Jinhua domestic frequency converter

Jinhua domestic frequency converter I. internal main circuit structure



low voltage frequency converter with "AC-DC-AC" structure, its internal main circuit is composed of rectifier and inverter, as shown in Figure 1. The three-phase alternating current input from R, s and t terminals is rectified to direct current by three-phase rectifier bridge (composed of diodes d1-d6), and the voltage is UD. Capacitors C1 and C2 are filter capacitors. Six IGBT transistors (insulated gate bipolar transistors) V1-V6 form a three-phase inverter bridge, which inverts the direct current into three-phase alternating current with adjustable frequency and voltage.

Figure 1 In Figure 1, the two ends of filter capacitor C1 and C2 are connected in parallel to each other to make the voltage on the two capacitors basically equal and prevent the capacitor from being damaged in operation (at present, due to the advancement of technology, most of the electrolytic capacitors of low-voltage (380V) inverter do not need to be used in series). A resistance R and a pair of contactor contacts km are connected between the rectifier bridge and the filter capacitor. The reason is that when the frequency converter is just connected to the power supply, the voltage on the filter capacitor is 0V, and the peak value of the rectifier voltage is 537V when the power supply voltage is 380V. In this way, there will be a large charging impulse current at the moment of connecting to the power supply, which may damage the rectifier diode. In addition, the filter with the terminal voltage of 0 The wave capacitor will reduce the rectifier voltage to 0V in a moment, and form the interference to the power supply network. In order to solve the above problems, in the rectifier bridge and filterA current limiting resistor R is connected between the wave capacitors, which can limit the charging current of the filter capacitors to a permissible range. However, if the current limiting resistor R is always connected in the circuit, its voltage drop will affect the output voltage of the converter and also reduce the power conversion efficiency of the converter. Therefore, after the filter capacitor is charged, the contactor km will short-circuit the current limiting resistor R to make it out of operation.

3、 The external connection terminals of the main circuit

are roughly the same as those of the main circuit of various frequency converters, as shown in Figure 2. Among them, R, s and T are the power terminals of the frequency converter, which are connected to the three-phase AC power supply; u, V and W are the output terminals of the frequency converter, which are connected to the motor; P + is the + terminal of the rectifier bridge output. When leaving the factory, a copper strip with sufficient cross-section area is used for short connection between the P + terminal and the P terminal. When it is necessary to connect to the DC reactor DL, remove the copper strip and connect the DL between the P + and p; P and N are the + of the filtered DC circuit -The terminal can connect the braking unit and braking resistance; PE is the grounding terminal.

Fig. 2 external connection terminal of main circuit



IV. when the common DC bus of frequency conversion system

motor is in braking (power generation) state, the energy absorbed by the frequency converter from the motor will be saved in the electrolytic capacitance of the DC link of the frequency converter, and the DC bus voltage in the frequency converter will rise. If the frequency converter is equipped with a braking unit and a braking resistor (these two elements are optional), the frequency converter can turn on the resistor for a short time, so that the regenerative electric energy can be consumed in a thermal way, which is called energy consumption braking. Of course, it can also be solved by adopting regenerative energy feedback schemeThe problem of renewable energy in variable frequency speed regulation system can achieve the purpose of energy saving. The standard general PWM inverter is not designed to make the regenerative energy feedback to the three-phase power supply. If the DC link of multiple inverters is interconnected by a common DC bus, the regenerative energy generated by one or more motors can be consumed and absorbed by other motors in an electric way. Alternatively, a set of braking units and braking resistors with a certain capacity are arranged on the DC bus to absorb the regenerative energy that cannot be absorbed by the electric state motor. If the common DC bus and energy feedback unit are combined, the redundant energy on the DC bus can be directly fed back to the grid, so as to improve the energy saving effect of the system. To sum up, in the frequency conversion and speed regulation system with multiple motors, it is a better scheme to select the common DC bus scheme and configure a group of braking units, braking resistance and energy feedback unit, so as to improve the system performance and save investment.

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

Fig. 3 common DC bus of frequency converter



1. Three phase AC power incoming line

the power input ends of each frequency converter are parallel connected to the same AC bus, and the power phase of each frequency converter input end is consistent. In Figure 3, circuit breaker QF is the incoming line protection device of each frequency converter. LR is the incoming reactor. When multiple inverters are operating in the same environment, the adjacent inverters will interfere with each other. In order to eliminate or reduce this interference, and to improve the power factor of the input side of the converter, it is necessary to connect LR.

2. DC bus

km is the control switch connecting DC link of frequency converter and common DC bus. Fu is a semiconductor fast fuse. Its rated voltage can be 700V. The current of the driving motor must be taken into account in the rated current. In general, 125% of the rated load current can be selected.

3. The regenerative energy fed back to the public DC bus by the public braking unit and / or energy feedback device can be consumed by the shared braking resistance when it can not be fully absorbed. If the energy feedback device is used, this part of the renewable energy will be fed back to the grid, so as to improve the efficiency of energy saving.

4. Control unit




according to the command of control unit, each frequency converter parallels its DC link to the common DC bus through km, or quickly disconnects with the common DC bus after the frequency converter fails.



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