JP2712418B2 - Pulse width modulation type inverter device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse width modulation type inverter device for obtaining an alternating current having a variable voltage and a variable frequency, and more particularly to a pulse width modulation type inverter device for controlling a carrier frequency to be high.

[Conventional technology]

FIG. 5 shows an example of a configuration of a conventional pulse width modulation type inverter device. In FIG. 5, (10) is a DC power supply, (20) is a diode which is connected to a controllable element and anti-parallel, and has a variable voltage. Inverter for converting into variable frequency alternating current (inverter), (30) a motor, (40) a reference voltage generator that outputs a reference voltage waveform that serves as a reference for the output frequency and output voltage of the inverter output, (50) Is a carrier generator that creates and outputs a carrier waveform of frequency fc with a waveform such as a triangular wave,
(60) is a reference voltage generator (40) and a carrier generator (50)
Pulse width modulation (hereinafter, PWM) that generates a firing signal (PWM signal) for the controllable element of the inverter (20) based on the output signal of
And (70) a drive circuit that receives the signal of the PWM circuit (60) and drives the controllable element of the inverter (20).

Next, the operation of the above configuration will be described with reference to the drawings. Sixth
The figure is a typical operation diagram of this type of PWM control, and is an explanation of the operation of the U, V, and W three-phase PWM inverters with respect to one U phase. First, the output voltage of the inverter, a reference voltage serving as a reference for the output frequency, and a signal for modulating the reference voltage,
For example, a comparison is made with a triangular carrier waveform, and a period during which the reference voltage is higher than the carrier wave is turned on, and a period during which the reference voltage is lower than the carrier wave is turned off, thereby _obtaining the PWM signal U _PO of the upper controllable element in the U phase. PWM of lower controllable element of U phase
Signal U _NO is obtained as an inverter signal U _PO.

Actually, in order to prevent short circuit of upper and lower elements, the ON timing is delayed by _Td time, and the short-circuit prevention processed PWM signal U
_P, at U _n, controllable element is driven, as a result, the output voltage of the U-phase as shown in FIG. 6 (d) of obtaining the output waveform V _UO that is pulse width modulated sinusoidally. The V phase and the W phase are obtained in the same manner.

5, the reference voltage generator (40) outputs the reference voltage waveform, the carrier generator (50) outputs the triangular carrier, whereby the PWM circuit (60) outputs the PWM signal. Create and drive circuit (70) PWM circuit (60)
The inverter controllable element of the inverter (20) is driven by the PWM signal. As a result, an AC having a variable voltage and a variable frequency is obtained from the inverter.

[Problems to be solved by the invention]

However, in the conventional PWM inverter device, when the electric motor is driven with the PWM waveform as described above, a high-frequency sound is generated due to the carrier frequency, which causes an increase in noise. As one means for avoiding this, there is a method of increasing the carrier frequency to an upper limit of the human audible frequency or a higher frequency.

That is, when the carrier frequency is increased, the noise level gradually decreases, and when the carrier frequency fc is set to 10 KHz to 15 KHz, the noise level approaches the upper limit of the audible range, and the noise level also significantly decreases. Further, when the frequency exceeds 20 KHz, the audible range is exceeded, and the frequency sound cannot be sensed by human hearing, and the noise characteristics are almost the same as when driven by a commercial power supply.

However, increasing the carrier frequency fc means increasing the number of switching times of the inverter controllable element, and changing the potential of each phase of the inverter output (in this case, either P or N of the inverter DC bus). That is, the number of times increases. Usually, on the inverter output side, there is a stray capacitance between the wiring connecting the motor and the inverter and the ground, between the primary winding of the motor and the motor frame, etc., so that the number of changes in the inverter output potential increases. Since more current flows to charge and discharge the stray capacitance, there is a problem that so-called leakage current increases.

Usually, since the power supply on the input side of the inverter is grounded, the above phenomenon forms a leakage current circuit through the ground. FIG. 7 shows an explanatory diagram of this phenomenon. In this figure, for the sake of simplicity of description, the stray capacitance is captured in a lumped manner, and is assumed to exist only as _Cl between the neutral point of the motor and the motor frame. In FIG. 7, (35) is a stray capacitance, (90) is an input power supply, (100) is a forward converter, and (110) is a smoothing circuit. Fig. 8 shows the results of PWM control.
State of motor neutral point potential change in carrier section and leakage current
This shows the state of i _l .

Here, considering the change in the motor neutral point potential,
Based on the PWM signal generation method shown in FIG. 6, the PWM signal of each phase is obtained by comparing the reference voltage waveform shown in FIGS. 9A and 9B with the carrier waveform. P obtained
The state of the WM signal can be expressed as the following three phases. U of inverter comprising a bridge circuit (20), V, phases controllable elements W is, P-side _{(U P, V P, W} P), N -side (U _N,
V _N , W _N ) are on and never on at the same time, so the three-phase PWM signal state is represented by the state of 2 ³ = 8. This is defined as follows. V ₀ = (000), V ₁ = (0
01), V ₂ = (010), V ₃ = (011), V ₄ = (100), V ₅ =
(101), V ₆ = (110), V ₇ = (111) Here, the meanings of the states are as follows. For example, V ₃ = (011) is U _P is U _N is on off, V _P is V ON
_N is off, W _P is on and W _N is off, and V ₄ = (1
00) is U _P is turned in U _N is turned off, V _P is the V _n in Og on, W _P is W _N off indicates the ON state.

Based on these definitions, inverter output phases U, V, W
The potential is determined by the PWM pattern shown in FIG. 9, 8 of the combination _{V 0, V 1, V 2} , V 3, V 4, V 5, V 6, V 7
There is a street. Here, at V ₀ , since the lower sides of all the U, V, and W phases are ON, the motor windings are all N-side (0) potentials of the inverter DC bus, and the motor neutral point potential is also N. V ₇ is U Conversely, V, W phase are both because upper ON, the motor neutral point potential P
(E _dc ). V ₁ , V ₂ , and V ₄ are N-side for two of three phases,
Since one phase is at the P-side potential, the neutral point of the motor is a potential of 1 / 3E _dc , and V ₃ , V ₅ , and V ₆ have two phases of the P-side and one phase at the N-side potential of three phases. Therefore, the motor neutral point has a potential of 2 / 3E _dc .

Therefore, in any case, 0 (N), 1 / 3E _dc , 2 / 3E _dc ,
Take one of the four potentials E _dc (P). These changes states PWM pattern at that time is determined by (the inverter output electrons, to achieve an output frequency), 3 when the arm control during motor so always select V _0, V ₇ vector 1 carrier segment FIG. 8A shows the change in the potential at the sex point. In the two-arm control, either V ₀ or V ₇ is selected in one carrier section, so that the change in the motor neutral point potential is V ₀ selection section in FIG. 8B and V V in FIG. 8C. _It looks like ₇ selected sections.

That is, the leakage current i _l is represented as a current that charges or discharges _Cl every time the motor neutral point potential changes.
Therefore, if the carrier frequency fc is high, the leakage current i _l increases,
Decrease if low. In the two-arm control, as shown in FIG. 9 (b), the PWM state is V ₀ every electrical angle of 60 °.
There period and V ₇ period is to remain present present. For example, 1 carrier segment in the example of V ₀ is changes as follows.
V ₀ (000), V ₄ (100), V ₆ (110), V ₄ (100), V ₀ (00
0). In the example of V _7, 1 carrier section changes as follows. V ₇ (111), V ₃ (011), V ₂ (010), V ₃ (01
_{1), V 7 (111)} .

Thus since the change in the interval where V ₀ In summary the change in the neutral point potential of the motor is present in each state is as Figure 8 (b), section V ₇ exists 8 ( c)
become that way.

Here, ignoring the ground impedance, this leakage current i _l becomes Where i _l increases as C _l increases and fc increases.

Therefore, in order to improve the noise characteristics, the carrier frequency fc
Increases the leakage current due to the ground stray capacitance, and has the following problems.

(A) Normally, an earth leakage breaker installed on the inverter input power supply side operates.

(B) When the motor frame is not grounded, an electric shock is likely to occur when the motor frame is touched.

(C) charge and discharge current to the floating capacitance C _l since superimposed upon switching of the inverter controllable elements, switching loss increases.

However, these greatly depend on the actual leakage circuit impedance.

The present invention has been made in order to solve the above-described problems, and detects a leakage current in an actual installation state, enables sufficiently low-noise operation, and a pulse that avoids an increase in the leakage current. It is intended to obtain a width modulation type inverter device.

[Means for solving the problem]

A pulse width modulation type inverter device according to a first aspect of the present invention includes a reference voltage generator that outputs a reference voltage waveform, a carrier generator that outputs a carrier waveform having a predetermined frequency, and the outputs of the reference voltage generator and the carrier generator. A pulse width modulation circuit that generates a pulse width modulation signal by comparison, a drive circuit that drives a controllable element of an inverter based on the pulse width modulation signal, and a variable voltage variable frequency that is driven by the drive circuit. In a pulse width modulation type inverter device having an inverter for obtaining a three-phase alternating current, a leakage current detector for detecting a leakage current of the inverter is provided, and when the leakage current level is smaller than a predetermined value, The reference voltage generator selects a reference voltage waveform at the time of controlling the three-phase arm, which is set so as to switch each arm of the three-phase bridge of the inverter at each carrier cycle. When the leakage current level is higher than a predetermined value, the on / off state of one of the arms of the three-phase bridge is fixed, and the switching control is performed on only the remaining two arms. The reference voltage waveform at the time of arm control is selectively transmitted by the reference voltage generator.

Further, a pulse width modulation type inverter device according to a second aspect of the present invention includes a reference voltage generator that outputs a reference voltage waveform, a carrier generator that outputs a carrier waveform having a predetermined frequency, and the above-described reference voltage generator and carrier generator. A pulse width modulation circuit for comparing outputs and generating a pulse width modulation signal, a drive circuit for driving a controllable element of an inverter based on the pulse width modulation signal, and a variable voltage variable driven by the drive circuit In a pulse width modulation type inverter device including an inverter for obtaining an alternating current of frequency, a leakage current detector for detecting a leakage current of the inverter is provided, and the carrier generator and the leakage current detector are provided. The carrier frequency is selectively output in response to the output.

[Action]

In the pulse width modulation type inverter device according to the first invention, the reference voltage generator is adapted to select a reference voltage waveform in response to the inverter output leakage current level, and when the leakage current level is low, there are many switching circuits. Selecting a reference voltage waveform for three-arm control to reduce noise generated from the motor, and selecting a reference voltage waveform for two-arm control with a small number of switching circuits when the reference voltage waveform is large, to reduce inverter output leakage current; In addition, it works so as not to significantly impair the noise characteristics generated from the electric motor.

Further, the carrier generator according to the second invention is adapted to select a carrier frequency in response to an inverter output leakage current level, and when the leakage level is low, increase the frequency to increase the number of switching circuits, and When the generated noise is reduced and the leakage current is large, the carrier frequency may be lowered to reach a predetermined leakage current level, thereby reducing the inverter output current and significantly impairing the noise characteristics generated by the motor. Act as if not.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment according to the first invention, in which a DC power supply (10), an inverter (20), a load motor (30), a carrier generator (50), a PWM circuit (60), Drive circuit (70)
Is the same as in the prior art. Reference numeral (80) denotes a leakage current detector that detects inverter output leakage current, particularly leakage current due to ground stray capacitance, and the reference voltage generator (40) that receives the detected value is the leakage current detector (80). A reference voltage waveform for two-arm or three-arm control for obtaining a predetermined sinusoidal output voltage and output frequency in accordance with the output is selectively output.

That is, when the leakage current level is smaller than the predetermined value, the reference voltage waveform at the time of the three-phase arm control determined so as to switch each arm of the three-phase bridge of the inverter (20) for each carrier cycle is selectively transmitted. In addition, when the leakage current level is higher than a predetermined value, a two-arm control is provided in which one of the arms of the three-phase bridge is fixed in an on / off state and only the remaining two arms are switching-controlled. The reference voltage waveform at the time is selectively transmitted.

Referring to FIG. 9 showing reference voltage waveform examples of two-arm control and three-arm control, three-arm control performs switching control for both three arms, and a typical example thereof is a sine shown in FIG. 9 (a). It is a wavy reference voltage waveform. this is,
Since the switching is performed without a pause every carrier cycle, the frequency of the high-frequency component included in the voltage component applied to the motor is high. Therefore, in the human auditory characteristic, the higher the frequency, the lower the noise level can be heard. As shown in a), the leakage current is large because the number of potential changes at the neutral point of the motor is large. In order to control the waveform of the line voltage, at least two arms may be switched and one arm may be stopped for each carrier cycle. A typical example is shown in FIG. 9 (b). In this example, 60 ° of the half cycle of each phase waveform is positively or negatively saturated (fixed) and the remaining phases are controlled so that the line voltage becomes a sine wave.
In this case, the switching is stopped for only 1/3 of one cycle of the electrical angle of each phase. Therefore, even if the frequency of the high frequency component included in the voltage component applied to the electric motor is the same as fc for the three arms, Slightly lower than the
Noise characteristics are reduced, but not significantly impaired,
As shown in FIGS. 8 (b) and (c), the leakage current level
Since the number of potential changes at the motor neutral point is 2/3 And there is a remarkable effect in reducing the leakage current.

Here, the carrier frequency fc is set to a sufficiently high frequency so as to significantly improve the motor noise characteristics. As the modulation method, the three-arm control method is selected, and the operation is started.

That is, in the leakage current detector (80) of FIG.
Assuming that the inverter output leakage current level is lower than a predetermined value, the reference voltage generator (40) receives the output of the leakage current detector (80) and receives a reference voltage waveform for a three-arm control having good noise characteristics. (For example, FIG. 9 (a)) is determined corresponding to the output voltage and output frequency at that time and is output to the PWM circuit (60). The PWM operation after the PWM circuit (60) is the same as the conventional example. At this time, the motor noise is sufficiently small due to the three-arm control, and the three-arm control is continued because the inverter output leakage current is small.

Next, when the inverter output leakage current level is higher than a predetermined value, the reference voltage generator (40) receives the output of the leakage current detector (80) and performs two-arm operation as shown in FIG. A control reference voltage waveform is determined according to the output voltage and output frequency at that time, and is sent to the PWM circuit (60). Therefore, at this time, the leakage current level is significantly reduced, and the motor noise characteristics are not significantly impaired.

That is, for example, the operation is started with the three-arm control method as the initial modulation method, and if the leakage current level is equal to or less than the predetermined value, the operation is continued, and if the leakage current level is equal to or more than the predetermined value, the two-arm control is selected to reduce the leakage current. I do.

The initial modulation condition is two-arm control. If the leakage current level is equal to or more than a predetermined value, the operation is continued as it is, and if it is equal to or less than the predetermined value, the operation shifts to three-arm control. When transitioning from three-arm control to two-arm control or from two-arm to three-arm control, smooth transition can be realized if control is performed so that the inverter output line voltage phase continues.

In the above embodiment, as a circuit embodiment of the detector (80), a leakage current is detected as a zero-phase current component by a current detector such as a DCCT, and a comparator signal corresponding to the level is generated. What is necessary is just to send it to a reference voltage generator (40).

As a circuit for realizing the reference voltage generator (40),
As shown in FIGS. 8A and 8B, reference voltage waveforms for three-arm and two-arm control along predetermined voltages and frequencies are stored in advance in a ROM or the like, and a leakage current detector (80 ) May be selected according to the signal and output to the PWM circuit (60), or may be constantly calculated. After the PWM circuit (60), the digital signal may be converted into a digital signal for digital processing, or an analog signal may be converted for analog processing and output to the PWM circuit (60).

Furthermore, although the leakage current detector is installed on the inverter output side and a method of measuring a so-called zero-sequence current component has been described, it may be an inverter input side or an inverter DC bus part. Also, in the embodiment of the present invention, the method of switching between the three arms when the leakage current level is equal to or less than the predetermined value and to the two arms when the leakage current level is equal to or more than the predetermined value has been described. It is also possible to continuously control between 3 arms → 2 arms according to the current level.

9 (a) and 9 (b) show the reference voltage waveforms, but the present invention is not limited to these.

As described above, according to the inverter output leakage current level under the actual installation conditions, the reference voltage waveform is selected so that three-arm control is performed when the current is small and two-arm control is performed when the current is large. Thus, the motor noise is sufficiently low, and even when the leakage current is large, the inverter leakage current can be suppressed without significantly impairing the noise characteristics. Also, these controls can be easily realized since they can be handled by changing only the reference voltage waveform.
Since the carrier frequency can be kept constant, resonance avoidance design such as a filter added to the output side is also easy.

Next, FIG. 3 shows an embodiment according to the second invention.
In the figure, a carrier generator (50) is adapted to output a triangular carrier waveform having a frequency corresponding to the output of the leakage current detector (80), and a leakage current for detecting the leakage current. The detector (80) has the configuration shown in FIG.

That is, in FIG. 4, (81) is an amplifier for amplifying a leakage current detection signal detected by a current detector such as a DCCT using a Hall element to a predetermined level, and (82) is converting a true effective value into a DC signal. The RMS converter (83) compares the preset leakage current level (set value) with the current leakage current level detected by the RMS converter (82), and determines the current value from the set value. A comparator that outputs a signal to the carrier generator (50) in the direction of lowering the carrier frequency when the detected leakage current level is large, or in the direction of increasing the carrier frequency when the current detected leakage current level is smaller than the set value. is there.

The operation of the illustrated embodiment including the leakage current detector (80) and the carrier generator (50) having such a configuration will be described.

First, the set leakage current level input to the comparator (83) is set from the following viewpoint.

That is, (a) a level at which the earth leakage breaker installed on the inverter input power supply side does not malfunction, (b) a leakage current level that does not pose a danger to the human body even when the motor frame is touched, for example, and (c) as described above. The charge / discharge current to the stray capacitance _Cl increases the switching loss of the inverter controllable element, but the loss is at an acceptable level.

From each of these points (a) to (c), selection is made according to the purpose. Alternatively, it is needless to say that it may be set in a composite manner, and it is needless to say that the noise characteristic of the electric motor is considered.

In the state set based on the above viewpoint, when the inverter starts operating at, for example, the carrier frequency _fco ,
At this time, the leakage current determined by the conditions such as the ground stray capacitance of the inverter load system, the grounding processing state, the ground impedance, the grounding state of the inverter power supply system, and the like flows as shown in FIG. This current is detected as a zero-phase current by, for example, DCCT, amplified by an amplifier (81), converted to a DC signal corresponding to a true RMS value by an RMS converter (82), and output to a comparator (83). . The comparator (83) compares the set leak current level described above with the leak current output from the effective value converter (82). If the leak current is larger than the set value, the carrier generator reduces the carrier frequency. Output to (50). Carrier generator (50) receives this reduces the current carrier frequency f _co the f _co -.DELTA.f _c. Hereinafter, the control is continued by the same operation, and the state becomes the normal state when the leakage current and the set value match. If the leakage current is smaller than the set value, the carrier frequency is increased again, and a normal state is established when the leakage current matches the set value.

Therefore, if the inverter grounding condition is such that leakage current is unlikely to flow, raise the carrier frequency (the upper limit of the carrier frequency is determined separately and will not be made larger than necessary), and the noise characteristics from the motor will be sufficiently small. In other words, if the leakage current is likely to flow, the noise current is controlled to a level that satisfies the above (a), (b) or (c), so that noise characteristics are maintained within a range where no other problems occur. Can be realized.

In the embodiment shown in FIG. 3, the leakage current detecting portion is provided on the inverter output side and a method of measuring a so-called zero-phase current component is shown. However, the leakage current detecting portion may be on the inverter input side or the inverter DC bus portion. Also, the modulation method is described by taking the example of a three-arm control PWM method in which all three arms are switched, but a two-arm control PWM method in which switching is stopped for only 1/3 of one electrical angle period, or another PWM method. Even in the same application, the same effect can be obtained. Also,
The amplifier (81) may be, for example, a high-pass filter for separating a component due to stray capacitance from the leakage current. Furthermore, although the configuration example of the inverter input and output has three phases, it is needless to say that the configuration is not limited to three phases.

〔The invention's effect〕

As described above, according to the first invention, the inverter output leakage current under actual installation conditions is detected, and according to the level, three-arm control is performed when the leakage current is small, and two-arm control is performed when the leakage current is large. The reference voltage waveform is selected so that when the leakage current is small, the motor noise is sufficiently small, and when the leakage current is large, the noise characteristics can be suppressed without significantly impairing the noise characteristics. Noise characteristics and leakage current characteristics that are optimal for the set conditions can be obtained.

Further, according to the second aspect, the carrier frequency is determined according to the leakage current level, so that a sufficient noise reduction can be achieved under the condition where the leakage current is small, and the setting is made under the condition where the leakage current is large. Operation can be performed within the range of the leakage current without sacrificing noise characteristics, and operation according to the installation state of the inverter is possible. And an amplifier,
The above-described desired operation can be accurately performed according to the effective value of the leakage current by the leakage current detector including the effective value converter and the comparator.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment according to the first invention, FIGS. 2 (a) and 2 (b) are explanatory diagrams of a reference voltage generator (40) realizing circuit in the embodiment of FIG. 1, and FIG. 2 is a block diagram of an embodiment according to the invention, FIG. 4 is a block diagram of a leakage current detector in the embodiment of FIG. 2, FIG. 5 is a block diagram of a conventional example, and FIG. FIG. 7, FIG. 7, and FIGS. 8 (a), (b), (c) and FIG. 9 are diagrams for explaining the leakage current phenomenon. In the figure, (10) is a DC power supply, (20) is an inverter,
(30) is a load motor, (40) is a reference voltage generator, (50)
Is a carrier generator, (60) is a PWM circuit, (70) is a drive circuit, and (80) is a leakage current detector. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (2)

(57) [Claims] 1. A reference voltage generator for outputting a reference voltage waveform, a carrier generator for outputting a carrier waveform of a predetermined frequency, and comparing the outputs of the reference voltage generator and the carrier generator to generate a pulse width modulation signal. A pulse width modulation circuit to be generated, a drive circuit for driving a controllable element of the inverter based on the pulse width modulation signal, and an inverter driven by the drive circuit to obtain a three-phase alternating current with a variable voltage and variable frequency And a leakage current detector for detecting a leakage current of the inverter, and when the leakage current level is smaller than a predetermined value, a three-phase bridge of the inverter is provided. A reference voltage waveform at the time of three-phase arm control determined to switch each arm for each carrier cycle is selectively transmitted by the reference voltage generator, and the leakage current level is controlled. When it is larger than the predetermined value, the reference voltage waveform at the time of the two-arm control, which is set so that the on / off state of one of the arms of the three-phase bridge is fixed and only the remaining two arms are subjected to the switching control, is described above. A pulse width modulation type inverter device characterized in that the signal is selectively transmitted by a reference voltage generator. 2. A reference voltage generator for outputting a reference voltage waveform, a carrier generator for outputting a carrier waveform of a predetermined frequency, and comparing the outputs of the reference voltage generator and the carrier generator to generate a pulse width modulation signal. A pulse width modulation circuit to be generated, a drive circuit that drives a controllable element of the inverter based on the pulse width modulation signal, and an inverter that is driven by the drive circuit to obtain an alternating current with a variable voltage and variable frequency. In the pulse width modulation type inverter device provided with a leakage current detector for detecting the leakage current of the inverter,
The carrier generator and a configuration for selectively outputting a carrier frequency in response to the output of the leakage current detector, and
An amplifier for amplifying the leakage current signal to a predetermined level, an effective value converter for converting the output of the amplifier to a DC signal corresponding to the effective value of the leakage current, and a predetermined set leakage The current level is compared with the current leakage current level detected by the RMS converter, and if the current detection leakage current level is higher than the set value, the carrier frequency is decreased, and the current detection leak level is set from the set value. A pulse width modulation type inverter device comprising a comparator which outputs a signal to a carrier generator in a direction to increase a carrier frequency when a current level is small.