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Claims  |
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I claim:
1. An inverter comprising:
a switching circuit including a plurality of switching elements
sequentially energizing windings of a plurality of phases of an electric
motor;
a pulse width modulation circuit for obtaining a pulse width modulated
signal;
electrical quantity detecting means for detecting a quantity of electricity
supplied to the switching circuit, thereby generating an electrical
quantity signal;
position detecting means for detecting a rotational position of a rotor of
the motor, thereby providing information about the rotational position of
the rotor;
energization signal generating means for generating an energization signal
at a time according to a commutation timing necessary for the switching
circuit on the basis of the information about the rotational position of
the rotor, the energization signal generating means comprising means for
obtaining a reference commutation timing from the information about the
rotational position of the rotor, electrical quantity comparing means for
sequentially sampling the electrical quantity signals generated by the
electrical quantity detecting means to thereby obtain a mean electrical
quantity value and comparing the mean electrical quantity value with a
last mean electrical quantity value, and compensation means for
compensating the commutation timing so that the commutation timing
corresponds to a point of time preceding the reference commutation timing
by a predetermined compensation period of time, the compensation means
changing the predetermined compensation time period in accordance with the
result of comparison by the electrical quantity comparing means; and
drive means synthesizing the energization signal and the pulse width
modulated signal for driving each switching element.
2. An inverter comprising:
a switching circuit including a plurality of switching elements
sequentially energizing windings of a plurality of phases of an electric
motor;
a pulse width modulation circuit for obtaining a pulse width modulated
signal;
position detecting means for detecting a rotational position of a rotor of
the motor, thereby providing information about the rotational position of
the rotor;
speed detecting means for detecting a rotational speed of the rotor of the
motor;
means for determining a duty ratio of the pulse width modulated signal on
the basis of a result of comparison of the detected rotational speed of
the rotor with an externally supplied speed command;
energization signal generating means for generating an energization signal
at a time according to a commutation timing necessary for the switching
circuit on the basis of the information about the rotational position of
the rotor, the energization signal generating means comprising means for
obtaining a reference commutation timing from the information about the
rotational position of the rotor and compensation means for compensating
the commutation timing so that the commutation timing corresponds to a
point of time preceding the reference commutation timing by a
predetermined compensation period of time, the compensation means changing
the predetermined compensation time period on the basis of the detected
rotational speed of the rotor and the determined duty ratio of the pulse
width modulated signal; and
drive means synthesizing the energization signal and the pulse width
modulated signal for driving each switching element.
3. An inverter according to claim 2, which further comprises a memory for
storing tabular data of the compensation time periods in connection with
the rotational speed of the rotor and the duty ratio of the pulse width
modulated signal and wherein the energization signal generating means
selects the compensation time period corresponding both to the rotational
speed of the rotor and to the duty ratio of the pulse width modulated
signal from the tabular data to thereby determine the compensation time
period.
4. An inverter according to claim 1 or 2, wherein the position detecting
means supplies the energization signal generating means with, as the
information about the rotational position of the rotor, a time when a
terminal voltage of the motor winding intersects a reference voltage set
at a half of a direct current power supply voltage supplied to the
switching circuit and the energization signal generating means comprises a
first timer timing a period between the time when the winding terminal
voltage intersects the reference voltage and a subsequent time of
intersection, operational means for operating a period of time between the
time when the winding terminal voltage intersects the reference voltage
and a reference commutation timing on the basis of the time period
obtained by the first timer, thereby subtracting a compensation time
period from the obtained time period to obtain a post-compensation time
period, and a second timer initiating a timing operation at the time when
the winding terminal voltage intersects the reference voltage, the
commutation timing being determined to be a time when the second timer has
completed a timing operation for the post-commutation time period.
5. An inverter comprising:
a) a switching circuit including a plurality of switching elements
sequentially energizing windings of a plurality of phases of an electric
motor, the switching elements having respective diodes connected in
parallel thereto;
b) a pulse width modulation circuit for obtaining a pulse width modulated
signal;
c) position detecting means for detecting a rotational position of a rotor
of the motor, thereby providing information about the rotational position
of the rotor;
(d) commutation time period detecting means detecting an energization time
period of each diode due to release of energy stored in each motor winding
while the corresponding switching element is being commutated, the
detected energization time period of each diode serving as a commutation
time period of the corresponding switching element;
(e) energization signal generating means determining a commutation timing
on the basis of the information about the rotational position of the rotor
and the commutation time period of each switching element, thereby
generating an energization signal corresponding to the determined
commutation timing; and
(f) drive means synthesizing the energization signal and the pulse width
modulated signal for driving each switching element.
6. An inverter according to claim 5, wherein the commutation time period
detecting means compares the terminal voltage of each motor winding with a
reference voltage to detect the energization of each diode.
7. An inverter comprising:
a) a switching circuit including a plurality of switching elements
sequentially energizing windings of a plurality of phases of an electric
motor, the switching elements having respective diodes connected in
parallel thereto;
b) a pulse width modulation circuit for obtaining a pulse width modulated
signal;
c) position detecting means for detecting a rotational position of a rotor
of the motor, thereby providing information about the rotational position
of the rotor;
d) commutation time period detecting means comparing a terminal voltage of
each motor winding with a reference voltage for detecting an energization
time period of each diode due to release of energy stored in each motor
winding while the corresponding switching element is being commutated, the
commutation time period detecting means compensating the detected
energization time period of each diode on the basis of a period and duty
ratio of the pulse width modulated signal, thereby determining a
commutation time period of each switching element;
e) energization signal generating means determining a commutation timing on
the basis of the information about the rotational position of the rotor
and the commutation time period of each switching element, thereby
generating an energization signal corresponding to the determined
commutation timing; and
f) drive means synthesizing the energization signal and the pulse width
modulated signal for driving each switching element.
8. An inverter comprising:
a) a switching circuit including a plurality of positive side switching
elements each having a diode connected in parallel thereto between a
positive direct current power supply line and each winding terminal of an
electric motor and a plurality of negative side switching elements each
having a diode connected in parallel thereto between a negative direct
current power supply line and each motor winding terminal so that windings
of a plurality of phases of the motor are sequentially energized;
b) a pulse width modulation circuit for obtaining a pulse width modulated
signal;
c) position detecting means for detecting a rotational position of a rotor
of the motor, thereby providing information about the rotational position
of the rotor;
d) commutation time period detecting means comparing a terminal voltage of
each motor winding with a reference voltage to detect an energized state
of each diode due to discharge of energy stored in each motor winding
while the corresponding switching element is being commutated, thereby
determining a commutation time period of each switching element on the
basis of a time period of operation of detecting the energized state of
each diode;
e) energization signal generating means determining a commutation timing on
the basis of the information about the rotational position of the rotor
and the commutation time period of each switching element, thereby
generating an energization signal corresponding to the determined
commutation timing;
f) selection signal generating means for generating a selection signal so
that either the positive or the negative side switching elements are
selected to be thereby controlled to be turned on and off by the pulse
width modulated signal, the selection signal being changed for every
commutation timing; and
g) drive means for driving each switching element on the basis of the
energization signal, the pulse width modulated signal and the selection
signal so that the negative side switching elements are controlled in
accordance with on and off states of the pulse width modulated signal when
commutation is caused to occur between two of the positive side switching
elements and so that the positive side switching elements are controlled
in accordance with the on and off states of the pulse width modulated
signal when commutation is caused to occur between two of the negative
side switching elements.
9. An inverter according to claim 6, 7 or 8, wherein the reference voltage
with which the terminal voltage of each motor winding is compared for the
detection of the energized state of each diode is set at a half of a
direct current power supply voltage of the switching circuit.
10. An inverter according to claim 5, 7 or 8, wherein the energization
signal generating means obtains a reference commutation timing from the
information about the rotational position of the rotor and determines the
commutation timing so that the commutation timing corresponds to a time a
compensation period of time before the reference commutation timing, the
compensation period of time being set at a half of the commutation time
period.
11. An inverter according to claim 5, 7 or 8, wherein the position
detecting means compares a or the terminal voltage of each motor winding
with a or the reference voltage to obtain the information about the
rotational position of the rotor.
12. An inverter according to claim 5, 7 or 8, wherein the position
detecting means compares a or the terminal voltage of each motor winding
with a or the reference voltage to obtain the information about the
rotational position of the rotor and the reference voltage is set at a
half of a direct current power supply voltage of the switching circuit.
13. An inverter according to claim 5, 7 or 8, wherein the position
detecting means supplies the energization signal generating means with, as
the information about the rotational position of the rotor, a time when a
terminal voltage of the motor winding intersects a reference voltage set
at a half of a direct current power supply voltage supplied to the
switching circuit and the energization signal generating means comprises a
first timer timing a period between the time when the winding terminal
voltage intersects the reference voltage and a subsequent time of
intersection, operational means for operating a period of time between the
time when the winding terminal voltage intersects the reference voltage
and a reference commutation timing on the basis of the time period
obtained by the first timer, thereby subtracting a compensation time
period from the obtained time period to obtain a post-compensation time
period, the compensation time period corresponding to a half of the
commutation time period, and a second timer initiating a timing operation
at the time when the winding terminal voltage intersects the reference
voltage, the commutation timing being determined to be a time when the
second timer has completed a timing operation for the post-commutation
time period. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an inverter including a switching circuit for
sequentially energizing a plurality of windings of an electric motor such
as a brushless motor at a time according to a commutation timing
corresponding to a predetermined rotational position of a rotor of the
motor and further relates to an air conditioner controlled by such an
inverter as mentioned above.
2. Description of the Prior Art
Air conditioners and refrigerators have recently employed, as a compressor
motor, a brushless motor classified into a DC motor and an inverter
driving the brushless motor for the purpose of variable performance of a
compressor or saving electric power consumption. The brushless motor
usually necessitates one or more position sensors sensing a rotational
position of a rotor of the motor and generating a rotational position
signal so that a phase winding to be energized is determined. Since the
compressor motor is exposed to the refrigerant in the air conditioners and
the refrigerators, it is sometimes difficult to dispose the position
sensors in the brushless motor employed in the compressor. In view of this
drawback, the present inventor and others have developed a technique for
detecting a voltage induced in the motor winding and for electrically
processing the detected voltage into the rotational position signal. This
technique was applied for patent in Japan and the application was
published under Japanese unexamined patent application publication No.
64-8890 (1989).
The above-mentioned technique will be described as the prior art for the
present invention with reference to FIGS. 22 to 24. In the following
description, the pulse width modulation system is applied to the
technique. Referring to FIG. 22 showing the electrical circuit of an
inverter, a DC power supply circuit 2 connected to an AC power supply 1
comprises a full-wave rectifier circuit 3, a reactor 4a and a smoothing
capacitor 4b. A three-phase bridge circuit 13 serving as a switching
circuit is connected between a positive side DC power supply line 5 and a
negative side DC power supply line 6 of the DC power supply circuit 2. The
three-phase bridge circuit 13 comprises switching elements such as
switching transistors 7 to 12. Output terminals 14u, 14v and 14w of the
three-phase bridge circuit 13 are connected to terminals of windings 15u,
15v and 15w of a brushless motor 15 respectively. Three transistors 7, 9
and 11 are connected between the positive side DC power supply line 5 and
the respective output terminals 14u, 14v and 14 w to thereby serve as
positive side switching elements. The other three transistors 8, 10 and 12
are connected between the negative side DC power supply line 6 and the
respective output terminals 14u, 14v and 14w to thereby serve as negative
side switching elements. When these transistors 7-12 are controlled to be
turned on and off in a predetermined order, the windings 15u, 15v and 15w
of the brushless motor 15 are repeatedly energized sequentially with a
phase difference of 120 degrees (electrical angle), so that the brushless
motor is driven. In this case, each transistor is turned on in the period
of 120 degrees and off in the period of 240 degrees and furthermore, the
duty ratio is controlled in each "on" period by a pulse width modulated
(PWM) signal P.sub.1 as shown in FIG. 23(a). Consequently, the terminal
voltages V.sub.u, V.sub.v and V.sub.w of the windings 15u, 15v and 15w of
the brushless motor 15 have waveforms as shown in FIGS. 23(b), 23(c) and
23(d) respectively.
FIGS. 24(a) and 24(b) show waveforms of the terminal voltage V.sub.u and
the winding current I.sub.u of the winding 15u of the brushless motor 15
respectively without the pulse width modulation applied. In the waveform
of the terminal voltage V.sub.u, a positive or negative slope section
t.sub.a in the electrical angle of 60 degrees represents a voltage induced
in the winding 15u and elongated positive and negative pulses represent
pulse voltages due to diodes D1 to D6 connected in parallel to the
respective transistors 7-12 of the three-phase bridge circuit 13.
Reference symbol V.sub.0 represents a reference voltage provided by a
resistance type potential divider circuit 16 connected between the DC
power supply lines 5, 6. The reference voltage V.sub.0 is set at a half of
a voltage in the DC power supply circuit 2 of the three-phase bridge
circuit 13. As understood from FIGS. 24(a) and 24(b), a commutation timing
lags by about 30 degrees with respect to a time when the induced voltage
and the reference voltage V.sub.0 cross, which time will be referred to as
"zero crossing time."
The terminal voltages Vu, V.sub.v and V.sub.w are compared with the
reference voltage V.sub.0 by respective comparators 18 to 20 provided in a
position signal circuit 17 serving as the position sensing means, thereby
being converted to fundamental wave signals V.sub.u ', V.sub.v ' and
V.sub.w ' for discrimination of 180-degree sections of the terminal
voltages V.sub.u, V.sub.v and V.sub.w as shown in FIGS. 23(e), 23(f) and
23(g) respectively. These fundamental wave signals V.sub.u ', V.sub.v '
and V.sub.w ' serve as information about the rotational position of the
rotor of the brushless motor 15. The fundamental wave signals V.sub.u ',
V.sub.v ' and V.sub.w ' are then supplied to a waveform synthesizing
circuit 21 serving as energization signal generating means. The
fundamental wave signals V.sub.u ', V.sub.v ' and V.sub.w ' are collated
with the PWM signal P.sub.1 by the waveform synthesizing circuit 21 to be
converted to recognitive waveform signals U.sub.a, V.sub.a and W.sub.a
each comprising continuous square waves composed only of a positive pulse
component and having a period of 180 degrees in electrical angle. The
recognitive waveform signals U.sub.a, V.sub.a and W.sub.a are out of phase
with one another by 120 degrees. A rise point and a fall point of each
recognitive waveform signal correspond to the abovementioned zero crossing
point.
The waveform synthesizing circuit 21 is provided with first and second
timing functions. Six first phase segment patterns X1 to X6 are formed
from the three recognitive waveform signals U.sub.a, V.sub.a, W.sub.a by
the first timing function. Each of the first phase segment patterns X1-X6
has a period of 60 degrees in electrical angle. Six second phase segment
patterns Y1 to Y6 are formed by the second timing function. The second
phase segment patterns have start points same as those of the first phase
segment patterns X1-X6 respectively and each second phase segment pattern
has a period of 30 degrees in electrical angle. The waveform synthesizing
circuit 21 finally converts signals of the second phase segment patterns
to energization signals U.sub.p, U.sub.n, V.sub.p, V.sub.n, W.sub.p and
W.sub.n as shown in FIGS. 23(l) to 23(q) respectively.
The start points of the energization signals correspond to the end points
of second phase segment patterns Y1-Y6 and accordingly, lag behind the
zero crossing point by 30 degrees. Consequently, the phase patterns of the
energization signals correspond to the commutation timing patterns
required of the transistors 7-12 of the three-phase switching circuit 13.
On the other hand, a speed determination circuit 22 serving as speed
detecting means determines a speed deviation on the basis of a speed
command signal S.sub.c and the energization signal W.sub.n supplied
thereto from the waveform synthesizing circuit 21 as a speed detection
signal representative of the rotational speed of the brushless motor 15.
The speed determination circuit 22 generates a speed deviation signal
S.sub.d in accordance with the determined speed deviation, which signal is
supplied to a pulse width modulation circuit 23. The pulse width
modulation circuit 23 controls the duty ratio of the PWM signal P.sub.1 in
accordance with the magnitude of the speed deviation signal S.sub.d. The
PWM signal P.sub.1 whose duty ratio has been controlled as described above
is supplied to gates 25, 27 and 29 of a gate circuit 24 composing drive
means. The PWM signal P.sub.1 and the energization signals U.sub.p,
V.sub.p and W.sub.p are synthesized by the gates 25, 27, 29 or more
specifically, the PWM signal is ANDed with the respective energization
signals by the gates, for example, and resultant signals are supplied as
base control signals to the bases of the positive side transistors 7, 9
and 11 of the three-phase bridge circuit 13 such that the transistors are
on-off controlled in accordance with "on" and "off" modes of the PWM
signal P.sub.1. On the other hand, the energization signals U.sub.n,
V.sub.n and W.sub.n to which signals the pulse width modulation is not
applied are supplied to the bases of the negative side transistors 8, 10
and 12 via gates 26, 28 and 30 respectively so that the transistors 8, 10
and 12 are on-off controlled. Consequently, the transistors 7-12 are
on-off controlled by the energization signals U.sub.p, V.sub.p, W.sub.p,
U.sub.n, V.sub.n and W.sub.n in the patterns as shown in FIGS.
23(l)-23(q), thereby driving the brushless motor 15. Furthermore, the
speed of the brushless motor 15 is controlled by the control of the duty
ratio by the PWM signal P.sub.1 as shown in FIG. 23(a).
The above-mentioned "on" mode of the PWM signal P.sub.1 refers either to
the high or low level of the pulse signal at which level the transistors
are turned on. The transistors are turned on when the pulse signal is at
the high level in FIGS. 23(a)-23(q). The "off" mode of the PWM signal
P.sub.1 refers either to the high or low level of the pulse signal at
which level the transistors are turned off. The transistors are turned off
when the pulse signal is at the low level in FIGS. 23(a)-23(q).
As obvious from the foregoing, the windings 15u, 15v and 15w of each phase
are energized for the period of 120 degrees with the lag of 30 degrees
with respect to the zero crossing time. FIGS. 25(a), 25(b) and 25(c) show
the relations among the induced voltage, applied voltage and current
without the PWM control in the case of the winding 15u of phase U, for
example. The voltage of the DC power supply circuit 2 applied to the
winding 15u has a symmetrical waveform about the peak P.sub.t of the
induced voltage in the period of 120 degrees. On the other hand, the
current I.sub.u flowing into the winding 15u is gradually increased
slopewise upon application of the voltage and reaches the normal state
with the lag of time period T.sub.1 relative to the applied voltage. The
current I.sub.u is gradually decreased slopewise upon completion of
application of the voltage, reaching zero with the lag of time period
T.sub.2 equal to the time period T.sub.1. Accordingly, the current I.sub.u
flowing into the winding 15u takes a waveform unsymmetrical about the peak
T.sub.p of the induced voltage, resulting in a phase difference with
respect to the induced voltage. This phase difference also occurs when the
PWM control is applied.
A torque produced by an electric motor is generally shown by the product of
the induced voltage and the current. Since the current lags the induced
voltage in the prior art as described above, the efficiency of the motor
is reduced. In the air conditioners, particularly, a quick cooling or
warming operation is required at its maximum output under the limited
power supply capacity. Thus, an improvement in the motor efficiency in the
air conditioners or the like has been desired for the energy saving and
reduction of the running cost.
In view of the foregoing, it has been proposed that the commutation timing
be determined to take a time earlier by a predetermined electrical angle
than the time lagging behind 30 degrees the time when the induced voltage
and the reference voltage V.sub.0 cross, as shown in FIG. 25(d). The
predetermined electrical angle corresponds to a time period T.sub.d in
FIG. 25(d). In this case, the waveform of the current I.sub.u is
symmetrical about the peak T.sub.p of the induced voltage. Accordingly,
since a power factor is improved, the current I.sub.u can be reduced and
the motor efficiency can be improved.
However, the above-mentioned lag time periods T.sub.1 and T.sub.2 are not
fixed but are varied. For example, the lag time periods become long as the
load torque and that is, the current are larger while they are short as
the motor speed and that is, the induced voltage are higher. Consequently,
a sufficient improvement in the motor efficiency cannot be achieved even
when the commutation timing is determined to take a time earlier by the
predetermined electrical angle than the time lagging behind 30 degrees the
time when the induced voltage and the reference voltage V.sub.0 cross.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an inverter
wherein even when the lag time of the winding current starting from
initiation or completion of application of voltage to each motor winding
is varied by the load torque and the rotational speed of the motor, the
current can be caused to flow in each motor winding so as to be in phase
with the induced voltage, whereby the efficiency of the motor can be
improved.
Another object of the invention is to provide an air conditioner wherein a
compressor motor is controlled by the above-described inverter so that the
inverter can be operated efficiently.
In one aspect, the present invention provides an inverter comprising a
switching circuit including a plurality of switching elements sequentially
energizing windings of a plurality of phases of an electric motor, a pulse
width modulation circuit for obtaining a pulse width modulated signal,
electrical quantity detecting means for detecting a quantity of
electricity supplied to the switching circuit, thereby generating an
electrical quantity signal, position detecting means for detecting a
rotational position of a rotor of the motor, thereby providing information
about the rotational position of the rotor, energization signal generating
means for generating an energization signal at a time according to a
commutation timing necessary for the switching circuit on the basis of the
information about the rotational position of the rotor, and drive means
synthesizing the energization signal and the pulse width modulated signal
for driving each switching element. The energization signal generating
means comprises means for obtaining a reference commutation timing from
the information about the rotational position of the rotor, electrical
quantity comparing means for sequentially sampling the electrical quantity
signals generated by the electrical quantity detecting means to thereby
obtain a mean electrical quantity value and comparing the mean electrical
quantity value with a last mean electrical quantity value, and
compensation means for compensating the commutation timing so that the
commutation timing corresponds to a point of time preceding the reference
commutation timing by a predetermined compensation period of timed the
compensation means changing the predetermined compensation time period in
accordance with the result of comparison by the electrical quantity
comparing means.
A quantity of electricity supplied to the switching circuit during rotation
of the rotor, for example, the quantity of current is detected by the
electrical quantity detecting means. The information about the rotational
position of the rotor is obtained by the position detecting means. The
reference commutation timing is obtained from the information about the
rotational position of the rotor. The electrical quantity signals are
sampled, and the mean electrical quantity value is obtained. The mean
value is compared with the last mean electrical quantity value. The
commutation timing is compensated so as to correspond to the point of time
preceding the obtained reference commutation timing by the predetermined
compensation period of time. The predetermined compensation time period is
changed so as to take a value in accordance with the result of comparison
of the mean electrical quantity value with the last mean value. The
energization signal is generated at the time according to the compensated
commutation timing. The drive means synthesizes the energization signal
and the pulse width modulated signal and a resultant signal is supplied to
each switching element of the switching circuit, thereby driving it. Thus,
the plurality of motor windings are sequentially energized so that the
rotor is rotated. Since the quantity of electricity supplied to the
switching circuit is one of factors determining the commutation timing, it
can be determined such that the winding current is in phase with the
induced voltage of the winding even when the current is rendered small or
even when the motor speed or the load torque are changed.
In another aspect, the invention provides an inverter comprising a
switching circuit including a plurality of switching elements sequentially
energizing windings of a plurality of phases of an electric motor, a pulse
width modulation circuit for obtaining a pulse width modulated signal,
position detecting means for detecting a rotational position of a rotor of
the motor, thereby providing information about the rotational position of
the rotor, speed detecting means for detecting a rotational speed of the
rotor of the motor, means for determining a duty ratio of the pulse width
modulated signal on the basis of a result of comparison of the detected
rotational speed of the rotor with an externally supplied speed command,
energization signal generating means for generating an energization signal
at a time according to a commutation timing necessary for the switching
circuit on the basis of the information about the rotational position of
the rotor, and drive means synthesizing the energization signal and the
pulse width modulated signal for driving each switching element. The
energization signal generating means comprises means for obtaining a
reference commutation timing from the information about the rotational
position of the rotor and compensation means for compensating the
commutation timing so that the commutation timing corresponds to a point
of time preceding the reference commutation timing by a predetermined
compensation period of time, the compensation means changing the
predetermined compensation time period on the basis of the detected
rotational speed of the rotor and the determined duty ratio of the pulse
width modulated signal.
The information about the rotational position of the rotor being rotated is
obtained by the position detecting means. The rotational speed of the
rotor is detected by the speed detecting means. The pulse width modulation
circuit compares the detected rotational speed of the rotor with the speed
command, thereby obtaining the pulse width modulated signal whose duty
ratio is in accordance with the result of the comparison. The energization
signal generating means obtains the reference commutation timing from the
information about the rotational position of the rotor. The energization
signal generating means compensates the commutation timing so that the
commutation timing corresponds to the point of time preceding the obtained
reference commutation timing by the predetermined compensation period of
time. The energization signal generating means changes the predetermined
compensation time period on the basis of the detected rotational speed of
the rotor and the determined duty ratio of the pulse width modulated
signal. The energization signal is generated at the time according to the
compensated commutation timing. The drive means synthesizes the
energization signal and the pulse width modulated signal for driving the
switching elements. Consequently, the windings of the plurality of phases
of the motor are sequentially energized so that the rotor is rotated at a
speed in accordance with the speed command. The factors determining the
commutation timing include the rotational speed of the rotor and the duty
ratio of the pulse width modulated signal, which duty ratio has a
correlation with the load torque. Consequently, the commutation timing can
be so determined that the voltage induced in each winding is in phase with
each winging current even in variation of the rotational speed or load
torque.
In further another aspect, the invention provides an inverter comprising a
switching circuit including a plurality of switching elements sequentially
energizing windings of a plurality of phases of an electric motor, the
switching elements having respective diodes connected in parallel thereto,
a pulse width modulation circuit for obtaining a pulse width modulated
signal, commutation time period detecting means detecting an energization
time period of each diode due to release of energy stored in each motor
winding while the corresponding switching element is being commutated, the
detected energization time period of each diode serving as a commutation
time period of the corresponding switching element, energization signal
generating means determining a commutation timing on the basis of the
information about the rotational position of the rotor and the commutation
time period of each switching element, thereby generating an energization
signal corresponding to the determined commutation timing, and drive means
synthesizing the energization signal and the pulse width modulated signal
for driving each switching element.
The information about the rotational position of the rotor being rotated is
obtained by the position detecting means. The commutation time period of
each switching element is detected by the commutation time period
detecting means. The energization signal generating means determines the
commutation timing on the basis of the information about the rotational
position of the rotor and the commutation time period of each switching
element, thereby generating an energization signal corresponding to the
determined commutation timing. The drive means synthesizes the
energization signal and the pulse width modulated signal for driving the
switching element. Consequently, the windings of the plurality of phases
are sequentially energized so that the rotor is rotated.
The current flowing into the winding does not rise from zero to the normal
state upon start of the voltage application and does not fall from the
normal state to zero upon termination of the voltage application such that
a time lag occurs. The reason for this is that the storage and release of
the magnetic energy are not instantaneously performed under the influence
of a counter electromotive force. Accordingly, the lag time period of the
current at the time of start of the voltage application is approximately
equal to that of the current at the time of termination of the voltage
application. Furthermore, the commutation time period of each switching
element is approximately equal to a time period necessary for release of
the energy stored in the winding at the time of termination of voltage
application and that is, the lag time period in the case where the winding
current falls from the normal state to zero. The current flowing in this
case reflows through the diode connected in parallel to the switching
element. Thus, the commutation time period of the switching element can be
determined by detecting the energization time period of the diode.
Accordingly, the energization signal corresponding to the commutation
timing is formed on the basis of not only the information about the
rotational position of the rotor but also the commutation time period.
Consequently, the winding current can be in the phase with the induced
voltage.
In further another aspect, the invention provides an inverter comprising a
switching circuit including a plurality of switching elements sequentially
energizing windings of a plurality of phases of an electric motor, the
switching elements having respective diodes connected in parallel thereto,
a pulse width modulation circuit for obtaining a pulse width modulated
signal, position detecting means for detecting a rotational position of a
rotor of the motor, thereby providing information about the rotational
position of the rotor, commutation time period detecting means comparing a
terminal voltage of each motor winding with a reference voltage for
detecting an energization time period of each diode due to release of
energy stored in each motor winding while the corresponding switching
element is being commutated, the commutation time period detecting means
compensating the detected energization time period of each diode on the
basis of a period and duty ratio of the pulse width modulated signal,
thereby determining a commutation time period of each switching element,
energization signal generating means determining a commutation timing on
the basis of the information about the rotational position of the rotor
and the commutation time period of each switching element, thereby
generating an energization signal corresponding to the determined
commutation timing, and drive means synthesizing the energization signal
and the pulse width modulated signal for driving each switching element.
The above-described arrangement relates to a drawback resulting from the
comparison of the terminal voltage of the winding with the reference
voltage for the detection of the energization time period of the diode. In
this case, the variation of the winding terminal voltage does not appear
in synchronism with the termination of energization to the diode when the
switching elements are on-off controlled in accordance with the "on" and
"off" modes of the PWM signal. Consequently, the energization time period
of the diode cannot be accurately detected by the comparison of the
winding terminal voltage with the reference voltage. In the
above-described arrangement, however, the energization time period of the
diode obtained by the comparison of the winding terminal voltage with the
reference voltage is compensated by the period and duty ratio of the PWM
signal. The compensated energization time period of the diode can be
rendered approximately equal to the actual energization time period.
In further another aspect, the invention provides an inverter comprising a
switching circuit including a plurality of positive side switching
elements each having a diode connected in parallel thereto between a
positive direct current power supply line and each winding terminal of an
electric motor and a plurality of negative side switching elements each
having a diode connected in parallel thereto between a negative direct
current power supply line and each motor winding terminal so that windings
of a plurality of phases of the motor are sequentially energized, a pulse
width modulation circuit for obtaining a pulse width modulated signal,
position detecting means for detecting a rotational position of a rotor of
the motor, thereby providing information about the rotational position of
the rotor, commutation time period detecting means comparing a terminal
voltage of each motor winding with a reference voltage to detect an
energized state of each diode due to discharge of energy stored in each
motor winding while the corresponding switching element is being
commutated, thereby determining a commutation time period of each
switching element on the basis of a time period of operation of detecting
the energized state of each diode, energization signal generating means
determining a commutation timing on the basis of the information about the
rotational position of the rotor and the commutation time period of each
switching element, thereby generating an energization signal corresponding
to the determined commutation timing, selection signal generating means
for generating a selection signal so that either the positive or the
negative side switching elements are selected to be thereby controlled to
be turned on and off by the pulse width modulated signal, the selection
signal being changed for every commutation timing, and drive means for
driving each switching element on the basis of the energization signal,
the pulse width modulated signal and the selection signal so that the
negative side switching elements are controlled in accordance with on and
off states of the pulse width modulated signal when commutation is caused
among the positive side switching elements and so that the positive side
switching elements are controlled in accordance with the on and off states
of the pulse width modulated signal when commutation is caused to occur
between two of the negative side switching elements.
The switching elements to be on-off controlled by the PWM signal are
switched between the positive side and negative side switching elements.
Accordingly, even when the energization time period of the diode is
detected by the comparison of the winding terminal voltage with the
reference voltage, the variation of the winding terminal voltage appears
in synchronism with termination of energization to the diode, which can
provide accurate detection of the energization time period of the diode.
The energization signal generating means may obtain a reference commutation
timing from the information about the rotational position of the rotor and
determines the commutation timing so that the commutation timing
corresponds to a time a compensation period of time before the reference
commutation timing, the compensation period of time being set at a half of
the commutation time period.
The determination of the commutation timing on the basis of the reference
commutation timing may be determined by a software arrangement. More
specifically, the position detecting means may supply the energization
signal generating means with, as the information about the rotational
position of the rotor, a time when a terminal voltage of the motor winding
intersects a reference voltage set at a half of a direct current power
supply voltage supplied to the switching circuit. The energization signal
generating means may comprise a first timer timing a period between the
time when the winding terminal voltage intersects the reference voltage
and a subsequent time of intersection, operational means for operating a
period of time between the time when the winding terminal voltage
intersects the reference voltage and a reference commutation timing on the
basis of the time period obtained by the first timer, thereby subtracting
a compensation time period from the obtained time period to obtain a
post-compensation time period, the compensation time period corresponding
to a half of the commutation time period, and a second timer initiating a
timing operation at the time when the winding terminal voltage intersects
the reference voltage. The commutation timing is determined to be a time
when the second timer has completed a timing operation for the
post-commutation time period.
Other objects of the present invention will become obvious upon
understanding of the illustrative embodiments about to be described.
Various advantages not referred to herein will occur to those skilled in
the art upon employment of the invention in practice.
BRIEF DESCRIPTION OF THE DRAWINGS
Several embodiments of the present invention will be described with
reference to the accompanying drawings, in which:
FIG. 1 is an electrical circuit diagram showing the inverter of a first
embodiment in accordance with the present invention;
FIGS. 2(a) to 2(q) are waveform charts showing waveforms of various signals
and voltages shown in FIG. 1;
FIG. 3 is a flowchart showing an operation of a microcomputer incorporated
in the inverter;
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