JPS634428B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a power source for electrochemical surface treatment of metals, which has a low voltage and large current output with little rip current, using a current source inverter connected to a controllable rectifier via a DC reactor. This invention relates to an inverter circuit suitable for.

In this type of inverter circuit, multiphase rectification is desired for the rectifier in order to reduce ripples in the inverter current, and since the inverter has a large current, it is desired to improve the utilization rate of the thyristor.

Conventionally, voltage source inverters have been used for this type of power supply. However, in this application, an accurate rectangular current waveform is desired.
Therefore, voltage source inverters cannot meet this requirement. On the other hand, a normal current source inverter requires a large DC reactor to reduce ripple, and due to the large current, the number of parallel thyristor elements in the inverter increases, making it impossible to achieve perfect current balance. The current was reduced and the utilization rate was poor. In addition, the circuit becomes complicated because, for example, a balancing reactor is included in order to balance the current between the elements. In addition, since the output current of a single unit is usually several thousand amperes, it was necessary to devise a manufacturing method.

This invention eliminates the above drawbacks, divides the inverter into multiple parts, reduces current ripple with a relatively small DC reactor, and connects the output of each unit inverter in parallel, so that the output current of each unit inverter can be completely adjusted. The purpose is to easily obtain large current output in a balanced state.

FIG. 1 shows an example of a single-phase output current source inverter circuit to which the invention is applied, and FIG. 2 shows an example of operating waveforms in FIG. 1.

Figure 1 shows an example that is configured with two unit inverter circuits, rectifier Conv1 and rectifier Conv2.
consists of a thyristor bridge connected to an AC power supply voltage with a phase difference of 30 degrees through a secondary two-winding transformer Tr, and is operated with a phase difference of 30 degrees. On the other hand, rectifier Conv1 and
DC reactor DCL1 and Conv2 respectively
The inverter Inv1 and the inverter Inv2 connected via the DCL2 are constituted by self-commutated inverters having a commutation circuit, and their AC output terminals are connected to each other. Both are operated synchronously with no phase difference. In this case, as can be seen from the operation waveform diagram shown in Figure 2, Conv1 and Conv2
The output voltage waveform of is 3 as shown by Vd ₁ and Vd ₂ .
In the case of phase full-wave rectification, it pulsates at a period six times the power supply frequency fs, and the phases of the pulsating waves of Vd ₁ and Vd ₂ are shifted by 30 degrees from each other. Therefore, the DC currents Id ₁ and Id ₂ of each unit inverter flowing due to this voltage are also as shown in Fig. 2.
The waveforms are as shown in Id ₁ and Id ₂ , and the power supply frequency fs
It pulsates at a period six times that of the pulsating wave, and the phases of both pulsating waves are 30 degrees out of phase with each other. In more detail, this
The frequency component of pulsation included in Id ₁ and Id ₂ is 6kfs
(k=1, 2, 3...). Both inverters Inv
1, Inv2, AC output terminals are directly connected to each other,
Since the two units are operated synchronously, their combined output current I _L
has a magnitude of I _L =Id ₁ +Id ₂ . Therefore I _L
Regarding the pulsation component, Id ₁ and Id ₂ are at 30 degrees (1/
Since there is a phase difference of 12fs), the lowest pulsation (6fs
component) is no longer canceled out, only the pulsating component of 12 times the power supply frequency or more remains, and the pulsating component of I _L is greatly reduced. That is, each rectifier operates with 6-phase rectification, but it is combined with the output of the inverter and contains only the pulsation of 12-phase rectification. Furthermore, since the unit inverters in each group are individually controlled in current, they can be operated with Id ₁ =Id ₂ and the current burden on the inverter elements can be made equal.

In other words, if only thyristors are connected in parallel in one inverter, the forward voltage drop of the elements will be unbalanced, resulting in unbalanced current sharing between the elements, and a reactor will be required to balance the current between each thyristor element. In addition to this, it is also necessary to lower the current rating of the element. Therefore, if you try to increase the capacity of such an inverter by simply connecting elements in parallel, the utilization rate of the elements will be poor, resulting in a large number of elements, and the size of attached equipment such as reactors for current balance. .

On the other hand, according to the present invention, individual unit inverters are connected in parallel, and each unit inverter is controlled to have an equal current (Id ₁ =Id ₂ =Id).
The current sharing is completely equal and the utilization rate of the thyristor element is 100%. Further, in the present invention, by connecting in parallel unit inverter outputs each composed of a rectifier (for current adjustment), a DC reactor, and a self-excited inverter, it is possible to easily increase the output using conventional techniques. In this example, a self-excited inverter is used, but in the case of a capacitive load, etc.
Depending on the nature of the load, a load commutation type inverter may be used without a commutation circuit. Furthermore, it is also possible to operate the above-mentioned inverters by connecting a plurality of sets of AC outputs in parallel.

As described above, according to the present invention, since n (n≧2) unit current source inverters are connected in parallel at the inverter output terminals, it is possible to easily increase the size by using a plurality of unit inverters with the capacity that is easiest to manufacture. Capacity can be increased. Further, it is possible to effectively utilize the thyristor element (configuring one unit inverter in parallel). and,
By operating the rectifier with a phase shift of 60 degrees el/n, the output current ripple can be significantly reduced.

Although the above example has been explained using a single phase, the present invention can be applied to any application where ripple reduction is required using a three-phase rectangular wave. However, it is particularly effective as an inverter circuit for a single-phase output power source for metal surface treatment.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an inverter circuit according to the present invention, and FIG. 2 is an operational waveform diagram for explaining the embodiment of FIG. 1. Tr...power transformer, Conv1, Conv2...rectifier, DCL ₁ , DCL ₂ ...DC reactor, Inv ₁ ,
Inv ₂ ...Inverter, Z _L ...Load.

Claims (1)

[Scope of Claims] 1. In a power supply device for a load that requires a large rectangular wave current with little ripple, such as an electrochemical surface treatment facility, the AC output side is connected in parallel to each other and all are operated synchronously with no phase difference. n (n≧2) unit inverters are provided, and the DC input side of each unit inverter is connected to the DC output side of a separate controllable rectifier via a separate DC reactor. Input side: 60°/n each
An inverter circuit characterized in that the inverter circuit is connected to an alternating current power source insulated from each other via a transformer so that alternating current power source voltages having different phases are applied to each other.