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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to an inverter refrigerator which has a power circuit for rectifying alternating current to output a desired DC voltage and a motor control circuit for driving a motor.
Hitherto, a control unit for controlling the speed of a compressor motor by providing a rectifier circuit for rectifying AC to convert it into DC and by combining a power circuit, which suppresses higher harmonics of current and which controls
the DC voltage, with a driving circuit for driving the compressor motor, has been disclosed in PCTJP 97/13318 (First Document).
The First Document describes a motor control circuit comprising a rectifier circuit and a smoothing circuit for converting AC power to DC, a converter circuit having a chopper circuit for controlling the DC voltage by utilizing an energy storage
effect caused by switching operations and a reactor (inductance), a motor driving unit comprising an inverter circuit and a motor connected to the DC side of the converter circuit, an inverter control circuit for controlling the speed of the motor by
controlling the switching operations of the inverter circuit, a speed detecting circuit for computing the speed of the motor by detecting the position of the rotor of the motor, a speed control circuit for controlling the speed of the motor via the
inverter control circuit by taking in the computed value of speed and a value of a speed command, and a DC voltage control circuit for controlling the DC voltage via the converter control circuit by taking in an output signal of the speed control circuit
and effecting control in accordance with the output signal.
The inverter control circuit drives the motor by driving a switching element of the inverter circuit to apply a rotating magnetic field to the motor based on a position signal from the speed detecting circuit and a conduction ratio signal from
the speed control circuit. The speed detecting circuit detects the induced voltage of the motor to calculate the position of the rotor and outputs a pulse-like position detection signal. It also calculates the speed from the calculated position signal
and outputs it to the speed control circuit as a speed detected value. Then, the speed control circuit calculates the conduction ratio signal of the PWM pulse of the inverter so that a deviation between the speed command from the outside and the speed
detected value is zeroed. The speed of the motor is controlled by the inverter circuit, the motor, the speed detecting circuit, the inverter control circuit and the speed control circuit described above.
The converter control circuit drives the switching element of the chopper circuit in accordance with the signal from the DC voltage control circuit. The DC voltage control circuit detects the DC voltage and the output signal of the speed control
circuit, e.g., the conduction ratio signal, and controls the DC voltage so as to raise the DC voltage by a predetermined width when the conduction ratio signal reaches a predetermined value, e.g., at the upper limit within a certain range of the
conduction ratio, or controls the DC voltage so as to drop the DC voltage by a predetermined width when the conduction ratio signal reaches the lower limit value. The DC voltage control circuit of the converter is formed by the converter circuit, the
converter control circuit and the DC voltage control circuit and operates to control the DC voltage.
Although the motor control unit described in the First Document has not been described with regard to possible use for a refrigerator, one using so-called PAM control means for controlling a DC voltage as a motor control unit for driving a
refrigerator compressor has been described in JP-A-7-260309 (Second Document) and JP-A-7-218097 (Third Document).
Although the Second and Third Documents have suggested that energy may be saved by using the PAM inverter as a controller of a motor for driving a compressor of a refrigerator, they have provided so specific proposal for saving energy while
performing those functions required by a refrigerator. The structure described in the First Document has not been considered for use in a refrigerator, so that it provides no disclosure concerning energy saving.
The power voltage (AC voltage supplied to house-hold plugs) for driving the motor becomes .+-.7.5% of the reference value when an allowable variation prescribed by the electric utility law and a voltage drop within a home are taken into
consideration. The conventional controller of the motor using a voltage doubling circuit has a difference of voltage in the DC stage of 43 V between the maximum value and the minimum value of 260 V to 303 V, so that there is a situation in which the
motor is not activated when the voltage of the DC stage is low.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a refrigerator which is capable of achieving energy saving while those functions required of a refrigerator are performed.
A second object of the present invention is to provide a refrigerator which allows higher harmonics to be reduced while achieving an energy saving.
A third object of the present invention is to provide a refrigerator whose compressor can be activated even when the voltage of the power supply fluctuates.
The above-mentioned objects may be achieved by a refrigerator comprising a motor for driving a compressor; an inverter for rotating and controlling the motor; and a converter for inputting AC to supply DC of variable voltage to the inverter and
having a first operating mode for operating the motor in a speed range which is less than a first rotating speed and a second operating mode for operating the motor with a second speed which is faster than the first rotating speed.
The second object may be achieved by a refrigerator comprising a rectifier circuit for converting AC into DC; a boosting chopper for boosting the DC; a reactor provided between the rectifier circuit and the boosting chopper; an inverter provided
behind the boosting chopper for converting DC to AC; a motor which is rotated and controlled by the AC from the inverter and which drives a compressor; boosting chopper control means for controlling the boosting choppers so that the DC inputted to the
inverter becomes DC of a plurality of kinds; and inverter control means for controlling the inverter using pulse width modulation at each of the plurality of types of DC voltages. The refrigerator is arranged such that the reactor presents a large
inductance in a small current range and a small inductance in a large current range.
The above-mentioned third object may be achieved by a refrigerator comprising a motor for driving a compressor; an inverter for rotating and controlling the motor; and a converter for inputting AC to supply DC of variable voltage to the inverter,
and comprising further means for increasing the DC voltage supplied to the inverter relative to the value converted from AC to DC in activating the motor.
The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings in which like numerals refer to like parts.
BRIEF
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a controlling for a refrigerator according to an embodiment of the invention;
FIG. 2 is a circuit diagram showing the internal structure of a converter PAM voltage command generator;
FIG. 3 is a graph showing the characteristics of motor applied voltage and a conduction ratio versus rotating speed;
FIG. 4 is a chart showing the time constant change characteristics of the converter PAM voltage command generator;
FIG. 5 is a graph showing the characteristics of a reactor of a converter circuit;
FIG. 6 is a graph showing the motor efficiency when DC voltage switching and converter control are turned off;
FIG. 7 is a waveform chart showing currents when the pulse width is changed in the course of modulating the pulse width;
FIG. 8 is a characteristic diagram showing the efficiency of a control circuit when converter control is turned off and on;
FIG. 9 is a graph showing the transition of the DC voltage in activating the motor;
FIG. 10 is a longitudinal section view schematically illustrating the structure of the freezer-refrigerator according an embodiment of the invention;
FIG. 11 is a flowchart for explaining ice-making control of the freezer-refrigerator of the embodiment;
FIG. 12 is a circuit diagram showing the structure of circuits for switching over between full-wave rectification and double voltage rectification; and
FIG. 13 is a characteristic diagram showing the DC voltage characteristics in switching from the full-wave rectification to the double voltage rectification.
DETAILED DESCRIPTION OF THE INVENTION
A refrigerator is required to have a quick freezing capability for quickly freezing cooked foods for preserving them and storing them, a quick ice-making capability for making ice in a short time and an energy-saving capability for keeping the
annual charge for electricity low (reduce annual power consumption) because the refrigerator is used while being constantly plugged into the power source in ordinary homes. While the quick freezing and quick ice-making capabilities may be achieved by
increasing the rotating speed of the compressor to increase the amount of refrigerant circulating during the freezing cycle, it is necessary to drive the compressor at low speed to save energy. The following problems occur in trying to achieve both a
driving of the compressor at high speed and at low speed.
Presently, a brush-less motor, in which a permanent magnet is embedded in a rotator and the rotator is rotated by causing a stator to generate a rotating magnetic field produced by an inverter, is used often as a motor for operating the
compressor of the typical refrigerator (mainly a reciprocating-type). The rotating speed of this brush-less motor may be expressed by the following expression:
where, (N) denotes the rotating speed of the motor, (V) is the motor applied voltage, (I) is the motor current, (R) is the internal resistance of the motor, (k) is a coefficient, and (.phi.) is the flux density.
As it is apparent from the above expression, the greater the applied voltage V and the smaller the internal resistance R of the motor, the higher the rotating speed is. While 288 V, which is twice 144 V (about 250 V when a load is connected), of
DC voltage inputted to the inverter may be obtained by using a voltage doubling circuit, the internal resistance R of the motor varies depending on whether the specification of the motor is set at a high speed or a low speed.
When the specification is set at a high speed for example, the value of the internal resistance R is reduced by setting the number of turns of the stator of the motor at 120 turns, for example. However, there has been a problem when the
specification of the motor is set at the high speed side in that the efficiency of the motor drops remarkably in a low speed range.
Meanwhile, when the number of turns of the coils of the stator is set at 140 turns, for example, to adjust the specification of the motor to the low speed range (to enhance the efficiency in the low speed range), there arose a problem in that the
rotating speed necessary for the quick freezing and quick ice-making operations cannot be obtained because the motor applied voltage V is kept constant and the internal resistance R of the motor increases.
Thus, according to the present embodiment, the high speed rotation of the motor has been realized by adjusting the specification of the motor to the low speed range and by increasing the inverter inputted voltage in the high speed range. While
the increase of the inverter input voltage, i.e., the DC stage voltage, may be achieved by providing a boosting chopper (or a PWM controllable converter) behind a converter for converting AC to DC and by controlling this boosting chopper in chopping (PAM
control), there has been a problem when the boosting chopper is operated over the whole operational range of the motor, as described in the First Document, in that the efficiency of the motor drops in the low speed range where the voltage applied to the
motor is low. That is, when the inverter control is performed for the refrigerator, the rotating speed of the compressor motor is driven often at the present minimum rotating speed. In such a case, there has been a problem in that the efficiency of the
circuit drops due to switching loss of a power element when an input current is caused to forcibly flow to boost the voltage by a boosting chopper circuit within the converter circuit at this time by the switching operation of the power element and by an
energy storage effect of the reactor.
Further, even at the minimum rotating speed, the lowest voltage of the DC voltage must be controlled at 163 V or more including voltage fluctuation when the switching operation of the power element is implemented to boost the DC voltage in the
boosting chopper circuit within the converter circuit. Therefore, there has been a problem in that the compressor motor is designated for operation at a point where the DC voltage is high, i.e., it is not designed optimally, thus dropping the
efficiency.
This happens because the value of the DC voltage is about 144 V and the lowest DC voltage obtained by boosting it with the lowest conduction ratio is about 163 V in case of full-wave rectification not using a voltage doubling circuit, the pulse
width which is the PWM waveform is thinned, the value of the current flowing during the ON period of the inverter increases (the maximum value of the current flowing during the period when a switching element of a certain phase is ON) and the difference
with the lowest current in a circulating mode (the period during which current is flowing in a circulating diode in that phase) increases in trying to reduce the rotating speed. This differential current is proportional to the pulsating flux density,
and the greater then differential current, the greater the iron loss becomes.
In order to solve this problem, according to the present embodiment, the DC voltage is lowered further by turning off the boosting chopper in the low speed range. The pulse width of the PWM of the inverter may be widened by lowering the voltage
of the DC stage. The pulsating flux density may be reduced and the iron loss of the motor may be reduced as a result by thus widening the pulse width because the difference between the maximum current value and the minimum current value in one period of
the inverter switching element may be reduced.
In addition, while a lower voltage has been realized by turning off the boosting chopper in the low speed range of the motor to reduce the iron loss of the motor, as described above, there has arisen a problem in that high order higher harmonics
contained in the input current are increased by turning off the boosting chopper. Although the current waveform is sinusoidal and higher harmonic components may be reduced in the range in which the boosting chopper is operative by controlling the power
factor to almost 1, because the current command is created based on the input AC voltage, the current waveform is determined by the value of the inductance L of the reactor of an LC filter provided in the DC stage between the converter and the inverter
in the range in which the boosting chopper is not operative, and a sharp current whose peak value is large and whose width is small flows as the value of L is small and the current waveform becomes sinusoidal as the value of L is large. Then, although
the problem of the higher harmonics may be solved by increasing the inductance of the reactor, there arises a problem in that the size of the reactor which allows the higher harmonics to be reduced increases, and it cannot be easily housed in an electric
component box provided between a back plate and an inner plate of the refrigerator, for example.
In order to solve this problem, according to the present embodiment, an inductance variable reactor whose inductance increases in the low current range is used. The reactor is constructed so that a loop magnetic circuit is formed by winding a
coil around an iron or amorphous material and so that an air gap is provided at part of this magnetic circuit. While the use of a reactor on the DC side of the inverter having an iron core has been describe din JP-B-64-2029 and has an effect of reducing
higher harmonics, the greater the current flowing through the reactor, the smaller the inductance becomes in a reactor merely having an iron core. It poses a problem in that the necessary inductance value cannot be obtained over the whole range in which
the boosting chopper is turned off and higher harmonics increase in the low speed side in that range.
According to the present embodiment of the invention, the influence caused by the higher harmonics may be minimized over the whole operating range of the motor because the reactor is structured so as to have a gap at a part by forming the iron
core and other parts into a ring, and the constant inductance value L is maintained even if the current value increases from the start. Further, as will be described later, while the present embodiment is arranged so as to operate at one speed (lowest
speed) when the boosting chopper is turned off, the degree of freedom of design is widened without changing the reactor per machine type within the range because the approximately constant part exists in the inductance value of this reactor even when the
lowest speed is different per type of refrigerator.
One embodiment of the invention described above will be explained below with reference to the drawings. FIG. 1 is a block diagram for explaining of the whole structure of the motor controller comprising the converter circuit using the rectifying
circuit and the boosting chopper circuit, the inverter circuit and the compressor motor.
An AC power source 1 is supplied to a plug socket is general and the refrigerator receives electricity by inserting a plug of the refrigerator power cord into the plug socket. The received AC is connected to a converter circuit 2 to be converted
into DC. The converter circuit 2 outputs the current as a DC via diodes 21, 22, 23 and 24 composing a rectifier circuit, a rector 25, a diode 26 and a boosting chopper circuit within the converter circuit 2 is connected to the output side of the
rectifier circuit in the converter circuit 2 and boosts the voltage by forcibly causing the input current to flow by the switching operation of the transistor 27 and the energy storage effect of the reactor 25, as described above. The boosted DC voltage
is supplied to a smoothing capacitor 5 so as to be outputted as a stable DC voltage. While the boosting mechanism is well known, it will be explained briefly. When the diode 21 side is plus and the switching element 27 is ON, the current flows from the
AC power source 1, to the diode 21, the reactor 25, the switching element 27, the diode 24 and back to the AC power source 1, and electromagnetic energy is accumulated in the reactor 25. When the switching element 27 is turned off at this time, a
current flows through the smoothing capacitor 5 from the reactor 25 via the diode 26 for preventing a reverse current flow and the electromagnetic energy is shifted to the capacitor 5, thus boosting the voltage of the capacitor 5. Thereby, the DC stage
voltage is boosted. It is noted that the resistor 28 within the converter circuit 2 is a resistor for detecting current.
An inverter 3 which converts DC to AC and which generates a rotating magnetic field for rotating the motor 7 is connected to the capacitor 5. The inverter 3 is connected with the motor 7 for driving the compressor 4. Although the compressor 4
which is driven by the motor 7 is not shown in detail in the figure, it is mainly a reciprocating-type compressor housed in a closed container together with the motor 7. It may be also a rotary-type compressor.
The inverter 3 is a three-phase inverter in which IGBT (Insulated Gate Bipolar Transistor) devices 31a through 32c are used as switching elements in the present embodiment. Circulating diodes 33a through 34c are connected to those switching
elements in parallel, respectively. Then, the rotating speed of the motor 7 is controlled by controlling conduction of the DC supplied from the capacitor 5 in phases of 120 degrees based on the output of the rotating position detector of the motor 7 so
that a preset rotating speed is attained and by controlling the conduction ratio (pulse width control) in the conduction period in each phase.
It is noted that a resistor 6 is a resistor for detecting current. The value of this detected current is sent to an over-current protector 111, which outputs a signal for turning off all switching elements in the inverter 3, to a driver 110 when
the value of the detected current exceeds a threshold level. Then, the driver 110 turns off the switching elements. This is provided so as not to have a current minor loop in the control of the inverter.
A comparator 100 compares the freezer compartment present temperature, which is indicated by a signal from a temperature obtained from a freezer compartment temperature detector 138 and outputs the temperature deviation before reaching the
maximum speed command, the speed command becomes constant when the deviation is greater than that. Meanwhile, the induced voltage of the motor 7 is inputted to a position detector 102 to compute the position of the magnet from this induced voltage and a
signal representing the rotating speed of the motor 7 is outputted by a speed calculator 103 based on the position signal. A comparator 104 compares this detected speed with the above-mentioned speed command (in regard to this, a speed command limiter
136 and a selecting circuit 137 will be described later). The deviation of the speed is inputted to an inverter PWM duty commanding device 105, and a pulse train whose pulse width is determined so that the speed deviation is zeroed is generated by
proportional-plus-integral computation based on the speed deviation. The output signal of the position detector 102 is also inputted to a commutation output device 108 to output a pulse train which represents a commutation timing of the conduction of
120 degrees of the switching element of each phase (a pulse train which deviates by 120 degrees per each phase) per each switching element (the figure shows one switching element). The switching elements 32a, 32b and 32c composing a lower arm of each
phase turn on during this period of commutation timing and ON/OFF operation of the switching elements 31a, 31b and 31c composing the upper arm is controlled via a driver 110 by taking AND logic of the pulse train representing the commutation timing and
the pulse train representing the previous PWM signal using an AND circuit 109.
Next, the control of the DC stage voltage of the converter circuit 2 will be explained. The voltage of the DC stage in the present embodiment is controlled in three stages of high voltage (280 V), intermediate voltage (170 V) and low voltage
(120 V). The high voltage and the intermediate voltage are realized by controlling ON/OFF operation of the boosting chopper 27. The boosting chopper 27 is turned off to increase the output pulse width of the inverter 3 in the low voltage range, thus,
contributing to the energy saving.
The actual rotating speed of the motor 4 computed by the speed calculator 103 and the speed command computed by the speed command generator 101 are inputted to a converter PAM voltage command generator 106 and a converter operation judging device
107. The converter PAM voltage command generator 106 generates a high voltage or intermediate voltage command based on the inputted actual speed and the speed command. The value of this voltage command is compared with the DC voltage across the
capacitor 5 detected by the Dc voltage detecting circuit 50 to output a command having a current peak value so that the voltage across the capacitor 5 becomes the selected high voltage or intermediate voltage. When it is judged that the DC stage voltage
must be lowered based on the inputted actual speed and the speed command, the converter operation judging device 107 outputs a chopper off signal (PAM off signal) which operates to turn off the boosting chopper (hereinafter the switching element 27 may
be referred to as the boosting chopper 27).
A multiplier 201 multiplies the command of the current peak value from the converter PAM voltage command generator 106 with the voltage (pulsating current) detected by the voltage detector 29 and full-wave rectified by the diodes 21, 22, 23 and
24 to output an instantaneous current command. A comparator 202 compares the instantaneous current command with the actual instantaneous current detected by the current detecting resistor 28 and inputs the deviation thereof to a comparator 204 to
compare the deviation with a saw-toothed wave (chopping wave) generated by an oscillator 203 to obtain a pulse width modulated signal. This signal is inputted to and amplified by a driving circuit 205 to generate a gate signal of the boosting chopper
27. The phase of the input voltage almost is equal to that of the current and the power factor approaches 1 by effecting control so that the differences between the instantaneous current command and the instantaneous current is eliminated. It is
possible to suppress the higher harmonics by forming the current into a sinusoidal wave. It is noted that when a low voltage is required, the chopper off signal which is the output of the converter operation judging device 107, is inputted to the drive
circuit 205 to stop the switching operation of the boosting chopper 27 by blocking the gate signal thereof.
Each element surrounded by a dotted line A is packaged in one integrated circuit. It is noted that the output of the converter PAM voltage command generator 106 is the intermediate voltage command during the time when the chopper off signal is
outputted and the chopping signal based on the deviation of the instantaneous current is outputted to the drive circuit. At this time, they are formed in the integrated circuit so that the DC stage voltage rises gradually even if the chopper off signal
is released (not shown).
Many converter control circuits whose control is circuit-integrated (IC-ed) and which control a DC voltage by controlling an analog voltage have been manufactured recently.
It is noted that, although the voltage command and the value of the actual speed in the embodiment described above, it may be determined by the conduction ratio of the pulse width modulation signal of the inverter.
Next, details of the converter PAM voltage command generator 106 will be explained with reference to FIG. 2. The high voltage or the intermediate voltage is selected by processing a program based on the value of the speed command and the value
of the actual speed and a command is outputted in the form of an analog voltage which enables the selected voltage to be generated on the converter control circuit side. That is, a command for changing the DC stage voltage is changed depending on
whether the voltage determined by the value is changed depending on whether the voltage determined by the value of a partial potential of the resistors R2 and R3 is outputted or a voltage determined by the volume of the parallel resistance of the
resistors R3 and R4, which are connected in parallel, and the value of partial potential of the resistor R2 is outputted.
However, because a large change occurs in the DC voltage at the switching point and the stepwise voltage change causes ill-effects on the PWM duty of the inverter, the present embodiment is arranged such that the change gradually proceeds by an
analog circuit even when the voltage command changes by the above-mentioned program processing. This will be explained below.
The DC voltage command outputted by the program processing changes stepwise. Then, the voltage is caused to increase gradually by giving a time constant to the signal changing stepwise using the resistor R1 and a capacitor C. This voltage is
compared with a chopping wave formed in the oscillating circuit. The peak value of the output waveform of this oscillating circuit is set to a level below the voltage of the DC voltage command value. Because the output of the comparator is produced
when the output of the time constant circuit is large, a pulse train whose width is widened gradually as the output voltage of the time constant circuit gradually increases is outputted. Then, when the capacitor C is completely charged, the comparator
outputs a signal ON. When the output of the comparator is ON, the transistor T turns on, so that voltage produced by the value of the parallel resistance of the resistors R3 and R4 and the partial potential of the referred to as the first partial
potential) is outputted on the converter control circuit side (the partial potential (hereinafter referred to as the second partial potential) of the resistors R2 and R3 when the transistor T is off). The voltage of the first partial potential and the
second partial potential are outputted alternately. The voltage period of the first partial potential is prolonged as the voltage of the time constant circuit rises and the voltage of the first partial potential is outputted in the end. Thus, a
rectangular wave having a different duty width corresponding to the time constant is outputted to the converter control circuit side. Although not shown, an integration circuit is provided on the converter control circuit side to convert the rectangular
wave into an analog voltage. This output becomes the DC voltage command and the peak value command of the current is obtained by comparing that value with the actual DC voltage. These controls allow the pulsation of the rotating speed of the motor 7 to
be suppressed because changes of the voltage in switching the DC voltage may be suppressed. FIG. 4 shows the pulsation of the rotating speed when the time constant of the resistor R1 and the capacitor C is changed. It can be seen that the pulsation of
the rotating speed is small when the time constant is large.
It is noted that although the chopping wave is formed and the time constant circuit is formed by the resistor R1 and the capacitor C in the present embodiment, a rectangular wave having a different duty width corresponding to the time constant
may be outputted as the DC voltage command to the program (software-wise).
When the voltage of the DC stage changes, e.g., from the intermediate voltage to the high voltage, the peak value of the pulse train outputted from the inverter 3 becomes high and the rotating speed increases sharply as the terminal voltage of
the motor 7 increases as a result.
While the voltage of the DC stage is determined software-wise from the value of the speed command and the actual speed within the converter PAM voltage command generator 106 in order to suppress this phenomenon as described above, the command of
the DC stage changes stepwise and therefore an analog circuit is provided to weaken this change.
When the command changes stepwise, a speed feedback circuit on the motor control side drops the increased speed to the commanded speed, so that the pulse is thinned to deal with it. However, the speed feedback circuit cannot respond instantly to
the quick increase of the motor and the speed of the motor cannot but be increased. Then, when the stepwise change is weakened so as to allow the speed feedback circuit to respond to that, it is undeniable that the actual rotating speed becomes higher
than the command value. Although this is not a big problem, there is a problem in that abnormal noise is produced as the rotating speed changes.
Then, according to the present embodiment, the voltage command of the DC stage is sent from the converter PAM voltage command generator 106 to the inverter PWM duty command generator 105. When the voltage of the DC stage changes in the direction
of the increase upon receiving the change of the command, the PWM duty is narrowed down in a range not becoming 0% and it is increased in a range not becoming 100% when the voltage changes in the direction of reduction.
After that, the speed is controlled so as to follow the speed command even in the DC stage voltage which gradually changes. That is, it brings about an effect that the burden on the speed feedback circuit is reduced because the increase/decrease
of the duty which the speed feedback circuit must implement with respect to the DC stage voltage changing direction is implemented based on the increase/decrease of the DC voltage command in advance.
Next, the control provided for actuating the compressor (motor) will be explained. The voltage normally transmitted to homes is allowed to have a fluctuation range based on the applicable electric utility law. It then has a fluctuation width of
281.+-.7.5% (260 V to 303 V) in the double voltage representation when a voltage drop due to interior wiring is taken into consideration. Therefore, there is a case when it becomes difficult to start the motor as its rotating torque is insufficient when
the voltage is low because the starting torque of the compressor is also large. Thus, the present embodiment is arranged such that the DC voltage command is controlled to the high voltage level or the intermediate voltage level at first to obtain a DC
voltage which fluctuates less and the activation of the motor is started during eh time when the compressor is stopped or when the starting command is issued. Thereby, the stable DC voltage of 281.+-.3% is supplied, thus, starting the motor reliably.
That is, the converter PAM voltage command generator 106 takes in the actual speed, which is represented by the output of the speed calculator 103 and the speed command which is represented by the output of the selecting circuit 137, and when the
actual speed is 0 and the speed command is issued, it judges that the compressor is to be started and sets the DC voltage command as the high voltage command. Then, the inverter causes the switching operation to start the motor under this voltage as
shown in FIG. 9. After that, when the freezer temperature approaches the temperature present value, the converter PAM voltage command generator 106 commands the low voltage via the intermediate voltage to drive the motor under the low voltage condition. At this time, the DC voltage fluctuates due to the fluctuation of the power supply voltage as described before because switching in the boosting chopper is stopped. However, even if there is a fluctuation of the power supply voltage, the motor is
controlled so as to follow the speed command because the induced voltage of the motor has risen, the speed feedback control has been established and the pulse width of the PWM for maintaining that speed is attained. It is noted that when the PAM is
turned off to lower the voltage, while the inverter controls the motor so that the speed command is attained by increasing the pulse width, the lowest speed is selected such that the conduction ratio becomes 100% when the voltage is the lowest in the
fluctuation width of the AC power supply voltage.
Here, a quick freezing operation, a quick ice-making operation and a save operation will be explained. The home freezing performance of a household refrigerator makes it possible to suppress the growth of ice crystals within the cellular
structures during freezing, to suppress the effluence of fluid from food (juice containing flavors and nutrition) during defrosting due to the destruction of cells in the food and to freeze with a high grade by minimizing the time of passing a maximum
ice crystal produced zone (-1.degree. C. to -5.degree. C.) where most of the moisture in foods is frozen. In order to realize that, a quick freezing button (quick ice-making button) 134 is provided on a door of the refrigerator so that the quick
freezing button (quick ice-making) operation is started when the button 134 is pressed. Beside the one provided on the door of the refrigerator, the quick freezing button 134 may be a relay contact or an electronic switch which may be closed by a remote
controller.
When the quick freezing button 134 is pressed, a timer within a quenching circuit 133 is activated and the quick freezing operation of two hours at most is conducted until the quick freezing button 134 is manually released or the timer turns off. The quenching circuit 133 sends a speed command for setting the rotating speed of the motor at 4,200 turns/minute (fixed) to a selecting circuit 137. The selecting circuit 137 selects the speed command from the quenching circuit 133 and outputs it to a
comparator 104. When the speed command of the motor is fixed, the deviation of the temperature may become large returning it, so that a temperature command is set at a value lower than the normal one by -7.degree. C. Therefore, the temperature command
is set by adding -7.degree. C. to an output from a temperature setting device 130 by an adder 135. The deviation of this temperature and the actual temperature is outputted from a comparator 100. Taking in this temperature deviation, the quenching
circuit 133 prevents the refrigerator and vegetable compartments, other than the freezer compartment, from being overcooled when the freezer temperature becomes lower than the preset freezer temperature (normally -18.degree. C.) by 7.degree. C. during
the quick freezing operation by setting the speed command from the fixed value of the motor 4,200 turns/min. to 1 turns/min. When the freezer temperature rises and exceeds the preset temperature which is lower than the normal temperature by 7.degree. C.
(by having hysteresis), the quenching circuit 133 issues the speed command of the motor again to start the quick freezing operation. This action is repeated until the timer is turned off.
The quick freezing (ice-making) operation described above has made it possible to shorten the maximum time of passing the ice crystal generating zone to 30 minutes or less, thereby to freeze products with a high quality.
Energy-saving in a refrigerator has been advocated lately from governmental demands as a measure for preventing global warming. In order to respond to this demand, according to the present embodiment, a save button 132 is provided on the door of
the refrigerator to realize an energy-saving mode.
When the save button 132 is pressed, the saving operation circuit 131 is activated. In order to raise the preset temperature (temperature command) by 1.degree. C., the saving operation circuit 131 outputs a signal for adding 1.degree. C. to
the output of the temperature setting device 130. An adder 135 does the adding and an output of the adder 135 is set as a temperature command during the save operation. Saving operation circuit 131 also outputs a signal to a speed command limiter 136
so that no speed command of 3,000 turns/min. or more is outputted to the rear stage even if the speed command at the output of the speed command generator 101 exceeds 3,000 turns/min. Thus, the temperature deviation is reduced by raising the preset
temperature. Therefore, the low voltage of the DC stage voltage of the main circuit is more likely to be selected and the pulse width of the inverter PWM waveform is increased, the iron loss of the motor is reduced and the power consumption may be
reduced as described before. Further, because the DC stage voltage can be controlled, the operable minimum rotating speed may be set between 1,600 turns/min. to 2,000 turns/min. Therefore, the rotating speed of the motor will not be increased
unnecessarily even though the temperature deviation is small, so that the power consumption may be reduced. Still more, the maximum speed is suppressed to 3,000 turns/min., even when the temperature deviation is very large, so that the rotating speed
will not become high unnecessarily and the power consumption may be reduced when this save button 132 has been pressed.
By the way, saving operation circuit 131 takes in the output of the freezer temperature detector 138 and releases the save control by detecting when the temperature of the freezer compartment exceeds -10.degree. C. When the load is so large that
the intra-freezing temperature rises even if the operation is continued at the rotating speed of the motor of 3,000 turns/min., the saving operation circuit 131 releases the save operation to return to the normal operation and to cool the compartment to
keep the temperature of the foods stored in the compartment at an adequate temperature.
It is noted that because the quick freezing operation is not compatible with the save operation, the system operates such that one of the operations is nullified even when the button of one operation is pressed during the time when the other
button of the other operation is operative.
Next, the operations for switching the DC voltage and for turning on/off the converter will be explained by reference to FIG. 3. FIG. 3 is a graph in which the horizontal axis represents the rotating speed of the compressor motor and the
vertical axis represents voltage applied to the motor and the conduction ratio. FIG. 3 shows a case when the load is constant.
A speed command is large and actual speed is also large in a state in which the refrigerator compartments are not cool even though the compressor is operative because the temperature difference is large. While a command is issued for setting the
DC voltage to a high voltage based on the both of them, this signal initially is 0 V and is outputted to the time constant circuit. The partial potentials across the resistors R2 and R3 are supplied to the converter control circuit side and the DC
voltage is set at the high voltage of 280 V, for example. The motor 4 is controlled at high speed when the DC voltage is 280 V. While a variable width of the rotating speed is controlled within a range of 2,700 turns to 4,200 turns, the drive signal is
created on the inverter control circuit side based on the conduction ratio signal from the deviation of the speed as described above to drive the switching elements, e.g., the transistors, of the inverter 3 to control the speed of the motor 4. The
conduction ratio corresponding to the variable width of the rotating speed is controlled within a range of 45% to 95% for example.
When the compartment of the refrigerator is cooled down and the temperature thereof approaches the preset temperature, the rotating speed of the compressor motor 4 drops. When the command of the rotating speed of the motor 4 is less than 2,700
turns for example and the actual rotating speed also falls below 2,700 turns (the conduction ratio is 4.5% or less for example when the value of command is decided by the conduction ratio to the switching element of the inverter 3), the output to the
time constant circuit is set to HIGH, and the resistors R3 and R4 are connected in parallel to change the value of the partial potential and to set the DC voltage to the intermediate voltage of 170 V (point B). At this time, the conduction ratio becomes
95% for example.
Here, since the product of the DC voltage and the conduction ratio must coincide before and after the switching, it is necessary to set the conduction ratio at a value not exceeding 100% when the DC voltage is lowered. The variable width of the
rotating speed is controlled within a range of 1,600 to 2,700 turns and the conduction ratio is 55% for example when the rotating speed is 1,600 turns.
When the compartment of the refrigerator is cooled down further and the temperature thereof approaches the preset temperature, the rotating speed of the motor 7 is set at the lowest rotating speed of 1,600 turns for example. When the conduction
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