JP2827986B2 - Induction motor 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 an apparatus for controlling an induction motor by an inverter capable of obtaining an arbitrary output voltage by PWM control.

[0002]

2. Description of the Related Art When starting a three-phase induction motor using a PWM control inverter, it is difficult 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. It is known.

[0003]

When the induction motor is accelerated to a predetermined rotational speed by V / f constant control, and the output frequency is kept constant at the predetermined rotational speed, the operation of the induction motor is continued. The motor cannot be operated at the highest efficiency. That is, the primary motor voltage V1 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 current I1. FIG. 10 shows changes in the primary current I1 and the efficiency η when the primary voltage V1 is changed from 228V to 114V while the rotation speed ωm of the induction motor is kept constant. As is apparent from FIG. 10, the primary voltage Vm for minimizing the primary current I1 and the primary voltage V for obtaining the highest efficiency.
η almost coincides with η. Therefore, the primary voltage V1 is set so that the primary current I1 is almost minimized for the highest efficiency operation.
Can be controlled, but there is no method and apparatus for easily achieving such control.

Accordingly, an object of the present invention is to provide a method and an apparatus for controlling an induction motor that can easily achieve operation at or near the highest efficiency.

[0005]

According to the present invention, there is provided a method for controlling an induction motor by an inverter capable of obtaining an arbitrary output voltage by PWM control using a PWM control circuit. An output voltage command value for commanding the output voltage of the inverter is prepared, and a sine wave having a frequency sufficiently lower than the output frequency of the inverter and an amplitude lower than the steady-state output voltage of the inverter is provided. An AC signal is prepared, the sine wave AC signal is superimposed on the output voltage command value, the input current of the induction motor is detected, and the phase angle of the sine wave AC signal superimposed on the output voltage command value is 0. In a section from degrees to 180 degrees, a first value consisting of a definite integral value of the detected value of the input current is obtained, and the phase angle of the sine wave AC signal is from 180 degrees. In a section of up to 60 degrees determined a second value consisting of a definite integral value of the detected value of the input current, wherein when the first value is greater than the second value is a constant value K
As a constant value, and when the first value is smaller than the second value, a negative constant value is obtained as the constant value K. When the first value is the same as the second value, the constant value is obtained. Obtain 0 or a positive constant value or a negative constant value as the numerical value K, obtain the integral value of the constant value by integrating the constant value K,
A modulation output voltage command value is obtained by subtracting the integral value of the constant value from the output voltage command value before or after the sine wave AC signal is superimposed, and the sine wave AC signal is superimposed and the integration of the constant value is performed. The present invention relates to a control method of an induction motor, wherein the modulated output voltage command value from which the value has been subtracted is given to the PWM control circuit as a voltage command value. It is also possible to perform the acceleration control by gradually increasing the output voltage together with the output frequency of the inverter, and then to perform the PWM control based on the modulated output voltage command value. According to a third aspect of the present invention, there is provided an apparatus for controlling an induction motor by an inverter capable of obtaining an arbitrary output voltage by PWM control using a PWM control circuit, wherein the inverter controls an output voltage of the inverter. Output voltage command value generating means for generating an output voltage command value,
A sine wave AC signal generating means for generating a sine wave AC signal having a frequency sufficiently lower than the output frequency of the inverter and having a lower amplitude than the steady state output voltage of the inverter; and the output voltage command value Means for superimposing the sine-wave AC signal on the output of the induction motor, means for detecting the input current of the induction motor, and a section in which the phase angle of the sine-wave AC signal superimposed on the output voltage command value is from 0 to 180 degrees. A first constant integration means for obtaining a first value consisting of a constant integration value of the detection value of the input current; and a detection of the input current in a section where the phase angle of the sine wave AC signal is from 180 degrees to 360 degrees. A second constant integration means for obtaining a second value consisting of a constant integration value of the value, and outputting a positive constant value as a constant value K when the first value is larger than the second value;
When the first value is smaller than the second value, a negative constant value is output as the constant value K. When the first value is the same as the second value, 0 or plus Comparing means for outputting a constant value or a negative constant value;
Constant integration means for integrating the constant value K to obtain an integral value of the constant value; and a modulation output by subtracting the integral value of the constant value from the output voltage command value before or after superimposing the sine wave AC signal. Subtraction means for obtaining a voltage command value; and means for giving the modulated output voltage command value obtained by superimposing the sine wave AC signal and subtracting the integral value of the constant value to the PWM control circuit as a voltage command value. It relates to a control device for an induction motor. The output voltage command value, sine wave AC signal, input current detection value, first and second values, constant value K
Etc. may be either analog or digital.

[0006]

In the invention of each claim of the present application, when a PWM pulse is formed based on a modulated output voltage command value obtained by superimposing a sine wave AC signal on an output voltage command value and the inverter is PWM-controlled, The output current, that is, the input current (primary current) of the induction motor oscillates including a sine wave AC component. The phase of the sine wave AC component of the input current changes depending on changes in the input current and the output voltage (primary voltage). In the present invention, the direction of increase or decrease of the output voltage for reducing the input current to the minimum is known based on the relationship between the phase of the sine wave of the modulation output voltage command value and the phase of the sine wave AC component of the input current, and the input current is minimized. The output voltage command value is controlled as described above. As a result, it is possible to easily achieve operation at or near the maximum efficiency with the input current being minimized or near this. As described in claim 2, when the induction motor is gradually increased in voltage along with the frequency to perform acceleration control, and then shifts to control based on a modulation output voltage command value, acceleration is quickly achieved, and thereafter, a high-efficiency operation state is established. Obtainable.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a method and an apparatus for controlling an induction motor according to an embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, a control device for an induction motor includes a three-phase rectifying / smoothing circuit 2 connected to a three-phase AC power supply terminal 1, and a three-phase inverter circuit 3 connected between the pair of DC output terminals 2a and 2b. , A three-phase induction motor 4 connected to the inverter circuit 3, an inverter control circuit 5, and current detectors 6a, 6
b, 6c, a full-wave rectifier circuit 6 connected to the current detectors 6a, 6b, 6c, a frequency determination circuit 7, a voltage determination circuit 8, a minimum current follow-up control circuit 9 according to the present invention, It comprises second changeover switches 10 and 11 and a switch control circuit 12.

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 an output terminal 2a of a rectifying / smoothing circuit 2 as a DC power supply terminal. 2b, and output lines 3a, 3b, 3c are derived from the midpoint of each phase arm.

The induction motor 4 has primary windings 4a, 4b, 4
A rotor (not shown) is provided in addition to the stator made of C. For example, a fan, a pump, or the like, which requires an energy-saving operation in accordance with a change in load, is connected to the rotor. In this embodiment, the primary windings 4a, 4b, 4
c is Y-connected, and output lines 3a,
3b, 3c.

The inverter control circuit 5 performs three-phase PWM control on the switches Q1 to Q6 of the inverter circuit 3,
/ F = constant (where V is the inverter output voltage and f is the inverter output frequency). The details will be described later.

The current detectors 6a, 6b, 6c are coupled to the output line 3a of the inverter circuit 3 and detect an inverter output current, that is, a primary motor current (input current). The rectifier circuit 6 connected to the current detectors 6a, 6b, 6c is a means for performing full-wave rectification on the outputs of the current detectors 6a, 6b, 6c and sending this value to the lowest current tracking control circuit 9. The primary current Idc obtained from the rectifier circuit 6 can be A / D converted and sent to the lowest current follow-up control circuit 9 as a digital signal.

The frequency determination circuit 7 includes an output frequency setting device 7
a, an acceleration gradient setting unit 7b, and an output frequency generator 7c, and generates an analog or digital output frequency command value F. The output frequency setting device 7a sets an analog value (for example, a voltage value) or a digital value indicating a target output frequency of the inverter corresponding to the target rotation speed of the electric motor 4. The acceleration gradient setter 7b outputs a frequency signal for controlling acceleration to a target rotational speed, that is, a target output frequency. Note that the acceleration gradient of the motor 4 corresponds to the increase width of the output frequency of the inverter per unit time. The output frequency generator 7c selects the output of the acceleration gradient setter 7b when acceleration control such as at startup is required, and selects the output of the output frequency setter 7a during steady state. The command value F is given to the inverter control circuit 5 and the voltage determination circuit 8.

The voltage determining circuit 8 as an output voltage command value generating means generates an output voltage command value Vin in response to the output frequency command value F given from the output frequency generator 7c. Output frequency command value F and output voltage command value Vin
Is as shown in the following equation. Vin = aF + b (a and b are constants) It is also possible to set Vin = aF by setting the constant b to zero.

In the conventional control method, the output voltage command value Vin obtained from the voltage determination circuit 8 is always
However, in the method according to the present invention, the output voltage command value Vin is selectively sent to the inverter control circuit 5 by the changeover switches 10 and 11. That is, when the motor 4 is started or accelerated, the first and second switches 10 and 11
Are turned on, the voltage determination circuit 8 is connected to the inverter control circuit 5, and in the steady state when acceleration is completed, the second contacts b of the changeover switches 10 and 11 are respectively turned on and the voltage determination circuit 8 is turned on. 8 is connected to the lowest current tracking control circuit 9, and the output line of the lowest current tracking control circuit 9 is connected to the inverter control circuit 5 via the second switch 11. The minimum current follow-up control circuit 9 superimposes the sine wave AC signal SINθ on the output voltage command value Vin, subtracts the offset voltage Voff, and modulates the output voltage command value Vou.
t is created and sent to the inverter control circuit 5. The details of the minimum current tracking circuit 9 will be described later.

The changeover switch control circuit 12 discriminates between the acceleration control section and the steady control section based on the output of the voltage determination circuit 8 and controls the on / off of the contacts a and b of the changeover switches 10 and 11. The determination of the acceleration control section and the steady control section by the changeover switch control circuit 12 may be performed based on the output frequency command value F of the output frequency determination circuit 7 or another signal indicating the acceleration control section.

FIG. 2 shows details of the inverter control circuit 5 of FIG. The inverter control circuit 5 carries out PWM control and V / f = constant control known in, for example, Japanese Patent Application Laid-Open No. 57-40369. The inverter control circuit 5 includes a three-phase sine wave generator 13 and a triangular wave carrier generator. 14 and 3
Comparators 15, 16, 17 and gate drive circuit 1
8 The three-phase sine wave generator 13 is connected to the second changeover switch 11 and the frequency determination circuit 7 of FIG. 1, and has a three-phase sine wave voltage Vsu having a frequency specified by the frequency command value F obtained from the frequency determination circuit 7. , Vsv, and Vsw are generated as shown in FIG. The amplitudes of the voltages Vsu, Vsv, and Vsw are determined by the output voltage command value Vin of the voltage determination circuit 8 or the modulation output voltage command value Vout determined by the minimum current tracking control circuit 9.
Is controlled in proportion to. Triangular wave carrier oscillator 1
4, generates a triangular wave voltage Vt having a frequency (for example, (20 kHz)) sufficiently higher than the frequencies of the sine wave voltages Vsu, Vsv, and Vsw (for example, 0 to 50 Hz). Compares the sine wave voltages Vsu, Vsv, and Vsw with the triangular wave voltage Vt, and calculates PW of FIGS. 4B, 4C, and 4D.
Outputs M pulses. 4 (B), (C), and (D) obtained from the comparators 15, 16, and 17.
Of the first, third and fifth switches Q1, Q5
3, while giving to the gate (control electrode) of Q5,
(B) The PWM signals of the opposite phases of (C) and (D) are applied to the second, fourth and sixth switches Q2, Q4 and Q6. In this embodiment, the three-phase sine wave generator 13 includes a memory and a D / A converter. This memory stores a large number of voltage level sine wave data, and the output voltage command value V
Sinusoidal data having an amplitude specified by in or the modulation output voltage command value Vout is read out by a clock corresponding to the frequency command value F, and is converted into a sine wave by D / A conversion.

FIG. 3 shows the minimum current tracking control circuit 9 of FIG. 1 in detail by analog analogy. The minimum current tracking control circuit 9 is formed by a microcomputer, but equivalently, a sine wave signal generator 21, a constant generator 22,
Multiplier 23, adder 24, first and second constant integrators 2
5, 26, comparator 27, integrator 28, first limiter 2
9, a subtractor 30, and a second limiter 31.

The sine wave generator 21 has a frequency (for example, 1/100 to 1/1 of the output frequency of the inverter 3) sufficiently lower than the output frequency of the inverter 3 shown in FIG.
5000). This sine wave S
INθ corresponds to the envelope indicated by the dotted line in FIG. The constant generator 22 supplies a constant A to the multiplier 23. The multiplier 23 multiplies the sine wave SINθ of the sine wave generator 21 by a constant A to form a sine wave AC signal A × SINθ whose amplitude level is adjusted. This sine wave AC signal A × S
The amplitude of INθ is set to a level sufficiently lower than the steady-state output voltage command value Vin. The adder 24 adds or superimposes the sine wave AC signal A × SINθ on the output voltage command value Vin to form a waveform having minute vibration. The subtractor 30 is a means for subtracting the offset voltage Voff limited by the limiter 29 from the output of the adder 24.
The modulated output voltage command value Vout shown in FIG. In addition,
The output voltage command value Vi on the input side of the adder 24
n lines. The modulation output voltage command value Vout obtained from the subtractor 30 is sent to the changeover switch 11 through the limiter 31. The limiters 29, 3
Reference numeral 1 is provided to suppress instability of rotation of the electric motor 4 due to excessive increase / decrease of the output voltage command value due to voltage modulation, torque fluctuation, or overcurrent due to overexcitation. Therefore, if the modulation output voltage command value does not change abnormally, the limiter 2
Either one or both of 9 and 31 can be omitted.

When a modulation output voltage command value Vout as shown in FIG. 7A is given from the lowest current follow-up control circuit 9 to the PWM inverter control circuit 5 in FIG. 1, as shown in principle in FIG. The envelope of the peak value of the inverter output voltage waveform changes so as to correspond to the sine wave AC component of the modulated output voltage command value Vout. As a result, the full-wave rectified waveform of the output current of the inverter 3, that is, the primary current I1 of the motor is also shown in FIG.
, And this envelope corresponds to the sine wave SINθ. The relationship between the voltage in FIG. 5A and the current in FIG. 5B shows a relationship in a light load state, and the phases of both envelopes are almost coincident. Note that FIG.
A full-wave rectified waveform of only one phase of the secondary current I1 is shown. The variation due to the sine wave SINθ in the three-phase full-wave rectified waveform also occurs in the same manner as in FIG.

The first and second constant integrators 25 and 26 shown in FIG .
Input the full-wave rectified current Idc shown in FIG. 5B or its envelope detection signal. The first constant integrator 25 synchronizes the sine wave SINθ of the sine wave generator 21 with the first constant integral value (first value) Ia of the rectified current Idc from 0 degrees to 180 degrees, that is, π. Is what you want. The second constant integrator 26 synchronizes with the sine wave SINθ by 180 degrees, that is, π.
To obtain a second constant integral value (second value) Ib of the rectified current Idc from 360 ° to 2π. The first and second constant integration values Ia and Ib obtained from the first and second constant integrators 25 and 26 are such that the current primary voltage V1 (effective value) reduces the primary current I1 in FIG. Voltage value Vm
Used to determine if it is higher (left) or lower (right).

Next, the principle for determining whether the primary voltage V1 is at a value that minimizes the primary current I1 will be described. When the primary voltage V1 is changed during the steady operation of the motor 4, the primary current I1 also changes. This primary current I1
Changes depending on the state of the load. At light load, when the primary voltage V1 is increased, the primary current I1
Increase, and when the primary voltage V1 decreases, the primary current I1 also decreases. Therefore, the region where the primary voltage V1 is higher than the primary voltage value Vm that minimizes the primary current I1 in FIG. 10 indicates a light load state. At a heavy load, the primary current I1 decreases when the primary voltage V1 increases, and the primary current I1 increases when the primary voltage V1 decreases. Therefore, the region where the primary voltage V1 is lower than the primary voltage value Vm that minimizes the primary current I1 in FIG. 10 indicates a heavy load state. In the present embodiment, the sine wave SINθ is superimposed on the output voltage command value Vin to create a modulated output voltage command value Vout, and the inverter 3 is controlled by this. Therefore, the output voltage of the inverter 3, that is, the primary voltage V1 changes at a large cycle (several tens of seconds) as shown in FIG. In the section where the output voltage command value Vout is increasing based on the sine wave SINθ and in the section where the output voltage command value Vout is decreasing, the light load state or the heavy load state is set based on the change state of the primary current I1. You can determine if there is. For example, due to the superimposed sine wave SINθ, the primary voltage V1 becomes V11 in FIG.
, The operation is the same as increasing the primary voltage V1 up to the voltage Va during the positive half-wave of the sine wave, and the primary current I1 increases at this time. Then, during the period of the negative half-wave, the operation becomes the same as when the primary voltage V1 is lowered to the voltage Vb. At this time, the primary current I1 decreases. On the other hand, if the primary voltage V1 changes from Vc to Vd around V12 on the right side of FIG. 10, it is the same as raising the primary voltage V1 to the voltage Vc during the positive half-wave of the sine wave. At this time, the primary current I1 decreases. During the negative half-wave period, the operation becomes the same as when the primary voltage V1 is lowered to Vd. At this time, the primary current I1 increases. Therefore, the sine wave SINθ included in the modulation output voltage command value Vout
Of the primary currents I1 and π (18
Compared with the primary current in the section from 0 ° to 2π (360 °), the primary current I1 is V V centered on the voltage V11 in FIG.
It can be known whether the value is a value corresponding to a to Vb or a value corresponding to Vc to Vd centering on the voltage V12.

FIG. 8 shows a light load state, that is, the voltage Va to FIG.
The modulated output voltage command value Vout and the rectified current Idc of the primary current I1 when the primary voltage V1 is sinusoidally modulated between Vb.
FIG. 9 shows a heavy load condition, that is, the voltage V in FIG.
The relationship between the modulated output voltage command value Vout and the rectified current Idc of the primary current I1 when the primary voltage V1 is sinusoidally modulated between c and Vd is shown. The phase of the AC component of the current Idc in FIG. 8B is shifted by 180 degrees from the phase of the AC component of the current Idc in FIG. 9B. The phase of the AC component of the current Idc also changes due to the change in the load state, and when the primary voltage Vm at which the primary current I1 in FIG. 10 is minimized is generated, the phase of the modulation output voltage command value Vout is changed with respect to the AC component. Thus, the AC component of the current Idc has a phase difference of 90 degrees.

The first and second constant integrators 25 and 26 shown in FIG.
The first and second integrated values Ia and Ib obtained by
Can be obtained, an output indicating that Ia> IbIa <IbIa = Ib can be obtained. The comparator 27 outputs a constant Vk = K when Ia> Ib, a constant Vk = -K when Ia <Ib, and a constant Vk = 0 when Ia = Ib. It should be noted that the constant Vk can be set to K or -K when Ia = Ib. An integrator 28 connected to the comparator 27 functions as a low-pass filter, that is, a smoothing circuit, and outputs an offset voltage Voff corresponding to a smoothed value of the comparison output, and supplies this to a subtractor 30 via a limiter 29. I do. The positive input terminal of the subtractor 30 is Vin + A
× SINθ is input, and Voff is input to the negative input terminal, so that an output of Vout = Vin + A × SINθ−Voff is obtained from the subtracter 30, and this is output to the PWM inverter control circuit 5 To be used as a voltage command.

Now, in the steady state, the phase relationship between the modulation output voltage command value Vout and the rectified current Idc is shown in FIGS. 8A and 8B.
In the case of (1), an output of a constant Vk = K indicating Ia> Ib is obtained from the comparator 27, and the offset voltage Voff increases. As a result, the average level of the modulated output voltage command value Vout decreases, the output voltage of the inverter 3, that is, the primary voltage of the motor 4 also decreases, and the primary current I1 also decreases toward the minimum value. Conversely, when the phase relationship between the modulated output voltage command value Vout and the rectified current Idc is as shown in FIGS. 9A and 9B, an output of a constant Vk = −K that indicates Ia <Ib is obtained from the comparator 27. The offset voltage Voff decreases, and the modulation output voltage command value Vou
The average level of t increases, the output voltage of the inverter 3, that is, the primary voltage V1 of the motor 4 also increases, and the primary voltage V1 increases toward the minimum value Vm of the primary current I1. By repeating the above-described operation, the primary voltage V1 becomes the value Vm at which the primary current I1 is minimized or near the value Vm. When the primary voltage V1 is at or near the value Vm that minimizes the primary current I1, the efficiency .eta. As a result, high-efficiency operation, that is, energy-saving operation of the electric motor 4 can be easily achieved.

In this embodiment, acceleration control is performed until the electric motor 4 is brought into a steady state. That is, at the time of startup or acceleration, an acceleration gradient output frequency command value for increasing the output frequency with a predetermined gradient is generated from the acceleration gradient setter 7b, and the command value is added to the inverter control circuit 3 and the voltage determination circuit is set. Add to 8. Further, the contact point a of the first and second changeover switches 10 and 11 is turned on to turn on the voltage determination circuit 8.
Is directly applied to the inverter control circuit 5. The output voltage command value Vin is gradually increased as shown in FIG. 6A, and the output frequency command value F is changed as shown in FIG.
When the voltage is gradually increased as shown in (1), the output voltage (primary voltage) V1 and the output frequency are increased in the same manner, acceleration control is achieved, and the target rotation speed is achieved in a short time. When the target rotational speed is reached at time t1 in FIG. 6, the changeover switch control circuit 12 in FIG.
At time t2, the contacts b of the changeover switches 10 and 11 are controlled to be turned on, and the control is shifted to the control by the minimum current follow-up control circuit 9. FIGS. 6B and 7B show changes in the primary current I1, and FIGS. 6C and 7C show changes in the load torque T1. When the control by the minimum current follow-up control circuit 9 according to the present invention is started from time t2 in FIG.
The modulation output voltage command value Vout changes so as to enable almost the highest efficiency operation at the load torque T1 in the section t3,
The primary current I1 reaches its minimum value some time after t2. When the load torque gradually increases at t3 in FIG. 7, the modulated output voltage command value Vout changes so that the operation can be performed at almost the maximum efficiency in this torque state, and the primary current I1 becomes the minimum value at this load torque. Change. That is, when the load torque T1 is gradually changed, 1
An operation for minimizing the next current I1 follows, and operation at almost the highest efficiency can be continued. Therefore,
This is advantageous when the load torque fluctuates as in a motor of a fan or a pump. In the method shown in FIG.
Since there is only the secondary current I1, the configuration of the control device is simplified, and the cost of the device is reduced.

[0026]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The rectifier circuit 6 can be changed to a rectifier circuit of single-phase full-wave, single-phase half-wave, three-phase half-wave, two-phase full-wave or two-phase half-wave. (2) The control circuit 9 shown in FIG. 3 can be constituted by an individual circuit or an analog circuit without using a microcomputer. (3) Instead of driving the voltage determination circuit 8 by the output of the output frequency determination circuit 7, the output frequency command value F can be generated based on the output of the voltage determination circuit 8. (4) The three-phase sine wave generator 13 generates sine wave data with digital values, and the triangular wave carrier oscillator 14 generates triangular wave data with digital values.
6 and 17 can be digital comparators. (5) Connect switches Q1 to Q6 to MOSF other than IGBT.
Various semiconductor switches such as ET can be used. (6) An effective value detection circuit may be provided instead of the rectifier circuit 6, and the detected effective value may be input to the constant integration circuits 25 and 26.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a control device for an induction motor according to an embodiment.

FIG. 2 is a block diagram illustrating an inverter control circuit of FIG. 1 in detail.

FIG. 3 is a block diagram illustrating a minimum current tracking control circuit of FIG. 1 in detail;

FIG. 4 is a waveform chart showing a state of each part of the inverter control circuit of FIG. 2;

FIG. 5 is a waveform diagram showing in principle a primary voltage of a motor for one phase and a rectified waveform of a primary current.

FIG. 6 is a waveform diagram showing Vin, I1, T1, and F during acceleration control.

FIG. 7 is a waveform diagram showing Vout, I1, T1, and F in a steady state.

FIG. 8 is a waveform chart showing Vout and Idc at a light load.

FIG. 9 is a waveform diagram showing Vout and Idc under heavy load.

FIG. 10 is a diagram showing a relationship among a primary voltage, a primary current, an efficiency, and a rotation speed of a motor.

[Explanation of symbols]

 3 Inverter 4 Induction motor 5 PWM inverter control circuit 9 Minimum current tracking control circuit

Claims (3)

(57) [Claims] 1. A method for controlling an induction motor by an inverter capable of obtaining an arbitrary output voltage by PWM control using a PWM control circuit, comprising: an output voltage command value for commanding the output voltage of the inverter; Preparing a sine wave AC signal having a frequency sufficiently lower than the output frequency of the inverter and having a lower amplitude than the steady-state output voltage of the inverter; An AC signal is superimposed, an input current of the induction motor is detected, and a phase angle of the sine wave AC signal superimposed on the output voltage command value is in a range from 0 degrees to 180 degrees. A first value consisting of a definite integral value is obtained, and the phase angle of the sine wave AC signal is changed from 180 degrees to 360 degrees.
A second value consisting of a definite integral value of the detected value of the input current is obtained in a section up to a degree. When the first value is larger than the second value, a positive constant value is obtained as a constant value K, The first value is equal to the second value
When the first value is equal to the second value, a negative constant value is obtained as 0 or a positive constant value or a negative constant value as the constant value K when the first value is the same as the second value. Integrating the constant value K to obtain an integral value of the constant value, subtracting the integral value of the constant value from the output voltage command value before or after superimposing the sine wave AC signal to obtain a modulation output voltage command value A method of controlling an induction motor, wherein the modulated output voltage command value obtained by superimposing the sine wave AC signal and subtracting the integral value of the constant value is given to the PWM control circuit as a voltage command value. 2. When accelerating the induction motor, gradually increasing the output frequency of the inverter and gradually increasing the output voltage of the inverter, and controlling the rotation speed of the induction motor to a predetermined rotation speed by the acceleration control. 2. The control method for an induction motor according to claim 1, wherein the PWM control based on the modulated output voltage command value is performed while the output frequency of the inverter is kept constant after the temperature has reached a predetermined value. 3. An apparatus for controlling an induction motor by an inverter capable of obtaining an arbitrary output voltage by PWM control using a PWM control circuit, wherein an output voltage command value for commanding the output voltage of the inverter is provided. Output voltage command value generating means for generating; and a sine wave AC signal having a frequency sufficiently lower than the output frequency of the inverter and generating a sine wave AC signal having an amplitude lower than the steady-state output voltage of the inverter. Signal generating means, means for superimposing the sine wave AC signal on the output voltage command value, means for detecting the input current of the induction motor, and phase of the sine wave AC signal superimposed on the output voltage command value A first value for obtaining a first value consisting of a definite integral value of the detected value of the input current in a section where the angle is from 0 degree to 180 degrees;
And a second constant integration means for obtaining a second value consisting of a constant integration value of the detected value of the input current in a section where the phase angle of the sine wave AC signal is 180 degrees to 360 degrees. When the first value is larger than the second value, a positive constant value is output as the constant value K. When the first value is smaller than the second value, a negative constant value is output as the constant value K. Comparing means for outputting 0, a positive constant value, or a negative constant value as the constant value K when the first value is the same as the second value; and a constant value obtained by integrating the constant value K. Constant integration means for obtaining an integral value of a numerical value; subtraction means for subtracting the integral value of the constant value from the output voltage command value before or after superimposing the sine wave AC signal to obtain a modulated output voltage command value; The sine wave AC signal is superimposed and the constant Control device for an induction motor and a means for providing the modulated output voltage command value that the integrated value obtained by subtracting the value as a voltage command value to the PWM control circuit.