JP4488415B2 - Power converter - Google Patents

Description

  The present invention relates to a power converter for driving an electric motor, and more particularly to a power converter having improved control characteristics.

There are several methods for stably controlling the motor load. For example, it is possible to detect the output current of the inverter that drives the AC motor, pay attention to the change in this current, and correct the AC motor in the deceleration direction when the current increases and the AC motor in the acceleration direction when the current decreases. Conceivable. However, in this case, for example, when the current increases, there is a risk of erroneous control unless it is distinguished whether the current is to be accelerated or decelerated. For this reason, a proposal has been made to convert the three-phase output current of the inverter into a three-phase to two-phase, convert it into an effective current and a reactive current, and correct in the deceleration direction when the effective current increases. In other words, an increase in effective current means an increase in torque and acts in the direction of accelerating the load. Therefore, when the effective current increases, the frequency reference is lowered to compensate for the direction of suppressing the AC motor speed (for example, patents). Reference 1).
JP-A-2-276490 (page 2-3, FIG. 1)

  In the case of the correction shown in the above-mentioned Patent Document 1, the stability of control may be a problem when starting and accelerating to be completed at a low speed or when restarting in a low speed range. When the acceleration is completed after startup, the effective current corresponding to the acceleration disappears, and the control circuit corrects in the acceleration direction. Thereafter, the correction amount is decreased by the time constant in the control circuit. When the deceleration rate due to this reduction amount is faster than the free-run characteristic of the AC motor, the AC motor changes from the power running mode to the regeneration mode. If the inverter does not have a regenerative function, the regenerative energy cannot be consumed and the inverter becomes unstable.

  In addition, when restarting at a low speed due to an instantaneous power failure or the like, a current flows due to the influence of the phase shift immediately after the restart, and the DC voltage and the output voltage decrease. The control circuit is corrected in the decelerating direction by the current, and thereafter, the correction amount decreases with a time constant in the control circuit, so that the control circuit is accelerated. At this time, the output voltage decreases and the frequency reference increases, so the voltage / frequency ratio decreases. In particular, the low speed range is easily affected by the voltage drop of the cable and the internal impedance drop of the AC motor, so when the voltage / frequency ratio decreases, the torque becomes insufficient and becomes unstable.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a power converter capable of stably controlling an AC motor when performing transient operation such as restart.

To achieve the above object, a power conversion device according to a first aspect of the present invention includes a converter that converts alternating current into direct current, a smoothing capacitor that smoothes the output of the converter, and converts the smoothed direct current into alternating current. The inverter that drives the AC motor, the current detection means that detects the output current of the inverter and converts it into effective current and reactive current, the control means that controls the inverter, and the control of the control means is corrected And a stabilization control means for subtracting a signal obtained by integrating the output of the signal from a signal obtained by multiplying the difference between the effective current and the reactive current by a gain, and the stabilization control means. When the output change rate of the pseudo-differential circuit becomes larger than the change rate limit value that defines the upper limit value of the deceleration rate according to the speed of the AC motor, the output of the pseudo-differential circuit is changed to the change rate. A change rate limit circuit that limits the rate limit value and holds the integral value of the pseudo-differential circuit, and adds and corrects the output of the change rate limit circuit to the frequency reference of the control means. It is characterized by that.

Moreover, the power converter device which is 2nd invention of this invention, The converter which converts alternating current into direct current,
A smoothing capacitor for smoothing the output of the converter; an inverter for driving the AC motor by converting the smoothed direct current to alternating current; and a current for detecting the output current of the inverter and converting it into an active current and a reactive current Detection means, voltage detection means for detecting the output voltage or DC voltage of the inverter, two sets of control means for normal and standby systems for controlling the inverter, and stabilization control means for correcting the control of the control means The stabilization control means subtracts a signal obtained by integrating the output of the signal from a signal obtained by multiplying the difference between the active current and the reactive current by a gain, and calculates the frequency reference of the control means based on the calculation result. And an integration hold means for holding the integrated value of the pseudo differentiation circuit, and the control means is switched from the normal system to the standby system and restarted. When, sets the integration-hold means, the output of the voltage detecting means is characterized in that so as to reset the integrator hold means when a predetermined value or more.

  According to the present invention, since the control characteristics are improved so as to be stable, it is possible to provide a power converter that can stably control the AC motor when performing transient operation such as restart.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a circuit configuration diagram showing a first embodiment of a power converter according to the present invention.

  The alternating current supplied from the three-phase alternating current power supply 1 is converted into direct current by the converter 2 and the smoothing capacitor 3 and supplied to the inverter 4. The inverter 4 converts the direct current into alternating current again to drive the alternating current motor 5. A current detector 6 is provided on the output side of the inverter 4, and a voltage detector 7 is provided on the DC link portion.

  The converter 2 and the inverter 4 are constituted by a main circuit in which power devices are bridge-connected, and these power devices are turned on / off by gate pulses from the main control unit 8.

  The main control unit 8 receives an external speed command and uses this as one input of the adder 81. The other input of the adder 81 is given from a stabilization control circuit 9 described later. The output of the adder 81 becomes the frequency reference of the AC motor 5 and becomes the input of the phase calculator 82. The phase calculator 82 calculates the output phase of the inverter 4 for stably driving the AC motor 5. The output of the phase calculator 8 gate-drives the power device of the inverter 4 via the gate control circuit 83.

  Further, the speed command makes a voltage reference via the V / f converter 84, and this voltage reference is compared with the DC voltage from the voltage detector 7 by the differentiator 85, and the error output thereof is input to the phase calculator 86. Become. The output of the phase calculator 86 drives the power device of the converter 2 through the gate control circuit 87.

  The output of the current detector 6 is given to the three-phase / two-phase converter 10. The three-phase / two-phase converter 10 converts the output current of the inverter 4 into an effective current and a reactive current according to the phase reference determined by the phase calculator 82 and outputs the converted current. The effective current and the reactive current are input to the stabilization control circuit 9.

  Hereinafter, the internal configuration of the stabilization control circuit 9 will be described.

  The difference between the above-described effective current and reactive current is calculated by the differentiator 91 and multiplied by K by the gain 92. The output of the gain 92 becomes the addition input of the difference unit 93, and the value obtained by integrating the output of the difference unit 93 by the integrator 94 is the subtraction input of the difference unit 93. Here, the gain 92, the difference unit 93, and the integrator 94 constitute a pseudo-differential circuit, and respond instantaneously to the step input with an output of K times, and return to the original value by the integration time constant of the integrator 94. Has characteristics.

  The output of the pseudo-differential circuit is input to the adder 81 described above via the change rate limit circuit 95. The change rate limit circuit 95 fixes the output at this upper limit value when there is an input exceeding the upper limit value of the deceleration rate set by the change rate limit value setting circuit 96 in accordance with the frequency reference value described above, and At that time, the operation of the integration circuit 94 is held.

  With the above circuit configuration, it is possible to perform stabilization correction in the acceleration direction when the effective current decreases and in the deceleration direction when the effective current increases, and the rate of change of the correction signal in the deceleration direction does not exceed a predetermined value. Thus, it becomes possible to perform the stabilization control smoothly. Further, the reactive current is input to the stabilization control circuit 9 with the reverse polarity, so that the reactive current can be kept almost constant during normal operation, and stable control can be performed during a transition such as control switching.

  In FIG. 1, the voltage control is performed by controlling the phase of the converter 2. However, the converter 2 may be a diode converter, and the output voltage may be controlled by PWM control of the inverter 4. .

  A power converter according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 2 is a circuit configuration diagram showing Embodiment 2 of the power conversion apparatus of the present invention. About each part of this Example 2, the same part as each part of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The difference between the second embodiment and the first embodiment is that the change rate limit value setting circuit 96 </ b> A is set according to the free-run deceleration characteristic of the AC motor 5.

  If the change rate limit value of the change rate limit value setting circuit 96A is obtained in this way, the amount of deceleration greater than the free-run deceleration rate of the load machine and the AC motor 5 is not corrected and added. This will not cause any problems. When the free-run deceleration rate of the AC motor 5 varies depending on the operating conditions, the change rate limit value setting circuit 96A may be set on the safe side where the deceleration rate is the smallest.

  The operation and effect at the start-up in this case will be described with reference to FIG. FIG. 3 is a time chart showing an output current at the time of startup and a correction amount which is an output of the stabilization control circuit 9. As shown in FIG. 3, the startup current flows at the start of load application, and therefore the correction amount is shifted to the negative side. However, since the frequency reference at the time of activation is fixed to the activation frequency, it is not decelerated by this correction amount.

  Thereafter, when the frequency is increased at a predetermined rate according to the acceleration start condition, a constant acceleration current flows. In this acceleration state, the output of the pseudo-differential circuit decreases toward zero with the time constant of the integrator 94. When the acceleration is completed, the output current decreases and the correction amount swings to the plus side. The AC motor 5 can be decelerated upon completion of acceleration. However, if the deceleration rate exceeds the deceleration rate determined by the free-run characteristics of the AC motor 5 due to some control disturbance, the correction amount is positive as shown in the figure. Since it is held in the state and the frequency reference is corrected in the acceleration direction, there is no problem that the control becomes unstable.

  FIG. 4 is a circuit configuration diagram showing Embodiment 3 of the power conversion device of the present invention. About each part of this Example 3, the same part as each part of the power converter device which concerns on Example 2 of FIG. 2 is shown with the same code | symbol, and the description is abbreviate | omitted. The third embodiment is different from the second embodiment in that a deceleration signal given from the main control circuit is provided, and a switching circuit 97 for selecting a deceleration rate given from the main control circuit and a differentiator 98 are provided and changed. The output of the switching circuit 97 is subtracted by the differentiator 98 from the change rate limit value that is the output of the rate limit value setting circuit 96A, and this output is given to the change rate limit circuit 95 as the change rate limit value. .

  With the above configuration, even when the AC motor 5 is decelerating, the change rate limit value of the change rate limit circuit 95 is determined by subtracting this deceleration rate from the value determined by the change rate limit value setting circuit 96A. Thus, as in the case of the second embodiment, it becomes possible to prevent the control from becoming unstable.

  A power converter according to a fourth embodiment of the present invention will be described with reference to FIGS. 5 and 6. FIG. 5 is a circuit configuration diagram showing Embodiment 4 of the power converter of the present invention. About each part of this Example 4, the same part as each part of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The fourth embodiment is different from the first embodiment in that the change rate limit circuit 95 and the change rate limit value setting circuit 96 are omitted, and the correction output of the differentiator 93 is directly input to the adder 81. The point where the voltage detector 11 for detecting the output voltage of the inverter 4 is provided, the comparison circuit 99 and the flip-flop 100 are provided, and the flip-flop 100 is set by the restarting signal given from the main control circuit 8 to integrate the integrator The integration value of 94 is held, the level corresponding to the integration hold value and the output voltage of the voltage detector 11 are compared by the comparator 99, and the integration hold of the integrator 94 is canceled when the output voltage becomes larger. This is the point.

  As described above, when restarting, the integration hold signal is set, and when the output voltage exceeds a predetermined value, resetting the integration hold signal will reduce the voltage / frequency ratio at the time of restart. Instability can be prevented.

  Note that the above-described restart may be performed in the case of a power failure restart or when the control system is doubled with the normal system and the standby system, and the normal system is switched to the standby system. In the former case, the main control circuit 8 has a function of detecting a power failure, stopping the inverter 4, and turning on an appropriate phase upon power recovery detection and restarting it, but is not shown in FIG. . In the latter case, the main control circuit 8 has a dual configuration of a normal system and a standby system and can be switched, but the illustration is also omitted. In addition to the main control circuit 87, the stabilization control circuit 9 and the three-phase / two-phase converter 10 may be included in the dual configuration of the normal system and the standby system.

  The operation and effect at the time of restart of the fourth embodiment will be described with reference to FIG. FIG. 6 is a time chart showing frequency reference, output voltage, and output current at the time of restart. As shown in FIG. 6, when a power failure or a failure occurs in the regular control system, the inverter 4 is stopped and the rotational phase of the AC motor 5 is calculated from the output voltage to prepare for restart. When the power can be restored or switched to the standby system and can be restarted, the inverter 4 is restarted by adjusting the output phase of the inverter 4 to the estimated phase of the AC motor. Immediately after the restart, current flows due to the influence of a slight phase shift between the AC motor 5 and the inverter 4 and the influence of the difference between the DC voltage and the output voltage. Thereafter, when the AC motor 5 rotates, the energy of the DC circuit is consumed and the DC voltage decreases. At this time, since the reactive current decreases, the frequency reference is corrected in the deceleration direction.

  According to the present invention, when the output voltage is lower than the integration hold level in the above state, the integrator 94 is held, the frequency reference is made constant, and the voltage / frequency ratio at the time of restarting decreases and becomes unstable. To prevent becoming. Without such a measure, the correction amount is reduced by the integration time constant of the integrator 94, that is, corrected in the acceleration direction, so that the voltage is lowered and the frequency reference is in the acceleration direction. There is a risk that the voltage / frequency ratio is lowered and the output current is greatly pulsated and unstable.

  As described above, stable control can be performed if the frequency-based correction amount is held at a constant value when a power failure occurs or when the control system is switched.

  It is obvious that a DC voltage by the voltage detector 7 may be used instead of the output voltage of the inverter 4 by the voltage detector 11.

The circuit block diagram which shows Example 1 of the power converter device which concerns on this invention. The circuit block diagram which shows Example 2 of the power converter device which concerns on this invention. 9 is a time chart for explaining the operation of the second embodiment. The circuit block diagram which shows Example 3 of the power converter device which concerns on this invention. The circuit block diagram which shows Example 4 of the power converter device which concerns on this invention. 9 is a time chart for explaining the operation of the fourth embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 3 phase alternating current power supply 2 Converter 3 Smoothing capacitor 4 Inverter 5 AC motor 6 Current detector 7 Voltage detector 8 Main control circuit 81 Adder 82 Phase calculator 83 Gate controller 84 V / f converter 85 Difference unit 86 Phase calculation Unit 87 Gate controller 9 Stabilization control circuit 91 Difference unit 92 Gain 93 Difference unit 94 Integrator 95 Change rate limit circuit 96, 96A Change rate limit value setting circuit 97 Switcher 98 Difference unit 99 Comparator 100 Flip-flop 10 3 phase / 2 phase converter 11 Voltage detector

Claims (5)

A converter that converts alternating current to direct current,
A smoothing capacitor for smoothing the output of the converter;
An inverter that converts the smoothed direct current into alternating current to drive an alternating current motor; and
Current detection means for detecting an output current of the inverter and converting the current into an effective current and a reactive current;
Control means for controlling the inverter;
Stabilization control means for correcting the control of the control means,
The stabilization control means includes
A pseudo-differential circuit that subtracts a signal obtained by integrating the output of this signal from a signal obtained by multiplying the difference between the effective current and the reactive current by a gain;
When the output change rate of the pseudo-differential circuit becomes larger than a change rate limit value that defines an upper limit value of the deceleration rate according to the speed of the AC motor, the output of the pseudo-differential circuit is limited to the change rate limit value. And a change rate limit circuit configured to hold an integral value of the pseudo-differential circuit,
A power conversion apparatus characterized in that the output of the change rate limit circuit is added and corrected to the frequency reference of the control means.   The power conversion device according to claim 1, wherein the change rate limit value is a deceleration rate determined by a free-run characteristic of the AC motor. The stabilization control means includes
According to the deceleration signal obtained from the control means and the deceleration rate at that time,
During deceleration, the value obtained by subtracting the deceleration rate from the deceleration rate determined by the free-run characteristic of the AC motor is the change rate limit value,
2. The power conversion device according to claim 1, wherein a deceleration rate determined by a free-run characteristic of the AC motor is set as the change rate limit value at other times. A converter that converts alternating current to direct current,
A smoothing capacitor for smoothing the output of the converter;
An inverter that converts the smoothed direct current into alternating current to drive an alternating current motor; and
Current detection means for detecting an output current of the inverter and converting the current into an effective current and a reactive current;
Voltage detection means for detecting the output voltage or DC voltage of the inverter;
Two sets of control means, a normal system and a standby system, for controlling the inverter;
Stabilization control means for correcting the control of the control means,
The stabilization control means includes
A pseudo-differential circuit that subtracts a signal obtained by integrating the output of this signal from a signal obtained by multiplying the difference between the effective current and the reactive current by gain, and corrects the frequency reference of the control means by the calculation result;
An integration hold means for holding the integrated value of the pseudo-differential circuit;
When the control means is switched from the normal system to the standby system and restarted,
Set the integral hold means,
The power conversion apparatus according to claim 1, wherein the integration hold means is reset when the output of the voltage detection means becomes a predetermined value or more. A converter that converts phase AC to DC,
A smoothing capacitor for smoothing the output of the converter;
An inverter that converts the smoothed direct current into alternating current to drive an alternating current motor; and
Current detection means for detecting an output current of the inverter and converting the current into an effective current and a reactive current;
Voltage detection means for detecting the output voltage or DC voltage of the inverter;
Control means for controlling the inverter and having a power failure restarting function;
Stabilization control means for correcting the control of the control means,
The stabilization control means includes
A pseudo-differential circuit that subtracts a signal obtained by integrating the output of this signal from a signal obtained by multiplying the difference between the effective current and the reactive current by gain, and corrects the frequency reference of the control means by the calculation result;
An integration hold means for holding the integrated value of the pseudo-differential circuit;
When the power failure is restarted by the control means,
Set the integral hold means,
The power conversion apparatus according to claim 1, wherein the integration hold means is reset when the output of the voltage detection means becomes a predetermined value or more.

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