The main circuit components of the frequency converter, the physical diagram of the internal circuit of the frequency converter

Frequency conversion energy saving is mainly reflected in the application of fans and water pumps. In order to ensure the reliability of production, various production machinery has a certain margin when designing and using power transmission. When the motor cannot run at full load, in addition to meeting the requirements of power transmission, the excess torque increases the consumption of active power, resulting in a waste of electrical energy.

The main circuit of the inverter is composed of the following four parts:

Rectifier circuit

DC intermediate circuit

Inverter circuit

Auxiliary circuit

Frequency converter is a power control device that uses frequency conversion technology and microelectronics technology to control AC motors by changing the frequency of the motor's working power supply.

Using the "AC-DC-AC" structure of the low-voltage inverter, its internal main circuit is composed of rectifier and inverter, as shown in Figure 1. The three-phase alternating current input from the R, S, and T terminals is rectified into a direct current through a three-phase rectifier bridge (consisting 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～V6 form a three-phase inverter bridge, inverting direct current into three-phase alternating current with adjustable frequency and voltage.

Figure 1 Main circuit inside the inverter

In Figure 1, a resistor is connected in parallel to both ends of the filter capacitors C1 and C2 in order to make the voltages on the two capacitors basically equal to prevent the capacitors from being damaged during work. (At present, due to technological progress, the low voltage (380V) inverter Most electrolytic capacitors do not need to be used in series). A resistor R and a pair of contactor contacts KM are connected between the rectifier bridge and the filter capacitor. The reason is that the voltage on the filter capacitor is 0V when the inverter is just connected to the power supply, and the rectified voltage when the power supply voltage is 380V The peak value is 537V, so there will be a large charging impulse current when the power is turned on, which may damage the rectifier diode; in addition, a filter capacitor with a terminal voltage of 0 will instantly reduce the rectifier voltage to 0V, which will cause interference to the power supply network. . In order to solve the above problems, 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 will also reduce the power conversion efficiency of the inverter. Therefore, after the filter capacitor is charged, the current-limiting resistance is set by the contactor KM. Short-circuit R to make it exit operation.

The external connection terminals of the main circuit of various inverters are roughly the same, as shown in Figure 2. Among them, R, S, T are the power supply terminals of the inverter, which are connected to the AC three-phase power supply; U, V, W are the output terminals of the inverter, which are connected to the motor; P+ is the + terminal of the rectifier bridge output, and the P+ terminal is at the factory. Use a piece of copper with a large enough cross-sectional area to short-circuit with the P terminal. When the DC reactor DL needs to be connected, 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 the braking resistor; PE is the grounding terminal.

Figure 2 External connection terminals of the main circuit

When the motor is in the braking (generating) state, the energy absorbed by the inverter from the motor will be stored in the electrolytic capacitor of the DC link of the inverter, and cause 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 accessories), the inverter can turn on the resistor for a short time to consume the regenerative electric energy in a thermal manner, which is called dynamic braking. Of course, adopting the regenerative energy feedback scheme can also solve the regenerative energy problem of the variable frequency speed regulation system, and can achieve the purpose of saving energy. The standard general-purpose PWM inverter is not designed to feed the regenerative energy back to the three-phase power supply. If the DC links of multiple frequency converters are interconnected through a common DC bus, the regenerative energy produced by one or more motors can be consumed and absorbed by other motors in an electric manner. Or, set a set of braking units and braking resistors with a certain capacity on the DC bus to absorb the regenerative energy that cannot be absorbed by the electric motor. If the shared DC bus is combined with the energy feedback unit, the excess energy on the DC bus can be directly fed back to the grid, thereby improving the energy-saving effect of the system. To sum up, in a variable frequency speed regulation system with multiple motors, it is better to use a shared DC bus scheme and configure a set of braking units, braking resistors and energy feedback units to improve system performance and save investment. Program.

Figure 3 shows the widely used common DC bus scheme, which includes the following parts.

Figure 3 Common DC bus of the frequency converter

1. Three-phase AC power inlet

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

2. DC bus

KM is a control switch that connects the DC link of the inverter to the public DC bus. FU is a semiconductor fast fuse. Its rated voltage can be 700V. The rated current must consider the maximum current of the driving motor during electric or braking. Generally, 125% of the rated load current can be selected.

3. Common 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 shared braking resistor. If an energy feedback device is used, this part of the regenerative energy will be fed back to the grid, thereby improving the efficiency of energy saving.

4. Control unit

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