JPH09117152A - Current controller for voltage type pwm inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current controller for voltage type PWM inverter suitable for attaining highly accurate and stabilized current control which can be synchronized easily with a carrier signal even when the frequency is varied depending on the operating conditions of inverter during operation. SOLUTION: The current controller for voltage type PWM inverter detects the output current from a PWM inverter 1 at a predetermined current control period and produces a PWM signal by comparing an instantaneous voltage command value, obtained by operating the detected value, with a carrier signal. The current controller for voltage type PWM inverter comprises means 9 for measuring the current control period, means 14 for setting a minimum necessary value of current control period, and means 12, 10, 15 for outputting a signal at a moment of time when the carrier signal has a maximum amplitude after the count of measuring means 9 exceeded the minimum necessary value. Current detection and current control processing is performed in synchronism with the outputted signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for a voltage type PWM inverter controlled by a PWM signal.

[0002]

2. Description of the Related Art An output current of a voltage type PWM inverter PWM-controlled by a pulse width modulation signal (PWM) obtained based on a modulated wave signal and a carrier wave signal pulsates into a fundamental wave component in response to switching of a power element. It becomes a current waveform in which the components are superimposed. As a method of detecting the fundamental wave component as much as possible from such output current using a microcomputer,
For example, the technique described in Japanese Patent Publication No. 4-47554 is known. This method is a method of detecting a current at the time of maximum amplitude of a carrier signal, that is, a method of detecting and controlling an output current in synchronization with a carrier signal, thereby detecting a fundamental wave component of an output current of an inverter. be able to. Here, the frequency of the carrier signal is
Due to the use of IGBTs for power elements and the like, it is becoming possible to operate at higher frequencies up to about 10 kHz. By increasing the frequency of the carrier wave signal, current ripple due to switching of the power element can be reduced, and electromagnetic noise due to current ripple can be reduced. On the other hand, there are problems such as an increase in switching loss and an increase in noise at the time of switching due to the higher frequency of the carrier signal. Therefore, it is required to change the frequency of the carrier signal according to the inverter operating conditions, and to lower the frequency of the carrier signal to operate under conditions where low loss is required.

[0003]

In the method of detecting and controlling the output current in synchronism with the carrier wave signal as in the prior art, the current control cycle changes with the frequency change of the carrier wave signal. In particular, when the frequency of the carrier wave signal becomes high, it is necessary to shorten the current control cycle, and for that purpose, a faster microcomputer and A / D converter are required. Moreover, the current control cycle is determined from the required current control response, and even if the current control cycle is made shorter than necessary, the control accuracy is deteriorated and the control performance cannot be improved. Further, when the control processing is executed by a signal at a constant cycle by the timer, in order to synchronize the carrier signal with the control cycle, when the frequency of the carrier signal changes, the microcomputer outputs the cycle of the signal output by the timer accordingly. Need to change. This places a heavy burden on the microcomputer.

An object of the present invention is to achieve stable and highly accurate current control by easily synchronizing the current control with the carrier signal when the frequency of the carrier signal is changed according to the operating conditions of the inverter during operation of the inverter. Another object of the present invention is to provide a current control device for a voltage-type PWM inverter suitable for the purpose.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a measuring means for measuring a current control cycle, a means for setting a necessary minimum value of the current control cycle, and a means for detecting a time point of maximum amplitude of a carrier signal. A means for raising a flag is provided, and at the same time as the current detection and the current control processing are executed at the time of the maximum amplitude of the carrier signal, the measuring means starts counting, and then when the count value becomes equal to the required minimum value, the flag is turned on. This is accomplished by standing up and subsequently counting by the measuring means and starting the next current detection and current control process at the first maximum amplitude of the carrier signal after the flag is raised. Here, after the count value of the measuring means is read at the time of the first maximum amplitude of the carrier wave signal, the count value is initialized and the flag is cleared. Further, a means for actually measuring the current control cycle is provided, the count value of the measuring means for measuring the current control cycle at the time when the flag is set is read, the previous current control cycle is calculated, and the current control cycle is used. This is achieved by modifying the constants of the control system, such as constants.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a current control device for a voltage-type PWM inverter and a power conversion device using the same according to an embodiment of the present invention. In FIG. 1, the power converter is P
It consists of a WM inverter 1, a forward converter 3, and a capacitor 4. An AC voltage of an AC power supply 2 is converted into a direct current by the forward converter 3, and a DC voltage smoothed by the capacitor 4 is input to the PWM inverter 1. The DC voltage is pulse-width modulated and converted into a three-phase AC voltage, which is applied to the induction motor 5. Further, the current control device of the voltage type PWM inverter includes a current detector 6, an A / D converter 7, a microcomputer 8, a counter 9, a flip-flop 10, a speed command circuit 11, a carrier signal generation circuit 12, a comparator 13, and a minimum. Value discrimination circuit 1
4, an AND circuit 15, and a latch circuit 16.

The operation of this current control device will be described with reference to the time schedule shown in FIG. In FIG.
(1) is a carrier signal having a frequency of 10 kHz, (2) is a signal f output from the carrier signal generating circuit 12 when the carrier signal of (1) is at the maximum amplitude point, and (3) is a counter 9
(4) is the output signal e of the minimum value determination circuit 14,
(5) shows the output of the flip-flop 10, (6) shows the output signal g of the AND circuit 15, and (7) shows the current control execution status. Now, a three-phase current flows from the PWM inverter 1 to the induction motor 5, two of the three-phase currents are detected by the current detector 6, and the A / D converter 7 outputs a current detection signal from the microcomputer 8. When receiving a, the A / D converter 7
Converts the current value detected by the current detector 6 into a digital value and inputs it to the microcomputer 8. The microcomputer 8 is P
After detecting the current of the WM inverter 1, control processing is performed so that the value of the current flowing through the induction motor 5 responds to the current command value obtained as described later (FIG. 2 (7)).
In this control process, a voltage command value for the PWM inverter 1 is calculated, and instantaneous voltage signals (modulation wave signals) c for three phases having a sine waveform are output according to the voltage command value. The PWM signal for controlling the PWM inverter 1 is obtained by comparing the modulated wave signal c and the triangular wave carrier wave signal d (FIG. 2 (1)) output from the carrier wave signal generating circuit 12 by the comparator 13. . Apply this PWM signal to the PWM inverter 1,
The induction motor 5 is controlled to the speed command value given by the speed command circuit 11. In such a control system, the carrier signal generation circuit 12 generates the carrier signal d (see FIG. 2) generated by itself.
The signal f ((2) in FIG. 2) at the maximum amplitude point in (1)) is input to the AND circuit 15. On the other hand, the counter 9 continues to count up during execution of the above-described current detection and control processing. A count value corresponding to the time required for current control is preset in the minimum value determination circuit 14. For example, A / D
It takes 30 μs to convert all the current values (current values for two phases) that the converter 7 needs to detect into a digital amount and output the converted value to the microcomputer 8, and 170 μs for the control process executed thereafter. In this case, at least 200 μs is required from the current detection to the end of the control process. Therefore, considering the margin, the minimum required time of the current control cycle is set to 260 μs. Here, the reason why the time required for current control is overestimated with a margin is that the microcomputer 8 processes the speed command value input from the speed command circuit 11 in addition to the control process at regular intervals, in the present embodiment. This is because, for example, since it is performed every 5 ms, time for that is required. When the counter 9 counts up, for example, every 1 μs, the minimum value determination circuit 14 has a count value 260 of 260 μs.
Set. After the control process ends, when the count value of the counter 9 becomes equal to the set value 260 of the minimum value determination circuit 14 (260 μs after the start of current detection), the minimum value determination circuit 1
4 outputs the compare match signal e to the flip-flop 10 (FIG. 2 (4)). When the compare match signal e is input, the flip-flop 10 changes the output to the AND circuit 15 from the "0" level to the "1" level (flag is set) (FIG. 2 (5)). The counter 9 continues to count up. The carrier signal generation circuit 12 outputs the signal f to the AND circuit 15 when the carrier signal d is at the maximum amplitude point (FIG. 2 (2)).
When the output “1” from the flip-flop 10 is input to the AND circuit 15, the AND circuit 15 outputs the current control start signal g to the microcomputer 8 (FIG. 2 (6), 300 μ in this embodiment).
s). When the microcomputer 8 receives the current control start signal g, it outputs the current detection signal a to the A / D converter 7,
When the conversion is completed, that value is fetched as a current detection value and the next current control is started (FIG. 2 (7)). Further, the microcomputer 8 outputs the current detection signal a to the A / D converter 7 and at the same time outputs the reset signal b to the counter 9 and the flip-flop 10 to initialize the count value of the counter 9 and output the flip-flop 10. To "0" level. The counter 9 counts up after inputting the reset signal b ((3) in FIG. 2). The current control start signal g output from the AND circuit 15 is also output to the latch circuit 16. When the current control start signal g is input, the latch circuit 16 receives the count value (300 μm) of the counter 9 at that time.
s) is detected and retained. The above is the operation executed within one current control cycle. The control cycle at this time is three cycles (300 μs) of the carrier wave signal.

Next, at the moment when the microcomputer 8 outputs the current detection signal a to the A / D converter 7 by the above-mentioned current control start signal g, the frequency of the carrier signal changes from 10 kHz to 8 kHz, for example, depending on the operating condition of the inverter. Suppose The operation at this time will be described using the time schedule shown in FIG. In FIG. 3, (1) is a carrier signal,
(2) shows the operation of the counter 9, and (3) shows the current control execution status. When the frequency of the carrier signal is 10 kHz, the control cycle is three cycles (300 μs) of the carrier signal,
As described in FIG. 2, the microcomputer 8 uses the counter value of the counter 9 and the signal f at the maximum amplitude point of the carrier wave signal d.
Current control start signal g from AND condition (circuit 15) with
When (after 300 μs) is received, the current detection signal a is output to the A / D converter 7, and the next current control is started. Here, it is assumed that the frequency of the carrier signal changes to 8 kHz. In this case also, as in FIG. 2, the counter 9 counts up, and the count value is the set value 260 of the minimum value determination circuit 14.
Then, the minimum value determination circuit 14 outputs the compare match signal e to the flip-flop 10, and the flip-flop 10 outputs “1” to the AND circuit 15. After that, when the signal f representing the maximum amplitude point of the carrier signal is input from the carrier signal generation circuit 12 to the AND circuit 15, the AND circuit 15 causes the current control start signal g (312.
5 μs later) is output to the microcomputer 8, and the microcomputer 8 starts the next current control, and at the same time, the latch circuit 16 detects and holds the count value of the counter 9 at that time. The control cycle at this time is 2.5 cycles of the carrier signal (31
2.5 μs). This means that even if the frequency of the carrier signal is changed during the operation of the inverter, the current control is synchronized with the carrier signal. As described above, in the present embodiment, when the frequency of the carrier signal is changed according to the operating condition of the inverter during the operation of the inverter, in particular, the frequency of the carrier signal is increased and the frequency of the carrier signal is decreased depending on the operating condition of the inverter. Even if it is changed to, the current control can be easily synchronized with the carrier signal.

Next, the microcomputer 8 outputs the current detection signal a to the A / D converter 7, then reads the value (300 μs) held by the latch circuit 16 and calculates the previous current control cycle. (300 × 1 μs = 300 μs), and the constants of the control system such as the time constant used in the current control process are corrected based on the calculation result. The control process is performed on the assumption that the control cycle is known in advance, but in the present embodiment, the control cycle changes as the frequency of the carrier signal changes.
The control cycle cannot be known in advance. Therefore, the constants of the control system such as the time constant are corrected based on the previous control cycle, and the control process is executed. In this example, the control cycle this time is 312.5 μs, but this value cannot be known at the time of execution of the control processing this time. Therefore, the constants of the control system such as the time constant are corrected based on the previous control cycle of 300 μs, and the control process is executed. In this way, these constants include an error of 12.5 μs, which is the difference between the current control cycle and the previous control cycle.
When operating the WM inverter, the frequency of the carrier signal is not changed abruptly as in this example. Therefore, even when the frequency of the carrier wave signal is changed according to the operating condition of the inverter, the cycle difference of continuous current control is small and it can be said that the control system is not affected.

FIG. 4 shows the structure of the carrier signal generating circuit 12. The clock pulse h output from the clock pulse generator 24 is counted by the up / down counter 25. The count value of the counter 25 is the maximum value determination circuit 27.
Is compared with the set maximum value (corresponding to the positive maximum amplitude value of the carrier signal), and the overflow signal i is output when the count value becomes equal to the set maximum value. Signal i is an OR circuit 2
It is added to the flip-flop 30 through 9. The flip-flop 30 changes its output from "1" level to "0" level. The counter 25 switches between up-counting and down-counting according to the output state of the flip-flop 30, and when the count value of the counter 25 reaches the maximum value, it switches to down-counting. Similarly, when the count value becomes equal to the minimum value (corresponding to the maximum negative amplitude value of the carrier signal), the minimum value determination circuit 28 outputs the underflow pulse j. When the underflow pulse j is input, the counter 25 switches from down count to up count. The counter 25 repeats such an operation and outputs a triangular wave-shaped count value that changes between the maximum value and the minimum value. The count value is converted into an analog amount by the D / A converter 26 to obtain the carrier signal d. On the other hand, the output signal f of the OR circuit 29 is generated when the carrier signal has the maximum positive and negative amplitude values, and is output to the AND circuit 25 of FIG. When the output frequency of the clock pulse generator 24 is changed, the counting speed of the counter 25 is changed and the frequency of the carrier wave signal d can be changed.

Next, FIG. 5 shows the control processing contents of the microcomputer 8. In the embodiment shown in FIG. 1, the current flowing through the induction motor 5 is decomposed into a component parallel to the rotating magnetic flux of the induction motor 5 (excitation current component) and a component orthogonal thereto (torque current component), and each component is independently controlled. By doing so, high response control similar to that of a DC machine is realized. The inside of the broken line in FIG. 5 is the processing content performed by the microcomputer 8. As described above, the microcomputer 8 performs the control process after detecting the current at the timing of the maximum amplitude point of the carrier wave signal d. The processing will be described below with reference to the flowchart of FIG. First, the A / D conversion of the A / D converter 7 is started by the conversion process 7, and the current values of the two phases detected by the current component decomposition process 35 are set as an exciting current component parallel to the rotating magnetic flux of the induction motor 5. It is decomposed into a torque current component that goes straight to it.
The current values of the two components after the decomposition are called the exciting current and the torque current, which are represented as Id and Iq, respectively. A speed detection value Wr is calculated from the torque current Iq by the speed calculation process 31.
The speed control value 3 is calculated by calculating the difference between the speed detection value Wr and the speed command value f * read from the speed command setting circuit 11 in FIG.
Execute Step 2. As a result, the current command value Iq * of the torque current component is obtained. Further, the difference between the torque current command value Iq * and the torque current Iq is calculated, the torque current control process 33 is executed, and the frequency f ₁ of the voltage applied to the induction motor 5 is obtained. Next, the difference between the pre-given excitation current command value Id * and the excitation current Id is calculated, the excitation current control process 34 is executed, and the current command value Idcom of the excitation current component is obtained. Then, the excitation current component command value Idcom and the torque current I
Using q, the frequency f ₁ and constants such as the internal resistance of the induction motor 5, the voltage command value calculation processing 36 determines the voltage command values of the three phases. In response to this voltage command value, instantaneous voltage signals (modulation wave signals) c for three phases having a sine waveform are output. This completes the current control once, and the result of the control process is output as an instantaneous voltage signal (modulated wave signal) c. Then, the same processing is performed again after the next current detection.

Next, an example of correcting the constants of the control system such as the time constants used in the control process using the actually measured current control cycle will be described. The excitation current control processing 34 of FIG. 6 among the control processing contents of the microcomputer 8 is shown in a block diagram in FIG. 7. Exciting current command value I given in advance
Calculate the difference between the exciting current Id of the detected current and d *, and PI
The control is performed and the exciting current component command value Idcom is output. If only I (integration) control is taken out of these, FIG.
become. When this is z-converted at the sampling cycle (current control cycle) t, it is expressed as shown in FIG. As a result, the integration constant y is obtained as in the following equation. y = t × Ki This current control cycle t is obtained by reading the value held by the latch circuit 16 by the microcomputer 8 (hereinafter,
The same is true. ). The filter process is used for removing the noise of the current detected by the current component decomposition process 35 of FIG. 6 and the like, and is represented by the block diagram of FIG. 10 when the time constant is T. When this is z-converted at the sampling cycle (current control cycle) t, it is expressed as shown in FIG. As a result, the filtering constant y is obtained by the following equation. y = t / (t + T) Further, the voltage command value calculation processing 36 of FIG. 6 outputs the instantaneous voltage signal c having a sine waveform, and therefore calculates the phase. The phase calculation constant used at this time is y, the current control cycle is t,
Assuming that the output frequency of the PWM inverter 1 is f, the phase calculation constant y can be obtained by the following equation. y = 2π × t × f As described above, among the various constants of the control system used in the control process, the integration constant, the filtering constant, and the phase calculation constant are obtained from the current control cycle t. In the present embodiment, since the current control cycle t is measured every time as described above, the current control cycle measured for each control process is applied to the above equation, each constant is calculated, and the result is used for the control process. As a result, even if the frequency of the carrier wave signal changes during the inverter operation and the current control cycle changes, advanced current control is executed without degrading control accuracy.

In the above embodiments, the case where the modulated wave signal for generating the PWM signal and the carrier wave signal are asynchronous types has been described. However, the present invention has the same relationship when they are synchronous types. Applicable. In the above embodiment, the maximum amplitude point of the carrier signal is both the maximum value and the minimum value of the carrier signal, but only at the maximum value, or
By outputting the signal f at the maximum amplitude point of the carrier signal only to the minimum value to the AND circuit 15, the current control can be performed in synchronization with the maximum value of the carrier signal or in synchronization with the minimum value. .

[0014]

As described above, according to the present invention,
When the frequency of the carrier signal is changed according to the operating conditions of the inverter during operation of the inverter, in particular, even if the frequency of the carrier signal is increased and changed to a lower frequency depending on the operating conditions of the inverter, current control is performed. Can be easily synchronized with the carrier signal,
Highly accurate current control can be realized. Further, according to the present invention, each time the current control cycle is measured, the constants of the control system are calculated using the measured current control cycle for each control process, and the result can be used for the control process. Even if the frequency of the carrier signal changes during operation and the current control cycle changes, advanced current control can be executed without degrading control accuracy.

[Brief description of the drawings]

FIG. 1 is a current control device of a voltage-type PWM inverter showing an embodiment of the present invention and a power conversion device using the same.

FIG. 2 is a time schedule showing the operation of the present invention.

FIG. 3 is a time schedule showing the operation of the present invention when the frequencies of carrier signals are different.

FIG. 4 is an explanatory diagram of a carrier signal generation circuit.

FIG. 5 is a block diagram of control processing.

FIG. 6 is a flowchart of control processing.

FIG. 7 is a block diagram of the exciting current control process.

FIG. 8 is a block diagram of integral control.

FIG. 9: Z conversion of integral control

FIG. 10 is a block diagram of filtering processing.

FIG. 11: Z transformation of filtering

[Explanation of symbols]

 1 PWM Inverter 2 Power Supply 3 Forward Converter 5 Induction Motor 6 Current Detector 7 A / D Converter 8 Microcomputer 9 Counter 10 Flip Flop 11 Speed Command Circuit 12 Carrier Wave Signal Generation Circuit 13 Comparator 14 Minimum Value Discrimination Circuit 15 AND Circuit 16 Latch circuit

Claims (5)

[Claims] 1. A voltage-type PWM controlled by a PWM signal obtained by detecting an output current of a PWM inverter at every predetermined current control cycle and comparing an instantaneous voltage command value calculated from the detected value with a carrier signal. In the current control device of the inverter, measuring means for measuring the current control cycle, means for setting a necessary minimum value of the current control cycle, and a carrier signal after the time when the count value of the measuring means exceeds the necessary minimum value. A current control device for a voltage-type PWM inverter, characterized in that means for outputting a signal at the time of maximum amplitude is provided, and current detection and current control processing are executed in synchronization with the signal. 2. A voltage-type PWM inverter controlled by a PWM signal by detecting an output current of a PWM inverter for each predetermined current control cycle, comparing an instantaneous voltage command value calculated from the detected value with a carrier signal, and controlling the PWM signal. The current control device is provided with a measuring means for measuring the current control cycle, a means for setting the required minimum value of the current control cycle, and a means for detecting the maximum amplitude time point of the carrier signal, and the current at the maximum amplitude time point of the carrier signal. At the same time performing the detection and current control process,
The measuring means starts counting, and when this count value becomes equal to the required minimum value, a flag is set, then the measuring means continues counting, and the first maximum of the carrier signal after the flag is set. A current control device for a voltage-type PWM inverter, which starts the next current detection and current control processing at the time of amplitude. 3. The clock pulse generator according to claim 2, wherein the means for detecting the time point of maximum amplitude of the carrier wave signal comprises a clock pulse generator, an up / down counter, a maximum value discriminating circuit and a minimum value discriminating circuit. Or, compare the down count value with the setting value corresponding to the maximum positive or negative amplitude value of the carrier signal, and when the up or down count value exceeds the positive or negative setting value, the maximum carrier signal Voltage type P characterized by detecting the time point of amplitude
Current control device for WM inverter. 4. The maximum amplitude time point of the carrier signal according to claim 1, wherein both the maximum value and the minimum value of the carrier signal, or only the maximum value time point, or the minimum value time point. A current control device for a voltage-type PWM inverter, which is set only at a time point. 5. The method according to claim 1, further comprising means for measuring the current control cycle, and correcting the constant of the control system used for the current control process by using the measured current control cycle. The current control device of the characteristic voltage type PWM inverter.