JPH07264869A - Inverter and current change detecting device for inverter - Google Patents

Abstract

PURPOSE:To easily detect a change in a current by controlling an inverter part so as to high efficiently operate an induction motor by increasing (decreasing) AC voltage, in the case of increasing (increasing) an inverter input current, when decreasing (increasing) the AC voltage output from the inverter part. CONSTITUTION:A current change detecting device 71 is connected to a shunt resistor 65 provided between one end of an AC power supply 80 and a rectifier 61, to detect a change in an inverter input current flowing from the AC power supply 80 to the rectifier 61. Based on an increase/decrease signal from the current change detecting device 71, a voltage control device 72 decreases AC voltage in the case of increasing of an inverter input current when increasing AC voltage output from an inverter part 63, and on the other hand, increases the AC voltage in the case of increasing of the inverter input current when decreasing the AC voltage output from the inverter part 63, thus controlling the inverter part 63 so as to highly efficiently operate an induction motor 70.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current change detecting device for an inverter used for constantly controlling operation of an induction motor or the like with high efficiency without being affected by changes in operating frequency and load.

[0002]

2. Description of the Related Art Conventionally, as an inverter for driving an induction motor, there is an inverter described in Japanese Patent Publication No. 63-49478. This inverter, as shown in FIG.
A rectifier 51 connected to a phase AC power supply, a chopper section 52 connected to the DC voltage output of the rectifier 51 and varying the DC voltage, and a smoothing capacitor provided between the positive and negative poles of the DC voltage output of the chopper section 52. 53 and an inverter section 54 connected to the DC voltage output of the chopper section 52 via a smoothing capacitor 53, and a voltage control section 55 for controlling the output voltage of the chopper section 52.
And a voltage control command section 56 that outputs a voltage control signal to the voltage control section 55. Then, the induction motor 60 is connected to the output terminal of the inverter unit 54, and the current sensor CT is provided between the output terminal of the inverter unit 54 and the induction motor 60.

The voltage control command section 56 of the inverter is
Based on the signal representing the current from the current sensor CT, that is, the signal representing the current flowing through the induction motor 60, the DC voltage of the chopper section 52 is controlled via the voltage control section 55 to increase or decrease the AC voltage output of the inverter section 54. Then, the current flowing from the inverter unit 54 to the induction motor 60 is minimized. In this way, the current flowing through the induction motor 60 is minimized so that the induction motor 60 is always operated with high efficiency.

[0004]

In the above inverter, the current sensor CT is used to detect the current flowing in the induction motor 60, or the current sensor C is used.
Instead of T, a DC current sensor DCCT (FIG. 9) for detecting an input current flowing from the chopper unit 52 to the inverter unit 54.
Is indicated by a dotted line). Therefore, the signal representing the current from the current sensor CT or the direct current sensor DCCT is converted into digital data so that it can be processed.
Since it is necessary to provide a / D (analog / digital) converter, there is a drawback that the cost becomes high.

Further, recently, when a microcomputer (hereinafter referred to as a microcomputer) is used for control and the voltage control command section 56 is constituted by a microcomputer, the above A / D converter is controlled to perform A / D conversion. There is the inconvenience that the microcomputer must take in the digital data composed of the plural pieces of bit information. Furthermore, since the difference between the A / D-converted digital data is sequentially calculated to detect the increase / decrease in the current flowing through the induction motor 60, there is a problem that the load on the microcomputer increases.

Further, since the current sensor CT or the direct current sensor DCCT is provided, there is a problem that the cost is high.

Therefore, an object of the present invention is to provide a current change detecting device for an inverter, which can easily detect an increase / decrease in current flowing through a load and can reduce cost by using an overcurrent detecting shunt resistor. is there.

[0008]

In order to achieve the above object, a current change detecting device for an inverter according to a first aspect of the invention is a rectifier to which an AC power source is connected, and a DC voltage output from the rectifier is converted into an AC voltage. And an inverter section for driving the induction motor, the inverter input current flowing from the AC power source to the rectifier, the inverter DC current flowing from the rectifier to the inverter section, and the inverter flowing from the inverter section to the induction motor. Current detecting means for detecting any one of the output currents, smoothing means connected in parallel to the current detecting means, differentiating means for differentiating a voltage between both terminals of the smoothing means, and smoothing means for the smoothing means. The voltage detected by the current detecting means is determined according to the positive / negative of the differential signal from the differentiating means that differentiates the voltage between both terminals. Based on the increasing signal and the decreasing signal from the converting means, the converting means that outputs an increasing signal and a decreasing signal that represent increase and decrease of the flow, and the current is increased when the alternating voltage output from the inverter part is increased. When increasing, the AC voltage is decreased, while when the AC voltage output from the inverter unit is decreased and the current increases, the AC voltage is increased to operate the induction motor with high efficiency. And a control means for controlling the inverter section.

The inverter current change detecting device according to a second aspect of the invention is an inverter current change detecting device having an overcurrent detecting shunt resistor between the DC power supply and the inverter part. Smoothing means connected in parallel to the resistor, differentiating means for differentiating the voltage across the terminals of the smoothing means, and the positive or negative sign of the differential signal obtained by differentiating the voltage across the terminals of the smoothing means from the differentiating means. The present invention is characterized by comprising conversion means for outputting an increase signal and a decrease signal representing an increase / decrease in current flowing from the DC power supply to the inverter section.

According to another aspect of the present invention, there is provided an inverter current change detecting device in which an overcurrent detecting shunt resistor between a DC power source and an inverter portion and a current flowing through an induction motor connected to the inverter portion are minimized. And a control means for controlling the induction motor with high efficiency by varying the AC output voltage of the inverter part, the current change detecting device for the inverter, which is connected in parallel to the shunt resistor for detecting the overcurrent. The smoothing means, the differentiating means for differentiating the voltage between both terminals of the smoothing means, and the DC power source according to the positive / negative of the differential signal obtained by differentiating the voltage between the both terminals of the smoothing means from the differentiating means. Is provided with a conversion means for outputting an increase signal and a decrease signal indicating an increase or decrease in the current flowing from the inverter to the inverter.

According to a fourth aspect of the present invention, there is provided an inverter current change detection device according to any one of the first to third aspects of the present invention, wherein the conversion means comprises:
A first photocoupler which outputs the increase signal when the differential signal from the differentiating means represents an increase in current, and a differential signal from the differentiating means represents a decrease in the current,
A current change detection device for an inverter, comprising: a second photocoupler that outputs the decrease signal.

[0012]

According to the inverter current change detecting device of the present invention, the inverter input current flowing from the AC power source to the rectifier, the inverter DC current flowing from the rectifier to the inverter unit, and the inverter flowing from the inverter unit to the induction motor. When the current detecting means detects any one of the output currents, the voltage generated across the current detecting means is smoothed by the smoothing means connected in parallel to the current detecting means. That is, a signal representing a smooth current is removed by removing fine fluctuations in the signal representing the current detected by the current detecting means, for example, by removing high frequency components due to the carrier frequency of the PWM (pulse width modulation) system. To get Then, the voltage between both terminals of the smoothing means is differentiated by the differentiating means. At this time, by differentiating the voltage between both terminals of the smoothing means by the differentiating means as a signal representing the current from which the high frequency component has been removed, it is possible to obtain a differential signal representing the increase / decrease in the current. The smoothing means prevents the high-frequency component from being differentiated by the differentiating means so as to prevent detection of increase / decrease in required current. When the differential signal obtained by differentiating the voltage between the both terminals of the smoothing means from the differentiating means is positive, the converting means outputs the increase signal representing the increase of the current detected by the current detecting means, while the differential signal is outputted. Is negative, it outputs a decrease signal indicating a decrease in the current detected by the current detecting means. The inverter input current and the inverter DC current are substantially proportional to the inverter output current flowing from the inverter section to the load, and the increase signal and the decrease signal in each case represent increase and decrease of the current flowing to the load. Then, based on the increase signal and the decrease signal from the conversion means, the control means decreases the AC voltage when the current increases when the AC voltage output from the inverter unit is increased. If the current increases when the AC voltage output from the inverter unit is decreased, the AC voltage is increased to control the inverter unit so as to operate the induction motor with high efficiency.

Therefore, since the increasing signal and the decreasing signal from the converting means are processed by the controlling means, it is not necessary to provide an A / D converter for A / D converting the signal representing the current from the current detecting means. Further, when a microcomputer is used as the control means, it is not necessary to control the A / D converter by the microcomputer, and the increase signal and the decrease signal can be taken in from the input port of the microcomputer and easily processed, and the load on the microcomputer is lightened. can do.

Further, according to the inverter current change detection device of the second aspect, when a current flows from the DC power supply to the inverter unit, a voltage is generated across the overcurrent detection shunt resistor. The voltage generated across the overcurrent detecting shunt resistor is used to detect the overcurrent flowing through the inverter unit. On the other hand, the voltage generated across the overcurrent detecting shunt resistor is smoothed by the smoothing means connected in parallel to the overcurrent detecting shunt resistor. That is,
A fine fluctuation of a signal representing a current flowing from the DC power supply to the inverter is removed, for example, a high frequency component caused by a carrier frequency of a PWM (pulse width modulation) system is removed to obtain a signal representing a smooth change of current. . Then, the voltage between both terminals of the smoothing means is differentiated by the differentiating means. At this time, the voltage between both terminals of the smoothing means,
By differentiating by a differentiating means as a signal representing the current from which the high frequency component has been removed, a differential signal representing the increase or decrease in the current can be obtained. The smoothing means prevents the high frequency component from being differentiated by the differentiating means so as to prevent detection of increase / decrease in required current. When the differential signal obtained by differentiating the voltage between the terminals of the smoothing means from the differentiating means is positive, the converting means outputs an increase signal representing an increase in the current flowing from the DC power supply to the inverter section, while the differential signal is outputted. When is negative, it outputs a decrease signal indicating a decrease in current flowing from the DC power supply to the inverter section. The current flowing from the DC power supply to the inverter section is substantially proportional to the current flowing from the output of the inverter section to the load, and the increase signal and the decrease signal indicate increase and decrease of the current flowing to the load.

Therefore, the existing shunt resistor for overcurrent detection is used without adding a new current detector,
The smoothing means, differentiating means, and converting means can easily detect an increase or decrease in the current flowing from the inverter to the load.

Further, according to the inverter current change detection device of the third aspect, when a current flows from the DC power supply to the inverter section, a voltage is generated across the overcurrent detection shunt resistor. The voltage generated across the overcurrent detecting shunt resistor is used to detect the overcurrent flowing through the inverter unit. On the other hand, the voltage generated across the overcurrent detecting shunt resistor is smoothed by the smoothing means connected in parallel to the overcurrent detecting shunt resistor. That is,
A signal representing a smooth change of current is obtained by removing small fluctuations of the signal representing the current flowing from the DC power supply into the inverter unit, for example, by removing high frequency components due to the carrier frequency of the PWM (pulse width modulation) system. . Then, the voltage between both terminals of the smoothing means is differentiated by the differentiating means. At this time, the voltage between both terminals of the smoothing means,
By differentiating the signal representing the current from which the high frequency component has been removed by the differentiating means, a differential signal representing the increase / decrease in the current can be obtained. The smoothing means prevents the high-frequency component from being differentiated by the differentiating means so as to prevent detection of increase / decrease in required current. When the differential signal obtained by differentiating the voltage between the terminals of the smoothing means from the differentiating means is positive, the converting means outputs an increase signal indicating an increase in the current flowing from the DC power supply to the inverter section, while the differential signal is outputted. When is negative, it outputs a decrease signal indicating a decrease in current flowing from the DC power supply to the inverter section. The current flowing from the DC power supply to the inverter unit is substantially proportional to the current flowing from the inverter unit to the load, and the increase signal and the decrease signal represent increase and decrease in the current flowing to the load. Then, based on the increase signal and the decrease signal, the control means changes the AC output voltage of the inverter unit so that the current flowing through the induction motor becomes a minimum, and controls the operation of the induction motor with high efficiency.

Therefore, the existing overcurrent detecting shunt resistor is used without adding a new current detector,
The smoothing means, differentiating means, and converting means can easily detect an increase or decrease in the current flowing from the inverter to the load. Further, since the increasing signal and the decreasing signal are processed by the control means, it is not necessary to provide an A / D converter for A / D converting the signal representing the current from the current detector. Further, when a microcomputer is used as the control means, it is not necessary to control the A / D converter by the microcomputer, and the increase signal and the decrease signal can be taken in from the input port of the microcomputer and easily processed, and the load on the microcomputer is lightened. can do.

According to a fourth aspect of the present invention, there is provided an inverter current variation detecting device according to any one of the first to third aspects, wherein the first photocoupler is the differential circuit. The second photocoupler outputs the decrease signal when the differentiated signal from the means represents an increase in current, while the second photocoupler outputs the decrease signal when the differentiated signal from the differentiator means represents a decrease in current. .

Therefore, the differential signal from the differentiating means can be converted into an increasing signal and a decreasing signal by the first and second photocouplers. That is, the analog signal (differential signal) is converted into a digital signal (increase signal and decrease signal). Further, the increase signal and the decrease signal can be easily insulated from the shunt resistor for detecting the overcurrent, the smoothing means and the differentiating means.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An inverter current change detecting device of the present invention will be described in detail below with reference to an embodiment.

FIG. 1 is a block diagram of an inverter using a current change detecting device according to an embodiment of the present invention.
Is a rectifier composed of a diode bridge to which a three-phase AC power source is connected, 2 is a chopper section that varies the DC voltage of the DC voltage output from the rectifier 1, and 3 is a smoothing capacitor that smoothes the DC voltage output from the chopper section 2. Reference numeral 4 denotes an inverter unit that converts the smoothed DC voltage from the smoothing capacitor 3 into AC voltage, and 5 is provided between the smoothing capacitor 3 and the inverter unit 4, and the rectifier 1 to the chopper unit 2 and the smoothing capacitor 3 are provided. It is a shunt resistor for detecting an overcurrent that detects an overcurrent flowing through the inverter unit 4 via the. Further, 6 is a current change detecting device which receives a signal representing the current from the shunt resistor 5 and detects an increase / decrease in the current, and 7 is an increase signal and a decrease signal representing the increase / decrease in the current from the current change detecting device 6. In response, the voltage control command unit 8 outputs a voltage control signal, and the voltage control unit 8 receives the voltage control signal from the voltage control command unit 7 and outputs a gate signal to the chopper unit 2. The induction motor 10 is connected to the output terminal of the inverter unit 4.
The chopper unit 2, the voltage command control unit 7, and the voltage control unit 8 constitute a control unit 11 as an example of control means, and the voltage command control unit 7 is constituted by a microcomputer.

The shunt resistor 5 for detecting the overcurrent is used.
The peak value of the current detected by the above is compared with a predetermined overcurrent detection value, and when the peak value of the current exceeds the overcurrent detection value, it is judged as an overcurrent and the overcurrent operation is performed. That is,
In order to protect the circuit elements used in the inverter unit 4, the current should not exceed a predetermined current value,
This prevents the inverter unit 4 from being damaged by heat or the like generated by overcurrent.

FIG. 2 shows a circuit diagram of the current change detecting device 6, in which the shunt resistor 5 for detecting the overcurrent is used to connect the rectifier 1 to the chopper section 2 and the smoothing capacitor 3.
The change in the current flowing through the inverter unit 4 is detected via the. The current change detection device 6 includes a smoothing capacitor C ₁ as an example of a smoothing unit, which is connected in parallel to the shunt resistor 5 and whose positive electrode side is connected to one end of the shunt resistor 5 on the load side.
One end of the smoothing capacitor C _{1 on} the positive electrode side is a capacitor C ₂
It is connected to the inverting input through, the other end of the negative electrode side of the smoothing capacitor C ₁ is connected to the non-inverting input, an amplifier IC1 connected to the output and the inverting input via a resistor R _1, the amplifier IC1 It is the connection output through a resistor R ₂ to a cathode of the light emitting diode, a first photo coupler PC1 having an anode connected to the other end of the power supply side of the shunt resistor 5,
An anode is connected to the cathode of the first photocoupler PC1 and a second photocoupler PC2 is connected to the anode of the first photocoupler PC1.

The above first and second photocouplers PC1 and PC2
A power supply V _DD is connected to the collectors of the transistors through resistors R ₃ and R _4, and the emitters of the transistors of the first and second photocouplers PC1 and PC2 are connected to the ground GND. Then, the collector of the first photocoupler PC1 outputs an increase signal (negative logic), and the collector of the second photocoupler PC2 outputs a decrease signal (negative logic). The first constitutes a conversion circuit 21 as an example of the conversion means at a second photocoupler PC1, PC2 and a resistor _{R 2, R 3, R 4} .

Further, the amplifier IC1 is connected to the power source V _DD and the power sources + V _CC and -V _CC insulated from the power source V _DD . The amplifier IC1, constitute a differentiating circuit 20 as an example of a differentiator in the capacitor C ₂ and resistor R _1. Since the differentiating circuit 20 differentiates and inverts and amplifies the voltage across the smoothing capacitor C ₁ , the positive and negative logic of the output differential signal is inverted.

According to the current change detecting device 6 having the above structure,
When the current flowing through the inverter unit 4 changes, the voltage across the terminals of the shunt resistor 5 becomes the smoothing capacitor C.
_The input signal smoothed by ₁ is removed from the rectifier 1 through the chopper unit 2 and the smoothing capacitor 3 to the inverter unit 4, and the high frequency component is removed from the input signal (see FIG. The value shown in a) is input. Then, the differentiating circuit 20 differentiates the input signal, that is, the voltage between both terminals of the smoothing capacitor C ₁ , and outputs a differential signal (shown in FIG. 3B). Then, when the current increases, the differential signal of the output of the differentiating circuit 20 becomes negative, while
When the current decreases, the differential signal of the differentiating circuit 20 becomes positive, and the amplitude of the differential signal is substantially proportional to the slope of the change in the current.

Therefore, when the current flowing from the rectifier 1 through the chopper unit 2 and the smoothing capacitor 3 to the inverter unit 4 is substantially constant and does not change, the output of the differentiating circuit 20 becomes substantially zero, and the first and second 2 photo couplers PC1 and P
No current flows through the light emitting diodes of C2, and the transistors of the first and second photocouplers PC1 and PC2 are turned off. Therefore, the first and second photocouplers PC1 and P1
C2 output increase signal (shown in Figure 3 (c)) and decrease signal (Figure (d))
Are shown as H).

When the current flowing from the rectifier 1 through the chopper section 2 and the smoothing capacitor 3 to the inverter section 4 increases, the differential signal of the output of the differentiating circuit 20 becomes negative, and the differential signal from the power source side of the shunt resistor 5 becomes 1 through the light emitting diode of the photocoupler PC 1 and the resistor R ₂
A current flows through the output of 1 to turn on the transistor of the first photocoupler PC1. The first photo coupler PC1
When the transistor of is turned on, a current flows from the power source V _DD to the ground GND through the resistor R ₃ and the increase signal (Fig. 3 (d)).
Indicates the L level. On the other hand, no current flows through the light emitting diode of the second photocoupler PC2, the transistor of the second photocoupler PC2 is turned off, and the decrease signal (shown in FIG. (C)) becomes H level.

On the other hand, when the current flowing from the rectifier 1 through the chopper section 2 and the smoothing capacitor 3 to the inverter section 4 decreases, the differential signal of the output of the differentiating circuit 20 becomes positive, and the second differential signal from the output of the amplifier IC1. through the light emitting diode and a resistor R ₂ of the photocoupler PC2, a current flows to the power supply side of the shunt resistor 5, a transistor of the second photocoupler PC2 is turned on. The second photo coupler PC2
When the transistor is turned on, a current flows from the power supply V _DD to the ground GND via the resistor R ₄ , and the decrease signal (shown in FIG. (C)) becomes L level. On the other hand, no current flows through the light emitting diode of the first photocoupler PC1, the transistor of the first photocoupler PC1 is turned off, and the increase signal
(Shown in FIG. 3 (d)) becomes H level.

FIG. 4 is a flow chart showing the operation of the voltage command control unit 7 composed of a microcomputer. The voltage command control unit 7 controls to increase or decrease the output voltage of the inverter unit 4 according to the change of the input current. 1 and the inverter unit 4 according to the operating frequency when the operating frequency is changed.
The process of control 2 for increasing or decreasing the output voltage is performed. In addition,
This process is repeated at predetermined intervals.

The process of the voltage control command unit 7 will be described below with reference to FIG.
It will be described in accordance with the flowchart of.

First, in step S1, it is determined whether or not the operating frequency matches the command frequency. If it is determined that the operating frequency matches the command frequency, the process proceeds to step S2.
Next, in step S2, it is determined whether or not a predetermined t _w seconds have elapsed after the operating frequency matches the command frequency,
When t _w seconds have elapsed, the process proceeds to step S3. On the other hand, t _w
If the second has not elapsed, the processing is terminated, and steps S1 and S2 are repeated until _tw seconds have elapsed. Then, in step S3, it is determined whether or not the control 1 is the first time, and the control 1
If it is the first time, the process proceeds to step S4, and the voltage correction width ΔV = c · f _out + d (c, d: constant,
f _out : operating frequency) is calculated. Then, in step S5, the voltage correction width ΔV is added to the reference output voltage V ₀ at the maximum efficiency point at the operating frequency f _out Hz under the standard load condition +
After correction, the process proceeds to step S10. That is, the output voltage V ₁ = V ₀ + ΔV is calculated, and the first time of the processing of the control 1 is the inverter unit 4
The voltage control command unit 7 outputs a voltage command signal such that the output voltage of V ₁ becomes V ₁ , and the voltage control unit 8 outputs a gate signal to the chopper unit 2 based on the voltage command signal. The reference output voltage V _{0 at} the maximum efficiency point at the operating frequency f _out Hz under the standard load condition is obtained by the reference output voltage V ₀ = a · f _out + b (a, b: constant).

On the other hand, when it is not the first time of the control 1 in step S3, the process proceeds to step S6 to determine the change of the input current. That is, it is determined that the input current has increased when the increase signal is at the L level, while the input current has decreased when the decrease signal is at the L level, based on the increase signal and the decrease signal from the current detection device 6. When the increase signal and the decrease signal are at H level, it is determined that the input current does not change. If it is determined in step S6 that the input current has decreased, the process proceeds to step S7, in which the output voltage increases (+),
If it is determined that the input current has increased while maintaining the ± polarity that determines the direction of decrease (−), the process proceeds to step S8, and ±
Reverse the polarity. That is, when the output voltage of the inverter unit 4 is in the increasing (+) direction, it is in the decreasing (-) direction, and when the output voltage of the inverter unit 4 is in the decreasing (-) direction, it is in the increasing (+) direction. To do. Also, step S6
If there is no change in the input current, the processing is terminated and the processing is repeated from step S1.

Next, in step S9, control 1 is performed based on the ± polarity determined in step S7 or step S8.
Process. That is, V _{n + 1} = V _n ± ΔV (V _n : previous output voltage, V _{n + 1} : current output voltage) is calculated so that the output voltage of the inverter unit 4 becomes V _{n + 1} . The voltage control command unit 7 outputs a voltage command signal, and the voltage control unit 8 outputs a gate signal to the chopper unit 2 based on the voltage command signal. Therefore, in the first step of the process of control 1 after the operating frequency matches the command frequency in step S1, it is determined that the output voltage is + polarity in the increasing (+) direction and it is determined that the input current has increased ± The output voltage is gradually increased from the reference output voltage V ₀ until the polarity is reversed.

Next, in step S10, the control voltage rate K = V _{n + 1} / V ₀ is calculated, and this processing is ended. The initial value of the control voltage ratio K is 1.

If it is determined in step S1 that the operating frequency does not match the command frequency, the process proceeds to step S11, control 2 is performed, and V _m = K · (a · f _out + b) (a, b: Constant) is calculated, and a voltage command signal is output from the voltage control command unit 7 so that the output voltage of the inverter unit 4 becomes V _m , and the voltage control unit 8 outputs the voltage command signal based on the voltage command signal. The gate signal is output to. That is, while gradually changing the operating frequency to the command frequency, the inverter unit 4 outputs the output voltage V _m corrected by the control voltage ratio K from the output voltage V ₀ at each operating frequency at that time.

Therefore, if the command frequency is changed and the operating frequency fluctuates, the control 2 of step S11 is performed.
The process controls the output voltage and the operating frequency is coincident with the command frequency, shifts to the processing control 1 after waiting t _w seconds, increase or decrease the output voltage, the input current to operate at maximum efficiency point of minimum is there.

FIG. 5A shows the output voltage characteristic with respect to the operating frequency. When the torque is larger than the standard torque with respect to the characteristic under the standard load condition, that is, the standard torque, the output is made according to the magnitude of the torque. If the torque is smaller than the standard torque while the voltage is larger than the standard torque, the output voltage is made smaller than the standard torque according to the magnitude of the torque, and the maximum efficiency point is always obtained according to the load condition. To do so.

For example, as shown in FIG. 5B, the relationship between the output voltage of the inverter unit 4 and the input current to the induction motor 10 when the operating frequency is constant is represented by a U-shaped characteristic curve. It Then, when the operating frequency is a predetermined frequency, the output voltage V _{0 at} the maximum efficiency point at the operating frequency f _out Hz under the standard load condition is increased by ΔV in + polarity to obtain the output voltage V ₁ . Then, the input current gradually increases. Therefore, if the output voltage is decreased by ΔV in the negative polarity and the output voltages are V ₂ and V ₃ , the input current gradually decreases. Furthermore, from the output voltage V ₃ −
When the output voltage is decreased by ΔV in polarity to the output voltage V ₄ , the input current gradually exceeds the minimum value and gradually increases. At this time, the polarity is reversed, the output voltage is increased by ΔV in + polarity, and the output voltage returns to V ₃ . In this way, the output voltage is controlled so that the maximum efficiency point, that is, the input current is minimized by repeatedly increasing and decreasing the output voltage. Therefore, the input current to the induction motor 10 can be minimized, and the operation of the induction motor 10 can always be controlled with high efficiency.

Further, as shown in FIG. 5 (C), when the command frequency is changed from the processing state of control 1 for variably controlling the output voltage, the processing state moves to control 2 and the output voltage V at the maximum efficiency point is changed. _{From m} to the commanded frequency, the output voltage of the inverter unit 4 is lowered according to the output voltage characteristic with respect to the operating frequency under the standard load condition. Then, after the operation frequency coincides with the command frequency, the process of control 1 for varying the output voltage is performed after _tw seconds have elapsed.

As described above, the existing overcurrent detecting shunt resistor 5 is utilized without adding a new input current detector, and the smoothing capacitor C ₁ , the differentiating circuit 20 and the first,
The second photocouplers PC1 and PC2 can detect an increase or decrease in the input current flowing from the rectifier 1 to the inverter unit 4 with a simple configuration. Therefore, the input current flowing from the rectifier 1 to the inverter unit 4 is substantially proportional to the input current flowing to the induction motor 10, so that the rectifier 1 to the inverter unit 4 can be used.
The increase / decrease in the current flowing through the induction motor 10 can be detected by detecting the increase / decrease in the input current flowing through the induction motor 10. Further, since the voltage command control unit 7 uses a microcomputer to process the increase signal and the decrease signal from the current change detection device 6 by the voltage command control unit 7, the conventional input current is
It is not necessary to provide an A / D converter for D / D conversion. Further, it is not necessary to control the A / D converter, and the increase signal and the decrease signal can be fetched from the input port of the microcomputer, easily processed, and the load on the microcomputer can be reduced.

Further, the first and second photocouplers PC
The differential signal from the differentiating circuit 20 can be converted into an increasing signal and a decreasing signal by 1 and PC2 in accordance with the sign. Further, the increase signal and the decrease signal can be easily insulated from the shunt resistor 5 for detecting the overcurrent, the smoothing capacitor C ₁ and the differentiating circuit 20.

In the above embodiment, the rectifier 1 is connected to the three-phase AC power source.
However, the AC power supply is not limited to this, and a single-phase AC power supply may be used to obtain a DC voltage by a rectifier for single-phase AC.

Further, although the voltage control command section 7 is composed of a microcomputer, it goes without saying that it may be composed of a logic circuit or the like.

Although the first and second photocouplers PC1 and PC2 are used as the converting means, the converting means is not limited to this, and the differential signals from the differentiating means are compared by a comparator to obtain the differential signal. Positive and negative may be converted into an increasing signal and a decreasing signal.

Further, a shunt resistor 5 for detecting an overcurrent for detecting an overcurrent flowing from the rectifier 1 to the inverter section 4 is provided.
Although the current detection means is not limited to this,
Alternatively, it may be provided separately.

That is, as shown in FIG.
The rectifier 61 connected to 0, the smoothing capacitor 62 that smoothes the DC voltage output from the rectifier 61, and the smoothed DC voltage from the smoothing capacitor 62 is converted into an AC voltage to drive the induction motor 70. And an inverter section 63 that operates, and a shunt resistor 64 for detecting an overcurrent is provided between one end of the smoothing capacitor 62 and the inverter section 63. Then, one end of the AC power source 80 and the rectifier 61
A shunt resistor 65 as a current detecting means is provided between the and. The same current change detection device 71 as that shown in FIG. 2 is connected to the shunt resistor 65, and the current change detection device 71 detects an increase or decrease in the inverter input current flowing from the AC power supply 80 to the rectifier 61. An average value circuit (not shown) is provided between the shunt resistor 65 and the current change detection device 71. Then, based on the increase signal and the decrease signal from the current change detection device 71, the voltage control device 72 as the control means increases the inverter input current when the AC voltage output from the inverter unit 63 is increased. In this case, while reducing the AC voltage, while increasing the inverter input current when the AC voltage output from the inverter unit 63 is reduced, the AC voltage is increased to increase the efficiency of the induction motor 70. The inverter unit 63 is controlled to operate at. The voltage control device 72 controls the inverter unit 63 by the pulse width modulation method. That is, the voltage control device 72 increases or decreases the AC voltage of the inverter unit 63 by increasing or decreasing the pulse width of the control signal for controlling the inverter unit 63.

Further, instead of the shunt resistor 65 for detecting the inverter input current, a shunt resistor 66 for detecting the inverter DC current is provided between one end of the DC voltage output of the rectifier 61 and the inverter unit 63, or the inverter unit. A shunt resistor 67 for detecting the inverter output current is provided between the induction motor 63 and the induction motor 70, and the shunt resistor 6 is provided.
The current change detection device 71 may be connected to the 6 or the shunt resistor 67. The current detecting means is not limited to the shunt resistors 65, 66, 67, and it goes without saying that a current transformer, a Hall element or the like may be used.

[0049]

As is apparent from the above, the inverter according to the invention of claim 1 drives the induction motor by converting the rectifier to which the AC power source is connected and the DC voltage output from the rectifier into the AC voltage. In an inverter having an inverter unit, any one of an inverter input current flowing from the AC power source to the rectifier, an inverter DC current flowing from the rectifier to the inverter unit, and an inverter output current flowing from the inverter unit to the induction motor. One is detected by the current detecting means, the voltage between both terminals of the smoothing means connected in parallel to the current detecting means is differentiated by the differentiating means, and the voltage between both terminals of the smoothing means is differentiated from the differentiating means. Depending on whether the differential signal is positive or negative, the conversion means is configured to increase and decrease signals representing increase and decrease of the current detected by the current detection means. Based on the increase signal and the decrease signal from the conversion means, the control means decreases the AC voltage when the current increases when the AC voltage output from the inverter unit increases. When the current increases when the AC voltage output from the inverter unit is decreased, the AC voltage is increased to control the inverter unit so as to operate the induction motor with high efficiency.

Therefore, according to the inverter of the present invention, since the increasing signal and the decreasing signal from the converting means are processed by the control means, the signal representing the current from the current detecting means is A / D converted. The / D converter need not be provided. Further, when a microcomputer is used as the control means, it is not necessary to control the A / D converter by the microcomputer, and the increase signal and the decrease signal can be easily processed by being taken in from the input port of the microcomputer, thereby reducing the load on the microcomputer. Can be lightened.

According to a second aspect of the present invention, there is provided an inverter current change detection device which has an overcurrent detection shunt resistor between the DC power supply and the inverter part. The voltage between both terminals of the smoothing means connected in parallel to the shunt resistor is differentiated by the differentiating means, and the converting means determines the positive / negative of the differential signal obtained by differentiating the voltage between both terminals of the smoothing means from the differentiating means. The output signal is an increase signal and a decrease signal that indicate an increase or decrease in the current flowing from the DC power supply to the inverter unit.

Therefore, according to the current change detecting device for an inverter of the second aspect of the invention, when a current flows from the DC power source to the inverter section, the voltage generated across the shunt resistor for detecting the overcurrent is the overcurrent. Smoothing is performed by the smoothing means connected in parallel to the detecting shunt resistor, and the voltage between both terminals of the smoothing means is differentiated by the differentiating means. When the differential signal obtained by differentiating the voltage between both terminals of the smoothing means from the differentiating means is positive, the converting means outputs an increase signal indicating an increase in current flowing from the DC power supply to the inverter section, while When the differential signal is negative, it outputs a decrease signal indicating a decrease in the current flowing from the DC power supply to the inverter unit. Therefore, it is possible to easily detect the increase or decrease of the current flowing from the DC power supply to the inverter unit by the smoothing means, the differentiating means and the converting means by using the existing overcurrent detecting shunt resistor without adding a new current detector. . Therefore, since the current flowing from the DC power supply to the inverter unit is substantially proportional to the current flowing from the inverter unit to the load, it is possible to detect an increase or decrease in the current flowing from the inverter unit to the load.

According to another aspect of the present invention, there is provided an inverter current change detecting device, in which an overcurrent detecting shunt resistor is provided between a DC power source and an inverter portion, and a current flowing through an induction motor connected to the inverter portion is minimal. By changing the AC output voltage of the inverter unit so that the current change detection device of the inverter having a control means for operating the induction motor with high efficiency, the overcurrent detection shunt resistor According to the positive / negative of the differential signal obtained by differentiating the voltage between both terminals of the smoothing means connected in parallel by the differentiating means and differentiating the voltage between the both terminals of the smoothing means from the differentiating means, the converting means changes the DC power supply to the inverter. It outputs an increase signal and a decrease signal indicating increase and decrease of the current flowing through the section.

Therefore, according to the current change detecting device for an inverter of the third aspect of the invention, the smoothing means, the differentiating means, and the differentiating means can be used without adding a new current detector by using the existing overcurrent detecting shunt resistor. The conversion means can easily detect an increase or decrease in the current flowing from the DC power supply to the inverter unit. Therefore, since the current flowing from the DC power supply to the inverter unit is substantially proportional to the current flowing from the inverter unit to the load, it is possible to detect an increase or decrease in the current flowing from the inverter unit to the load. Further, since the increasing signal and the decreasing signal are processed by the control means, it is not necessary to provide an A / D converter for A / D converting the signal representing the current from the current detector. Further, when a microcomputer is used as the control means, the A / D converter does not need to be controlled by the microcomputer, and the increase signal and the decrease signal can be easily processed by being taken in from the input port of the microcomputer, thus reducing the load on the microcomputer. Can be lightened.

According to a fourth aspect of the present invention, there is provided an inverter current change detection device according to any one of the first to third aspects of the present invention.
A first photocoupler which outputs the increase signal when the differential signal from the differentiating means represents an increase in current, and a differential signal from the differentiating means represents a decrease in the current,
And a second photocoupler that outputs the reduction signal.

Therefore, according to the current change detecting device for an inverter of the present invention, the differential signal from the differentiating means can be converted into the increasing signal and the decreasing signal by the first and second photocouplers. it can. That is, the differential signal which is an analog signal is converted into an increasing signal and a decreasing signal which are digital signals. Further, the increase signal and the decrease signal can be easily insulated from the overcurrent detecting shunt resistor, the smoothing means and the differentiating means.

[Brief description of drawings]

FIG. 1 is a block diagram of an inverter using a current change detection device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of the current change detection device.

3 (a) to 3 (d) are diagrams showing signals of respective parts of the current change detection device.

FIG. 4 is a diagram showing an operation of a microcomputer of the inverter.

5A is a diagram showing a relationship between an operating frequency and an output voltage, FIG. 5B is a diagram showing a relationship between an output voltage and an input current, and FIG. 5C is a command. It is a figure which shows the relationship between an operating frequency and output voltage when changing a frequency.

FIG. 6 is a block diagram of an inverter according to another embodiment of the present invention.

FIG. 7 is a block diagram of an inverter according to another embodiment of the present invention.

FIG. 8 is a block diagram of an inverter of another embodiment of the present invention.

FIG. 9 is a block diagram of a conventional inverter.

[Explanation of symbols]

1 ... Rectifier, 2 ... Chopper part, 3 ... Smoothing capacitor, 4
... Inverter unit, 5 ... shunt resistor, 6 ... current change detecting device, 7 ... voltage control instruction unit, 8 ... voltage control unit, 10 ... induction motor, 20 ... differential circuit, C ₁ ... smoothing capacitor, C ₂
... _{capacitors, R 1, R 2, R} 3, R 4 ... resistance, IC1 ... amplifier, PC1, PC2 ... photo coupler.

Claims (4)

[Claims] 1. A rectifier (6) to which an AC power source (80) is connected.
1) and an inverter unit (63) for converting the DC voltage output from the rectifier (61) into an AC voltage to drive the induction motor (70), the inverter from the AC power supply (80) Which of the inverter input current flowing through the rectifier (61), the inverter DC current flowing through the inverter unit (63) from the rectifier (61), and the inverter output current flowing through the induction motor (70) from the inverter unit (63). Of the current detecting means (65, 66, 67) for detecting one of them, the smoothing means (C ₁ ) connected in parallel to the current detecting means (65, 66, 67), and the smoothing means (C ₁ ). According to the positive / negative of the differential signal from the differentiating means (20) for differentiating the voltage between both terminals and the differentiating means (20) for differentiating the voltage between both terminals of the smoothing means (C ₁ ), the current detection is performed. Detected by means (65, 66, 67) And a conversion means (2) for outputting an increasing signal and a decreasing signal representing an increase / decrease in current.
1) and the increasing signal and the decreasing signal from the converting means (21), when the current increases when the AC voltage output from the inverter section (63) increases, the AC voltage is On the other hand, when the current increases when the AC voltage output from the inverter unit (63) is decreased, the AC voltage is increased to increase the induction motor (7).
0) to operate the inverter (0) with high efficiency.
And a control means (72) for controlling the inverter. 2. An inverter current change detection device having an overcurrent detection shunt resistor (5) between a direct current power supply and an inverter part (4), which is connected in parallel to the overcurrent detection shunt resistor (5). and connected to smoothing means (C _1), and differentiating means (20) for differentiating the voltage between both terminals of the smoothing means (C _1), said smoothing means from said differentiation means (20) of the (C ₁₎ There is provided a conversion means (21) for outputting an increase signal and a decrease signal representing an increase / decrease in the current flowing from the DC power supply to the inverter section (4) according to the positive / negative of the differential signal obtained by differentiating the voltage between both terminals. An inverter current change detection device characterized by the above. 3. An overcurrent detecting shunt resistor (5) between the DC power supply and the inverter section (4), and the inverter section (4).
A control means for highly efficiently controlling the operation of the induction motor (10) by varying the AC output voltage of the inverter section (4) so that the current flowing in the induction motor (10) connected to the inverter is minimized. And a smoothing means (C ₁ ) connected in parallel to the overcurrent detecting shunt resistor (5) and both terminals of the smoothing means (C ₁ ). According to the positive / negative of the differential signal (20) for differentiating the voltage and the differential signal obtained by differentiating the voltage between the terminals of the smoothing means (C ₁ ) from the differentiating means (20), the DC power source changes the inverter section ( 4) A current change detecting device for an inverter, comprising: a conversion means (21) for outputting an increase signal and a decrease signal representing an increase / decrease of the current flowing in 4). 4. The current change detecting device for an inverter according to any one of claims 1 to 3, wherein the converting means is configured to operate when the differential signal from the differentiating means (20) represents an increase in current. A first photocoupler (PC1) that outputs an increase signal and a second photocoupler (PC2) that outputs the decrease signal when the differential signal from the differentiating means (20) indicates a decrease in current are provided. An inverter current change detection device characterized in that