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What’s so magical about the main circuit inside the inverter?

Source: InternetPublisher:方世玉223 Keywords: Low voltage inverter frequency converter power supply and other power circuits Updated: 2021/11/17

1. Internal main circuit structure

The internal main circuit of a low-voltage inverter using an "AC-DC-AC" structure consists of two parts: rectifier and inverter, as shown in Figure 1.

The three-phase alternating current input from the R, S, and T terminals is rectified into direct current by the three-phase rectifier bridge (composed of diodes D1 to D6), and the voltage is UD. Capacitors C1 and C2 are filter capacitors. Six IGBT tubes (insulated gate bipolar transistors) V1 to V6 form a three-phase inverter bridge, which inverts DC power into three-phase AC power with adjustable frequency and voltage.

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Figure 1 Internal main circuit of the frequency converter

2. Voltage equalizing resistor and current limiting resistor

In Figure 1, a resistor is connected in parallel at both ends of the filter capacitors C1 and C2 in order to make the voltages on the two capacitors basically equal and prevent the capacitors from being damaged during work (currently, due to technological advancement, low-voltage (380V) frequency converters Most electrolytic capacitors do not need to be used in series).

There is a resistor R and a pair of contactor contacts KM connected between the rectifier bridge and the filter capacitor. The reason is: when the frequency converter is just connected to the power supply, the voltage on the filter capacitor is 0V, and the rectified voltage when the power supply voltage is 380V The peak value is 537V, so when the power is turned on, there will be a large charging impulse current, which may damage the rectifier diode; in addition, the filter capacitor with a terminal voltage of 0 will instantly reduce the rectified voltage to 0V, causing interference to the power network .

In order to solve the above problem, a current limiting resistor R is connected between the rectifier bridge and the filter capacitor to limit the charging current of the filter capacitor within an allowable range. However, if the current-limiting resistor R is always connected in the circuit, its voltage drop will affect the output voltage of the inverter and also reduce the power conversion efficiency of the inverter. Therefore, after the filter capacitor is charged, the contactor KM will Short-circuit R to stop running.

3. External connection terminals of the main circuit

The external connection terminals of the main circuits of various frequency converters are roughly the same, as shown in Figure 2. Among them, R, S, and T are the power terminals of the frequency converter, connected to the AC three-phase power supply; U, V, and W are the output terminals of the frequency converter, connected to the motor; P+ is the + terminal of the rectifier bridge output, and the P+ terminal is shipped from the factory. Use a copper piece with a large enough cross-sectional area to short-circuit the P terminal. When it is necessary to connect the DC reactor DL, remove the copper piece and connect DL between P+ and P; P and N are filtered DC circuits. The + and - terminals can be connected to the braking unit and braking resistor; PE is the grounding terminal.

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Figure 2 Main circuit external connection terminals

4. Common DC bus of frequency conversion system

When the motor is braking (generating power), the energy absorbed by the inverter from the motor will be stored in the electrolytic capacitor of the DC link of the inverter, causing the DC bus voltage in the inverter to increase. If the inverter is equipped with a braking unit and a braking resistor (these two components are optional), the inverter can turn on the resistor for a short time to consume the regenerated electric energy in the form of heat, which is called energy consumption braking.

Of course, adopting a regenerative energy feedback solution can also solve the problem of regenerative energy in the variable frequency speed control system and achieve the purpose of saving energy. The standard general-purpose PWM inverter is not designed to feed back the regenerative energy to the three-phase power supply. If the DC links of multiple frequency converters are interconnected through a common DC bus, the regenerative energy generated by one or more motors can be consumed and absorbed electrically by other motors. Alternatively, a set of braking units and braking resistors with a certain capacity can be installed on the DC bus to absorb regenerative energy that cannot be absorbed by the motor in the electric state. If the shared DC bus is combined with an energy feedback unit, the excess energy on the DC bus can be directly fed back to the power grid, thereby improving the energy-saving effect of the system.

To sum up, in a variable frequency speed regulation system with multiple motors, choosing a shared DC bus solution and configuring a set of braking units, braking resistors and energy feedback units is a better way to improve system performance and save investment. plan.

Figure 3 shows a widely used shared DC bus solution, which includes the following parts.

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Figure 3 Common DC bus of the frequency converter

1. Three-phase AC power incoming line

The power input terminals of each frequency converter are connected in parallel to the same AC bus, and the power phases of the input terminals of each frequency converter must be consistent. In Figure 3, circuit breaker QF is the incoming line protection device of each frequency converter. LR is an incoming line reactor. When multiple frequency converters are running in the same environment, adjacent frequency converters will interfere with each other. In order to eliminate or reduce this interference and improve the power factor on the input side of the frequency converter, LR is connected. necessary.

2. DC bus

KM is the control switch connecting the DC link of the frequency converter to the public DC bus. FU is a semiconductor fast fuse, and its rated voltage can be selected from 700V. The rated current must consider the maximum current of the drive motor during motoring or braking. In general, 125% of the rated load current can be selected.

3. Public braking unit and/or energy feedback device

If the regenerative energy fed back to the common DC bus cannot be completely absorbed, the unabsorbed regenerative energy can be consumed through the common braking resistor. If an energy feedback device is used, this part of the regenerated energy will be fed back to the power grid, thereby improving energy saving efficiency.

4.Control unit

According to the instructions of the control unit, each frequency converter connects its DC link in parallel to the common DC bus through KM, or quickly disconnects from the common DC bus after the frequency converter fails.

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