JPS62123965A - How to limit the output current of voltage source PWM inverter - Google Patents

Abstract

PURPOSE:To make it possible to implement the sequential control by detecting the deviation exceeding the predetermined current limit value of the inverter output current with its polarity and in accordance with this by correcting the output voltage command value. CONSTITUTION:An induction motor 2 is driven at variable speed with a voltage type PWM inverter 1 composed of transistors Tr. This variable speed drive is performed by carrying on the on-off control of the upper and bottom arm transistors Tr by means of a frequency setting device 10, an arithmetic circuit 11 operating the output voltage command value V*u-V*w, PWM pattern generation circuits 20-40 and a carrier signal generator 12. In this case, coefficient multipliers 25, 35 and 45 are provided as well as an adder 24 with which each output of the multipliers above-mentioned is added to the voltage command V*u-V*w from the voltage command arithmetic circuit 11. Thus if the positive current flows in the U-phase and its value comes up to more than the current limiting value, for example, then the coefficient multiplier 25 gives a positive signal to the adder 24 in proportion to the deviation exceeding this current limiting value from the zero output condition, and corrects the voltage command value to suppress the output current.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a current limiting method in a voltage source PWM (Pulse Width Modulation) inverter.

[Conventional technology]

As a conventional technique, a case will be described in which a voltage type PWM inverter is used to drive an induction motor at variable speed. FIG. 4 is a block diagram showing such a control system.

The main circuit consists of six switching elements (here, transistors 'I'rt to T'r6 are shown as an example, but they are not limited to this) as shown in the same figure (a), and processes reactive components. It is comprised of a normal voltage-type three-phase inverter 1 consisting of six diodes D1 to D6 for the purpose of controlling the power supply, and a three-phase induction motor 2 serving as a load, for example. Further, although current control is not particularly performed in such a system, a current detector 3 is usually provided to protect the device.

Incidentally, a constant V/F (voltage/frequency) control method is known as a general example of variable speed driving of an induction motor in such a system. Since this method is already well known, the details will be omitted here, but the basic operation will be explained with reference to FIG. 4(b).

In the same figure (b), first, when a frequency command is set by the frequency setter 1o, output voltage command values vU, vv, vw that give the magnitude and frequency of the three-phase voltage to be output from the inverter 1 are set accordingly. The calculation is performed by the calculation circuit 11, and the U, V, W phase PWM pattern generation circuits 20, 30 .
40 each will be powered by humans. PWM pattern generation circuit 2
0°30,. Reference numeral 40 is a voltage command value MU given from the voltage command value calculation circuit 11, which is composed of an adder 21, a comparator 22, an inverter 23, etc., respectively.

vv, vw are compared with the carrier signal from the carrier signal generator 12, and the output signal is sent to a pace drive circuit corresponding to each transistor (not shown) in order to control on/off the upper and lower arm transistors of the corresponding phase according to the result. give to

Now, the cause of so-called overcurrent, in which the output current of the PWM inverter exceeds the limit value, will be explained.

@4 Overcurrent can occur in a system like the one shown in Figure 4, broadly speaking, in the following two ways. One of them is when a fault current flows due to a short circuit in the output, and in this case, it is necessary to immediately turn off the transistor, stop the operation of the inverter, and reduce the current. The other case is when an overcurrent occurs during control, such as when the load on the motor suddenly changes. In this case, it is desirable to control the current so that it does not exceed the overcurrent level without reducing the current or stopping the inverter. As a control method, for example, when sudden changes in the set value occur,
One possible method is to once return the speed setting value to the value before the sudden change inside the calculation circuit 11, and then gradually change this setting value once the overcurrent is resolved. In addition, in response to a sudden change in load, in order to reduce the slip S of the induction machine 2 within the calculation circuit 11, the speed setting value is lowered once when S>0, and then the speed setting value is lowered as the overcurrent is resolved. A conceivable method is to gradually increase the value of S, while performing the opposite control when the slip S is So.

[Problem that the invention seeks to solve]

By the way, all of the above methods are required to be executed at a speed sufficiently commensurate with the change in current, but if the arithmetic circuit 11 is composed of, for example, a microprocessor, the control It takes a considerable amount of time to
In reality, there are many cases where overcurrent cannot be suppressed, so conventionally, even in such cases, it is common to pulse off all transistors to cut off the current, just as in the case of a failure. In other words, even if an overcurrent occurs due to control or load changes, the inverter is stopped, so the motor current immediately drops to zero, and when the inverter starts operating, the current rises again, resulting in repeated current interruptions. . Therefore, systems that do not like the occurrence or frequent repetition of such current interruptions have the problem of having to replace the inverter with a large capacity inverter.

Therefore, the present invention suppresses overcurrents that occur during control, such as sudden changes in set values or sudden changes in load, by suppressing them so that they do not exceed the overcurrent level. The purpose of this invention is to provide a control method that allows continuous control without intermittent lightning.

[Means for solving problems]

A detection means for detecting the deviation of the inverter output current exceeding a predetermined current limit value along with its polarity, and a correction means for correcting the output voltage command value according to the deviation and its polarity are provided.

[Effect]

This was done by focusing on the fact that in a voltage source PWM inverter, by changing the output voltage command value that is compared with the carrier signal, the current slope can be changed at a speed determined approximately by the carrier frequency, and overcurrent can be changed. When an overcurrent occurs, the overcurrent is suppressed by adding the current deviation from the current limit value and the offset according to its polarity to the output voltage command value until a voltage command value that does not cause an overcurrent is calculated. This is to suppress the increase in

〔Example〕

FIG. 1 is a block diagram showing an implementation eAJ of the present invention.

As is clear from the figure, this embodiment has a coefficient unit 25.
35.45 and their respective outputs to the voltage command calculation circuit 11.
The present invention is characterized in that it is provided with an adder 24 that adds the voltage command V♂l vV''I vW'' from the voltage command V♂lvV''IvW'' with the polarity as shown in the figure, and other aspects are the same as in FIG. In addition, PWM
Although only the U phase is specifically shown in the pattern generation circuit (see reference numeral 20), it goes without saying that the pattern generation circuit is similarly constructed for the 2 and W phases.

The coefficient multipliers 25, 35, and 45 are composed of amplifiers having a dead band corresponding to the current limit value, or those having an equivalent function, and each of them is connected to the current detector 3 that detects the output current of each phase of the inverter. It is connected. Therefore, the coefficient multipliers 25, 35, 45 detect the deviation of the inverter output current exceeding the above current limit value along with its polarity, and send the output to the PWM pattern generating circuit 20, 30, 40 of each phase.
It is applied to the adder 24 in the figure with the polarity shown. In addition,
The polarity of the current detected by each coefficient multiplier is made to match the polarity of the output voltage signal, and in this case, for example, it is assumed that the current flowing in the motor is positive.

In this way, for example, when a positive current flows in the U phase and its value exceeds the current limit value, the coefficient multiplier 25 generates a positive current proportional to the deviation exceeding the current limit value from the previous state of zero output. Output a signal. This output is given to the adder 24 as a negative offset, regardless of the polarity of the voltage command value YU itself. On the other hand, when the current in the negative direction exceeds the limit value, it is given as a positive offset to the voltage command value VU. Furthermore, this is V.

The same applies to the W phase. In this way, the voltage command value is corrected according to the current deviation and its polarity, and the output current is thereby suppressed by performing voltage control similar to the conventional method.

The reason why overcurrent can be suppressed by the above-described control will now be explained.

Generally, the main circuit of a PWM inverter is
It can be considered in terms of phase components, and its equivalent circuit is shown in Figure 2 (a).
It can be considered as a model of an induction motor represented by an AC voltage source vU (-vU), an axle L, and a back electromotive force eU. In addition, in this equivalent circuit, a current flows due to the voltage difference between Vll and eU, so the equivalent circuit in the same figure (a) can be further transformed into a circuit consisting of a differential voltage source V Ue U and a reactor L as shown in the same figure (b). can be converted. Note that the polarity of the 1jt flow is positive in the direction in which it flows to the motor, as in FIG. 1, so the direction of the arrow in FIG. 2 is positive.

Now, consider the case where the positive current exceeds its limit value. In this case, as described above, the negative offset is superimposed on the voltage command value vU4) via the coefficient multiplier 25, and as a result, the output voltage can be controlled at approximately the speed of the carrier frequency. The equivalent circuit of (b) is shown in Figure 3 (
You can think of it as (a). In other words, Figure 3 (
As shown in (a), the negative offset voltage ΔV acts in the direction (-) of decreasing the current by 7°C with respect to the positive current, so it is possible to suppress the increase in the current. Similarly, when the current in the negative direction exceeds the limit value, a positive offset amount ΔV is superimposed on the voltage command value vUK, so the equivalent circuit becomes as shown in FIG. 3(b). In this case as well, the positive off-cella)7jj pressure Δ
Since V works in the direction of decreasing the negative current,
With this, it is possible to limit the current mt.

In this way, it can be seen that the current can be limited only by the polarity of the output current, regardless of the magnitude and polarity of the differential voltage vU-eU. The fact that it is unrelated to the differential voltage also means that it is unrelated to the inverter's output voltage vU1 and the magnitude of the back electromotive force eU of the load (1 phase). This means that current can be limited regardless of the current.

〔Effect of the invention〕

As described above, according to the present invention, when the output current exceeds the limit value, further increase in current is almost instantaneously suppressed,
During this time, the arithmetic circuit calculates a voltage command value that provides a current value that does not exceed the limit value, which provides the advantage that normal control can be returned to without interrupting the current. Furthermore, when performing the above calculation,
In an induction motor, it is necessary to determine the polarity of the S, but this can be done at the same speed as the calculation of the voltage command value.
It goes without saying that this is completely possible.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing an equivalent circuit for one phase of an inverter in a steady state. Figure 3 is a circuit diagram showing an equivalent circuit for one phase of the inverter when current is limited, and Figure 4 is a configuration diagram showing a conventional example of a voltage-type PWM inverter control system. Code explanation 1:...-@Kugata PWM inverter, 2...Induction motor, 3...Electronic Chaos detector, 10...
・Frequency setter, 11...Medium voltage command value calculation circuit, 1
2. River... carrier signal generator, 20, 30, 40...
...PWM pattern generation circuit, 21, 24...
... Adder, 22 ... Humparator, 23 ...
...Inverter, 25, 35, 45...Coefficient unit. Agent Patent Attorney Akio Namiki Agent Patent Attorney Kiyota Matsuzaki 15! I1 WE2 figure (Rono figure 3 (b)

Claims (1)

[Claims] In a voltage-type PWM inverter that performs voltage control based on a pulse width modulation (PWM) signal obtained by comparing an output voltage command signal and a carrier signal that determines the modulation frequency, the deviation of the output current that exceeds a predetermined current limit value By providing a detection means for detecting the deviation and its polarity, and a correction means for correcting the output voltage command value according to the deviation and its polarity, and performing voltage control based on the corrected output voltage command value, An output current limiting method for a voltage source PWM inverter, which is characterized by limiting the output current to a predetermined value.