JPH01170376A - Pwm inverter with current limiting function - Google Patents

Abstract

PURPOSE:To rapidly provide a current suppressing effect by conducting the current limiting operation of the output current of an inverter by the operating speed of a logic level in a decoder. CONSTITUTION:Upper and lower arms for forming the main circuit M of an inverter are turned ON, OFF and 3-phase AC is supplied to a load. This controller 4 has comparators 11A-11C, a triangular wave oscillator 12, and the comparator 11A inputs U-phase sine wave voltage command and a triangular wave, compares the command with the triangular wave, and produces outputs A (c), (a), (b). Other comparators 11B-11C similarly output, and each has, in addition to a main circuit M, a comparator 15, a level setter 16, a decoder 17, a timing clock generator 13, D-type flip-flops 14A-14C. The decoder 17 judges the igniting state of the switching element in the main circuit M up to that time, selects a circulation mode in response to it, and leads the main circuit M to the operating state of its mode.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a PWM (pulse width modulation) inverter that controls the speed of a load such as an induction motor by supplying a variable voltage output to the load. More specifically, a current limiting function that automatically limits the output current of the inverter to prevent the switching elements that make up the inverter from being destroyed when the load current suddenly rises and exceeds a certain limit. This relates to a PWM inverter.

[Prior Art] FIG. 2 is a circuit diagram showing a general pwm inverter that supplies voltage to a load.

In the figure, 1 is a DC voltage source consisting of a capacitor, P is a positive voltage, N is a negative voltage, Tr is a transistor, D is a diode, 3 is a load such as an induction motor, and 4 is a controller. The main circuit runs from the U phase, ■ phase, and W phase to a certain upper arm and
It is composed of a lower arm consisting of a phase, a Y phase, and a Z sum, and each phase is composed of a parallel-connected circuit of a transistor Tr and a diode D, each of which is a self-extinguishing switching element. On/off firing pulses (firing commands) for the transistors Tr as switching elements of each phase are generated and supplied by the controller 4, thereby supplying the load 3 with a three-phase AC voltage.

FIG. 3 is a waveform diagram showing the process of creating such on/off ignition pulses (ignition commands) for one phase. That is, a triangular carrier wave and a sine wave signal wave as shown in Fig. 3(a) are input to the controller l, and by comparing the two and making a decision, the signal wave as shown in Fig. 3(b) is input. An on-command with a modulation width like this is created and output.

Incidentally, since the above explanation is already well-known and well-known, there is no need to explain it in further detail.

[Problem that the invention seeks to solve]

Now, in the conventional inverter circuit as shown in Fig. 2, the current supplied to the load 3 is indirectly controlled by the output voltage from the inverter, and the current is not directly controlled. do not have. Even if a current regulator is installed, it will detect the load current and if it exceeds the limit value, it will adjust the output voltage of the inverter so that the load current is below the limit value, so it is still a direct It does not control current. As a result, there is a time delay before the current value is actually controlled, and during this time the current value may go too far and become excessive, which may destroy the transistors that serve as switching elements that make up the inverter's main circuit. Ta.

FIG. 4 is a waveform diagram showing an example of the waveform of such a load current.

If you look at the overall waveform as a sine wave, it is controlled to be a sine wave on average, but if you look at it more closely, you will notice that the waveform looks like a triangular wave superimposed on a sine wave. Dew.

This is because when the current value rises and exceeds the limit, the ignition pulse switches from Pl to P2, and therefore the conduction pattern of the switching element changes and the current value decreases, but in reality, this control is as described above. This is because voltage control is used as described above, which causes a time delay in current control, and as a result, the current value goes too far and becomes excessive. As a result, there was a risk that the transistor as a switching element would be destroyed.

Conventionally, countermeasures have been taken to suppress the current rate of change by inserting an AC reactor between the inverter and the load, but in this case, the inverter's current-carrying capacity must be increased relative to the load capacity. This causes problems such as increased costs.

An object of the present invention is to solve the problems of the prior art, to have a function of quickly limiting the output current when it exceeds a certain tolerance value, and to use a switching element without the need to insert an AC reactor. An object of the present invention is to provide a PWM inverter with a current limiting function that can prevent damage to the PWM inverter.

[Means for solving problems]

In order to achieve the above object, the present invention provides a PWM inverter that variably controls the voltage output to a load.
A first circulation mode in which an off state is input and only the switching elements belonging to the upper arm are turned on and the current circulates between the load and the bridge; and a first circulation mode in which only the switching elements belonging to the lower arm are turned on. Among the second freewheeling modes in which the current circulates between the load and the bridge, which freewheeling mode can be transitioned to by changing the state of the minimum number of switching elements from OFF to ON? a decoding circuit that outputs a state change command from off to on to a switching element that needs to change its state from an off state to an on state in order to shift to the free circulation mode; activating the decoding circuit when the amplitude of the output current exceeds a certain limit while monitoring the output current;
Equipped with a start-up recovery circuit that restores the power when it returns to within the limit.

[Effect]

Referring again to FIG. As is already well known,
When supplying three-phase AC from the inverter to the load 3, the upper arm (U, V, W phases) and lower arm (X, Y, Z phases) that make up the inverter's main circuit are connected to each other that belongs to the upper arm. Either two phases and one phase belonging to the lower arm are turned on, or two phases belonging to the lower arm and any one phase belonging to the upper arm are turned on, and three phases are applied to the load. AC is supplied.

This on/off state is shown in FIG. 2A as a conductive element pattern. In other words, a conductive element (switching element) that turns on
As shown in FIG. 2A, there are eight combination patterns, of which patterns 1lhl to 6 belong to the mode in which current is supplied from the inverter to the load.

However, only the three phases belonging to the upper arm were all turned on.
If none of the phases belonging to the lower arm turn on and remain off (pattern 7 in Figure 2A), or only three phases belonging to the lower arm turn on and none of the phases belonging to the upper arm turn on. If the inverter does not turn on and remains off (pattern 8 in Figure 2A), no current is supplied from the inverter to the load 3, and the current simply circulates between the inverter and the load 3. The operating state is in recirculation mode.

This will be explained using a specific example. Now, the three phases belonging to the upper arm (
Suppose that all phases (U, V, W phases) are turned on, and all phases belonging to the lower arm are turned off.

In this case, it is assumed that immediately before that, the current flowing into the load 3 from the R terminal flows back to the inverter side from the S terminal and the T terminal. Since there is inductance in the circuit, the current tends to continue its current flow.

As a result, the current circulates through the circuit of the R terminal, the load 3, the S terminal, the -phase diode D, and the U-phase transistor T r % R terminal. Also, the current is R terminal, load 3, T
terminal, W-phase diode D, U-phase transistor Tr,
It also flows into the R terminal circuit.

When operating in such a freewheeling mode, the flowing current necessarily decreases. The same is true when all three phases (X, Y, and Z phases) belonging to the lower arm are turned on and all phases belonging to the upper arm are turned off.

The present invention utilizes such a perfusion mode of operation. That is, when it is detected that the load current is excessive, the main circuit of the inverter is forcibly shifted to the freewheeling mode operating state to reduce the current, and when the load current is no longer excessive, the operating state of the inverter main circuit is changed. It returns it to its original state. Thereby, destruction of the switching element can be prevented without requiring insertion of an AC reactor.

Although it is called circulation mode, the three phases belonging to the upper arm (U
, V, W phases) are all on and all phases belonging to the lower arm are off, and the first circulation mode is where all three phases (X, Y, Z phases) belonging to the lower arm are on and the upper arm Since there is a second reflux mode in which all phases belonging to the reflux mode are turned off, it is necessary to separately judge which reflux mode it is advantageous to shift to.

That is, the firing status of the switching elements in the main circuit up to that point is 3 phases (U, V, W) belonging to the upper arm.
phase), two phases are on, and three phases belonging to the lower arm (X
, Y, Z phase) is on (see lines 4 to 6 in Figure 2A), the method is to turn on all three phases belonging to the upper arm and shift to the first circulation mode. However, this is more advantageous than turning on all three phases belonging to the lower arm and transitioning to the second circulation mode, since the number of switching elements that change from off to on can be reduced.

Similarly, if the firing status of the switching elements in the main circuit up to that point is that two of the three phases belonging to the lower arm are on and one of the three phases belonging to the upper arm is on. (11m1 to 3 in Figure 2A), it is better to turn on all three phases belonging to the lower arm and shift to the second circulation mode, or to turn on all three phases belonging to the upper arm and shift to the first circulation mode. This is advantageous because fewer switching elements are required to change from off to on than when switching from off to on.

In this way, the above-mentioned decoding is used to judge the firing status of the switching elements in the main circuit up to that point, select the more advantageous freewheeling mode accordingly, and guide the main circuit to the operating state of the freewheeling mode. It is a circuit. Moreover, since such a decoding circuit only needs to operate when the amplitude of the inverter's output current exceeds a limit, the above-mentioned startup recovery circuit monitors the amplitude of the inverter's output current and decodes it when necessary. Only operate the decoding circuit.

〔Example〕

Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

In the figure, 4 is a controller, and comparators 11A, IIB, 11C and a triangular wave oscillator 12 are shown as part of its circuit configuration.

The comparator 11A receives the U-phase sine wave voltage command and the triangular wave as input, compares them, and outputs a high (H level) or low (L level) output A. The other comparators 11B and 11C are set to high (H level) in exactly the same way.
Alternatively, low (L level) outputs B and C are output.

M indicates the main circuit side. In addition, 15 is a comparator,
16 is a level setter, 17 is a decoder, 13 is a timing clock generator, and 14A, 14B, and 14C are D-type flip-flops.

The comparator 15 is supplied with the amplitude value of the output current of the inverter, and compares this with the set level set in the level setter 16. outputs high (H level), but when it exceeds this, it outputs low (L level) and supplies it to the input terminal of the decoder 17.
Start up 7.

FIG. 1A shows the input side (A, B, etc.) of the decoder 17.

C) and output side (XI, X2.X3.X4.X5゜X6)
FIG. 2 is an explanatory diagram showing the relationship between truth values.

In the same figure, the mode of classification patient 1 corresponds to U, Y, Z (!1hl in Fig. 2A) of the conductive element pattern, and at this time, when the signal of Xi on the output side becomes high (H), An interrupt is generated from the R input terminal (reset input terminal) of the D-type flip-flop 14A, and the Q output signal of the flip-flop 14A is forcibly changed from high (H) to low (L), causing the conduction element to change. pattern x, y
, z (second freewheeling mode) to suppress the inverter output current. When the inverter output current decreases and the output of the comparator 15 returns to the high (H) level, D
The interrupt signal to the R input terminal of the flip-flop 14A returns to low (L) level, and the conductive element pattern also changes to X, Y.
, return from Z to U, Y, Z.

The same holds true for the other classification modes in FIG. 1A.

〔Effect of the invention〕

According to the present invention, the current limiting operation of the inverter output current is performed at the logic level operation speed in the decoder, so there is less delay in the operation from current detection to current limiting, and as a result, the current suppressing effect is reflected quickly. Particularly when a transistor with a small overcurrent withstand capacity is used as a switching element, its protection is effective.

In addition, since current can be suppressed without connecting an AC reactor between the inverter and the load, it is only necessary to select an inverter capacity that matches the motor capacity, which has the effect of not increasing costs. .

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, Fig. 1A is an explanatory diagram showing the relationship between truth values on the input side and output side of a decoder, and Fig. 2 is a general circuit diagram for supplying voltage to a load. A circuit diagram showing a PWM inverter, Fig. 2A is an explanatory diagram showing a conducting element pattern, Fig. 3 is a waveform diagram showing the process of creating ON/OFF firing pulses (ignition commands) for only one phase, and Fig. 4 is a waveform diagram showing an example of the waveform of load current. Explanation of symbols 4...Controller, 11...Comparator, 12...
Triangular wave generator, 13... timing clock generator,
14A, 14B, 14C...D type flip-flop,
15... Comparator, 16... Level setter, 17...
·decoder

Claims (1)

[Claims] 1) In a PWM inverter that variably controls the voltage output to a load, switching elements belonging to the upper and lower arms of the bridge that constitute the main circuit of the inverter are turned on and off.
A first circulation mode in which an off state is input and only the switching elements belonging to the upper arm are turned on and the current circulates between the load and the bridge; and a first circulation mode in which only the switching elements belonging to the lower arm are turned on. Among the second freewheeling modes in which the current circulates between the load and the bridge, which freewheeling mode can be transitioned to by changing the state of the minimum number of switching elements from OFF to ON? a decoding circuit that outputs a state change command from off to on to a switching element that needs to change its state from an off state to an on state in order to shift to the free circulation mode; activating the decoding circuit when the amplitude of the output current exceeds a certain limit while monitoring the output current;
A PWM inverter with a current limiting function, characterized in that it is equipped with a startup recovery circuit that restores the power when it returns to within a limit;