive electric current, and the remaining six
thyristors SCRk- thereof forming another bridge circuit
(hereinafter referred to as "the negative converter") BC2 for
delivering negative electric current.
[0049] When the three-phase generator with twenty-four poles (three
of them are used to generate synchronizing signals for control of
respective gates of the thyristors SCRK.+-.) connected to the
cycloconverter CC is driven by the engine, eight cycles of
three-phase alternating current are supplied to the cycloconverter
CC per one revolution of the crankshaft of the engine. If the
rotational speed of the engine is set to a range of 1200 rpm to
4500 rpm (equivalent to a frequency range of 20 Hz to 75 Hz), the
frequency of the three-phase AC output from the generator is 160 Hz
to 600 Hz, eight times as high as the rotational speed of the
engine.
[0050] Referring again to FIG. 1, the three output terminals U, V,
and W of the three-phase main coil 1 are connected to the
respective input terminals U, V and W of each of the positive and
negative converters BC1 and BC2 of the cycloconverter CC, as
mentioned above. The output side of the cycloconverter CC is
connected to an LC filter 3 for removing harmonic components of
output current from the cycloconverter CC. The output side of the
LC filter 3 is connected to an output voltage-detecting circuit 5
for detecting a voltage of the output current with the harmonic
components removed, delivered from the LC filter 3. The output
voltage-detecting circuit 5 has a positive input terminal thereof
connected to the LC filter 3 and a negative input terminal thereof
connected to a ground GND of the control system of the power unit
whereby a single-phase output is obtained from the positive and
negative input terminals of the output voltage-detecting circuit
5.
[0051] The output voltage-detecting circuit 5 has an output side
thereof connected to an effective voltage value-calculating circuit
8 for calculating an effective value per repetition period of the
output voltage from the circuit 5. The circuit 8 has an output side
thereof connected to a negative input terminal of a comparator 9.
Connected to a positive input terminal of the comparator 9 is a
reference effective voltage value-generating circuit 10 for
generating an effective value of a reference voltage to be
generated by the power unit(reference effective voltage value). The
comparator 9 has an output side thereof connected to a control
function-calculating circuit 11 which calculates a control
function, such as a linear function, based on results of the
comparison by the comparator 9.
[0052] The control function-calculating circuit 11 has an output
side thereof connected to one input side of an amplitude control
circuit 12 which controls the amplitude of a target wave output
from a target wave-forming circuit 14. The amplitude control
circuit 12 has the other input side thereof connected to the output
side of a comparator 20 which delivers the difference (or a value
corresponding to the difference) between a sinusoidal wave having
e.g. a commercial frequency of 50 Hz or 60 Hz delivered from a
reference sinusoidal wave-forming circuit 13 and the output voltage
detected by the output voltage-detecting circuit 5. The amplitude
control circuit 12 delivers an amplitude control signal for
controlling the amplitude of the target wave in response to the
control function delivered from the control function-calculating
circuit 11 and the difference delivered from the comparator 20.
[0053] The amplitude control circuit 12 has an output side thereof
connected to the target wave-forming circuit 14 which generates a
target wave in response to the amplitude control signal from the
circuit 12. The target wave-forming circuit 14 has an output side
thereof connected to the positive input terminal of a comparator
21. On the other hand, a rotational fluctuation-detecting circuit
22 that detects fluctuations in the rotational speed of the engine
based on synchronizing signals from a synchronizing signal-forming
circuit 18, referred to hereinafter, has an output side thereof
connected to a gain-adjusting circuit 23. The gain-adjusting
circuit 23 adjusts the level of the output from the rotational
fluctuation-detecting circuit 22, i.e. generates a reverse
characteristic signal which is formed by adjusting the amplitude of
the output from the rotational fluctuation-detecting circuit 22
such that the resulting reverse characteristic signal has a
characteristic reverse to a characteristic of a fluctuation in the
engine rotational speed, for use in changing the amplitude of the
target wave, and delivers the reverse characteristic signal to a
negative input terminal of the comparator 21. The comparator 21 has
an output side thereof connected to a firing angle control device
15 for controlling the firing angle of a gate of each of the
thyristors SCRk.+-. constituting the cycloconverter CC, as well as
to a positive input terminal of a comparator 16.
[0054] The firing angle control device 15 is comprised of a
positive gate control device 15a for controlling the firing angles
of gates of the thyristors SCRk+ of the positive converter BC1
(hereinafter referred to as "the positive gates") and a negative
gate control device 15b for controlling the firing angles of gates
of the thyristors SCRk- of the negative converter BC2 (hereinafter
referred to as "the negative gates").
[0055] The positive and negative gate control devices 15a, 15b each
have six comparators, not shown, each of which compares the target
wave with a synchronizing signal (reference sawtooth wave),
referred to hereinafter, and fires a corresponding gate when the
two waves agree with each other The comparator 16 has a negative
input terminal thereof connected to the output side of the output
voltage-detecting circuit 5 and an output terminal thereof
connected to the positive gate control device 15a and the negative
gate control device 15b. The comparator 16 compares the voltage
delivered from the output voltage-detecting circuit 5 with the
target wave, and selectively delivers a high (H) level signal and a
low (L) level signal depending upon results of the comparison.
[0056] When the H level signal is delivered from the comparator 16,
the positive gate control device 15a is enabled while the negative
gate control device 15b is disabled. On the other hand, when the L
level signal is delivered from the same, the positive gate control
device 15a is disabled while the negative gate control device 15b
is enabled.
[0057] The output sides of the three-phase sub coils 2 are
connected to the synchronizing signal-forming circuit 18.
[0058] FIG. 4 shows the construction of the synchronizing
signal-forming circuit 18. As shown in the figure, the circuit 18
is formed of six photocouplers PCk (k=1, 2, . . . 6) and six diodes
Dk (k=1, 2, . . . , 6).
[0059] Components of the three-phase alternating current (i.e.
U-phase current, V-phase current, and W-phase current) obtained
from the three-phase sub coil 2 are supplied to a three-phase
full-wave bridge rectifier FR formed by primary light-emitting
diodes (LED's) of the respective six photocouplers PCk and the six
diodes Dk. Direct current components of the three-phase alternating
current full-wave rectified by the three-phase full-wave rectifier
FR are transformed into light by the primary light-emitting diodes,
and then the light is converted into electric current by secondary
photosensors, not shown, associated with the primary light-emitting
diodes of the photocouplers PCk. In short, electric current
corresponding to the three-phase alternating current full-wave
rectified by the three-phase full-wave rectifier FR is delivered
from the secondary photosensors of the photocouplers. The electric
currents are used to form a synchronizing signal having e.g. a
sawtooth waveform for controlling a phase control angle (firing
angle) .alpha. of a gate of each of the thyristors SCRk.+-., as
described in detail hereinafter.
[0060] FIG. 5 shows changes in line-to-line voltages applied
between respective pairs of the U, V, and W phases of the
three-phase AC power shown in FIG. 3 or 4 and timing of "turn-on"
of the photocouplers PCk.
[0061] Assuming that the line-to-line voltages (U-V, U-W, V-W, V-U,
W-U, and W-V) change as shown in FIG. 5, the waveform of a
full-wave rectified output from the three-phase full-wave rectifier
FR has a repetition period of one sixth of that of the waveform of
each line-to-line voltage obtained from the main coil. For example,
when the U-V voltage is in a phase angle range of 60.degree. to
120.degree. where the U-V voltage is the highest of all the
line-to-line voltages, the photocouplers PC1 and PC5 are turned on
in pair (the other photocouplers are held off), whereby the
three-phase full-wave rectifier circuit FR delivers electric
current at a voltage corresponding to the U-V voltage. That is, the
three-phase full-wave rectifier FR delivers electric current at a
voltage corresponding to the maximum value of all the line-to-line
voltages, so that the repetition period of the output voltage
corresponds to a phase angle of 60.degree., and hence is equal to
one sixth of the repetition period of the three-phase output
voltage of the main coil, which corresponds to a phase angle of
360.degree..
[0062] FIG. 5 also shows a controllable range of timing of firing
(turn-on) of the gate of each of the thyristors SCRk.+-., which is
set to a phase angle range of 120.degree. to 0.degree. of a
corresponding line-to-line voltage with two examples of timing of
firing of each gate which are indicated by hatched portions (i.e.
firing angles of 120.degree. and 60.degree.) described
hereinafter.
[0063] According to this timing, each gate of the positive
converter BC1 is fired (turned on) to deliver electric current
therefrom, and each gate of the negative converter BC2 is turned on
to absorb electric current thereto.
[0064] Needless to say, the gates are not required to be
continuously held on over a selected portion of the controllable
range, but the application of a predetermined pulse at timing
indicated by the hatched portion (e.g. corresponding to one of the
firing angles of 120.degree. and 60.degree.) enables the same
operation as above to be performed.
[0065] FIGS. 6A to 6D show examples of waveforms of the output of
the cycloconverter obtained when the thyristors SCRk.+-. of the
positive and negative converters BC1 and BC2 are fired at
respective firing angles of 120.degree. and 60.degree..
[0066] FIG. 6A shows an output waveform of the cycloconverter CC
obtained when each thyristor SCRk+ of the positive converter BC1 is
turned on at a firing angle .alpha. of 120.degree., and FIG. 6B an
output waveform of the same obtained when each thyristor SCRk- of
the negative converter BC2 is turned on at a firing angle .alpha.
of 120.degree.. On the other hand, FIG. 6C shows an output waveform
of the same obtained when each thyristor SCRk+ of the positive
converter BC1 is turned on at a firing angle .alpha. of 60.degree.,
and FIG. 6D an output waveform of the cycloconverter CC obtained
when each thyristor SCRk- of the negative converter BC2 is turned
on at a firing angle .alpha. of 60.degree..
[0067] When each thyristor SCRk+ of the positive converter BC1 is
turned on at the firing angle .alpha. of 120.degree., the output
waveform of the cycloconverter CC presents a full-wave rectified
current waveform as shown in FIG. 6A. When each thyristor SCRk+ of
the positive converter BC1 is turned on at a firing angle .alpha.
of 60.degree., the output waveform contains a lot of harmonic
components as shown in FIG. 6C. These harmonic components, however,
can be removed by a low-pass filter connected to the output side of
the cycloconverter CC, so that electric current is output at an
averaged voltage. As described hereinabove, assuming that the power
supply to the cycloconverter is a three-phase generator having
twenty-four poles, and the rotational speed of the engine is set to
3600 rpm, the frequency of a basic wave of the harmonic components
is given by the following equation:
60 Hz (=3600 rpm).times.8(-th harmonic).times.3 (phases).times.2
(half waves) (=1 full wave)=2.88 kHz
[0068] Further, by varying the firing angle .alpha. of each
thyristor of the positive converter BC1 within a range of 0.degree.
to 120.degree., the cycloconverter CC is capable of generating a
positive voltage as desired which has an average voltage within a
range of 0 V to a positive full-wave rectified voltage. By varying
the firing angle .alpha. of each thyristor of the negative
converter BC2 in the same manner, the cycloconverter CC is capable
of generating a negative voltage as desired which has an average
voltage within a range of 0 V to a negative full-wave rectified
voltage.
[0069] Next, the manner of controlling the firing angle .alpha.
will be described.
[0070] FIG. 7 shows reference sawtooth waves generated for
controlling the firing angle a of the thyristors of the
cycloconverter. The reference sawtooth waves shown in the figure
are generated based on respective electric currents detected by
i.e. taken out from the secondary photosensors of the photocouplers
PCk shown in FIG. 4.
[0071] A reference sawtooth wave for control of the thyristor SCR1+
of the positive converter BC1, for instance, is one which changes
in voltage within a phase angle range of 120.degree. to -60.degree.
and assumes 0 V at a phase angle of 0.degree.. Reference sawtooth
waves each having a phase difference of 60.degree. from adjacent
ones sequentially correspond to the thyristors SCRk+, i.e. SCR1+,
SCR6+, SCR2+, SCR4+, SCR3+, and SCR5+, respectively.
[0072] On the other hand, a reference sawtooth wave for control of
the thyristor SCR1- of the negative converter BC2, for instance, is
one which is symmetrical with the sawtooth wave for the thyristor
SCR1+ with respect to a horizontal zero voltage line, i.e. which
has a phase difference of 180.degree. from the sawtooth wave for
the thyristor SCR1+. Similarly to the positive converter BC1,
reference sawtooth waves each having a phase difference of
60.degree. from adjacent ones sequentially correspond to the
thyristors SCRk-, i.e. SCR1-, SCR6-, SCR2-, SCR4-, SCR3-, and
SCR5-, respectively.
[0073] Thus, the twelve sawtooth waves provide respective reference
waveforms for control of the thyristors SCRk.+-. of the positive
and negative converters BC1, BC2. These sawtooth waves are compared
with a target waveform r by the use of comparators, not shown,
provided in twelve channels, and a point of intersection of each
sawtooth wave with the target waveform determines a firing angle of
each corresponding thyristor SCRk.+-..
[0074] By employing a sinusoidal wave as the target wave to thereby
sinusoidally varying the firing angle .alpha., it is possible to
obtain a sinusoidal output wave from the cycloconverter CC.
[0075] In FIG. 7, the controllable range of the firing angle is
expanded from the range of 120.degree. to 0.degree. shown in FIG. 5
to a range of 120.degree. to -60.degree.. The reason for thus
expanding the controllable range of the firing angle is as
follows:
[0076] In the conventional cycloconverter CC in which the firing
angle is controlled within a range of 120.degree. to 0.degree., if
the output voltage the cycloconverter CC is controlled to decrease
when a capacitive load is connected to an output terminal thereof
and at the same time a positive potential exists on the load side,
there occurs a discontinuity in the relationship between the firing
angle of each thyristor SCRk.+-. and the output voltage, which can
make it impossible to stabilize the output voltage. That is, to
decrease the output voltage when a positive potential exists on the
load side, it is required to absorb the positive charge on the load
side. In the conventional cycloconverter, however, since the firing
angle a is controlled within the limited range of 120.degree. to
0.degree., it is impossible for the positive converter BC1 to
absorb the positive charge on the load side, and therefore the
negative converter BC2 has to absorb it. When the negative
converter BC2 absorbs the positive charge, since the output voltage
from the negative converter BC2 can change from the negative
full-wave rectified voltage to 0 V as described above, the positive
charge on the load side suddenly drops to 0 V, causing a
discontinuity in the output voltage. If the controllable range of
the firing angle is expanded to 120.degree. to -60.degree., it is
possible to absorb the positive charge by the negative converter
BC2 such that a positive output voltage is achieved, so that no
discontinuity occurs in the output voltage, thereby making it
possible to secure stability of the control.
[0077] However, if the controllable range is thus expanded to the
negative side, as shown in FIG. 8, the output ranges of the
positive and negative converters BC1, BC2 overlap with each other,
so that there exist two intersecting points TO1 and TO2 between the
target wave r and each sawtooth wave, and hence it is impossible to
judge which of the positive and negative converters BC1 and BC2
should be selected for firing the gate of a corresponding one of
the thyristors SCRk.+-. thereof. To solve this problem, in the
present embodiment, one of the positive and negative converters BC1
and BC2 is selected according to results of the comparison by the
comparator 16, as described above.
[0078] Referring again to FIG. 1, the output side of the
synchronizing signal-forming circuit 18 is connected to the
positive gate control device 15a and the negative gate control
device 15b. Connection lines between the synchronizing
signal-forming circuit 18 and the positive and negative gate
control devices 15a, 15b are each formed by six signal lines which
are connected to respective corresponding ones of the six
comparators of each of the gate control devices 15a and 15b for
supplying them with the respective sawtooth waves having extended
sawtooth portions described above with reference to FIG. 7 at
timing shown in FIG. 7.
[0079] The output sides of the six comparators of the positive
control device 15a are connected to the gates of respective
corresponding ones of the thyristors SCRk+ of the positive
converter BC1, while the output sides of the six comparators of the
negative control device 15b are connected to the gates of
respective corresponding ones of the thyristors SCRk- of the
negative converter BC2.
[0080] Although in the present embodiment, the synchronizing
signal-forming circuit 18 is constructed such that it forms
synchronizing signals (reference sawtooth waves) in response to the
three-phase output from the three-phase sub coil 2, this is not
limitative, but a single-phase sub coil may be employed in place of
the three-phase sub coil 2 to form a synchronizing signal in
response to the single-phase output.
[0081] Next, the operation of the power unit constructed as above
will be described.
[0082] As the rotor R is driven for rotation by the engine,
voltages are produced between the three-phase output terminals of
the three-phase main coil 1 as described above. Then, as the gate
of each of the thyristors SCRk.+-. is fired by the firing angle
control device 15, the cycloconverter CC delivers electric current,
and the filter 3 removes harmonic components from the electric
current. The output voltage-detecting circuit 5 detects the voltage
of the electric current. The effective voltage value-calculating
circuit 8 calculates an effective value of the voltage per
repetition period thereof based on the voltage thus detected and
generates a signal indicative of the calculated effective
value.
[0083] The comparator 9 compares the effective value per repetition
period with the reference effective voltage value delivered from
the reference effective voltage value-generating circuit 10, and
the control function-calculating circuit 11 calculates the control
function (linear function) based on results of the comparison to
deliver the calculated function. More specifically, the control
function-calculating circuit 11 calculates the linear function such
that a proportional coefficient (constant of proportionality) of
the linear function is increased as the difference between the
reference effective voltage value from the reference effective
voltage value-generating circuit 10 and the effective value per
repetition period from the effective voltage value-calculating
circuit 8 is larger.
[0084] The comparator 20 delivers the difference between the
reference sinusoidal wave output from the reference sinusoidal
wave-forming circuit 13 and the output voltage detected by the
output voltage-detecting circuit 5 to the amplitude control circuit
12.
[0085] The amplitude control circuit 12 generates a control signal
for controlling the amplitude of the target wave (sinusoidal wave
of 50 Hz or 60 Hz) to be delivered from the target wave-forming
circuit 14, based on the difference delivered from the comparator
20 and the control function calculated by the control
function-calculating circuit 11 as described above, and the target
wave-forming circuit 14 forms the target wave based on the control
signal and delivers the same to the comparator 21.
[0086] When the target wave delivered from the target wave-forming
circuit 14 contains components ascribable to a fluctuation in the
rotational speed of the engine, the comparator 21 removes the
components from the target wave based on the reverse characteristic
signal input to the negative input thereof from the gain-adjusting
circuit 23. Then, the comparator 16 compares the target wave
removed of components ascribable to a fluctuation with the detected
voltage from the output voltage-detecting circuit 5. When the
former is higher in voltage than the latter, a high level (H)
signal is delivered from the comparator 16 to enable the positive
gate control device 15a, whereas when the former is lower in
voltage than the latter, a low level (L) signal is delivered from
the comparator 16 to enable the negative gate control device
15b.
[0087] The comparators of a selected one of the positive gate
control device 15a and the negative gate control device 15b each
compare the target wave from the target wave-forming circuit 14
with a corresponding sawtooth wave from the synchronizing
signal-forming circuit 18, and when the target wave agrees with or
intersects the sawtooth wave, a one-shot pulse having a
predetermined wavelength is delivered from the gate control device
15 to the gate of a corresponding one of the thyristors SCRk.+-. to
control the firing angle thereof.
[0088] FIGS. 9A to 9D are diagrams which are useful in explaining
grounds of the method employed by the present embodiment for
removing fluctuations (pulsation) in the output voltage caused by
fluctuations in the engine rotational speed. FIG. 9A shows an
example of a fluctuation in the engine rotational speed caused by
an explosion stroke of the engine. FIG. 9B shows an example of a
fluctuation in an output voltage obtained from one predetermined
phase of the three-phase main coil 1 appearing in FIG. 1, which is
ascribable to the fluctuation in the engine rotational speed shown
in FIG. 9A, while FIG. 9C shows an example of a fluctuation in a
single-phase output voltage detected by the output
voltage-detecting circuit 5 appearing in FIG. 1, which is
ascribable to the fluctuation in the engine rotational speed shown
in FIG. 9A. FIG. 9D shows an example of a characteristic of the
reverse characteristic signal generated by the gain-adjusting
circuit 23, based on which the target wave is corrected to cope
with the fluctuation in the engine rotational speed shown in FIG.
9A.
[0089] As shown in FIG. 9A, the explosion stroke of the engine
carried out during a time period from a time t1 to a time t2 causes
a change in the rotational speed in a range of .+-.150 rpm with a
sharp gradient of rise with respect to a rated rotational speed of
3600 rpm. This change causes the output current obtained from the
three-phase main coil 1 to pulsate as shown in FIG. 9B, and as a
result, the output voltage also pulsates as shown in FIG. 9C.
[0090] Therefore, by changing the amplitude of the target wave
based on the characteristic of the reverse characteristic signal
shown in FIG. 9D, which is reverse to the characteristic of the
change (fluctuation) in the rotational speed of the engine, it is
possible to remove the pulsation of the output voltage shown in
FIG. 9B and that of the signal-phase output voltage shown in FIG.
9C.
[0091] To this end, the gain-adjusting circuit 23 supplies the
negative input terminal of the comparator 21 with the reverse
characteristic signal formed by adjusting the amplitude of the
output from the rotational fluctuation-detecting circuit 22 such
that the resulting reverse characteristic signal has a
characteristic reverse to a characteristic of a fluctuation in the
engine rotational speed, as described hereinbefore with reference
to FIG. 1.
[0092] As described above, according to the present embodiment,
effective value-based feedback control (feedback control carried
out mainly by the blocks 8 to 11 appearing in FIG. 1) is applied to
control of slow fluctuations in the input voltage or the output
voltage, while waveform-based feedback control (feedback control
basically carried out by the blocks 5, 13 and 20 appearing in FIG.
1) is applied to control of rapid fluctuations in the output
voltage. Further, to control fluctuations in the output voltage due
to fluctuations in the engine rotational speed, the method is
employed in which the amplitude of the target wave is changed based
on the reverse characteristic signal having a characteristic
reverse to that of the fluctuation in the engine rotational speed.
Therefore, it is possible to carry out more stable feedback control
by reducing the feedback gain, and at the same time sufficiently
cope with fluctuations in the output voltage occurring in
accordance with fluctuations in the input voltage ascribable to
fluctuations in the engine rotational speed.
[0093] Further, the output frequency of the three-phase generator
can be controlled to a predetermined frequency by the
cycloconverter CC irrespective of the output frequency of the
three-phase generator, that is, the output frequency of the power
unit does not depend upon the rotational speed of the drive source,
such as an engine, similarly to the inverter generator according to
the prior art described above. Therefore, it is possible to obtain
a high output from the generator driven by the drive source at a
fairly high rotational speed, whereby the generator can be reduced
in size and weight.
[0094] Furthermore, according to the present embodiment, it is
possible to directly convert a high-frequency output of the AC
generator to an AC output having a predetermined lower frequency,
such as a single-phase commercial frequency. Therefore, the number
of power circuit component parts can be largely reduced, which
largely contributes to large reduction of the manufacturing
cost.
[0095] Moreover, when a magneto generator having multiple poles is
used as the generator, the voltage delivered to the cycloconverter
varies largely depending on whether the power unit is under a
no-loaded condition or under a loaded condition. However, the
present power unit is capable of effectively controlling
fluctuations in the output voltage. Moreover, the use of the
magneto generator simplifies the formation of synchronizing
signals.
[0096] Still further, the rotor R of the generator can be also used
as the fly wheel of the engine. This permits the whole power unit
to be designed further compact in size.
[0097] Although in the above embodiment, an effective voltage value
per repetition period is calculated so as to enable the power unit
to cope with fluctuations in the output voltage as quickly as
possible, this is not limitative, but if top priority is given to
further stable control rather than quick responsivity, an effective
voltage value may be calculated over a plurality of repetition
periods.
* * * * *