Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2001-05-24
2004-02-03
Berhane, Adolf D. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S098000, C363S132000
Reexamination Certificate
active
06687139
ABSTRACT:
This is a continuation of International PCT Application No. PCT/JP99/06643 filed Nov. 29, 1999.
TECHNICAL FIELD
The present invention in general relates to an inverter control apparatus used in air conditioners. More particularly, this invention relates to an inverter control apparatus for controlling a compressor motor.
BACKGROUND ART
The air conditioner comprises an indoor unit (placed inside a house) and an outdoor unit (placed outside the house) Conventionally, in the outdoor unit of the air conditioner, there has been provided an inverter control apparatus for controlling a compressor motor (an induction motor, a DC brushless motor, etc. ) that is driven by an output AC voltage. The output AC voltage is obtained as follows. A converter and a smoothing capacitor once converts a commercial AC voltage into a DC voltage, and the inverter again converts the DC voltage into the output AC voltage by a PWM (Pulse Width Modulation) control.
The conventional converter is provided with a diode bridge circuit, and this converter converts a commercial AC voltage into a DC voltage. The smoothing capacitor is connected in parallel to the converter, and smoothes the DC voltage that has been obtained by the conversion of the converter. The inverter is composed of a switching element like a witching transistor. This inverter is a three-phase inverter that converts the DC voltage into an output AC voltage of a three-phase AC having a predetermined frequency by the PWM control for ON/OFF controlling the switching element at a predetermined timing. This inverter is input with a PWM switching pattern for instructing a switching timing of the switching element.
This PWM switching pattern is generated based on a target voltage corresponding to the operation frequency of the compressor motor by a known V/F (Voltage/Frequency) control. The operation frequency takes a value according to an air-conditioning load. For example, the operation frequency takes a large value when there is a large difference between a set temperature of the air conditioner and a room temperature.
The output AC voltage from the inverter is expressed as a surface area (i.e. surface area S) that is a product of a DC voltage V
DC
and a PWM width W as shown in FIG.
7
. In the PWM control, the PWM width W is controlled so that the output AC voltage coincides with the target voltage. The DC voltage V
DC
shown in this drawing is a DC voltage that has been smoothed by the smoothing capacitor, and the PWM width W corresponds to a period during which the switching element of the inverter is ON.
When the commercial AC voltage is supplied to the converter, the converter full-wave rectifies this commercial AC voltage to convert this voltage into the DC voltage. Then, the smoothing capacitor smoothes this DC voltage, and supplies the smoothed DC voltage to the inverter.
In this case, the PWM width W is calculated from the DC voltage V
DC
and the target voltage (i.e. the surface area S) shown in FIG.
7
. In other words, the PWM width W is a result of dividing the target voltage by the DC voltage V
DC
. In this calculation of the PWM width W, the DC voltage V
DC
is handled as a constant value. A PWM switching pattern corresponding to the PWM width W is input to the inverter.
Based on the above arrangement, the inverter ON/OFF controls the switching element at a predetermined timing according to the PWM switching pattern, thereby to convert the DC voltage into the output AC voltage having a predetermined PWM width. This output AC voltage is supplied to the compressor motor so that the compressor motor is driven.
As mentioned above, in the conventional inverter control apparatus, the DC voltage V
DC
is handled as a constant value for calculating the PWM width shown in FIG.
7
. However, in reality, the commercial AC voltage varies, therefore, the DC voltage V
DC
also varies.
Therefore, according to the conventional inverter control apparatus, there arises a difference between the PWM width W calculated and a theoretical value for carrying out an optimum control, when the DC voltage V
DC
has varied. In other words, the PWM width is calculated as a constant value regardless of a variation in the DC voltage V
DC
shown in
FIG. 7
following the variation in the commercial AC voltage. As a result, there arises a situation that the output AC voltage (corresponding to the area S) actually supplied from the inverter to the compressor motor cannot follow the target voltage.
FIGS.
8
(A) and (B) show a case where an output AC voltage V
OUT
′ from the inverter varies following the variation in the DC voltage V
DC
. FIG.
8
(A) shows a state that although it is desirable that the DC voltage V
DC
takes a constant value, the DC voltage V
DC
increases along lapse of time and then decreases, due to the influence of the variation in the commercial AC voltage. When the DC voltage V
DC
has varied like this, an output AC average voltage VA
OUT
′ that is a time-averaged output AC voltage V
OUT
′ also varies as shown in FIG.
8
(B).
As explained above, according to the conventional inverter control apparatus, the PWM width W is calculated based on the DC voltage V
DC
as a constant value, despite the fact that the V
DC
varies every moment from a DC voltage V
DC1
to a DC voltage V
DC2
, . . . , and to a DC voltage V
DC5
, as shown in the drawing. Therefore, surface areas S
1
to S
5
of output AC voltage elements V
1
′ to V
5
′ also take different values respectively.
As a result, the conventional inverter control apparatus has had the following problem. When the DC voltage V
DC
has varied following the variation in the commercial AC voltage, the output AC voltage V
OUT
′ that is supplied from the inverter to the compressor motor is deviated from the target voltage. As a result, it has not been possible to carry out an optimum operation of the compressor motor.
Particularly, when the commercial AC voltage has decreased suddenly, the output AC voltage V
OUT
′ becomes less than a minimum rated voltage of the compressor motor, which is a voltage shortage state. This results in an occurrence of a stalling. On the other hand, when the commercial AC voltage has increased suddenly, the output AC voltage V
OUT
′ exceeds a maximum rated voltage of the compressor motor, which is an overvoltage state. This results in a flow of an excess current to operate the protection circuit, and stops the operation of the compressor motor (a stop due to an overcurrent).
The power source situations (rated values, and stability, etc. of a commercial AC voltage,) in the world are different between the countries (regions). Therefore, in countries where the stability of the commercial AC voltage is low, the use of the conventional inverter control apparatus can easily invite the occurrence of the above-described voltage shortage and overvoltage. Therefore, the risk of a frequent occurrence of the stalling and a stop due to an overcurrent becomes very high. In other words, according to the conventional inverter control apparatus, there has been a problem that the stability of the control of the compressor motor is easily controlled by the power source situation.
A DC current I
DC
shown in FIG.
9
(A) includes a ripple IR
1
, as the inverter control apparatus uses a low-cost circuit for reducing the cost of. This DC current I
DC
is a voltage that has been smoothed by the smoothing capacitor. The size of the ripple I
R1
is determined by a circuit constant and the load.
According to the conventional inverter control apparatus, the DC current I
DC
that includes the ripple I
R1
shown in FIG.
9
(A) is switched by the PWM control. Therefore, an output AC current I
OUT
′ from the inverter shown in FIG.
9
(B) also includes a ripple I
R2
′. A peak value of this ripple I
R2
′ corresponds to a peak value of the ripple I
R1
(reference FIG.
9
(A).
From the above, the conventional inverter control apparatus has had also the following problems. It is necessary to use an overcurrent protection circuit that breaks a DC w
Ishigami Takahiro
Iwasaki Yoshihiro
Kawasaki Isao
Kikkawa Yoshihiko
Minejima Ichiro
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