JP3596694B2 - Filter circuit for PWM inverter - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a drive device for an AC motor using a PWM inverter.
[0002]
[Prior art]
When the AC motor is driven by the PWM inverter, a steep pulse voltage is applied to the motor winding in order to turn on / off a semiconductor switching element forming the inverter at a carrier frequency of several kHz to several tens of kHz. As a result, a ground voltage that is twice as large as that of the power supply is generated due to the distribution constants of the motor and the cable, and insulation is deteriorated. In addition, a voltage is generated between the frame and the shaft, that is, between the inner ring and the outer ring of the bearing due to the capacitance between the stator winding and the rotor, and this voltage destroys the oil film of the bearing and damages the bearing itself. Can lead to
As a solution to such a problem, a method of inserting a filter into an output circuit of an inverter is disclosed in Japanese Patent Application Laid-Open No. H2-254979.
[0003]
[Problems to be solved by the invention]
The technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 2-254979 is basically based on an LC filter, but slows the voltage rising speed, that is, dv / dt. Since it is set higher, the resonance phenomenon by LC continues. In order to suppress the continuation of the resonance, a resistor is inserted to obtain a damping effect. Therefore, there is a drawback that a loss occurs, and it is necessary to determine constants of L, C, and R for each application case.
Therefore, the problem to be solved by the present invention is to reduce the common mode voltage of the carrier frequency component to large radiation, reduce the occurrence of loss due to damping, and reduce the reactor and capacitor constants used in the filter to the inverter specifications only. And to prevent bearing damage due to shaft voltage.
[0004]
[Means for Solving the Problems]
In order to solve the above problems, a filter circuit for a PWM inverter according to the present invention includes a common mode reactor inserted between a PWM inverter and an AC motor driven by the inverter, and a common mode reactor including the common mode reactor and the AC motor. A three-phase star-connected reactor connected to a connection point, and a capacitor for grounding a neutral point of the reactor, the resonance frequency of the common-mode reactor and the capacitor, the output fundamental frequency of the inverter and the carrier frequency It is characterized in that the inductance of the common mode reactor and the capacitance of the capacitor are selected so as to be located in the middle and sufficiently separated from any of them.
Embodiments of the present invention will be described below.
Instead of the three-phase reactor connected to the connection point between the common-mode reactor and the AC motor, one of the three single-phase reactors with two sets of windings is star-connected, and the neutral point is connected by a capacitor. Ground and delta-connect the other winding.
Instead of a three-phase reactor connected to the connection point between the common mode reactor and the AC motor, one winding of a three-phase reactor having two sets of windings is star-connected, and the neutral point is grounded with a capacitor. At the same time, the other winding is delta-connected.
Instead of grounding the neutral point of the star-connected reactor via a capacitor, a capacitor is inserted in each phase winding of the star connection.
A neutral point of a circuit in which a combination of a reactor and a capacitor is staggered is grounded via another capacitor.
Instead of grounding the neutral point of the three phases via a capacitor, the neutral point is connected to the neutral point of the DC circuit of the inverter.
Common mode reactor and normal mode reactor connected in series between a PWM inverter and an AC motor driven by the inverter, and a three-phase star connected to a connection point between these reactors and the AC motor A connection capacitor, and another capacitor for grounding a neutral point of the capacitor, wherein the resonance frequency of the common mode reactor, the three-phase capacitor, and the ground capacitor, and the resonance frequency of the normal mode reactor and the three-phase capacitor, The inductance and the capacitance are selected so as to be located between the fundamental frequency and the carrier frequency of the above and to be sufficiently separated from both.
Instead of grounding the neutral point of the three phases via a capacitor, the neutral point is connected to the neutral point of the DC circuit of the inverter.
[0005]
[Action]
According to the present invention, basically, a low-pass LC filter for removing a common mode voltage of a carrier frequency component is inserted between an inverter and a motor. The LC resonance frequency is selected so as to be sufficiently lower than the carrier frequency and sufficiently higher than the inverter output fundamental frequency. Since the output fundamental wave frequency can be easily set to several tens of Hz for general use and the carrier frequency to ten and several kHz by applying IGBT, such a resonance frequency can be selected. Since the component of the carrier frequency is attenuated by the low-pass filter, the surge voltage and the bearing voltage are suppressed. Further, since the carrier frequency and the output fundamental wave frequency are separated from the LC resonance frequency, it becomes difficult to become a voltage source for exciting the resonance circuit.
By connecting the neutral point of the three phases to the neutral point of the DC circuit of the inverter instead of grounding via a capacitor, there is no current flowing to the ground through this filter. Ground current flowing through the capacitance of the motor and the cable is suppressed, so that leakage current and electromagnetic noise can be reduced.
[0006]
【Example】
An embodiment will be described below with reference to the drawings.
FIG. 1 is a circuit diagram showing a basic operation of the present invention. In this circuit, a common mode reactor 1 is inserted between a voltage type PWM inverter 11 and an AC motor 12 driven by the inverter. The common mode reactor 1 is provided with three sets of windings (in the case of three phases) on the same iron core, and shows an inductance for a current flowing between terminals of each winding, but shows an inductance for a normal three-phase current. In other words, since the magnetic fluxes generated by the windings cancel each other and exhibit no inductance, the basic characteristics of the motor are not affected. Reference numeral 3 denotes a three-phase reactor, which acts as a normal inductance between the respective phases U ₁ , V ₁ , and W ₁ , but a combined common mode inductance between each phase and the neutral point n 1 cancels out magnetic flux. It is zero in principle by action. Therefore, in order to examine the common-mode voltage can be ignored three-phase reactor 3, the circuit can be regarded as a low-pass filter according to the capacitance C ₁ of the inductance L ₁ and the capacitor 2 of the common mode reactor 1.
This filter circuit is a secondary low-pass filter, and the resonance frequency _fr is
_fr = 1 / (2π (L ₁ C ₁ ) ^1/2 )
Can be represented by
The resonant frequency f _r in the present invention, placed in between the carrier frequency f _c and the fundamental frequency f ₀ as shown in FIG. 2, and away as possible from both, for example so as to be near the geometric mean value of the two Is set. As a result, the carrier frequency component can be greatly attenuated. Further, the resonance circuit is hardly excited by the carrier frequency and the fundamental wave frequency.
[0007]
The three-phase reactor 3 shown in FIG. 1 has a configuration as shown in FIG. 3A. However, if the magnetic coupling between the phases is incomplete due to leakage magnetic flux, a common mode voltage drop occurs, and the filter is attenuated. The characteristics deteriorate. As shown in FIG. 3B, the magnetic coupling can be improved by using two windings for each phase, one for star connection, and the other for delta connection. In the figure, three single-phase reactors are used, but three-phase reactors may be used. Further, the two windings of the same phase are formed into a bifilar winding in which two electric wires are simultaneously wound together to obtain better results.
FIG. 4 shows the ground capacitor 2 of FIG. 1 dispersed in each phase of the three-phase reactor 3 and inserted as 4a, 4b, and 4c. If the capacitance of each capacitor is reduced to one third of the case of FIG. The characteristics as a common mode filter become equivalent. In this method, since the capacitors 4a, 4b, and 4c perform a current-limiting action on the normal mode current flowing through the reactor 3, the size of the reactor 3 can be reduced. When braking the motor driven by the inverter, a direct current may flow. In such a case, the capacitors 4a, 4b, and 4c also have an advantage of having a function of blocking the direct current flowing to the reactor 3.
FIG. 5 is obtained by further adding a ground capacitor 2 to the circuit of FIG. The reactor 3 and the capacitors 4a, 4b, 4c can set the normal mode current of the reactor, and the common mode reactor 1 and the capacitors 4a, 4b, 4c, and 2 can set the characteristics of the common mode filter, thereby improving the design flexibility.
Figure 6 shows another method of obtaining the neutral point n _1. The three-phase reactor 5 and the capacitors 6a, 6b, 6c star-connected to the output side form a normal mode filter, and the resonance frequency is set to an intermediate value between the carrier frequency and the fundamental wave frequency, similarly to the common mode filter. For example, a line waveform close to a sine wave can be obtained. Capacitors 6a was star connection, 6b, the neutral point n ₁ of 6c if grounded via a capacitor 2, it is possible to form a common mode filter. Its resonance frequency is the capacitance _C 1, _{C 3} of the inductance _{L 1} and the capacitor of the common mode reactor 1, is determined.
[0008]
Since the resonance frequency of the above filter circuit is determined by the product of the common mode inductance of the series component and the common mode capacitance of the parallel component, the degree of freedom of the combination is infinite. On the other hand, in this circuit, the ground current flows through the inductance and the capacitance, and if the ground current is large, the earth leakage breaker installed in the power supply may operate. To reduce the ground current, it is necessary to set the inductance to be large and the capacitance to be small. Figure 7, instead of grounding one end of the capacitor 2 in the circuit of FIG. 1, which are connected to the neutral point n ₂ of inverter DC bus to make a capacitor 7,8. Since the current flowing through the common mode filter circulates only inside the inverter and does not become the leakage current seen from the power supply, the most economical combination of inductance and capacitance is used as long as the load on the main circuit element is not affected. can do. This method of connecting to the DC neutral point can be applied to all of the filter circuits. 8, instead of grounding one end of the capacitor 2 of the circuit of FIG. 6, which are connected to the neutral point n ₂ of inverter DC bus to make a capacitor 7,8.
The capacitors 7 and 8 are assumed to have a sufficiently large capacitance compared to the capacitor 2 in order to obtain a stable DC neutral point potential, but C ₄ = (１ ／) C ₁ Alternatively, the capacitors 7 and 8 may have a filter function, and the capacitor 2 may be omitted.
[0009]
【The invention's effect】
(1) The common mode voltage of the carrier frequency component can be greatly reduced by the effect of the low-pass filter. Since the secondary filter attenuates at 40 dB / dec, if the ratio between the resonance frequency and the carrier frequency is 1:10, the radiation is reduced to 1/100.
(2) The constants of the reactor and capacitor used for the filter can be determined only by the specifications of the inverter, and the effect is hardly affected by the characteristics and installation conditions of the motor.
(3) As a result, no surge of the ground voltage occurs.
(4) Similarly, bearing damage due to shaft voltage can be prevented.
(5) In the method shown in FIG. 6, the voltage ripple of the line voltage can be reduced to a large value, so that problems such as a decrease in insulation of the motor can be prevented.
(6) In the circuit systems of FIGS. 7 and 8, leakage current is eliminated and electromagnetic noise can be reduced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a basic operation of the present invention.
FIG. 2 is an amplitude characteristic diagram of an LC filter.
3A and 3B show a connection method of a three-phase reactor, in which FIG. 3A shows a conventional method and FIG. 3B shows a method in which magnetic coupling is improved.
FIG. 4 is a system diagram showing an example in which ground capacitors are dispersedly inserted in each phase of a three-phase reactor.
FIG. 5 is a system diagram in which a ground capacitor is further added to the circuit of FIG. 4;
6 is a system diagram illustrating another method for obtaining the neutral point n _1.
FIG. 7 is a system diagram showing another example of connection of a capacitor.
FIG. 8 is a system diagram showing another example of connection of a capacitor and an inductance.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Common mode reactor, 2 capacitors, 3 phase reactor, 4a, 4b, 4c capacitor, 5 phase reactor, 6a, 6b, 6c capacitor, 7, 8 capacitor, 11 voltage type PWM inverter, 12 AC motor

Claims (8)

A common mode reactor inserted between the PWM inverter and an AC motor driven by the inverter; a three-phase star-connected reactor connected to a connection point between the common mode reactor and the AC motor; A capacitor for grounding a neutral point, wherein the resonance frequency of the common mode reactor and the capacitor is located between the output fundamental wave frequency and the carrier frequency of the inverter, and is sufficiently separated from any of them. A filter circuit for a PWM inverter, wherein an inductance of a mode reactor and a capacitance of the capacitor are selected. Instead of the three-phase reactor connected to the connection point between the common mode reactor and the AC motor, one of the three single-phase reactors with two sets of windings is star-connected, and the neutral point is connected by a capacitor. 2. The filter circuit for a PWM inverter according to claim 1, wherein the other windings are connected in a delta connection while being grounded. Instead of a three-phase reactor connected to the connection point between the common-mode reactor and the AC motor, one winding of a three-phase reactor with two sets of windings is star-connected, and the neutral point is grounded with a capacitor. 2. The filter circuit for a PWM inverter according to claim 1, wherein the other winding is delta-connected. The capacitor according to any one of claims 1 to 3, wherein a capacitor is inserted into each phase winding of the star connection instead of grounding a neutral point of the star-connected reactor via a capacitor. Filter circuit for PWM inverter. 5. The filter circuit for a PWM inverter according to claim 4, wherein a neutral point of a circuit in which a combination of a reactor and a capacitor is staggered is grounded via another capacitor. 6. The PWM inverter filter circuit according to claim 1, wherein the three-phase neutral point is connected to the neutral point of the DC circuit of the inverter instead of being grounded via a capacitor. . Common mode reactor and normal mode reactor connected in series between a PWM inverter and an AC motor driven by the inverter, and a three-phase star connected to a connection point between these reactors and the AC motor A connection capacitor, and another capacitor for grounding a neutral point of the capacitor, wherein the resonance frequency of the common mode reactor, the three-phase capacitor, and the ground capacitor, and the resonance frequency of the normal mode reactor and the three-phase capacitor, A filter circuit for a PWM inverter, wherein an inductance and a capacitance are selected so as to be located between the fundamental frequency and the carrier frequency and to be sufficiently separated from any of them. 8. The PWM inverter filter circuit according to claim 7, wherein the three-phase neutral point is connected to the neutral point of the DC circuit of the inverter instead of being grounded via a capacitor.

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