First, the internal main circuit structure
The low-voltage inverter adopting the "AC-DC-AC" structure has its internal main circuit consisting of two parts: rectification 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 to V6 form a three-phase inverter bridge, which inverts direct current into three-phase alternating current with adjustable frequency and voltage.
Figure 1 Internal main circuit of the inverter
Second, voltage equalizing resistor and current limiting resistor
In Figure 1, a resistor is connected in parallel between the filter capacitors C1 and C2, so that the voltages on the two capacitors are substantially equal to prevent the capacitor from being damaged during operation (currently, due to advances in technology, low-voltage (380V) inverters Most of the 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 when the inverter is just turned on, the voltage on the filter capacitor is 0V, and the rectification voltage when the power supply voltage is 380V. The peak value is 537V, so there will be a large charging surge current at the moment of power-on, which may damage the rectifier diode. In addition, the filter capacitor with terminal voltage of 0 will instantaneously reduce the rectified voltage to 0V, forming 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 to 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 resistor is used by the contactor KM. R is shorted to make it run out.
Third, the external connection terminal of the main circuit
The external connection terminals of the main circuits of various inverters are roughly the same, as shown in Figure 2. Among them, R, S, T is the power supply terminal of the inverter, connected to the AC three-phase power supply; U, V, W are the output terminals of the inverter, connected to the motor; P+ is the + end of the rectifier bridge output, P+ terminal at the factory A short copper plate with a large cross-sectional area is connected to the P terminal. When the DC reactor DL ​​needs to be connected, the copper piece is removed, and the DL is connected between P+ and P; P and N are filtered DC circuits. The + and - terminals can be connected to the brake unit and the braking resistor; PE is the grounding terminal.
Figure 2 main circuit external connection terminal
Fourth, the common DC bus of the frequency conversion system
When the motor is in the braking (power generation) 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 the DC bus voltage in the inverter will rise. If the frequency converter is equipped with a brake unit and a braking resistor (these two components are optional), the inverter can turn on the resistor for a short time, so that the regenerative energy is consumed in a thermal manner, which is called energy braking. Of course, the regenerative energy feedback scheme can also solve the regenerative energy problem of the variable frequency speed control system, and can achieve the purpose of saving energy. The standard general-purpose PWM inverter does not have the function of feeding back the regenerative energy to the three-phase power supply. If the DC link of multiple inverters is interconnected through a common DC bus, the regenerative energy generated by one or more motors can be absorbed and absorbed by other motors in an electric manner. Alternatively, a set of braking units and braking resistors of a certain capacity are provided on the DC bus to absorb regenerative energy that cannot be absorbed by the motorized 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 into the power grid, thereby improving the energy saving effect of the system. In summary, in a variable frequency speed control system with multiple motors, a common DC bus scheme is adopted, and a group of braking units, braking resistors and energy feedback units are configured, which is a better system performance and investment saving. Program.
Figure 3 shows a widely used shared DC bus solution, which includes the following parts.
Figure 3 Common DC bus of the inverter
1. Three-phase AC power supply line
The power input terminals of each inverter are connected in parallel on the same AC bus, and the phase of the power supply at the input of each inverter is consistent. In Figure 3, the circuit breaker QF is the line protection device for each inverter. LR is a line reactor. When multiple inverters are running in the same environment, adjacent inverters will interfere with each other. In order to eliminate or mitigate such interference, and to improve the power factor on the input side of the inverter, access to LR is necessary.
2. DC bus
KM is the control switch for connecting the DC link of the inverter to the common DC bus. FU is a semiconductor fast-blow fuse with a rated voltage of 700V. The rated current must take into account the maximum current of the drive motor during electric or braking. In general, 125% of the rated load current can be selected.
3. Common brake unit and/or energy feedback device
The regenerative energy fed back to the common DC bus can consume unrecovered regenerative energy through a common braking resistor if it is not fully absorbed. If an energy feedback device is used, this part of the regenerative energy will be fed back into 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 to the common DC bus through KM, or disconnects from the shared DC bus quickly after the inverter fails.Displacement sensor, also known as linear sensor, is a linear device belonging to metal induction. The function of the sensor is to convert various measured physical quantities into electricity. In the production process, the measurement of displacement is generally divided into measuring the physical size and mechanical displacement. According to the different forms of the measured variable, the displacement sensor can be divided into two types: analog and digital. The analog type can be divided into two types: physical property type and structural type. Commonly used displacement sensors are mostly analog structures, including potentiometer-type displacement sensors, inductive displacement sensors, self-aligning machines, capacitive displacement sensors, eddy current displacement sensors, Hall-type displacement sensors, etc. An important advantage of the digital displacement sensor is that it is convenient to send the signal directly into the computer system. This kind of sensor is developing rapidly, and its application is increasingly widespread.
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