JPH02136072A - Pulse width modulation type inverter device - Google Patents

Abstract

PURPOSE:To improve noise characteristic and leak current characteristic by detecting inverter output leak current under actual installation conditions and selecting the reference voltage waveform according to the level of the output leak current such that 3-arm control is made for low leak current while 2-arm control is made for high leak current. CONSTITUTION:In a leak current detector 80, a reference voltage generator 40 determines a 3-arm control reference voltage waveform having good noise characteristic based on the output voltage and output frequency at that time if the inverter output leakage current is lower than a predetermined level and provides the reference voltage waveform to a PWM circuit 60. At this time, motor noise is quite low and the leak current is also low because of the 3-arm control. If the inverter output leakage current level is higher than the predetermined level, the reference voltage generator 40 determines 2-arm control reference voltage waveform based on the output voltage and output frequency at that time and provides the reference voltage waveform to the PWM circuit 60 thus lowering the leakage current level.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a pulse width modulation type inverter device that obtains alternating current of variable voltage and variable frequency, and particularly to a pulse width modulation type inverter device that controls a carrier frequency to be high.

[Prior Art] Figure 5 shows an example of the configuration of a conventional pulse width modulation type inverter device, in which (1o) is a DC power supply, (20) is
is an inverter (inverter) consisting of a controllable element and a diode connected in antiparallel, and converts it into alternating current with variable voltage and variable frequency; (30) is the electric motor; (40) is 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); An inverse converter (20) based on the output signal of the carrier generator (50)
(70) is a pulse width modulation (hereinafter referred to as PWM) circuit that generates a firing signal (PWM signal) for the controllable element of
This is a drive circuit that receives signals from the M circuit (60) and drives the controllable elements of the inverse converter (20).

Next, the operation related to the above configuration will be explained with reference to the drawings. FIG. 6 is a typical operation diagram of this type of PWM control, and is an explanation of the operation for one phase of a PWM inverter of three phases, for example, U, ■, and W. First, a reference voltage, which is the reference for the output voltage and output frequency of the inverter, is compared with a signal for modulating this, such as a triangular carrier waveform. The PWM signal Llpo of the upper controllable element of the U phase is determined by turning it off during a period smaller than the carrier wave. The PWM signal UN0 of the lower controllable element of the U phase is obtained as an inverter signal of U2°.

In fact, in order to prevent short circuits between the upper and lower elements, the PWM is processed to delay the ON timing by 14 hours to prevent short circuits.
The controllable element is driven by the signals U3 and Un, and as a result, as shown in FIG. 6(d), the U-phase output voltage obtains an output waveform vU-0 which is sinusoidally pulse-width modulated. ■ phase,
The W phase can also be obtained in the same manner.

In the configuration shown in FIG. 5, the reference voltage generator (4o) outputs the reference voltage waveform, and the carrier generator (5o) outputs the triangular wave carrier, thereby causing the PWM circuit (6
0) creates the above PWM signal, and the drive circuit (70) generates the PWM signal.
WM circuit (60) P WM Inverse converter (20
) drives an inverter controllable element. As a result, an alternating current of variable voltage and variable frequency is obtained from the inverter.

[Problems to be Solved by the Invention] However, in the conventional PWM type inverter device, as described above (when a motor is driven with a PWM waveform, high-frequency sound is generated due to the carrier frequency, causing an increase in noise. One way to avoid this is to raise the carrier frequency to the upper limit of human audible frequencies or to a higher frequency.

That is, as the carrier frequency is increased, the noise level gradually decreases, and when the carrier frequency fc is increased from 10 KHz to 1
If it is set to 5KH, it will approach the upper limit of the audible range and the noise level will drop significantly. Furthermore, when H2 is made to exceed 20, the audible range is exceeded, and the frequency sound becomes undetectable to human hearing, resulting in noise characteristics that are almost the same as when driven by a commercial power source.

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

Since the power supply on the input side of the inverter is normally grounded, the above-mentioned phenomenon forms a leakage current circuit via the ground.

FIG. 7 shows an explanatory diagram of this phenomenon. In this figure, to simplify the explanation, stray capacitance is treated as a lumped constant, and is assumed to exist only as Cα between the motor neutral point and the motor frame.

In addition, in Figure 7, (35) is stray capacitance, (90)
is a human power source, (100) is a forward converter, and (110) is a smoothing circuit. In addition, Fig. 8 shows the state of motor neutral point potential change and leakage current i in one carrier section due to PWM control.
This shows the state of α.

Now, considering the change in motor neutral point potential, based on the PWM signal creation method shown in FIG.
PWM signals of each phase can be obtained by comparing the reference voltage waveforms shown in a) and (b) with the carrier waveforms. The state of the obtained PWM signal can be expressed in terms of three phases as follows. U, V of the inverse converter (20) consisting of a bridge circuit,
Each phase controllable element of W is P side (Up, vp, Wp), N
Either side (ON, ■2, WN) is on, but they are never on at the same time, so the three-phase PWM signal state is 23=
8 states. This is defined as follows.

Vo= (000), V,= (001), V2=
(010), V3= (011), V4= (100
), l/s= (101), V6= (+10)
, V7= (111) Here, the meaning of the state is as follows. For example, V3= (011) is when UP is off and ON is on, vP is on and VN is off, WP is on and WN is off, and V4= (100) is when UP is on and IJs are off. VP is off and vn is on,
This shows a state in which WP is off and WN is on.

Based on these definitions, the potentials of the inverter output phases U, ■, and W are determined by the PWM pattern shown in FIG. 9, and the combinations are vo, V,, V2, v3, v4, v5.
, v6, and v7. Here, in VO, since the lower sides of the U, V, and W phases are ON, all the motor windings are at N-side (0) potential of the inverter DC bus, and the motor neutral point potential is also N. On the contrary, for v7, the upper side is ON for all U, ■, and W phases.
Therefore, the motor neutral point potential is P(EdC). vl, v
2. For v4, two of the three phases are N side and one phase is P (lII
Since the motor neutral point is at HEde) potential, two of the three phases of v3, ■6, and v8 are on the P side, and 1
Since the phase is at the N side potential, the motor neutral point is at the potential of HEdc.

Therefore, in any case, O(N), HEdc, HEdc
, Edc(P). The states of these changes are determined by the PWM pattern (achieving the inverter output potential and output frequency) at that time, and 3
When using arm control, be sure to check ■ in one carrier section. ,■7
Since the vector is selected, the motor neutral point potential change is as shown in FIG. 8(a). Also, in 2-arm control, V in 1 carrier section. , or v7, the motor neutral point potential change is V in FIG. 8(b). The selected section is like the selection section (7) in FIG. 8(c).

That is, the leakage current i11 is represented by the current that charges or discharges Cα every time the motor neutral point potential changes. Therefore, if the carrier frequency fc is high, the leakage current il
l increases, and if it is low, it decreases. In the two-arm control, as shown in Figure 9(b), the PWM state is at an electrical angle of 60
The period in which vo exists and the period in which ■7 exists changes for each degree. For example, in the example of vo, one carrier interval changes as follows.

Vo (000), V4 (100), V, (110), V
4 (+00), Vo (000), and in the v7 example, 1
The career interval changes as follows. V7 (111)
, V3 (011), V2 (010), V3 (011),
V? (111).

Since it changes in this way, the change in the neutral point potential of the motor in each state can be summarized as V. Figure 8 (
b), and the section where v7 exists is shown in Figure 8(c).
become that way.

Here, if the earth impedance is ignored, this leakage current 12 is expressed as iq a C4-rT"c, and the larger Cα or fc is, the more j
α becomes larger.

Therefore, in order to improve the noise characteristics, the carrier frequency fc
Increasing the leakage current due to ground stray capacitance increases, resulting in the following problem.

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

(b) If the motor frame is not installed, touching the motor frame can easily cause an electric shock.

(C) Since the charging/discharging current to the stray capacitance CIl is superimposed upon switching of the inverter controllable element, switching loss increases.

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

This invention was made to solve the above-mentioned problem, and detects the leakage current in the actual installation state, enables sufficiently low-noise operation, and avoids an increase in the leakage current. The purpose is to obtain a pulse width modulation type inverter device.

(Means for Solving the Problems) A pulse width modulation type inverter device according to a first aspect of the invention includes: a reference voltage generator that outputs a reference voltage waveform; a carrier generator that outputs a carrier waveform of a predetermined frequency; A pulse width modulation circuit that compares the outputs of a generator and a carrier generator to generate a pulse width modulation signal, a drive circuit that drives a controllable element of an inverse modulator based on the pulse width modulation signal, and this drive circuit. A pulse width modulation type inverter device is provided with a leakage current detector for detecting a leakage current of the inverter, and a leakage current detector is provided to detect leakage current of the inverter, and a leakage current detector is provided to detect leakage current of the inverter. is smaller than a predetermined value, the reference voltage generator selects and sends out a reference voltage waveform during three-phase arm control, which is determined to switch each arm of the three-phase bridge of the inverter every carrier period. When the above leakage current level is larger than a predetermined value, two-arm control is performed, in which the on/off state of one arm of each arm of the three-phase bridge is fixed and only the remaining two arms are controlled to switch. The reference voltage waveform is selectively transmitted by the reference voltage generator.

Further, a pulse width modulation type inverter device according to a second invention includes a reference voltage generator that outputs a reference voltage waveform, a carrier generator that outputs a carrier waveform of a predetermined frequency, and a combination of the reference voltage generator and the carrier generator. A pulse width modulation circuit that compares outputs to generate a pulse width modulation signal, a drive circuit that drives a controllable element of an inverse modulator based on the pulse width modulation signal, and a variable voltage variable drive circuit that is driven by this drive circuit. In a pulse width modulation type inverter device comprising an inverter for obtaining alternating current frequency, a leakage current detector for detecting leakage current of the inverter is provided, and the carrier generator is connected to the oil source current detector. The carrier frequency is selectively output in response to the output of the carrier frequency.

(Function) In the pulse width modulation type inverter device according to the first invention, the reference voltage generator selects the reference voltage waveform in response to the inverter output leakage current level, and when the leakage current level becomes small, the reference voltage generator selects the reference voltage waveform in response to the inverter output leakage current level. Select a reference voltage waveform for 3-arm control with a large number of switching times to reduce noise generated from the motor, and select a reference voltage waveform for 2-arm control with a small number of switching times to reduce inverter output leakage current. It works to reduce noise and not significantly impair the noise characteristics generated by the motor.

Further, the carrier generator according to the second invention selects the carrier frequency in response to the inverter output leakage current level, and when the leakage level is small, the frequency is increased to increase the number of switching times, and the electric motor is To reduce the noise generated, and when the leakage current is large, lower the carrier frequency to reach the specified leakage current level, reduce the inverter output current, and reduce the noise generated from the motor.
It acts so as not to significantly impair the sound characteristics.

(Example) Hereinafter, each example of the present invention will be described with reference to the drawings. 1st
The figure shows an embodiment according to the first invention, in which a DC power supply (1O), an inverter (20), a load motor (30)
, a carrier generator (50), a PWM circuit (60), and a drive circuit (70) are the same as those of the conventional one. (80) is a leakage current detector that detects inverter output leakage current, especially leakage current caused by ground stray capacitance, and a base I! that receives the detected value. The voltage generator (40) is a leakage current detector (80)
2 arms or 3 arms to obtain a predetermined sinusoidal output voltage and output frequency according to the output of the
Selects and outputs the reference voltage waveform for arm control.

In other words, when the leakage current level is smaller than the predetermined value,
A reference voltage waveform for three-phase arm control, which is determined to switch each arm of the three-phase bridge of the inverter (20) every carrier period, is selected and sent, and when the leakage current level is larger than a predetermined value, , the reference voltage waveform for two-arm control, which is determined to fix the on/off state of one arm out of each arm of the three-phase bridge and control the switching of only the remaining two arms, is selectively transmitted. There is.

To explain this with reference to FIG. 9, which shows examples of reference voltage waveforms for two-arm control and three-arm control, three-arm control is one that performs switching control on all three arms, and a typical example thereof is shown in FIG.
This is the sinusoidal reference voltage waveform shown in a). This is because the carrier switches without pause every carrier cycle, so the frequency of the high-frequency component included in the voltage component applied to the motor is high, and due to the human auditory characteristics, the higher the frequency, the lower the noise level can be heard. As shown in FIG. 8(a), the leakage current is large because the number of potential changes at the motor neutral point is large. In order to waveform control the line voltage, at least two arms may be switched and one arm may be stopped from switching for each carrier cycle. A typical example of this is shown in Figure 9 (b
). In this example, 60° of each half cycle of the sum waveform is saturated (fixed) to positive or negative, and the remaining phases are controlled so that the line voltage becomes a sine wave.

In this case, switching is stopped only during the station period of the electrical angle-period of each phase, so even if the frequency of the high-frequency component included in the voltage component applied to the motor is the same as fc in the case of 3 arms, it is compared with that in the case of 3 arms. As it becomes lower, it will be slightly lower.
Noise characteristics are reduced, but not significantly impaired.
As shown in Figure 8 (b) and (C), the leakage current level tends to change the number of times the potential at the motor neutral point changes, so it is /1.
This has a significant effect on reducing leakage current.

Here, the carrier frequency fc is set to a sufficiently high frequency to significantly improve motor noise characteristics. Select the 3-arm control method as the modulation method and start operation.

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

Next, if the inverter output leakage current level is higher than a predetermined value, the reference voltage generator (40) detects the leakage current detector (
80), a reference voltage waveform for two-arm control as shown in FIG. 9(b) is determined according to the output voltage and output frequency at that time, and is sent to the PWM circuit (60). Therefore, in this case, the leakage current level is significantly reduced, and the motor noise characteristics are not significantly impaired.

That is, for example, operation is started using the 3-arm control method as the initial modulation method, and if the leakage current level is below a predetermined value, operation is continued, and if the leakage current level is above the predetermined value, 2-arm control is selected to reduce the leakage current. do.

The initial modulation condition is two-arm control, and if the leakage current level is above a predetermined value, the operation continues as it is, and if it is below the predetermined value, the control is switched to three-arm control. When changing from 3-arm control to 2-arm control or from 2-arm control to 3-arm control, a smooth transition can be achieved by controlling the inverter output line voltage phase to continue.

In the above embodiment, the circuit example of the detector (80) is a current detector such as a DCCT; detects leakage current as a phase current component, creates a comparator signal corresponding to the level, What is necessary is to send it to the reference voltage generator (40).

Further, as a circuit for realizing the reference voltage generator (40), the second
As shown in Figures (a) and (b), the reference voltage waveforms for 3-arm and 2-arm control in accordance with predetermined voltages and frequencies are stored in advance in a ROM, etc., and the leakage current detector (80 ) signal, select either one, and press PW
It may be output to the M circuit (60) or may be determined by constant calculation. From the PWM circuit (60) onward, if digital processing is required, the signal may be converted to a digital signal, or if analog processing is performed, the signal may be converted to analog and output to the PWM circuit (68).

Furthermore, as a leakage current detector, a method was shown in which the inverter output side is grounded and the so-called phase map current component is measured.
It may be the inverter input side or the inverter DC bus section. Furthermore, in the embodiment of the present invention, a method of switching to 3 arms when the leakage current level is below a predetermined value and to 2 arms when the leakage current level is above a predetermined value has been shown, but there is also an intermediate modulation method between 2 arms and 3 arms. Control may be performed continuously between the 3rd arm and the 2nd arm depending on the current level.

In addition, although the reference voltage waveforms are shown in FIGS. 9(a) and (b),
It is not limited to these.

As mentioned above, depending on the inverter output leakage current level under the actual installation conditions, by selecting the reference voltage waveform so that 3-arm control is used when the leakage current is small, and 2-arm control when it is large, it is possible to control the voltage when the leakage current is small. This is because the motor noise is sufficiently low, and even when the leakage current becomes large, the noise characteristics are not significantly impaired and the inverter leakage current is also suppressed. In addition, these controls can be easily implemented by changing only the reference voltage waveform, and since the carrier frequency can be kept constant, resonance avoidance designs such as filters added to the output side can be implemented. is also easy.

Next, FIG. 3 shows an embodiment according to the second invention, in which the carrier generator (50) has a triangular wave carrier having a frequency corresponding to the output of the leakage current detector (80). The fourth leakage current detector (80) outputs a waveform and detects leakage current.
It has the configuration shown in the figure.

That is, in FIG. 4, (81) is the leak detected by a current detector such as a DCCT using a Hall element (9, an amplifier that amplifies the current detection signal to a predetermined level, and (82) is the true effective value. RMS converter for converting to DC signal, (83)
is the predetermined set leakage current level (set value)
The current leakage current level detected by the effective value converter (82) is compared, and if the current detected leakage current level is larger than the set value, the carrier frequency is lowered and the current level is lower than the set value. If the detected leakage current level is small,
Carrier generator (50) in the direction of increasing the carrier frequency
This is a comparator that outputs a signal to the

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

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

In other words, (a) the level at which the earth leakage breaker installed on the inverter's power supply side does not malfunction, (b) the leakage current level at which there is no danger to the human body even if the motor frame is touched, and (C) as mentioned above. Although the charging/discharging current to the floating capacitor ff1clL increases the switching loss of the inverter controllable elements, the loss is at an acceptable level.

Selection is made from the viewpoints of each of these (a) to (c) depending on the purpose. Alternatively, it is of course possible to set the value in consideration of multiple factors, and it goes without saying that the noise characteristics of the electric motor should be taken into consideration.

When the inverter is started to operate at a carrier frequency of 10° in a state set based on the above viewpoints,
A leakage current determined by conditions such as the ground stray capacitance of the inverter load system, the grounding condition, the ground impedance, and the grounding condition of the inverter power supply system at that time flows as shown in FIG. This current is detected as a phase current using, for example, a DCCT, is amplified by an f amplifier (81), and is then amplified by an effective value converter (81).
2) converts it into a DC signal corresponding to the true effective value and outputs it to the comparator (83). The comparator (83) compares the aforementioned set leakage current level with the leakage current output from the effective value converter (82), and if the leakage current is larger than the set value, the carrier generator is activated to lower the carrier frequency. (5
0). The carrier generator (50) receives this and outputs the current carrier frequency f. is reduced to fco - Δfc. Thereafter, control is continued in the same manner, and the normal state is reached when the leakage current and the set value match. If the leakage current is smaller than the set value, the carrier frequency is increased, and when the leakage current matches the set value, the normal state is reached.

Therefore, if the inverter grounding condition is such that it is difficult for leakage current to flow, raise the carrier frequency (the upper limit of the carrier frequency is determined separately and do not increase it more than necessary) to sufficiently reduce the noise characteristics from the motor. If the condition is such that leakage current flows easily, the above (a) and (b)
Alternatively, since the leakage current level is controlled to a level that satisfies the viewpoint (C), it is possible to realize an operation that maintains the noise characteristics within a range where no other problems occur.

In the embodiment of FIG. 3, the leakage current detection part is installed on the inverter output side, and the so-called phase current component is measured, but it may be on the inverter input side or on the inverter DC bus. In addition, the modulation method is a three-arm control PWM method in which all three arms switch, but it is also possible to use a two-arm control PWM method in which switching is stopped only during the station period of the electrical angle period, or other PWM methods.
Even if the M method is applied in the same way, the same effect can be achieved. Further, the amplifier (81) may be a bypass filter for separating, for example, a component of leakage current due to stray capacitance. Furthermore, although an example of a three-phase configuration has been shown for both the inverter manual power and the output, it is needless to say that the configuration is not limited to three phases.

〔Effect of the invention〕

As described above, according to the first invention, the inverter output leakage current under actual installation conditions is detected, and depending on the level, 3-arm control is performed when the leakage current is small, and 2-arm control is performed when the leakage current is large. Since the reference voltage waveform is selected so that when the leakage current is small, the motor noise is sufficiently small, and even when the leakage current is large, the inverter leakage current can be suppressed without significantly impairing the noise characteristics. It is possible to obtain noise characteristics and leakage current characteristics that are optimal for the setting conditions.

Further, according to the second invention, since the carrier frequency is determined according to the leakage current level, sufficient noise reduction is achieved under conditions where the leakage current is small, and when the leakage current is large, the carrier frequency is determined according to the leakage current level. It can be operated within the current range without sacrificing noise characteristics, and can be operated according to the installation situation of the inverter.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of an embodiment according to the first invention, Figure 2 (a)
, (b) is the reference voltage generator (40
) An explanatory diagram of a realization circuit, FIG. 3 is a configuration diagram of an embodiment according to the second invention, FIG. 4 is a configuration diagram of a leakage current detector in the embodiment shown in FIG. 2, and FIG. 5 is a configuration diagram of a conventional example. Figure 6 shows the conventional P
WM inverter operating waveform explanatory diagrams, Figures 7 and 8 (a
), (b), (C) and FIG. 9 are diagrams explaining the leakage current phenomenon. In the figure, (lO) is a DC power supply, (2o) is an inverter, (30) is a load motor, (4o) is a reference voltage generator,
(50) is a carrier generator, (60) is a PWM circuit, (
7o) is a drive circuit, and (80) is a leakage current detector. In the figures, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] (1) A reference voltage generator that outputs a reference voltage waveform, a carrier generator that outputs a carrier waveform of a predetermined frequency, and a pulse width modulation signal is generated by comparing the outputs of the reference voltage generator and the carrier generator. A pulse width modulation circuit, a drive circuit that drives a controllable element of an inverse modulator based on the pulse width modulation signal, and an inverse converter that is driven by the drive circuit to obtain a three-phase alternating current of variable voltage and variable frequency. In the pulse width modulation type inverter device, a leakage current detector is provided to detect the leakage current of the inverter, and when the leakage current level is smaller than a predetermined value, each arm of the three-phase bridge of the inverter is The reference voltage generator selects and sends out a reference voltage waveform during three-phase arm control, which is determined to switch at each carrier cycle, and when the leakage current level is greater than a predetermined value, each arm of the three-phase bridge The reference voltage generator is configured to selectively send out a reference voltage waveform during two-arm control, which is determined to fix the on/off state of one arm and control the switching of only the remaining two arms. Characteristic pulse width modulation type inverter device. (2) A reference voltage generator that outputs a reference voltage waveform, a carrier generator that outputs a carrier waveform of a predetermined frequency, and a pulse width modulation signal is generated by comparing the outputs of the reference voltage generator and the carrier generator. A pulse width modulation circuit, a drive circuit that drives a controllable element of an inverse modulator based on the pulse width modulation signal, and an inverse converter that is driven by the drive circuit to obtain alternating current of variable voltage and variable frequency. In the pulse width modulation type inverter device, a leakage current detector for detecting leakage current of the inverter is provided, and the carrier generator selectively outputs a carrier frequency in response to the output of the leakage current detector. A pulse width modulation type inverter device characterized by the following.