JP3269521B2 - Inverter control method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a device for controlling an inverter suitable for driving an induction motor.

[0002]

2. Description of the Related Art PWM
When starting a three-phase induction motor using a control inverter, it is known to employ a control method for gradually increasing the output frequency of the inverter and gradually increasing the output voltage, that is, a V / f constant control method. In this method, if the induction motor is controlled to accelerate to a predetermined rotation speed, and the output frequency is kept constant when the rotation speed reaches the predetermined rotation speed, the induction motor cannot be operated at the highest efficiency. The primary motor voltage at which the highest efficiency can be obtained when the induction motor is driven under a certain load state is a voltage that substantially minimizes the primary motor voltage. Focusing on this point, the present applicant has disclosed a method for controlling the output voltage of the inverter so that the output current of the inverter becomes minimum after the motor reaches a predetermined rotation speed, according to Japanese Patent Application No. 7-263549. Proposed. However, this method has a problem that the response time until the output current of the inverter becomes minimum is long.

As another method for performing high-efficiency operation, that is, energy-saving operation of a motor, there is known a method of controlling the output of an inverter so that the power factor of the motor becomes a constant reference value. This method is a method of performing an energy-saving operation by reducing the output voltage of the inverter so that the power factor does not decrease at a light load in order to prevent the power factor of the motor from decreasing to a light load and reducing the efficiency. However, with this method alone, if the load suddenly increases in a stepped manner when the output voltage is low at a light load, the speed of the motor is reduced by losing the load and then gradually returns to a predetermined rotation speed, or an extreme In that case, there is a problem of stopping.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and an apparatus for controlling an inverter capable of performing energy-saving driving of a load whose power factor changes like an electric motor and having excellent responsiveness at the time of a sudden load change. It is in.

[0005]

SUMMARY OF THE INVENTION In order to solve the above problems and to achieve the above object, the present invention provides an output voltage command value for commanding an output voltage of an inverter, which involves a power factor fluctuation. Prepare a signal indicating the reference power factor of the load, detect the output current of the inverter, calculate the power factor of the load based on the detected value of the output current and a value indicating the output voltage of the inverter, A first correction value indicating a difference between the calculated power factor value and the reference power factor is calculated, and changes in accordance with a change in the output current value in at least a part of a change range of the output current value. Determining a correction coefficient, and obtaining a second correction value for changing an output voltage of the inverter according to the magnitude of the output current based on the output voltage command value and the correction coefficient; The invar indicated by the correction value of When the reduction amount of the output voltage of the inverter is higher than the reduction amount of the output voltage of the inverter indicated by the second correction value, the output voltage command value is corrected based on the second correction value. A first corrected output voltage command value, the inverter is controlled by the first corrected output voltage command value, and the reduction amount of the output voltage of the inverter indicated by the first correction value is equal to the second corrected output voltage command value. When the output voltage command value is lower than the reduction amount of the output voltage of the inverter indicated by the correction value, the output voltage command value is corrected by the first correction value to create a second corrected output voltage command value,
The present invention relates to a method of controlling an inverter, wherein the inverter is controlled by the second corrected output voltage command value. The correction coefficient is obtained by dividing the output current by a reference current (preferably a rated current), subjecting the output current to a limiter process, and a value corresponding to the output frequency or a first value of a constant value. Is divided by the output of the limiter process or a second value consisting of a value corresponding to the output. Further, it is desirable that the second value is a value of the square of the value subjected to the limiter processing. In addition, the first value is a constant value (for example, 1) when the output frequency is higher than a predetermined value (for example, 10 to 15 Hz), and is output when the output frequency is lower than the predetermined value. It is desirable that the value be a value corresponding to the frequency. Further, the first and second correction voltage command values are obtained by subjecting the first correction value to a limiter value using the second correction value as a limiter value, and outputting the output of the limiter process as an output voltage command value. It is desirable to subtract from the above. It is desirable to configure an apparatus for performing the method of claim 1 as described in claim 6.

[0006]

According to the present invention, not only the inverter output voltage is controlled so that the power factor approaches the reference power factor, but also the inverter output voltage is controlled according to the magnitude of the inverter output current. I do. Therefore, it is possible to prevent the output voltage from becoming too low when the load is small.
That is, the power factor is simply controlled to approach the reference power factor for energy saving. As a result, if the output voltage of the inverter is reduced, the responsiveness is deteriorated when the load is rapidly increased. On the other hand, in the present invention, the degree of reduction of the output voltage of the inverter is changed according to the output current value. As a result, it is possible to achieve both improvement in responsiveness at the time of load change and improvement in efficiency (energy saving). According to claims 2 to 6, the method of claim 1 can be achieved more reliably.

[0007]

Embodiments and Examples Next, embodiments and examples of the present invention will be described with reference to FIGS. An inverter device for driving the induction motor 4 shown in FIG. 1 includes a three-phase rectifying / smoothing circuit 2 connected to a three-phase AC power supply terminal 1 and a three-phase rectification smoothing circuit 2 connected between the pair of DC output terminals 2a and 2b. An inverter circuit 3, a PWM pulse forming circuit 5, current detectors 6a, 6b, 6c, an effective value calculator 6 connected to the current detectors 6a, 6b, 6c, and an output frequency command generator 7
, A reference power factor generator 8, and an output voltage command value calculator 9. Note that most of the PWM pulse forming circuit 5, most of the current effective value calculator 6, output frequency command generator 7, reference power factor generator 8, and output voltage command value calculator 9 are a microprocessor (microcomputer). Or, it is constituted by a digital circuit composed of a DSP, but is equivalently shown by blocks for easy understanding.

The inverter circuit 3 is a well-known three-phase bridge type inverter circuit, in which six IGBTs, that is, first to sixth switches Q1 to Q6 comprising insulated gate bipolar transistors are connected in a three-phase bridge. Switch Q
Feedback diodes D1 to D6 are connected in anti-parallel to 1 to Q6. That is, the first and second switches Q1,
A first phase arm comprising a series circuit of Q2;
A second phase arm composed of a series circuit of switches Q3 and Q4, and a third phase arm composed of a series circuit of fifth and sixth switches Q5 and Q6 as a DC power supply terminal.
Are connected to the output terminals 2a, 2b, respectively, and output lines 3a, 3b, 3c are derived from the midpoint of each phase arm.

Induction motor 4 as a load having a variable power factor
Has a rotor (not shown) in addition to a stator composed of primary windings 4a, 4b, and 4c, and is required to perform energy-saving operation according to a change in load. Etc. are combined. In this embodiment, the primary windings 4a, 4b, 4c are Y-connected and connected to the output lines 3a, 3b, 3c of the inverter circuit 3.

The PWM pulse forming circuit 5 forms a PWM (pulse width modulation) control signal for controlling the switches Q1 to Q6 of the inverter circuit 3, and converts the PWM control signal to the switches Q1 to Q6.
It is supplied to the gate (control terminal) of Q6.
Is equivalently shown in FIG. The PWM pulse forming circuit 5 includes first, second, and third phase control signal forming circuits 5a, 5b, and 5c, and a distribution circuit 10. The first phase control signal generation circuit 5a includes a sine wave signal generation unit 11, a triangular wave signal generation unit 12, a comparator 13, and a voltage control multiplier 14. The second and third phase control signal generation circuits 5b and 5c have substantially the same configuration as the first phase control signal generation circuit 5a. However, the second phase and the third phase
The sine wave signal generating means 11 of the first phase control signal generation circuit 5a in the phase control signal generation circuits 5b and 5c is a sine wave having a phase difference of 120 degrees and 240 degrees with respect to the first phase sine wave. Generate a signal. In the second and third phase control signal generation circuits 5b and 5c, those corresponding to the triangular wave signal generating means 12 are omitted, and the first phase triangular wave signal generating means 12 is also used. Of course, the second-phase and third-phase control signal generation circuits 5b and 5c can also be provided with triangular wave signal generating means independently, and can synchronize them with the first-phase triangular wave signal generating means 12. Further, instead of providing the sine wave signal generating means independently in the second and third phase control signal generating circuits 5a and 5b, a delay means is provided for the sine wave generated from the first phase sine wave signal generating means 11. With a delay of 120 degrees and 240 degrees
A phase sine wave can be obtained.

The sine wave signal generating means 11 generates a sine wave signal when the conversion circuit 2 operates as an inverter. This sine wave signal is, for example, a ROM (Read Only Memory)
By reading the sine wave data stored in. The output of the sine wave signal generator 11 is input to the comparator 13 via the multiplier 14. The multiplier 14 outputs the output voltage command value V of the output line 9a of the output voltage command value calculator 9 in FIG.
The multiplication of the sine wave signal by 0 'is sent to the comparator 13 and to the output voltage command value calculator 9 of FIG. From the multiplier 14, a sine wave voltage Vac whose amplitude is controlled by the output voltage command value V0 'is obtained. This sine wave voltage Vac is compared with the triangular wave voltage Vt shown in FIG. As a result, the PWM pulse shown in FIG. 5B, that is, the switch control signal is obtained from the comparator 13. The repetition frequency of the triangular wave voltage Vt is set sufficiently higher than the frequency of the sine wave voltage Vac. Since the amplitude of the sine wave voltage Vac of FIG. 5 is controlled by the output voltage command value V0 'of the line 9a, the width of the PWM pulse of FIG. 3, the output voltage changes. The sine wave voltages of the second and third phase control signal generation circuits 5b and 5c have delays of 120 degrees and 240 degrees with respect to the sine wave voltage Vac of FIG. Therefore, the first phase,
The second and third sine waves correspond to the reference phase voltages Vu, Vv, Vw of the inverter shown in FIG. When the comparator 13 is a digital comparison means, a digital / analog converter is provided at this output stage. When the comparator 13 is an analog voltage comparator, a digital / analog converter is provided on both input lines.

The distribution circuit 10 distributes output pulses of the first, second and third phase control signal generation circuits 5a, 5b and 5c to the switches Q1 to Q6 of the inverter circuit 3. For details,
FIG. 6 shows the PWM pulses of the first phase control signal generation circuit 5a.
(B) As shown in (C), the first and second switches Q1
, Q2 and the PW of the second phase control signal generation circuit 5b.
As shown in FIGS. 6D and 6E, the M pulse is applied to the third and fourth pulses.
And the PWM pulse of the third phase control signal generation circuit 5c to the fifth and sixth switches Q5 and Q6 as shown in FIGS. 6 (F) and 6 (G). In FIG. 6, each of the switches Q1 to Q6 is controlled only for a 180-degree period in one cycle. Can be configured. When controlling for the entire period of one cycle, the first and second switches Q1, Q2 are operated in opposite directions, the third and fourth switches Q3, Q4 are also operated in opposite directions, and the fifth and sixth switches are operated. Switches Q5 and Q6
Also operate in the opposite direction.

The current effective value calculator 6 shown in FIG. 1 includes current detectors CT1, CT2, and C composed of a magnetoelectric conversion element or a current transformer.
The current detected at T3 is converted from analog to digital, the instantaneous value is output to a line 6b, and the effective value of the output current of each phase is determined based on the instantaneous value and output to a line 6a. The current detectors CT1, CT2, CT3
Can be omitted, and the current of the omitted phase can be calculated from the currents of the other two phases. In this embodiment, the average of the effective values of the three-phase output currents of the inverter is calculated and sent to the output line 6a, which is used for output voltage control. If the output voltage command value calculator is provided independently for each phase, the effective values of the first, second, and third phase currents are sent to the respective output voltage command value calculators.

The output frequency command generator 7 sends an output frequency command to a line 7a in response to a frequency setting signal based on an operation switch (not shown) on an operation panel or the like. Since the rotation speed of the induction motor 4 corresponds to the output frequency of the inverter, the output frequency command is changed when changing the rotation speed.

The reference power factor generator 8 generates a signal indicating a reference power factor (for example, 0.8).
Sends this reference power factor to the output voltage command value calculator 9. The reference power factor is used for control for driving the electric motor 4 for energy saving.

The output voltage command value calculator 9 outputs the electric motor value based on the data indicating the output current value given from the lines 6a and 6b, the output frequency command given from the line 7a, and the signal showing the reference power factor given from the line 8a. In addition to the energy-saving driving of the motor 4, an output voltage command value that can suppress the fluctuation of the rotation speed of the motor 4 even when the load (not shown) torque of the motor 4 sharply increases is generated. It is configured as shown. More specifically, the output voltage command value calculator 9 is equivalently equivalent to an output voltage command generator 21, a subtractor 22, and a power factor calculator 23.
, A first correction value generator 24, and a correction value controller 25.

The output voltage command generator 21 generates an output voltage command V0 proportional to the output frequency command f0 of the line 7a. The characteristic line A in FIG. 7 shows the relationship between the change in the output frequency command f0 and the output voltage command V0. In order to surely rotate the motor 4 at a low frequency, that is, at a low rotation speed, a constant voltage V1 is used as an output voltage command in a region lower than the frequency f1 (for example, 10 to 15 Hz). In this embodiment, the output voltage command V0 obtained from the output voltage command generator 21 is output through the subtracter 22 without being sent to the output voltage command output line 9a as it is. The subtractor 22 subtracts the correction value Vc of the output line 25a of the correction value controller 25 from the output voltage command V0 obtained from the output voltage command generator 21, and outputs a corrected output voltage command value V0 '. The correction value Vc
Has a value smaller than the output voltage command value V0.
It changes according to the power factor and the input current. Further, in this embodiment, the corrected output voltage command value V0 'takes a value in a region between the characteristic line A and the characteristic line B in FIG.

The power factor calculator 23 calculates the inverter output current value I0 on line 6a and the corrected output voltage command value V0 on line 9a.
′, The power factor cos θ of the electric motor 4 is calculated based on the instantaneous value of the output current and the instantaneous value of the output voltage. Inverter circuit 3
Operates normally, an inverter output voltage corresponding to the corrected output voltage command value V0 'can be obtained. Therefore, this information can be obtained from the corrected output voltage command value V0' without directly detecting the output voltage of the inverter circuit 3. be able to. However, an output voltage detector of the inverter circuit 3 can be provided as necessary, and the voltage obtained from the output voltage detector can be input to the power factor calculator 23. As is well known, the power factor cos .theta. Can be obtained from P / V0 'I0. Here, the power P is obtained from the instantaneous value of the output voltage of the inverter and the instantaneous value of the output current. The instantaneous value of the output voltage is obtained by the line 9b from the output stage of the multiplier 14 of FIG. The output voltage of the inverter circuit 3 can be detected, and the instantaneous value of the output voltage can be obtained from the output voltage.
The instantaneous value of the inverter output current is obtained from the current effective value calculator 6 from the line 6b containing the information indicating the current instantaneous value.

The first correction value generator 24 shown in FIG.
The actual power factor cos of the electric motor 4 obtained from the power factor calculator 23 from the reference power factor cos θr (for example, 0.8) obtained from
The first correction value Va is obtained by subtracting θ. That is, the correction value generator 24 calculates Va = cos θr−cos θ, and can be called a power factor controller.

The correction value controller 25 is connected to the first correction value generator 24 by a line 24a, is connected to the current effective value line 6a, is connected to the frequency command line 7a, and is connected to the output voltage command line 21a. Generator 2
1 connected. The correction value controller 25 controls the first correction value Va obtained from the first correction value generator 24 based on the output current I0 and the frequency f0, and has a structure as shown in FIG. Have been. That is, the correction value controller 25 includes a limiter 31, a reference current generator 32,
A first divider 33, a lower limiter 34, a first multiplier 35, a second divider 36, an upper limiter 37,
It comprises a gain adjuster 38 and a second multiplier 39.

The limiter 31 limits (processes) the first correction value Va of the line 24a with the second correction value Vb obtained from the multiplier 39, and outputs an output voltage correction value Vc. . The first correction value Va is a power factor control signal, and changes according to the load change of the electric motor 4, and the second correction value Vb also changes according to the load change. Therefore, the limiter level in the limiter 31 is not constant. FIG. 8 shows the input / output relationship of the limiter 31 under a light load. The range of the first correction value Va from 0 to Va4 is the second level as the limiter level.
, The output of the limiter 31 changes in proportion to the first correction value Va. First correction value Va
Is equal to or higher than the second correction value Vb, that is, Va4 equal to Vb2 at the time of light load, the output of the limiter 31 becomes the second correction value Vb.
(Vb2 at light load). In addition, at the time of light load, it operates mainly in the region where the first correction value Va is higher than Va4 in FIG. When the load of the motor 4 is heavy load or rated load, the first correction value Va decreases because the power factor is improved, and the inverter output current I0 increases.
The second correction value Vb decreases in inverse proportion to this. As a result, the input / output relationship of the limiter 31 under heavy load is as shown in FIG. That is, when the first correction value Va is lower than Va4 in FIG.
When a2 is reached, the second correction value Vb = Vb1 is reached, and the output Vc of the limiter 31 is equal to or higher than Va2 and the second correction value Vb = Vb1.
become. The second correction value Vb1 under heavy load in FIG. 9 is lower than the second correction value Vb2 under light load in FIG. Since the output voltage correction value Vc of the output line 25a of the limiter 31 is input to the negative input terminal of the subtractor 22 in FIG. 3, the higher the output voltage correction value Vc output from the limiter 31, the higher the inverter output voltage, i. The output voltage command value V0 'thus obtained becomes lower.

Next, the means for generating the second correction value Vb shown in FIG.
FIG. 11 shows the state of each part when the load changes from a light load to a heavy load. The reference current generator 32 of FIG.
Generates a reference current value Ir corresponding to the rated current of the motor 4. The first divider 33 divides the output current I0 of the line 6a by the reference current Ir to obtain a value of Ia = I0 / Ir. The output Ia of the first divider 33 indicates the ratio of the output current I0 to the reference current Ir, and usually has a value smaller than 1. The lower limiter 34 connected to the first divider 33 determines a lower limiter value (for example, 0.25 to 1) of the division calculation force Ia.
This is indicated by L1 in (B). Therefore, when the input of the lower limiter 34 is lower than the lower limiter value L1 as shown before t1 in FIG. 11A, the output Ib of the lower limiter 34 becomes the lower limiter value L1 as shown in FIG. 11B. . After t1, the input Ia of the lower limiter 34
Is larger than the lower limiter value L1, the output Ib of the lower limiter 34 becomes the same as the input Ia. Since the inverter output voltage command value V0 'changes in proportion to the output Ib of the lower limiter 34, as is apparent from the relationship between FIG. 11B and FIG. 11E, the lower limiter 34 is controlled in the interval t0 to t1. The fact that the output is higher than the input means that the second correction value Vb has been corrected in a direction to increase the inverter output voltage. As a result, the lower limiter 34
When the load on the motor 4 is small, the inverter output voltage is
Of the characteristic line B.
The first multiplier 35 connected to the lower limiter 34
To adjust the gain of the denominator of the divider 36,
In this embodiment, the square value Ic of the output Ib of the lower limiter 34 is output. The second divider 36, to which the output Ic of the first multiplier 35 is input to the denominator, determines the correction coefficient K in consideration of the output frequency of the inverter. The upper limiter 37 connected to the line 7a of the output frequency f0 does not limit the frequency in the range where the frequency f0 is equal to or lower than f1 (for example, 15 Hz) as shown in FIG. (For example, 1).
The upper limiter 37 has a function of determining a substantial start frequency of the energy saving operation. If energy saving operation is performed with a significant decrease in output voltage in a frequency band lower than the relatively low limit frequency (for example, 15 Hz) determined by the upper limiter 37, smooth rotation of the motor 4 may not be possible. is there. Therefore, as shown in FIG. 7, for example, in a region lower than the frequency f1, the slope of the characteristic line B becomes gentle,
Although the voltage is relatively low, the voltage of the motor 4 is relatively high.
Supplied to Note that the upper limiter 37 is replaced with a comparator, and fa is used in a region lower than the frequency f1 in FIG.
Can be set to 0 and 1 in a region higher than f1. In this case, the output voltage of the characteristic line A in FIG. 7 is supplied to the motor 4 as it is in the section where the output frequency f0 is 0 to f1. Gain adjuster 38 connected to upper limiter 37
Is to adjust the gain so as to match the denominator of the second divider 36. In the second divider 36, FIG.
An operation is performed in which the limiter output fa indicated by 0 is gain-adjusted as a numerator and the output Ic of the multiplier 35 corresponding to the inverter output current I0 is the denominator. Therefore, when the output frequency f0 is equal to or higher than f1 (15 Hz), the numerator of a constant value is divided by the denominator Ic.
Output K varies inversely with the output current I0. The second multiplier 39 divides the inverter output voltage command value V0 by the divider 3
6 is multiplied by K to output a second correction value Vb = KV0. The second correction value Vb decreases as the output current I0 increases. When the output current I0 is constant and the output frequency f0 is higher than f1 (15 Hz), the divider 3
The correction coefficient K obtained from 6 has a constant value irrespective of a change in frequency. Even if the correction coefficient K is constant, the output voltage command value V0 changes according to the frequency command f0, so the characteristic line B in FIG. 7 showing the corrected minimum output voltage command value V0 'changes according to the frequency. are doing. However, the ratio of the variable range width between the characteristic lines A and B to the value of the characteristic line A is f1 to f1.
It is the same for each frequency in the f2 section.

Now, without providing the limiter 31 of FIG. 4, the first correction value Va of the line 24a is sent to the line 25a, and if Va = Vc, for example, at the output frequency f2 of FIG. An output voltage command V0 'indicated by, for example, V3 in FIG. 7 is generated from the subtractor 22. On the other hand, when the limiter 31 is provided according to the present embodiment, the correction by the second correction value Vb is added when the output frequency and the output current are the same as those in the conventional art, and the output voltage command V0 'indicated by V4 in FIG. 7 is obtained.
In FIG. 7, the output voltage variable range at the frequency f2 is V
2 to V5.

A change in control due to a load change will be described with reference to FIG. When the current (effective value) I0 sharply increases at the time t1 in FIG. 11, the output Ia of the divider 33 and the output Ib of the lower limiter 34 corresponding to the output current I0 are changed as shown in FIG.
(A) It becomes large as shown in (B). On the other hand, the second correction value Vb changes in inverse proportion to the output current I0 as shown in FIG. 11C, and falls from Vb2 to Vb1. Second correction value V
Since b is adjusted to be lower than the first correction value Va at the time of light load before t1, the limiting operation by the limiter 31 occurs, and the output Vc of the limiter 31 becomes the output of FIG.
As shown in (D), it becomes the second correction value Vb. When the output current I0 sharply increases at time t1, the first correction value (power factor control signal) Va decreases as shown in FIG. However, due to the response delay of the closed loop control, the first correction value Va
Does not drop sharply. Therefore, if the first correction value Va is used as it is as the output voltage correction value, the rotation speed is reduced due to the output voltage shortage. On the other hand, in the present embodiment, the second correction value Vb is changed from Vb2 to Vb1 at time t1.
As shown in FIG. 11D, the output Vc of the limiter 31 is maintained at the second correction value Vb = Vb1 in the interval from t1 to t2. Since Vb1 is lower than Vb2, the amount of subtraction in the subtractor 22 in FIG. 3 is reduced, and the inverter output voltage command value V0 'rises from V01 to V02 at time t1, as shown in FIG. Suppress the decrease in rotation speed due to a sudden increase in load. Since the load is relatively large, the first correction value V
When a crosses the second correction value Vb = Vb1 at time t2 as shown in FIG. 11C, the limiting operation by the limiter 31 is released, and the output Vc of the limiter 31 becomes as shown in FIG. 11D. The first correction value Va is output. Since the first correction value Va after t2 is lower than the second correction value Vb = Vb1, the output voltage command value V0 'is not changed after t2 in FIG.
As shown in (E), V higher than V02 in the section between t1 and t2.
It becomes 03. After t2, the output voltage is controlled so that the power factor becomes the reference power factor by the first correction value Va, that is, the power factor control signal, so that a high-efficiency operation state is achieved, and a reduction in power loss, that is, energy saving is achieved. You. In the control period using the second correction value Vb before t2, the inverter output voltage is lower than the characteristic line A, so that the effect of reducing power loss can be obtained.

As is apparent from the above description, according to the present embodiment, it is possible to achieve both improvement in energy saving and improvement in load response.

[0026]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The rectifying / smoothing circuit 2 can be changed to a single-phase full-wave, single-phase half-wave, three-phase half-wave, two-phase full-wave or two-phase half-wave rectification circuit. (2) The parts indicated by reference numerals 5 to 9 in FIG. 1 can be formed by individual circuits or analog circuits without being formed by a microcomputer. (3) Lower limiter 34 and upper limiter 37 in FIG.
Or both may be omitted. (4) The second correction value Vb can be changed in multiple steps without being continuously changed by the output voltage command value V0 and the output current I0.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an induction motor and an inverter according to an embodiment.

FIG. 2 is a block diagram illustrating a PWM pulse forming circuit of FIG. 1 in detail.

FIG. 3 is a block diagram showing an output voltage command value calculator of FIG. 1 in detail;

FIG. 4 is a block diagram illustrating a correction value controller of FIG. 3 in detail.

FIG. 5 is a waveform chart showing input and output of the comparator of FIG. 2;

6 is a waveform diagram showing output voltages of each phase of the inverter of FIG. 1 and a control section of each phase switch by a PWM pulse.

FIG. 7 is a characteristic diagram showing a relationship between an output frequency command and an output voltage command of the inverter.

8 shows input / output Va, Vb of the limiter of FIG. 4 under heavy load.
, Vc.

9 is a diagram showing input / output Va, Vb, Vc of the limiter of FIG. 4 at light load or no load.

FIG. 10 is a diagram showing inputs and outputs f0 and fa of the upper limiter of FIG. 4;

FIG. 11 is a waveform diagram showing a state of each unit in FIGS. 3 and 4;

[Explanation of symbols]

 Reference Signs List 3 inverter circuit 4 motor 5 PWM pulse forming circuit 6 current effective value calculator 9 output voltage command value calculator 25 correction value controller 31 limiter

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M ⁷ /42-7/98

Claims (6)

(57) [Claims] An output voltage command value for commanding an output voltage of an inverter is prepared, a signal indicating a reference power factor of a load accompanied by a power factor change is prepared, and an output current of the inverter is detected. Calculating a power factor of the load based on the detected value of the output current and a value indicating the output voltage of the inverter; a first correction indicating a difference between the power factor value obtained by the calculation and the reference power factor Determining a correction coefficient that changes in accordance with a change in the value of the output current in at least a part of a change range of the value of the output current, based on the output voltage command value and the correction coefficient, A second correction value for changing the output voltage of the inverter according to the magnitude of the output current is obtained, and the reduction amount of the output voltage of the inverter indicated by the first correction value is determined by the second correction value. The in indicated by the correction value When the output voltage is higher than the reduction amount of the output voltage of the inverter, the output voltage command value is corrected based on the second correction value to create a first corrected output voltage command value. Controlling the inverter according to the first correction value when the reduction amount of the output voltage of the inverter indicated by the first correction value is lower than the reduction amount of the output voltage of the inverter indicated by the second correction value. An output voltage command value that is corrected by the first correction value to generate a second corrected output voltage command value, and the inverter is controlled by the second corrected output voltage command value; Control method. 2. The correction coefficient is obtained by dividing the output current by a reference current, performing a limiter process on an output value of the division, and setting a first value corresponding to a constant value or an output frequency of the inverter.
2. The inverter control method according to claim 1, wherein the value is divided by the value subjected to the limiter processing or the second value corresponding to the value. 3. The inverter control method according to claim 2, wherein said second value is a value obtained by squaring a value subjected to said limiter processing. 4. The first value becomes a constant value when the output frequency of the inverter is higher than a predetermined frequency, and becomes a value corresponding to the output frequency when the output frequency is lower than the predetermined frequency. 4. The method for controlling an inverter according to claim 2 or 3. 5. Creating the first and second correction command voltage values based on the first and second correction values includes limiting the first correction value with the second correction value as a limit value. And a correction command voltage value obtained by subtracting the output value after the limiter processing from the output voltage command value.
Or the inverter control method according to 2 or 3 or 4. 6. An output voltage command value generating means for commanding an output voltage of an inverter, a reference power factor generating means for generating a signal indicating a reference power factor of a load accompanied by the power factor fluctuation, and the inverter Output power detecting means, power factor calculating means for calculating the power factor of the load based on the detected value of the output current and a value indicating the output voltage of the inverter, and a power obtained by the power factor calculating means. Calculating means for obtaining a first correction value indicating a difference between a rate value and the reference power factor; and a change in the value of the output current in at least a part of a change range of the output current detected by the output current detecting means. Means for calculating a correction coefficient that changes according to the following: a second correction for changing the output voltage of the inverter according to the magnitude of the output current based on the output voltage command value and the correction coefficient Operator to find the value And when the amount of reduction in the output voltage of the inverter indicated by the first correction value is higher than the amount of reduction in the output voltage of the inverter indicated by the second correction value. Outputting a correction value, and when the amount of reduction in the output voltage of the inverter indicated by the first correction value is lower than the amount of reduction in the output voltage of the inverter specified by the second correction value, Limiter means for outputting a correction value of 1; means for subtracting the output of the limiter means from the output voltage command value to obtain a corrected output voltage command value; and an output voltage corresponding to the corrected output voltage command value. Means for forming a control signal for a switch of the inverter as described above.