JP2002233187A - Apparatus and method for driving ac motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the conventional problem of a smoothing capacitor of small capacity being incapable of performing backup for an instantaneous power failure, in an apparatus for driving an induction motor with a circuit composed of a rectifying circuit, the smoothing capacitor and an inverter. SOLUTION: The induction motor 1 is driven by the rectifying circuit 5, the smoothing capacitor 6, and the variable-frequency variable-voltage inverter 7. The inverter 7 is constituted so as to be regeneratable. By reducing the output frequency and the output voltage of the inverter 7 at an instantaneous power failure, a regenerative operation is conducted, and the voltage of the smoothing capacitor 6 is controlled so as to be kept at a lower level than its normal one.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device and a method for driving an AC motor such as an induction motor, and more particularly, to a drive device comprising a combination of an AC-DC converter and a DC-AC converter. And a driving method.

[0002]

2. Description of the Related Art Induction motor driving devices comprising a combination of an AC / DC conversion circuit comprising a rectifier or a converter, a DC link capacitor or a smoothing capacitor, and a DC / AC conversion circuit comprising an inverter are widely used. .

[0003]

However, there is a case where the AC power supply is momentarily outaged or the voltage is lowered. If the capacity of the smoothing capacitor is increased, it is possible to cope with an instantaneous power failure, but this leads to an increase in cost and an increase in size. As a method of continuing the control of the inverter at the moment of a momentary power failure, the output frequency of the inverter is made lower than the rotational speed of the induction motor in the free-run state, the induction motor is operated as a generator, and the regenerative energy is passed through the DC-AC conversion circuit. A method of supplying the voltage to the smoothing capacitor is conceivable. However, even if the smoothing capacitor is charged with the regenerative energy of the induction motor,
The voltage of the smoothing capacitor becomes unstable due to a difference in environmental conditions such as a difference in inertia of the induction motor, and the control of the inverter cannot be stably continued. Further, there is a possibility that the inverter output will be in an overcurrent and overvoltage state after the power is restored.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and an apparatus for driving an induction motor capable of stably continuing the control operation of a DC-AC conversion circuit at the time of an instantaneous power failure or a drop in power supply voltage.

[0005]

SUMMARY OF THE INVENTION In order to solve the above problems and achieve the above object, the present invention provides an AC / DC conversion circuit connected to an AC power supply terminal, and a DC / DC converter of the AC / DC conversion circuit. A smoothing capacitor connected between the output terminals;
An AC motor is driven by a drive device including a regenerative DC-AC converter connected to the smoothing capacitor and a control circuit for controlling an output frequency and an output voltage of the DC-AC converter. The method, when the voltage of the AC power supply terminal becomes lower than a predetermined value,
The output frequency and output voltage of the DC-AC conversion circuit are maintained such that the voltage of the smoothing capacitor is maintained at a predetermined value between the normal value and zero by the regenerative operation of the AC motor and the DC-AC conversion circuit. And a method for driving an AC motor.

Further, when the voltage of the AC power supply terminal returns to a normal state after the voltage of the AC power supply terminal becomes lower than the predetermined value, the output frequency and the output voltage of the DC-AC conversion circuit may be further reduced. It is desirable to gradually increase one or both of them toward a normal value. Further, as set forth in claim 3, an AC-DC converter connected to an AC power supply terminal, a smoothing capacitor connected between a pair of DC output terminals of the AC-DC converter, and the smoothing capacitor. A regeneratively operable DC-AC conversion circuit connected to the
A drive device for an AC motor including a control circuit for controlling an output frequency and an output voltage of the DC-AC conversion circuit, wherein the control circuit detects a voltage of the smoothing capacitor. Power supply voltage detection means for detecting whether the power supply voltage of the AC power supply terminal is lower than a predetermined value, reference voltage generation means for generating a reference voltage indicating a reference of the voltage of the smoothing capacitor,
A difference signal forming means for forming a difference signal indicating a difference between the detected voltage obtained from the capacitor voltage detecting means and the reference voltage obtained from the reference voltage generating means, and that the power supply voltage is lower than a predetermined value. The output of the power supply voltage detecting means and the output of the difference signal forming means indicating that the detected voltage is lower than the reference voltage, the regenerative operation of the induction motor and the DC-AC conversion circuit, A frequency command and a voltage command for forming a command signal for controlling the output frequency and output voltage of the DC-AC conversion circuit so as to maintain the voltage of the smoothing capacitor at a predetermined value between the normal value and zero. It is desirable to have forming means. Further, as set forth in claim 4, the output frequency and the output voltage of the DC-AC conversion circuit are further changed in response to the output of the power supply voltage detecting means indicating that the power supply voltage has become higher than a predetermined value. It is desirable to have means for forming a frequency command and a voltage command for gradually increasing toward the normal output frequency and the normal output voltage. Further, as shown in claim 5, the frequency command and the voltage command for lowering the output frequency and the output voltage gradually decrease toward the respective lower limit values larger than zero, and decrease to the respective lower limit values. It is desirable that the lower limit be maintained at each time.

[0007]

According to the present invention, when the AC power supply voltage falls below a certain value due to an instantaneous power failure or voltage drop, the DC-AC conversion is performed so that the voltage of the smoothing capacitor is maintained at a predetermined value by the regenerative operation. Since the output frequency and output voltage of the circuit are controlled, the operation of the DC-AC conversion circuit can be stably continued. Further, according to the second or fourth aspect of the invention, it is possible to prevent a sudden change in the output current or voltage of the DC-AC conversion circuit after power recovery.

[0008]

Next, an embodiment of the present invention will be described with reference to the drawings.

A drive device 2 for a three-phase induction motor 1 according to an embodiment of the present invention shown in FIG. 1 has first, second and third input terminals 4a connected to a sine-wave three-phase AC power supply 3 of, for example, 50 Hz. , 4b, 4c, a rectifier circuit 5, a smoothing capacitor 6, which can also be called a DC link capacitor, a variable frequency variable voltage type inverter 7, a PWM control signal forming circuit 8, a capacitor voltage detecting circuit 9, And a frequency command and voltage command forming circuit 10.

As shown in FIG. 2, a rectifier circuit 5 as an AC-DC converter circuit includes first to sixth diodes connected to three-phase first, second and third input terminals 4a, 4b and 4c. It consists of three-phase bridge type rectifier circuits 5a to 5f. In addition,
The rectifier circuit 5 can be modified to a well-known converter circuit of a type for turning on / off a semiconductor switch, that is, a switching rectifier circuit.

The smoothing capacitor 6 is connected between a pair of DC lines 11 and 12 between a pair of DC output terminals of the rectifier circuit 5 and a pair of input terminals of the inverter 7. The smoothing capacitor 6 functions as a DC power supply for the inverter 7, the PWM control signal forming circuit 8, and the frequency command and voltage command forming circuit 10.

The inverter 7 is a well-known three-phase bridge type inverter. For example, as shown in FIG. 2, first, second, third, fourth, fifth and fifth transistors formed of three-phase bridge-connected transistors are provided. The sixth switch Q1, Q2, Q3,
Q4, Q5, Q6 and first, second, third, fourth, fifth, and sixth diodes D connected in parallel in the reverse direction.
1, D2, D3, D4, D5, D6. First to first
The sixth diodes D1 to D6 have a function of sending regenerative power of the induction motor 1 to the smoothing capacitor 6 and a function of protecting the first to sixth switches Q1 to Q6. The inverter 7 can control the output frequency and the output voltage.

Induction motor 1 connected to inverter 7
Drives the rotary load 13 having inertia.
When the rotor of the induction motor 1 rotates by inertia in a state where power supply from the inverter 7 to the induction motor 1 is stopped, the induction motor 1 operates as a generator and sends regenerative electric power to the inverter 7 side.

The PWM control signal forming circuit 8 is a bridge-connected first to sixth switch Q1 of the inverter 7 of FIG.
To generate a first to a sixth control signal for turning on and off Q6, and as shown in FIG. 3, a frequency command line 20, a voltage command line 21, and a voltage controlled oscillator or VCO 22. , A three-phase sine wave generating circuit 23, first, second and third multipliers 24, 25, 26, a sawtooth wave generating circuit 27, and first, second and third comparators 2.
8, 29, 30 and first, second, and third NOT circuits 3
1, 32, 33 and a dead time giving circuit 34.

VCO connected to frequency command line 20
Reference numeral 22 generates a clock signal having a frequency corresponding to the frequency command voltage Vf1 on the line 20. The three-phase sine wave generating circuit 23 connected to the VCO 22 synchronizes with the output of the VCO 22 to generate a three-phase sine wave voltage V having a phase difference of 120 degrees.
a, Vb, and Vc. Three-phase sine wave voltages Va, Vb
, Vc change in accordance with the output frequency of the VCO 22.

The first, second and third multipliers 24 connected to the voltage command line 21 and the three-phase sine wave generation circuit 23,
25, 26 are first, second and third sine wave voltages Va, V
b, Vc multiplied by the voltage command voltage V1 of the line 21 to obtain the first, second, and third phase voltage command values Va ', Vb', Vc '.
Is output.

The sawtooth wave generating circuit 27 has a three-phase sine wave voltage Va.
, Vb and Vc at a sufficiently high repetition frequency (for example, 20 to 150 kHz), a carrier signal is generated from the sawtooth voltage or the triangular wave voltage.

The first, second and third comparators 28,
29, 30 are first, second and third multipliers 24, 2
5, 26 and the sawtooth wave generating circuit 27, and the first and second
And the third-phase voltage command values Va ', Vb', Vc 'and the sawtooth voltage Vt are compared to determine whether the first, third and fifth switches Q1
Control signals Vg1, Vg3, Vg for Q3, Q3, Q5
Form 5 FIG. 6A shows the first comparator 28.
6A shows the first phase voltage command value Va 'and the sawtooth voltage Vt, and FIG. 6B schematically shows the first control signal Vg1 output from the first comparator 28. I have.
The second and third comparators 29 and 30 operate similarly to the first comparator 28. First, second and third N
The OT circuits 31, 32, and 33 are provided for the second, fourth, and sixth switches Q2, Q4, and Q6 each comprising a phase inverted signal of the output of the first, second, and third comparators 28, 29, and 30. Second, fourth and sixth control signals Vg2, Vg4, Vg6
To form The dead time providing circuit 34 is provided by a known method between the ON periods of the first and second switches Q1 and Q2, between the ON periods of the third and fourth switches Q3 and Q4, and between the fifth and fifth switches Q3 and Q4. A pause or dead time is provided between the on-periods of the switches Q5 and Q6 to prevent them from being simultaneously turned on. The first to sixth control signals Vg1 to Vg6 to which the dead time has been given are sent to the control terminals of the first to sixth switches Q1 to Q6. When the first to sixth switches Q1 to Q6 are turned on and off according to the first to sixth control signals Vg1 to Vg6, a three-phase sine wave output can be obtained from the inverter 7.

The frequency command and voltage command forming circuit 10
As shown in FIG. 4, a line 9a to which the output of the voltage detection circuit 9 for detecting the voltage of the smoothing capacitor 6 of FIG.
Reference voltage generation circuit 41, first subtractor 42, compensator 43, limiter 44, second subtractor 45, voltage command value forming circuit 46, power failure detection circuit 47, acceleration time determination circuit 48, a frequency determination reference value generation circuit 49, a correction value generation circuit 50, and a third subtraction circuit 51.

Next, the frequency command and voltage command generation circuit 10 of FIG. 4 will be described in detail with reference to FIG. The power failure detection circuit 47 is connected to the input terminals 4a and 4b, and as shown in FIG. 5A, the amplitude of the input voltage Vin between the input terminals 4a and 4b is t1 to t.
As shown in period 3, when it is lower than the predetermined level,
As shown in (C), a low level signal Ta indicating a power failure state is output. The reference voltage generation circuit 41 generates a reference voltage Vdcr corresponding to the detection voltage Vdc of the smoothing capacitor 6 at the time of normal operation, that is, at the time of no power failure. One input terminal of the first subtractor 42 is connected to the line 9a, and the other input terminal is connected to the reference voltage generation circuit 41. The output of the first subtractor 42 for calculating Vdcr-Vdc is zero or relatively small when the AC power supply 3 is normal, that is, when the power is on, and increases when the power supply is interrupted during t1 to t3.

A compensator 43 connected to the first subtractor 42
Consists of, for example, a proportional-integral (PI) compensator and a subtractor 4
2 is output as a smoothed signal. The limiter 44 connected to the compensator 43 limits the compensated difference signal ΔVdc to determine the lower limit value of the inverter output frequency at the time of a power failure, and outputs ΔVf. If the control according to the present invention is not performed, the capacitor detection voltage Vdc drops rapidly, and the operation of the inverter 7 and the PWM control signal forming circuit 8 cannot be continued. On the other hand, in the present embodiment, the voltage Vdc of the smoothing capacitor 6 is generated by the regenerative operation.
Is suppressed as shown in FIG. 5 (E). The output of the first subtractor 42 gradually increases from the power failure start time t1 in FIG. 5, but is limited to the maximum value Vmax by the operation of the limiter 44. If the power outage time is short, the power may be restored before the limiter 44 operates.

The frequency reference value generating circuit 49 is shown in FIG.
The frequency reference value Vfr shown is generated. This frequency reference value V
fr corresponds to a target value of the output frequency of the inverter 7 in a normal state. The output frequency of the inverter 7 changes by changing the reference value Vfr.

The correction value generation circuit 50 includes a power failure detection circuit 4
7, the acceleration time determination circuit 48, which is connected to the compensator 43, and which is determined based on the correction value Vfc shown in FIG.
Occurs. The correction value Vfc increases with a slope from the correction reference value, ie, zero, in synchronization with the start of the power failure period Ta shown at t1 to t3 in FIG. 5C, and has a slope from t3 to t4 after the end of the power failure. Fall. The period from t3 to t4 is arbitrarily determined by the acceleration time determination circuit 48. T1 of FIG.
The period t3 is a deceleration correction section, and the period T3 to t4 is an acceleration correction section. The value Vfacc of the acceleration period Tb indicated by t3 to t4 included in the correction value Vfc of FIG. Vfacc = K / (At) Here, K indicates a compensator coefficient provided from the compensator 43, t indicates an acceleration time up to a target frequency of the inverter, and A indicates an acceleration coefficient. The change in the correction value during the power outage period Ta can be represented by Kt. Note that the coefficient K has a value corresponding to the output level of the compensator 43.

The third subtractor 51 outputs the reference value V shown in FIG.
The correction reference value Vrc of FIG. 5I is created by subtracting the correction value Vfc of FIG. 5H from fr and sent to the second subtractor 45.

The second subtractor 45 subtracts the difference signal ΔVf shown in FIG. 5 (F) from the correction reference value Vfrc in FIG. 5 (I) to generate a frequency command voltage Vf1 shown in FIG. 5 (J). . This frequency command voltage Vf1 gradually decreases during the power failure period Ta from t1 to t3, and the acceleration period Tb from t3 to t4 after the power failure ends.
Gradually increase. The frequency command voltage Vf1 is supplied to the VCO 22 of FIG. The VCO 22 sends a clock having a frequency proportional to the frequency command voltage Vf1 to the three-phase sine wave generation circuit 23. Therefore, the inverter 7
Output frequency gradually decreases during the power outage period Ta, and the regenerative operation of the induction motor 1 rotating by the inertia of the load 13 becomes possible. In the acceleration period Tb after the power recovery, the frequency command voltage Vf1 gradually increases, so that the rotation speed Nr of the induction motor 1 also gradually increases as shown in FIG. 5B, and overcurrent and overvoltage are prevented. This prevents a trip operation, that is, an interruption of the inverter operation.

The voltage command forming circuit 46 of FIG. 4 includes a level converting circuit 52, a corrector 53, and a corrected voltage generating circuit 54.
And a CR time constant circuit 55. Level conversion circuit 5
2 is connected to the line 20 and converts the level of the frequency command voltage Vf1 so as to match the level of the voltage command voltage V1. By this conversion, a voltage Vv1 proportional to Vf1 is obtained. The correction voltage generating circuit 54 is a voltage command voltage V1 in a normal state.
A correction voltage Vc of a lower value is generated. This correction voltage Vc determines the lower limit value of the capacitor voltage Vdc at the time of power failure. The corrector 53 is connected to the level conversion circuit 52, the correction voltage generation circuit 54, and the power failure detection circuit 47.
And a switch circuit for selectively transmitting the correction voltage Vc during the period t1 to t3, and selectively transmitting the output Vv1 of the level conversion circuit 52 during the other periods. Therefore, the compensator 5
5 outputs Vv1 before t1 and after t3 in FIG. 5, and outputs Vc from t1 to t3. The RC time constant circuit 55 connected to the compensator 53 can be called a second compensator. The RC time constant circuit 55 converts the step-like signal obtained from the compensator 53 into a section between t1 and t2 in FIG. t3 to t4
This is for gradually changing with a time constant as shown in the section. The voltage command voltage V1 obtained from the time constant circuit 55 is applied to the first, second and third signals shown in FIG.
Are sent to the multipliers 24, 25, 26.

When the voltage command voltage V1 is changed as shown in FIG. 5K during the power failure period Ta and the acceleration period Tb after the power recovery,
The output voltage V0 of the inverter 7 also changes in proportion to this. If the output voltage of the inverter 7 is kept low during the power outage period Ta, the ON time width of each of the switches Q1 to Q6 of the inverter 7 becomes short, the power supply to the induction motor 1 as a load is limited, and the smoothing capacitor 6 When the induction motor 1 is rotated by inertia, the smoothing capacitor 6 is charged by the regenerative operation, and the voltage drop of the capacitor 6 is suppressed as shown in FIG. Is done. As a result, the operation of the inverter 7 can be continued even when a relatively long power failure or voltage drop occurs in a state where the inertia of the load 13 is relatively small. If a power failure occurs after the end of the regenerative operation, the capacitor voltage Vdc decreases, and the operation of the inverter 7 stops. Also,
The acceleration time Tb after power recovery is arbitrarily set by the acceleration time determination circuit 48, and the output frequency and the output voltage of the inverter 7 are suppressed from increasing sharply. As shown in the figure, the temperature rises gently, preventing overcurrent and also preventing overvoltage. As a result, the operation of the inverter 7 can be stably continued without interrupting the control of the inverter 7. Further, since the voltage command voltage V1 is created using the output of the second subtractor 45 for creating the frequency command voltage Vf1, the frequency command and the voltage command can be easily formed by a simple circuit.

[0028]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The inverter 7 can be another type of inverter such as a single-phase bridge type inverter. (2) Although FIG. 1 and FIG. 2 do not show a feedback control circuit based on the detection of the output voltage and frequency of the inverter 7, these can be added. (3) In FIG. 4, the frequency command voltage Vf1 obtained from the frequency command forming circuit is input to the voltage command forming circuit 46, but the voltage command voltage V1 may be formed without using the frequency command voltage Vf1. (4) The frequency command voltage Vf1 and the voltage command voltage V1 are
It can be formed by various circuits other than FIG. (5) The switches Q1 to Q6 can be semiconductor switches such as IGBTs or FETs. Further, the diodes D1 to D6 can be built-in diodes of the switches Q1 to Q6. (6) When power is restored, the frequency command voltage Vf1 and the voltage command voltage V
It is also possible to gradually increase only one of them.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a driving device for an induction motor according to one embodiment of the present invention.

FIG. 2 is a circuit diagram showing a rectifier circuit and an inverter of FIG. 1;

FIG. 3 is a circuit diagram illustrating a PWM control signal forming circuit of FIG. 1 in detail;

FIG. 4 is a circuit diagram showing a frequency command and voltage command forming circuit of FIG. 1;

FIG. 5 is a waveform chart showing a state of each unit in FIGS. 1 and 3;

FIG. 6 is a waveform chart showing input and output of the comparator of FIG. 2;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Induction motor 5 Rectifier circuit 6 Smoothing capacitor 7 Inverter 8 PWM control signal formation circuit 10 Frequency command and voltage command formation circuit

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[Procedure amendment]

[Submission Date] February 25, 2002 (2002.2.2)
5)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0024

[Correction method] Change

[Correction contents]

The third subtractor 51 outputs the reference value V shown in FIG.
The correction reference value Vfrc of FIG. 5I is created by subtracting the correction value Vfc of FIG. 5H from fr and sent to the second subtractor 45.

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference)

Claims (5)

[Claims] 1. An AC-DC converter connected to an AC power supply terminal, a smoothing capacitor connected between a pair of DC output terminals of the AC-DC converter, and a regenerative device connected to the smoothing capacitor. A method of driving an AC motor by a driving device including an operable DC-AC conversion circuit and a control circuit for controlling an output frequency and an output voltage of the DC-AC conversion circuit, wherein the AC power supply terminal When the voltage becomes lower than a predetermined value, the regenerative operation of the AC motor and the DC-AC conversion circuit keeps the voltage of the smoothing capacitor at a predetermined value between the normal value and zero. A method for driving an AC motor, comprising controlling an output frequency and an output voltage of the DC-AC conversion circuit. 2. The method according to claim 1, wherein when the voltage of the AC power supply terminal becomes lower than the predetermined value and then returns to a normal state, one or both of the output frequency and the output voltage of the DC-AC conversion circuit are changed. 2. The driving method according to claim 1, wherein the value is gradually increased toward a normal value. 3. An AC-DC converter connected to an AC power supply terminal, a smoothing capacitor connected between a pair of DC output terminals of the AC-DC converter, and a regenerative device connected to the smoothing capacitor. An AC motor driving device comprising: an operable DC-AC conversion circuit; and a control circuit for controlling an output frequency and an output voltage of the DC-AC conversion circuit, wherein the control circuit comprises: Capacitor voltage detecting means for detecting the voltage of the capacitor; power supply voltage detecting means for detecting whether the power supply voltage of the AC power supply terminal is lower than a predetermined value; and a reference indicating the reference of the voltage of the smoothing capacitor A reference voltage generating means for generating a voltage; and forming a difference signal indicating a difference between a detected voltage obtained from the capacitor voltage detecting means and a reference voltage obtained from the reference voltage generating means. Difference signal forming means, an output of the power supply voltage detecting means indicating that the power supply voltage is lower than a predetermined value, and an output of the difference signal forming means indicating that the detected voltage has dropped below the reference voltage. Based on the output frequency of the DC-AC conversion circuit, the voltage of the smoothing capacitor is maintained at a predetermined value between the normal value and zero by the regenerative operation of the induction motor and the DC-AC conversion circuit. And a frequency command and voltage command forming means for forming a command signal for controlling the output voltage. 4. An output frequency and an output voltage of the DC-AC conversion circuit in response to an output of the power supply voltage detecting means indicating that the power supply voltage has become higher than a predetermined value. 4. The driving device according to claim 3, further comprising means for forming a frequency command and a voltage command for gradually increasing the output voltage toward the output voltage. 5. The frequency command and the voltage command for lowering the output frequency and the output voltage gradually decrease toward respective lower limit values larger than zero and when reaching the respective lower limit values, The driving device according to claim 3, wherein the driving device is maintained at a lower limit.