CN1697307A - Pulse Width Modulation Frequency Converter with Passive Filter - Google Patents

Abstract

The frequency converter includes rectifier, inverter, smoothed filter C4, s from first inductor LI to seventh inductor LVII, first capacitor C1, second capacitor C2 and third capacitor C3, and resistance R. One end of LI, one end of LII, and one end of LIII is connected to inverter respectively. The other end of LI is connected to one end of LIV and one end of C1. The other end of C1 is connected to the other end of C2 and C3 and one end of R etc. through eliminating dv/dt, the invention eliminates harm on motors and quality of energy on power network caused by dv/dt.

Description

Pulse Width Modulation Frequency Converter with Passive Filter

Technical field:

The invention relates to a pulse width modulation (PWM) frequency converter with a passive filter used in systems such as electric drive and electric drive.

Background technique:

Frequency conversion speed regulation is an AC drive control technology with high benefit, good performance and wide application. As a control device for frequency conversion and speed regulation, the frequency converter has been widely used in various fields such as industry, agriculture and national defense, and will play an increasingly important role in the future world. However, frequency converters have made great contributions in terms of energy saving, improvement of human living environment, reduction of production costs, improvement of product quality, and improvement of industrial automation, while also producing some significant negative effects. With the rapid development of modern power electronic devices, the switching frequency of power switching devices, such as insulated gate bipolar transistors IGBT, can reach tens of kHz, and its fast turn-on and turn-off characteristics make the output of the inverter produce high dv/dt( voltage change rate). Excessively high dv/dt will cause a series of hazards to the drive system of the inverter, which can be summarized as follows: (1) During the high-speed on-off period of the power switching device, the high-frequency dv/dt will excite eddy currents in the motor core laminations and cause Heat loss will also cause the copper wire winding of the motor to consume more energy through the skin effect, aggravate the heat loss of the motor, increase the power loss of the motor, and reduce the efficiency, thereby affecting the performance of the motor. When the frequency of the high-frequency electromagnetic oscillation generated by the frequency converter is close to the natural oscillation frequency of the components of the motor, it will induce mechanical resonance and noise. (2) Due to the distributed capacitance of the power line and the parasitic capacitance inside the motor at high frequency, a charging and discharging current I _1g (I _1g is called leakage current, which is proportional to dv/dt) will be generated, which will flow into the ground wire and the leakage current will exceed It will cause malfunction of the motor protection circuit. (3) When the high-carrier frequency voltage-type PWM inverter drives the motor, when the high-frequency dv/dt acts on the parasitic capacitance inside the motor, not only will a charging and discharging current be generated, but also the rotor shaft will be damaged due to the cumulative effect of the capacitance. The voltage rises. Both of these will cause the breakdown of the lubricating oil film and produce EDM, which will cause premature damage to the motor bearing, increase the maintenance cost of the motor, and affect the normal operation of the system; (4) Leakage current with frequency ranging from 100kHz to several megabytes It flows back to the three-phase power supply of the system through the ground wire, generating high-frequency electromagnetic interference (EMI). It will lead to a decrease in power supply efficiency; it will also cause the relay protection device to malfunction due to interference, affect the normal operation of other electronic equipment on the grid, and even cause damage to the equipment; (5) When the motor and the frequency converter are inevitably When long-line transmission cables are used, such as in oil exploration, papermaking, mining and other fields, the distributed inductance and distributed capacitance due to long-line cables will produce reflected waves, which will double the dv/dt at the motor end. The doubling of the dv/dt of the common-mode voltage can further aggravate the above hazards.

Invention content:

The invention provides a pulse width modulation frequency converter with a passive filter, which can eliminate the dv/dt of the differential mode voltage and the dv/dt of the common mode voltage at the same time, thereby eliminating the harm of the dv/dt to the power quality of the motor and the grid . It includes a rectifier 1, an inverter 2 and a smoothing capacitor C4, one end of the smoothing capacitor C4 is connected to the positive output terminal of the rectifier 1 and the positive input terminal of the inverter 2, and the other end of the smoothing capacitor C4 is connected to the negative output of the rectifier 1 terminal and the negative input terminal of the inverter 2, it also includes the first inductance LI, the second inductance LII, the third inductance LIII, the fourth inductance LIV, the fifth inductance LV, the sixth inductance LVI, the seventh inductance LVII, the A capacitor C1, a second capacitor C2, a third capacitor C3 and a resistor R, one end of the first inductance LI, one end of the second inductance LII and one end of the third inductance LIII are respectively connected to the three-phase output terminals of the inverter 2 , the other end of the first inductance LI is connected to one end of the fourth inductance LIV and one end of the first capacitor C1, the other end of the second inductance LII is connected to one end of the fifth inductance LV and one end of the second capacitor C2, and the third inductance LIII is connected to One end of the sixth inductance LVI and one end of the third capacitor C3, the other end of the first capacitor C1 is connected to the other end of the second capacitor C2, the other end of the third capacitor C3 and one end of the resistor R, and the other end of the resistor R is connected to the second end One end of the seven inductance LVII, the other end of the seventh inductance LVII is connected to the negative or positive input end of the inverter, the fourth inductance LIV, the fifth inductance LV, the sixth inductance LVI and the seventh inductance LVII are the same and wound in the same phase On the same ring core T, the first capacitor C1 , the second capacitor C2 and the third capacitor C3 have the same capacitance values, and the first inductance LI, the second inductance LII and the third inductance LIII are the same. When the present invention works, the other ends of LIV, LV and LVI are respectively connected to the input terminals of the three-phase power supply of the motor, and LI, LII, LIII, C1, C2 and C3 form a differential mode filter to filter out the dv/dt of the differential mode voltage , C1, C2, C3, R, LI, LII, LIII, LIV, LV, LVI and LVII form a common-mode filter for filtering out the dv/dt of the common-mode voltage, and the filter in the frequency converter of the present invention can filter out the dv /dt, avoiding the harm of dv/dt to the power quality of the motor and the grid, and because the fourth inductance LIV, the fifth inductance LV, the sixth inductance LVI and the seventh inductance LVII have the same number of turns and are wound in the same ring in the same phase On the iron core T, the suppression effect on the common mode voltage dv/dt is basically not affected by the frequency change of the frequency converter. The invention has reasonable design, reliable operation and great popularization value.

Description of drawings:

Fig. 1 is a schematic diagram of the circuit structure of the present invention, Fig. 2 is a schematic diagram of the connection of the fourth inductance LIV, the fifth inductance LV, the sixth inductance LVI, the seventh inductance LVII and the ring core T winding, Fig. 3 is the differential mode filter of the present invention Figure 4 is the common-mode equivalent circuit diagram of Figure 1, Figure 5 is the equivalent circuit diagram of the transformer composed of LIV, LV, LVI and LVII, Figure 6 is the single-phase equivalent circuit diagram of Figure 5, Fig. 7 is the single-phase common-mode voltage equivalent circuit diagram of Fig. 4, and Fig. 8 is the equivalent circuit diagram after Fig. 7 eliminates mutual inductance, and Fig. 9 is the amplitude-frequency characteristic schematic diagram of the transfer function of the filter of the present invention, and Fig. 10 is experiment 1 A schematic diagram of the common-mode voltage at the motor end without a filter. FIG. 11 is a schematic diagram of the common-mode voltage at the motor end after applying the filter of the present invention in Experiment 1. FIG. 12 is a schematic diagram of the differential-mode voltage at the motor end without a filter in Experiment 2. Fig. 13 is a schematic diagram of the differential mode voltage at the motor end after applying the filter of the present invention in Experiment 2, Fig. 14 is a schematic diagram of the common mode voltage at the motor end without a filter in Experiment 2, and Fig. 15 is a schematic diagram of the motor end common mode voltage applied in Experiment 2 Schematic diagram of the common-mode voltage at the motor end after the filter.

Detailed ways:

Specific Embodiment 1: The present embodiment will be specifically described below with reference to FIG. 1 to FIG. 15 . In this embodiment, a rectifier 1, an inverter 2, a smoothing capacitor C4, a first inductor LI, a second inductor LII, a third inductor LIII, a fourth inductor LIV, a fifth inductor LV, a sixth inductor LVI, and a seventh inductor LVII, the first capacitor C1, the second capacitor C2, the third capacitor C3 and the resistor R, one end of the smoothing capacitor C4 is connected to the positive output terminal of the rectifier 1 and the positive input terminal of the inverter 2, and the other end of the smoothing capacitor C4 One end is connected to the negative output end of the rectifier 1 and the negative input end of the inverter 2, and one end of the first inductance LI, one end of the second inductance LII and one end of the third inductance LIII are respectively connected to the three-phase output end of the inverter 2 On, the other end of the first inductance LI connects one end of the fourth inductance LIV and one end of the first capacitor C1, the other end of the second inductance LII connects one end of the fifth inductance LV and one end of the second capacitor C2, and the third inductance LIII Connect one end of the sixth inductance LVI and one end of the third capacitor C3, the other end of the first capacitor C1 connects the other end of the second capacitor C2, the other end of the third capacitor C3 and one end of the resistor R, and the other end of the resistor R connects One end of the seventh inductance LVII, the other end of the seventh inductance LVII is connected to the negative or positive input end of the inverter, the fourth inductance LIV, the fifth inductance LV, the sixth inductance LVI and the seventh inductance LVII are the same and wound in the same phase Made on the same annular iron core T, the capacitance values of the first capacitor C1, the second capacitor C2 and the third capacitor C3 are equal, and the first inductance LI, the second inductance LII and the third inductance LIII are the same. The dv/dt suppression effect of the differential mode filter on the differential mode voltage is analyzed below: the output voltage of the three-phase voltage type PWM inverter contains positive and negative sequence components (ie, differential mode voltage) and zero sequence components (ie, common mode voltage) , which is commonly referred to as line voltage and phase voltage, because the voltage induced by the differential mode voltage output by inverter 2 on LVII is zero, which is equivalent to the resistance R being directly connected to the DC bus between the rectifier and the inverter, The resulting differential mode single-phase equivalent circuit is shown in Figure 3. The transfer function of the circuit shown in Figure 3 is:

$h=\frac{V_0}{{\mathrm{<figure-callout\; id="1"\; label="V"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">V</figure-callout>}}_1}=\frac{3\mathrm{RCs}+1}{\mathrm{LC}{\mathrm{the\; <figure-callout\; id="2"\; label="s"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+3\mathrm{RCs}+1}=\frac{2{\mathrm{\&zeta;}}_1{\mathrm{\&omega;}}_{1\mathrm{no}}\mathrm{the\; s}+{\mathrm{\&omega;}}_{1\mathrm{<figure-callout\; id="2"\; label="no"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">no</figure-callout>}}^2}{{\mathrm{the\; <figure-callout\; id="2"\; label="s"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+2{\mathrm{\&zeta;}}_1{\mathrm{\&omega;}}_{1\mathrm{no}}\mathrm{the\; s}+{\mathrm{\&omega;}}_{1\mathrm{no}}^2}$ (1)

Among them, ξ ₁ and ω _1n are the damping and resonant angular frequencies of the differential mode equivalent circuit, respectively, as shown in formula (2).

$\left\{\begin{array}{c}{\mathrm{\&zeta;}}_1=\frac{3R}{2}\sqrt{\frac{C}{L}}\\ {\mathrm{\&omega;}}_{1\mathrm{no}}=\frac{1}{\sqrt{\mathrm{LC}}}\end{array}\right.$ (2)

Equation (1) has low-pass performance. By selecting the circuit parameters, the damping ratio _ξ1 is greater than 1, that is, the over-damping circuit can effectively reduce the differential mode dv/dt, thereby eliminating the harm of overvoltage to the system. The role of the common-mode filter in reducing the dv/dt of the common-mode voltage analyzed below: the common-mode voltage is one-third of the sum of the three-phase phase voltages, and the common-mode voltage source Vcm is used to equivalently output the common-mode voltage of the PWM inverter. If the mode voltage is obtained, the common-mode equivalent circuit of Figure 1 is shown in Figure 4. For the convenience of analysis, the three-phase circuit shown in Figure 4 is simplified into a single-phase circuit as shown in Figure 6. In the complex frequency domain, the characteristic equation of the common-mode transformer in Figure 5 under zero initial conditions is:

$\left[\begin{array}{c}{V}_1\left(\mathrm{the\; s}\right)\\ V_2\left(\mathrm{the\; s}\right)\end{array}\right]=\left[\begin{array}{cc}3{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1\mathrm{the\; s}& 3\mathrm{Mrs.}\\ 3\mathrm{Mrs.}& ({\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">L</figure-callout>}}_2+2m)\mathrm{the\; s}\end{array}\right]\left[\begin{array}{c}{I}_1\left(\mathrm{the\; s}\right)\\ I_2\left(\mathrm{the\; s}\right)\end{array}\right]$ (3)

Among them: M is the mutual inductance among LIV, LV, LVI, LVII.

According to formula (3), the equivalent circuit of the four-winding transformer can be obtained, as shown in Figure 5. Considering the connection of R, L, and C, the single-phase common-mode equivalent circuit of Figure 5 can be obtained, as shown in Figure 6. Then get its equivalent circuit by eliminating the mutual inductance, as shown in Figure 7 and Figure 8. According to Figure 7 and Figure 8, its equation description is obtained by circuit theory:

$\left\{\begin{array}{c}\mathrm{<span\; class="notranslate">\; Vcm</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; Vco</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span>\\ \mathrm{<span\; class="notranslate">\; Vco</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; Cs</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span>\end{array}\right.$

_L1 is the inductance value of the seventh inductor LVII, and C is the capacitance value of the first capacitor C1, the second capacitor C2 and the third capacitor C3, then the transfer function is:

$h_{\mathrm{ov}}\left(\mathrm{the\; s}\right)=\frac{\mathrm{Vco}\left(\mathrm{the\; s}\right)}{\mathrm{Vcm}\left(\mathrm{the\; s}\right)}=\frac{3(L_1-m)C{\mathrm{the\; <figure-callout\; id="2"\; label="s"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+3\mathrm{RCs}+1}{(3{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L)C{\mathrm{the\; <figure-callout\; id="2"\; label="s"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+3\mathrm{RCs}+1}$ (4)

Among them, k and $m=k\sqrt{L_1{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">L</figure-callout>}}_2}$ are the coupling coefficient and mutual inductance of the common-mode transformer, L ₂ is the inductance value of the fourth inductor LIV, the fifth inductor LV, and the sixth inductor LVI, and R is the resistance value of the resistor R.

In the process of suppressing the common-mode voltage, it is desirable that the transfer function H _ov (s) has a low-pass characteristic to suppress high-frequency components in the common-mode voltage. According to formula (4), the conditions to meet the above requirements are:

$L_1-m=L_1-k\sqrt{L_1{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">L</figure-callout>}}_2}=0$ (5)

Then k=1 and L ₁ =L ₂ satisfy the above formula, which means that in order to meet the low-pass characteristics of the filter, the coupling of the common-mode transformer should be very good, and the toroidal core can meet this requirement, then the turns ratio of the common-mode transformer n is $\mathrm{no}=\sqrt{{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1/{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">L</figure-callout>}}_2}=1,$ Mutual inductance is $m=k\sqrt{L_1{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">L</figure-callout>}}_2}={\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1.$

Bringing the above conclusion into formula (4), we get:

$h_{\mathrm{ov}}\left(\mathrm{the\; s}\right)=\frac{3\mathrm{RCs}+1}{(3{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L)C{\mathrm{the\; <figure-callout\; id="2"\; label="s"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+3\mathrm{RCs}+1}=\frac{3\mathrm{RCs}+1}{(3{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L)C[{\mathrm{the\; <figure-callout\; id="2"\; label="s"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+2{\mathrm{\&zeta;}}_2{\mathrm{\&omega;}}_{2\mathrm{no}}\mathrm{the\; s}+{\mathrm{\&omega;}}_{2\mathrm{no}}^2]}$ (6)

Among them, ξ ₂ and ω _2n are the damping and resonant angular frequencies of the common-mode equivalent circuit, respectively, as shown in equation (7).

$\left\{\begin{array}{c}{\mathrm{\&omega;}}_{2\mathrm{no}}=\frac{1}{\sqrt{({3\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L)C}}\\ {\mathrm{\&zeta;}}_2=\frac{3R}{2}\sqrt{\frac{C}{(3{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L)}}\end{array}\right.$ (7)

It can be seen from formula (7) that k=1, n=1 can make the transfer function obtain low-pass characteristics, so that the high-frequency components produced by the common mode dv/dt can be suppressed.

The design process of the filter of the present invention can be divided into two steps: first, determine the inductance value L of the first inductance LI, the second inductance LII, the inductance L of the third inductance LIII and the first capacitance C1, the second inductance according to the needs of the differential mode dv/dt Capacitance C of the capacitor C2 and the third capacitor C3. This step is done separately because common-mode transformers have no effect on differential-mode dv/dt. Then proceed to the design of the common mode part.

First determine the cut-off angular frequency f _c : according to the definition of the cut-off angular frequency $\left|h\left(j{\mathrm{\&omega;}}_c\right)\right|=1/\sqrt{2}$ Thus the size of the cut-off angular frequency ω _1c is determined: ${\mathrm{\&omega;}}_{1c}={\mathrm{\&omega;}}_{1\mathrm{no}}\sqrt{(1+2{\mathrm{\&zeta;}}_1^2)+\sqrt{{(1+2{\mathrm{\&zeta;}}_1^2)}^2+1}}$ Since the dv/dt at the motor end is reduced by an over-damping circuit, an approximate expression of the cut-off angular frequency ω _1c is obtained: ${\mathrm{\&omega;}}_{1c}\&ap;{\mathrm{\&omega;}}_{1\mathrm{no}}\sqrt{2(1+2{\mathrm{\&zeta;}}_1^2)}$ From the above formula, it can be concluded that the minimum cut-off corner frequency for the filter to meet the performance requirements is

${\mathrm{\&omega;}}_{1c}=2.45{\mathrm{\&omega;}}_{1\mathrm{no}}=2.45/\sqrt{\mathrm{LC}}$ (8)

Determination of capacitor C: Usually, in order to make the filter have a higher output voltage, the value is 20-60 microfarads. According to formula (8), in order for the filter to filter out the high-frequency harmonic components in the output voltage of the inverter, the cut-off frequency should be much smaller than the switching frequency of the inverter, that is, ω _1c << 2πf _s , and the switching angle is generally taken as If the frequency is 0.2~0.4, the size of the inductance L is expressed by the following formula:

$L=(0.2~0.4)\frac{2(1+2{\mathrm{\&zeta;}}_1^2)}{{\left(2\mathrm{\&pi;}{f}_{\mathrm{the\; s}}\right)}^{2}C}$ (9)

Among them, the damping ratio generally takes a value ξ ₁ between 5 and 10, and f _s is the switching frequency of the inverter.

From formula (7), it is hoped that the larger _L1 is, the better the low-pass characteristic of the obtained transfer function will be. Since the main harmonic frequencies of the common-mode voltage are concentrated on the switching frequency and its multiples, equation (10) is obtained, so that the resonant frequency f _2n of the transfer function is much lower than the switching frequency f _s .

${\mathrm{\&omega;}}_{2\mathrm{no}}=2\mathrm{\&pi;}{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="A20051000991400101.png"\; state="\{\{state\}\}">f</figure-callout>}}_{2\mathrm{no}}=\frac{1}{\sqrt{(3{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L)C}}<\frac{1}{\sqrt{\mathrm{LC}}}<<2\mathrm{\&pi;}{f}_{\mathrm{the\; s}}$ (10)

According to the formula (10), theoretically, the resonance of the high-order harmonic cannot occur, but the resonance of the low-order harmonic can occur. However, the low-frequency common-mode voltage has little effect on the motor. At this time, L ₁ or L ₂ can be determined freely, which helps to reduce the loss, volume and cost of the common-mode transformer.

In the experiment, in order to obtain a higher line voltage at the motor end, the capacitance C often reaches the μF level, then L is calculated by the method mentioned in the previous section. Determine the value of _L1 according to the value of L, as shown in formula (4-23). All the above values should satisfy formula (10).

L ₁ ≈(2～3)L (11) The value of R satisfies formula (7):

$R=\frac{2{\mathrm{\&zeta;}}_2}{3}\sqrt{\frac{3{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20051000991400125.png,A20051000991400126.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L}{C}}$ (12)

experiment analysis:

The simulation conditions of experiment 1 are: PWM frequency is 4kHz; LI, LII and LIII are 9mH, C1, C2 and C3 are 60μF, then the differential mode resonance frequency of the filter is 217Hz; R is 11Ω, LIV, LV, LVI and LVII is 30mH, the common mode resonance frequency of the filter is 65Hz; the coupling coefficient is 1. Figure 9 shows the magnitude-frequency characteristics of the transfer function. As can be clearly seen from the figure, the filter proposed by the present invention has good low-pass characteristics, that is, its high-frequency band has good attenuation characteristics, such as f>1000Hz section, the inverter can be switched at the switching frequency (such as 4kHz), almost all of the common-mode voltage generated at the place is filtered, which is suitable for filtering the high-frequency components in the common-mode voltage generated by the inverter. Figure 10 shows the common-mode voltage at the motor end without any filter, and its effective value reaches 151V. Figure 11 shows the common-mode voltage at the motor end with a common-mode filter, and its effective value is 13V. Compared with Figure 10, the effective value is reduced by more than 90%, indicating that the proposed filter has a significant effect on reducing the common-mode voltage.

The experimental conditions of Experiment 2 are: the PWM switching frequency is 4kHz, and the cable length between the inverter and the motor is 100m. The values of inductance, capacitance, and resistance are the same as those in Experiment 1. Figure 12 is the waveform diagram of the differential mode voltage at the motor end without a filter. Since there is a 100m long cable connected between the inverter and the motor, there is a voltage reflection phenomenon at the motor end, and the line voltage value at the motor end is almost doubled, which will make The insulation life of the motor is shortened. When a filter is added between the motor and the inverter, as shown in Figure 13, the motor terminal line voltage waveform is filtered into a sine wave. Since the differential mode voltage dv/dt at the motor end is almost completely eliminated, the voltage reflection phenomenon does not exist, and the cable length between the motor and the frequency converter is not limited. At the same time, this result also shows that the common-mode transformer in the proposed filter has no effect on the differential-mode voltage. Figure 14 and Figure 15 are the actual measurement results of the common-mode voltage at the motor end. In Fig. 14, when no filter is added, the common mode voltage also produces voltage reflection phenomenon due to higher dv/dt, which will generate larger shaft voltage and bearing current and cause premature damage to the motor bearing. In Fig. 15, after using the filter of the present invention, the dv/dt of the common mode voltage is significantly reduced and when the carrier frequency changes, the effect of the filter in Fig. 15 is still obvious.

Claims (2)

1. A pulse width modulation frequency converter with a passive filter, which includes a rectifier (1), an inverter (2) and a smoothing capacitor (C4), one end of the smoothing capacitor (C4) is connected to the positive pole of the rectifier (1) The output terminal and the positive input terminal of the inverter (2), the other end of the smoothing capacitor (C4) connects the negative output terminal of the rectifier (1) and the negative input terminal of the inverter (2), and is characterized in that it also includes The first inductance (LI), the second inductance (LII), the third inductance (LIII), the fourth inductance (LIV), the fifth inductance (LV), the sixth inductance (LVI), the seventh inductance (LVII), the A capacitor (C1), a second capacitor (C2), a third capacitor (C3) and a resistor (R), one end of the first inductance (LI), one end of the second inductance (LII) and one end of the third inductance (LIII) One end is respectively connected to the three-phase output ends of the inverter (2), the other end of the first inductor (LI) is connected to one end of the fourth inductor (LIV) and one end of the first capacitor (C1), and the second inductor (LII ) is connected to one end of the fifth inductor (LV) and one end of the second capacitor (C2), the third inductor (LIII) is connected to one end of the sixth inductor (LVI) and one end of the third capacitor (C3), and the first The other end of the capacitor (C1) is connected to the other end of the second capacitor (C2), the other end of the third capacitor (C3) and one end of the resistor (R), and the other end of the resistor (R) is connected to the seventh inductor (LVII) One end, the other end of the seventh inductance (LVII) is connected to the negative or positive input end of the inverter, the fourth inductance (LIV), the fifth inductance (LV), the sixth inductance (LVI) and the seventh inductance (LVII ) are the same and wound on the same toroidal core (T) in the same phase, the capacitance values of the first capacitor (C1), the second capacitor (C2) and the third capacitor (C3) are equal, the first inductor (LI), the second The second inductance (LII) and the third inductance (LIII) are the same. 2. The pulse width modulation frequency converter containing passive filter according to claim 1, characterized in that the capacitance C of the first capacitor (C1), the second capacitor (C2) and the third capacitor (C3) is 20-60 microfarads, the inductance value L of the first inductance (LI), the second inductance (LII) and the third inductance (LIII) is: $L=(0.2~0.4)\frac{2(1+2{\mathrm{\&zeta;}}_1^2)}{{\left(2\mathrm{\&pi;}{f}_{\mathrm{the\; s}}\right)}^{2}C},$ The inductance values of the fourth inductance (LIV), the fifth inductance (LV) and the sixth inductance (LVI) are L ₁ ≈(2～3)L, and the value of the resistance (R) is $\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&zeta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 3</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 3</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}}<span\; class="notranslate">\; .</span>$

Images