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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a PWM (Pulse Width Modulation) inverter system
for producing a desired multi-phase ac output from a dc input by on-off
controlling a switching element such as, for example, a transistor, and
more particularly to production of a switching signal, that is, a PWM
signal for causing an on-off operation.
2. Prior Art
Conventionally, a typical method of producing an PWM signal involves
comparison of a sinusoidal wave signal which is each phase voltage
instruction with a triangular wave signal which is a modulating wave. The
principle of the method is described, for example, taking a single phase
inverter of a full bridge as an example in "Inverter Application Manual"
p.p. 28 to 34, published on Sept. 7, 1985 (hereinafter referred to as
prior art 1). Of late, a method is already known wherein a PWM signal of a
multi-phase inverter is treated as composition of space voltage vectors.
According to the method, on-off times of individual switching elements are
controlled such that a time average vector for a unit time may coincide
with an instruction value of a desired instantaneous voltage vector
(Japanese patent application No. 59-251001, which will be hereinafter
referred to as prior art 2).
FIG. 1 shows a circuit construction of a single phase inverter of the prior
art 1. In FIG. 1, reference symbols S.sub.1 to S.sub.4 denote each a
switching element such as a transistor, and FIG. 2 illustrates a process
of producing switching signals (on-off signals) to be coupled to the
switching elements. Referring to FIG. 2, (a) shows a relationship between
a sinusoidal wave signal and a triangular wave which is a modulating wave,
and a sinusoidal wave signal when the magnitude thereof is great, that is,
upon outputting of a high voltage, is indicated by a solid line while a
sinusoidal wave upon outputting of a low voltage is indicated by a chain
line. Meanwhile, (b) of FIG. 2 shows on-off signals of the individual
switching elements upon outputting of the high voltage while (c) of FIG. 2
shows on-off signals upon outputting of the low voltage.
Now, the prior art 2 will be described with reference to FIGS. 3 to 8.
Referring to FIG. 3, a main circuit 1 of a 3-phase inverter of the PWM
control method (hereinafter referred to simply as inverter) includes
switching elements S.sub.aP, S.sub.bP, S.sub.cP, S.sub.aN, S.sub.bN and
S.sub.cN such as transistors connected in a bridge circuit. The inverter 1
is connected to primary windings 2 of a 3-phase induction motor serving as
a load to the inverter 1, and also to a firing signal generating circuit 3
for producing a firing signal for turning the switching elements on and
off.
The voltage vectors V which are produced when a voltage to be applied to
the 3-phase load shown in FIG. 3 is supplied from the inverter 1 only
include such discrete voltage vectors V.sub.0 to V.sub.7 corresponding to
on-off states of the switching elements of the inverter 1 as seen in FIG.
4. It can be considered that the PWM inverter produces equivalent
continuous voltage vectors by switching such discrete voltage vectors at a
high speed. Symbols (0, 0, 0), (1, 0, 0), . . . , and (1, 1, 1) in FIG. 4
represent on-off states of switching element pairs S.sub.a (S.sub.aPLL ,
S.sub.aN), S.sub.b (S.sub.bP, S.sub.bN) and S.sub.c (S.sub.cP, S.sub.cN),
wherein symbol 1 represents that the switching element indicated by the
suffix P is on and the switching elements indicated by the suffix N is off
while symbol 0 represents an individually reverse state. However, (1, 1,
1) and (0, 0, 0) both represent a short-circuited condition wherein the
load terminals are short-circuited by the switching elements, and the
voltage vectors then are the zero vector having a magnitude equal to zero.
The prior art 2 describes such combinations and producing times of the
voltage vectors V.sub.0 to V.sub.7 that an average voltage vector in a
unit time T.sub.1 of discrete voltage vectors may coincide with a desired
arbitrary instantaneous voltage vector. When, for example, a voltage
vector V* of a magnitude .vertline.V*.vertline. is to be produced in a
region defined by and between the vectors V.sub.1 and V.sub.2 in FIG. 4,
the voltage vectors V.sub.1, V.sub.2 and 0 are produced with producing
times T.sub.a, T.sub.b and T.sub.o which are defined respectively by the
following expressions:
##EQU1##
where T.sub.a +T.sub.b +T.sub.o =T.sub.I, and .theta.', is an angle of V*
from V.sub.1 and K is a coefficient including an input dc voltage Ed.
The order of production of discrete voltage vectors V.sub.1, V.sub.2 and 0
are not limited at all from the principle of the average in time. In
particular, an average voltage vector becomes identical whether the
vectors are changed over in an order of V.sub.1 .fwdarw.V.sub.2 .fwdarw.0
or in another order of 0.fwdarw.V.sub.2 .fwdarw.V.sub.1 with the times
provided at the time T.sub.I by the equations (1) above, and accordingly
an identical average vector is obtained in an arbitrary changing over
order. In this instance, however, transitions of the on-off states
(switching transitions) of the switching elements are different from each
other. FIG. 5 shows an example of switching transitions of the individual
switching element pairs S.sub.a, S.sub.b and S.sub.c when the following
two changing over orders are employed in two successive unit times
2.multidot.T.sub.I.
0.fwdarw.V.sub.1 <V.sub.2 .fwdarw.0.fwdarw.V.sub.2 .fwdarw.V.sub.1 (a)
V.sub.1 .fwdarw.V.sub.2 .fwdarw.0.fwdarw.0.fwdarw.V.sub.2 .fwdarw.V.sub.1
(b)
Here, the 0 vector can take two switching states of (0, 0, 0) and (1, 1,
1). However, in case such switching operations as put in parentheses in
(a) and (b) of FIG. 5, the switching element pair S.sub.b makes twice
switching operations both in the case of (a) and (b). Accordingly, it is
not preferable in that the loss by the elements or the loss by the driving
circuit involved in the switching operations will increase or will
concentrate upon a particular phase. Further, comparison between the
switching transition views of (a) and (b) of FIG. 5 reveals that while
each phase switching element makes, in the case of (a), a switching
operation once within the period of 2.multidot.T.sub.I, the switching
element pair S.sub.a maintains the state of 1 in the case of (b) and is
not required to make a switching operation so that averaging is attained
by switching of each of the switching element pairs S.sub.b and S.sub.c
once. Accordingly, the case of (b) of FIG. 5 is advantageous from the
point of view of the loss described above.
Thus, switching signlas which cause such switching transitions required for
switching operations for all the three phases as in the case of the
switching transition view (a) of FIG. 5 will be hereinafter referred to as
3-phase demodulation switching signals and such 3-phase modulation as
described just above will be hereinafter referred to only as 3-phase
modulation while switching signals which do not require a switching
operation for one phase as in the case of the switching transition view
(b) of FIG. 5 will be hereinafter referred to as 2-phase modulation
switching signals and such 2-phase modulation will be hereinafter referred
to only as 2-phase modulation. Thus, when the triangular wave comparison
method of the prior art 1 is applied to a 3-phase inverter, the switching
transition then takes the transition pattern of (a) of FIG. 5 and makes
3-phase modulation. As well known in the art, such 2-phase modulation as
described above is recently more advantageous than comparison of a
sinusoidal wave signal with a triangular wave with respect to voltage
utilizing rate (ratio of a maximum ac output voltage to a dc voltage
input) and effective value of high frequency components.
If the switching elements of each of the switching element pairs S.sub.a
(S.sub.aP, S.sub.aN), S.sub.b (S.sub.bP, S.sub.bN) and S.sub.c (S.sub.cP,
S.sub.cN) are turned on at the same time, then the dc voltage Ed is
short-circuited to cause breakdown of the switching elements. Accordingly,
the switching elements of each switching element pair must be controlled
such that one of them may assume the on state when the other assumes the
off state. In this instance, however, when a switching element is turned
from the on state to the off state, there is some delay which arises in
the drive circuit for the switching element or in the switching element
itself. Accordingly, a short-circuiting preventing circuit is required for
delaying a signal for turning on the other element at a time other than
its regular timing. FIG. 6 shows an example of such a short-circuiting
preventing circuit for one phase. The short-circuiting preventing circuit
of FIG. 6 operates to delay the turning on signal for a switching element
connected thereto by a predetermined interval of time T.sub.d by means of
an on delay element 71. In FIG. 6, symbols S.sub.x represents a switching
signal, S.sub.xP * a positive side switching signal, S.sub.xN * a negative
side switching signal, S.sub.xP a positive side turning on signal, and
S.sub.xN a negative side turning on signal. Accordingly, the turning on
signal for each of the switching elements is modified by such a non-linear
element as described above.
In the case of the 2-phase modulation, there is a problem that the
influence of the modification is so great that the distortion rate of the
inverter output voltage becomes very high in a low voltage region wherein
the rate of the producing time of the zero vector 0 is high. Description
of this will be given below. Referringto the switching transition view for
the 2-phase modulation shown in (b) of FIG. 5, as the producing time
T.sub.0 of the zero vector 0 increases and the rate of the times T.sub.a
and T.sub.b decreases, the times when the switching elements S.sub.b and
S.sub.c continue the state of 0 decrease. Signals of the short-circuiting
preventing circuit when such switching signals (PWM signals) are requested
are illustrated in FIG. 7 wherein the turning on signal S.sub.xN of the
switching element on the negative side disappears due to the
short-circuiting preventing period T.sub.d and always presents the off
state. Further, since all of the turning on signals having a pulse width
smaller than the short-circuiting preventing period T.sub.d apparently
disappear, the distortion rate of the output voltage increases remarkably
due to the fact that the desired output voltage is low because it is
impossible to control the output voltage by fine adjustment of the pulse
width then. Due to this influence, particularly when power is supplied to
a reactance load such as an induction motor, the distortion of the load
current is so great that the induction motor cannot be operated stably
because the frequency and the voltage have a proportional relationship and
accordingly it is in a region wherein the reactance value is very small.
Further, even if a current minor loop for detecting a load current to
change a voltage instruction value to cause the deviation from a current
instruction value to be reduced to zero is added, disappearance of the
pulses will act as a kind of blind sector. Accordingly, an effect of
improvement in waveform by the minor loop cannot be anticipated and an
unstable element of the system may be provided to increase the current
distortion.
SUMMARY OF THE INVENTION
This invention has been made to eliminate the problems described above, and
it is an object of the present invention to provide a PWM inverter system
which has advantages in switching loss, voltage utilizing ratio and higher
harmonic effective value when switching signals are produced by 2-phase
modulation and wherein it is hardly influenced by modification of
switching signals by an influence of a short-circuiting preventing circuit
and so on and an output can be produced which is low in distortion rate
even in a low voltage output region.
In order to attain the object, according to the present invention, a PWM
inverter system comprises a first means for producing switching signals by
which, while the switching elements for each phase make a switching
operation once, the switching elements for another phase may be switched
at least once, a second means for producing switching signals by which
there is at least one phase for which a switching operation is not
effected only once while a switching operation is effected for any other
phase, and a change-over means for changing over between the first means
and the second means.
Accordingly, when the on-off duty ratio of switching signals by 2-phase
modulation increases and the influence by a short-circuiting preventing
circuit and so on increases, the PWM inverter system may be changed over
to 3-phase modulation. Accordingly, advantages of control by 2-phase
modulation and advantages of control by 3-phase modulation can be both
utilized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram showing a main circuit of a single phase
inverter;
FIG. 2 is a diagram illustrating a conventional PWM signal producing
method;
FIG. 3 is a circuit diagram showing a circuit of a 3-phase inverter system;
FIG. 4 is a voltage vector diagram of the 3-phase inverter system of FIG.
3;
FIGS. 5a to 5b are transition view of switching states;
FIG. 6 is a circuit diagram showing an example of short-circuiting
preventing circuit;
FIGS. 7 and 8 are waveform diagrams illustrating a waveform modifying
action of the short-circuiting preventing circuit of FIG. 6; and
FIG. 9 is a block circuit diagram showing an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, an embodiment of the present invention will be described with
reference to the drawings.
Referring to FIG. 9, a circuit iof a PWM inverter system of the present
invention include a vector composer 31 which calculates an instruction
value V* of an instantaneous voltage vector from phase voltage
instantaneous value instruction values V.sub.a *, V.sub.b * and V.sub.c *
and outputs a magnitude .vertline.V*.vertline. of the vector V* and an
angle .theta. of the vector from a reference direction. A latch circuit 32
is provided to latch the magnitude .vertline.V*.vertline., and another
latch circuit 33 is provided to latch the angle .theta.. The latch
circuits 32 and 33 take data in at intervals of a unit time T.sub.I and
hold the data for the time of T.sub.I to effect averaging of voltage
vectors. A PWM signal producing circuit 34 serving as a second means is
provided for producing a PWM signal for 2-phase modulation, and another
PWM signal producing circuit 36 serving as a first means is provided for
producing a PWM signal for 3-phase modulation. A data selector 36 and a
change-over signal generating circuit 37 which serves as a change-over
means are also provided. A firing signal generating circuit 30 of the PWM
inverter is thus constituted from the components 31 to 37.
Subsequently, operation of the system will be described.
The PWM signal producing circuit 34 for 2-phase modulation produces
switching signals which provide, for example, a switching mode making the
switching transition view indicated in (b) of FIG. 5. Meanwhile, the PWM
signal producing circuit 35 for 3-phase modulation produces switching
signals which provide, for example, a switching mode making the switching
transition view indicated in (a) of FIG. 5. The switching signals are
transmitted to the data selector 36 at the next stage. As described
hereinabove, modification of switching signals for 2-phase modulation to a
greater degree arises from the fact that there is a phase which is high in
on-off duty ratio. This means that the rate of the producing time T.sub.0
of the zero vector 0 increases, and is also equivalent to reduction in
magnitude of the inverter output voltage. Accordingly, the PWM system is
switched from 2-phase modulation to 3-phase modulation when
.vertline.V*.vertline. becomes lower than a particular value depending
upon the magnitude V* of a voltage vector instruction value corresponding
uniquely to the magnitude of the inverter output voltage. The change-over
signal generating circuit 37 delivers a change-over signal to the data
selector 36 when .vertline.V*.vertline. becomes lower than the particular
value. In response to the change-over signal, the data selector 36
delivers a change-over signal to a short-circuiting preventing circuit 7
to change over from 2-phase modulation to 3-phase modulation.
While an example of switching mode in 3-phase modulation is illustrated in
(a) of FIG. 5, here if a case where the rate of the producing time T.sub.0
of the zero vector 0 is examined, it can be recognized that the ratio
between the switching states 0 and 1 of the individual phases of the
switching element pairs S.sub.a, S.sub.b and S.sub.c approaches 1. This
can be seen from the fact that, depending upon comparison between (b) and
(c) of FIG. 2, the duty of switching signals approaches 50% as the voltage
becomes lower. Modification when such pulses as described above are
coupled to the short-circuiting preventing circuit 7 is illustrated in
FIG. 8. In the case of 3-phase modulation, disappearance of pulses as in
2-phase modulation (FIG. 7) does not occur because the on-off pulse width
increases as the voltage becomes lower. However, since turning on of the
switching elements S.sub.xP and S.sub.xN is delayed by T.sub.d, it cannot
be said that there is no influence of it. It is possible, however, to
finely adjust the widths of on and off pulses, and the influence can be
reduced considerably compared with 2-phase modulation wherein on pulses
having a width smaller than T.sub.d disappear completely. Further, the
possibility of such fine adjustment means that the influence can be
compensated for by addition of a current minor loop, and this is a great
advantage over the fact that 2-phase modulation produces a blind sector.
It is to be noted that while in the embodiment described above the method
of the prior art 2 is employed as a PWM modulation producing method, any
other method may be employed only if 3-phase and 2-phase modulating
signals are produced by the method.
Further, while the change-over signal generating circuit 37 receives and
monitors the magnitude .vertline.V*.vertline. of a voltage vector
instruction value, the switching is made for the object of avoiding, in
short, in 2-phase modulation that the widths of on pulses of individual
switching signals decrease by increase of the zero vector 0 and may be any
amount which corresponds to a switching state. For example, such a
constitution may also be available that various amounts such as a
frequency instruction signal or a zero vector producing time T.sub.0 are
received and monitored.
As apparent from the foregoing description, according to the present
invention, 2-phase modulation and 3-phase modulation are changed over from
one to the other or vice versa. Accordingly, the distortion ratio of the
output voltage of the inverter can be prevented from becoming extremely
high so that a stabilized operation of the PWM inverter system can be
attained. Further, the PWM inverter system is also advantageous in that
the ratio of the ac output voltage to the input dc voltage can be
increased and besides the switching loss can be reduced.
* * * * *
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