JP2003018853A - Common mode current reduction method - Google Patents

Abstract

(57) Abstract: A common mode current flowing in a power conversion device including a PWM rectifier circuit and a PWM inverter circuit is reduced at low cost. SOLUTION: Semiconductor elements Q1 and Q of a PWM rectifier circuit
3, Q5 and semiconductor elements Q7, Q of the PWM inverter circuit
9, Q11 on (or off) timing, or P
By making the off (or on) timing of the semiconductor elements Q2, Q4, Q6 of the WM rectifier circuit and the semiconductor elements Q8, Q10, Q12 of the PWM inverter circuit simultaneous, the change of the common mode voltage is reduced, and the common mode current is reduced. Reduce. As a result, the size of the common mode filter and the size of the cooling device of the common mode current suppressing device can be reduced, and the cost can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a common mode current reduction method in the case where a power conversion device including a PWM rectifier circuit and a PWM inverter circuit simultaneously operates both circuits.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional circuit configuration diagram in the case of driving an electric motor through a power conversion device composed of a PWM rectifying circuit and a PWM inverter circuit. Here, the PWM rectifier circuit has a full bridge configuration of six semiconductor switches in which a self-extinguishing arc semiconductor switch and a diode are connected in antiparallel, and a PWM inverter circuit similarly has the semiconductor switch 6
An example of a full bridge configuration is shown.

FIG. 3 shows a waveform diagram for explaining the operation of FIG. As shown in FIG. 3, in both the PWM rectifier circuit and the PWM inverter circuit, the gate of each semiconductor switch is controlled by comparing the triangular wave that is a carrier wave with the modulation signal wave (also referred to as a double edge modulation method) to perform PWM.
The rectifier circuit makes the input current sinusoidal with a power factor of 1 and outputs a desired DC voltage. On the other hand, the PWM inverter circuit outputs a three-phase AC voltage having a desired voltage and frequency by comparing the triangular wave, which is a carrier wave, with the modulation signal wave. In the switching mode of FIG. 3, "1" means that the switching elements of the upper arms of both conversion circuits are in the ON state,
“0” indicates that the switching element of the lower arm is on.

By the way, by switching each semiconductor switch, a common mode voltage and a common mode current are generated as shown in FIG. Here, the generation principle of the common mode voltage will be described below. Figure 2
Of the input terminals R, S, and T of the PWM rectifier circuit with respect to the midpoint N of the DC intermediate capacitors are v _RN , v _SN , and v, respectively.
Let _TN be the potential of the midpoint N of the DC intermediate capacitor with respect to the ground as v _NG , then the common mode voltage is
It can be expressed like an expression. v _NG =-(v _RN + v _SN + v _TN ) / 3 ≠ 0 (1)

As can be seen from the above equation (1), if the DC intermediate voltage is Ed, the common mode voltage v _NG of the PWM rectifier circuit is Ed / 3 each time the semiconductor switch of the PWM rectifier circuit is turned on and off. It will change. That is,
During one cycle of the triangular wave, three phases of the semiconductor switch of the PWM rectifier circuit are turned on and off, so that the common mode voltage generated by the PWM rectifier circuit changes six times.

On the other hand, in FIG. 2, the potentials of the inverter output terminals U, V, W with respect to the DC intermediate capacitor midpoint N are v _UN , v _VN , v _WN , respectively, and the neutral point S of the motor armature winding is shown. Assuming that the potential with respect to the capacitor midpoint N is v _SN , the common mode voltage can be expressed by the following equation (2). v _SN = (v _UN + v _VN + v _WN ) / 3 ≠ 0 (2) From this, the PWM inverter is generated by turning on and off three semiconductor switches of the PWM inverter circuit during one triangular wave period. Common mode voltage
It will change 6 times as above.

Therefore, the neutral point S of the motor armature winding is
The potential v _{SG with} respect to the ground G, that is, the common mode voltage, is as shown in the following formula (3), and when the modulation carrier of both conversion circuits is the same triangular wave, it changes 12 times in one cycle of the triangular wave. become. v _SG = v _SN + v _NG = (v _UN + v _VN + v _WN ) / 3- (v _RN + v _SN + v _TN ) / 3 (3)

Then, every time the common mode voltage expressed by the equation (3) changes, the common mode current for charging and discharging the stray capacitance between the windings of the electric motor and between the frames flows. The common mode current path is shown by a thick arrow in FIG. That is, three-phase PWM rectifier circuit and three-phase PWM
Due to the generation of common mode voltage due to the switching of the inverter circuit, the power supply → 3-phase PWM rectifier circuit → 3-phase PWM
The common mode current flows through the path from the inverter circuit to the motor to the ground wire. By the way, since this common mode current causes troubles such as malfunction of other devices, for example, a common mode filter as shown in FIG. 5 or a common mode current suppressing device as shown in FIG. The common mode current is prevented from flowing out to the power supply side.

The operating principle of the common mode current suppressing device will be described below. The common mode current suppressor includes a common mode transformer Tr and a field effect transistor F.
When a current flows in the positive direction (the direction from the power source to the electric motor via the inverter is positive) due to switching of the three-phase PWM rectifier circuit or the three-phase PWM inverter circuit, the current flows in Tr. The common mode current is detected, the FET 2 is turned on, and the common mode current is bypassed to the DC intermediate circuit. The same applies when the common mode current flows in the negative direction. In this case, the common mode current is bypassed to the DC intermediate circuit by turning on the FET1. That is, the common mode transformer Tr and the two FETs 1 and 2 constitute a feedback system so that the common mode current flowing to the power supply side (common mode transformer) becomes 0 even when the PWM rectifier circuit and the PWM inverter switch. , The common mode current suppressor operates.

[0010]

However, the one using the common mode filter as described above has a problem that the cost is increased due to the increase in size, and the one using the common mode current suppressor particularly reduces the generated loss. However, there is a problem that the cost is increased due to the increase in size of the cooling device. Therefore, an object of the present invention is to reduce the size of a common-mode filter or a cooling device for a common-mode current suppressor and reduce the common-mode current at low cost.

[0011]

In order to solve such a problem, in the invention of claim 1, a PWM rectifier circuit and a PWM rectifier circuit are provided.
When driving a load through a power conversion device including an inverter circuit, all the semiconductor elements of one of the upper and lower arms forming the PWM rectifier circuit and the semiconductor elements of one of the upper and lower arms forming the PWM inverter circuit One of the features is that all of the on and off timings are set at the same time to reduce the common mode current. In the invention of claim 1 above, the PWM
The carrier wave signal for PWM modulation of the rectifier circuit and the PWM inverter circuit can have the same waveform (claim 2).
Invention). In the invention of claim 1 or 2, the waveform may be a sawtooth wave (invention of claim 3).

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view of an embodiment of the present invention. This corresponds to FIG. 3 showing the driving method of FIG. As is clear from comparison with FIG. 1 and FIG. 1, it is a feature that the ON timing of each switching element of the PWM rectifier circuit and the PWM inverter upper arm or each OFF timing of the switching element of the lower arm are matched. . For that purpose, the carrier wave signal has the same waveform as shown in, and a sawtooth wave is particularly used here.

By doing so, the PWM rectifier circuit and the P
Since all the semiconductor switches of the WM inverter are turned on (or turned off) at the same time, the voltage change of the common mode voltage is 6 times in one cycle of the sawtooth wave (see FIG. 1), and the generation of the common mode voltage is the conventional one. It is reduced to half compared with the triangular wave comparison modulation method. The triangular wave comparison modulation method was a double edge modulation method,
The sawtooth wave comparison modulation method is a single edge modulation method.

[0014]

According to the present invention, since the common mode current can be reduced, it is possible to reduce the size of the common mode filter and the cooling device of the common mode current suppressor, and to reduce the cost. To be

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram showing a conventional example.

FIG. 3 is an operation explanatory diagram of FIG. 2;

FIG. 4 is a current path explanatory diagram of a common mode current.

FIG. 5 is a circuit diagram showing a connection example of a common mode filter.

FIG. 6 is a circuit diagram showing a connection example of a common mode current suppressing device.

[Explanation of symbols]

e _R , e _S , e _T ... 3-phase AC power supply, C _{Y1 to} C _Y3 ... Grounding capacitor, Cs ... Stray capacitance, Q _{1 to} Q ₁₂ ... Semiconductor switch, L _{1 to} L ₃ ... AC reactor, L _{m1 to} L _m3 … Motor armature winding, L _{c1 to} L _c3 … Common mode reactor, Tr
... Common mode transformer, e _U , e _V , e _W ... Motor induced voltage, C _d1 , C _d2 ... Electrolytic capacitor, FET1, FET
2 ... Field effect transistor.

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Claims (3)

[Claims] 1. When driving a load through a power conversion device including a PWM rectifier circuit and a PWM inverter circuit, the PWM inverter circuit and all of the semiconductor elements of either one of the upper and lower arms configuring the PWM rectifier circuit are configured. A common-mode current reduction method characterized in that the common-mode current is reduced by simultaneously turning on or off all the semiconductor elements of one of the upper and lower arms. 2. The common mode current reduction method according to claim 1, wherein the carrier wave signals for PWM modulation of the PWM rectifier circuit and the PWM inverter circuit have the same waveform. 3. The common mode current reduction method according to claim 1, wherein the waveform is a sawtooth wave.