What does the inverter main circuit consist of? Inverter main circuit diagram explanation

The main circuits inside different series of AC-DC-AC inverters are basically the same, and many phenomena that occur during the frequency conversion speed regulation process can be analyzed through the main circuits.

1. AC-DC conversion circuit

The AC-DC conversion circuit part of the AC-DC-AC inverter is composed of a rectifier circuit, a filter circuit, a current limiting circuit and a power indication circuit.

The AC-DC conversion circuit is a rectifier and filter circuit, whose task is to convert the three-phase (or single-phase) AC power of the power supply into a stable DC power. Since the DC voltage after rectification is high and is not allowed to be reduced, it has its own particularity in circuit structure.

1. Full-wave rectifier circuit

Most SPWM inverters use bridge full-wave rectifier circuits, and the rectifiers in medium and small capacity inverters use uncontrollable rectifier diodes or diode modules, as shown in the figure below, VD1~VD6 in the inverter AC-DC conversion circuit. When the three-phase line voltage is 380V, the peak voltage after rectification is 537V and the average voltage is 515V.

2. Filter circuit

In the figure above, the filter circuit refers to CF1 and CF2. Due to the limitations of the capacitance and withstand voltage of electrolytic capacitors, the filter circuit is usually composed of several capacitors connected in parallel into a group, and then connected in series by two capacitor groups CF1 and CF2. Because the capacitance of electrolytic capacitors has a large discreteness, the capacitance of capacitor groups CF1 and CF2 cannot be completely equal. As a result, the voltages UD1 and UD2 borne by each capacitor group are not equal, making the capacitor group on the side with higher voltage easily damaged. In order to make UD1 and UD2 equal, a voltage equalizing resistor RC1 and RC2 with equal resistance are connected in parallel next to CF1 and CF2.

3. Current limiting circuit

1) In the above figure, the current limiting circuit refers to a parallel circuit consisting of a current limiting resistor RL and a short-circuit switch SL connected in series between the rectifier bridge and the filter capacitor.

2) The function of the current limiting resistor RL is: before the inverter is connected to the power supply, the DC voltage UD on the filter capacitor CF (composed of CF1 and CF2 connected in series) is 0. Therefore, at the moment when the inverter is just connected to the power supply, a large impact current will flow to the filter capacitor through the rectifier, which may damage the rectifier bridge. If the capacity of the capacitor is large, it will also cause the power supply voltage to drop instantly and cause interference to the power grid. The current limiting resistor RL is connected in series between the rectifier bridge and the filter capacitor to weaken the impact current.

3) The function of the short-circuit switch SL is: if the current-limiting resistor RL is connected to the circuit for a long time, it will affect the DC voltage UD and the inverter output voltage. Therefore, when UD increases to a certain extent, the short-circuit switch SL is turned on to cut RL out of the circuit. SL is mostly composed of thyristors, and in inverters with smaller capacity, it is often composed of relay contacts.

4. Power indication circuit

The power indicator light HL not only indicates whether the power is on, but also has a very important function, that is, after the inverter cuts off the power, it indicates whether the charge on the filter capacitor CF has been released.

Since the capacity of CF is large and the power supply must be cut off when the inverter circuit stops working, there is no fast discharge circuit for CF, and its discharge time is often several minutes. Since the voltage on CF is high, it will pose a threat to personal safety if it is not fully discharged. Therefore, when repairing the inverter, you must wait until HL is completely extinguished before touching the conductive parts inside the inverter. Therefore, HL also has the function of prompting protection.

2. DC-AC conversion circuit

1. Three-phase inverter bridge circuit

The function of the inverter bridge circuit is to convert DC power into three-phase AC power. The inverter bridge circuit is composed of the switch devices V1~V6 in the figure below. At present, most of the switch devices in medium and small capacity inverters use IGBT tubes.

2. Freewheeling circuit

The freewheeling circuit is composed of VD7~VD12 in the figure above. Its functions are as follows:

1) Provide a path for the reactive current of the motor winding to return to the DC circuit.

2) When the frequency decreases and the synchronous speed decreases, a path is provided for the regenerative electric energy of the motor to be fed back to the DC circuit.

3) Provide a path for the parasitic inductance of the circuit to release energy during the inverter process.

3. Buffer circuit

When the inverter is turned off and on, the rate of change of its voltage and current is very large, which may damage the inverter. Therefore, a buffer circuit should be connected to each inverter to slow down the rate of change of voltage and current. The structure of the buffer circuit varies greatly due to the characteristics and capacity of the inverter. The figure below shows a typical buffer circuit (composed of R01R06, C01C06, VD01~VD06).

The functions of each component are as follows:

1) Capacitors C01~C06

Each time the inverter tube V1V6 switches from the on state to the off state, the voltage UCE between the collector and the emitter will rise very quickly from nearly 0V to UD. In this process, the voltage growth rate is very high, which can easily cause damage to the inverter tube. The function of C01C06 is to reduce the voltage growth rate of V1~V6 when it is turned off.

2) Resistors R01~R06

Each time V1V6 switches from the cut-off state to the on state, the voltage charged on C01C06 (equal to UD) will be discharged to V1V6. The initial value of the discharge current is very large and will be superimposed on the load current, causing damage to V1V6. Resistor R01R06 is used to limit the discharge current of C01C06 to V1~V6.

3) Diode VD01~VD06

The connection of current limiting resistor R01R06 will affect the effect of C01C06 in limiting the voltage growth rate when V1V6 is turned off. After VD01VD06 is connected, R01R06 will be ineffective during the shutdown process of V1V6.

3. Energy consumption braking circuit

1. Function of energy-consuming braking circuit

In the variable frequency speed regulation system, the motor speed reduction and shutdown are achieved by gradually reducing the frequency. At the moment the frequency is reduced, the synchronous speed of the motor decreases, but due to mechanical inertia, the motor speed remains unchanged. When the synchronous speed is lower than the rotor speed, the direction of the rotor winding cutting the magnetic lines of force is opposite, and the phase of the rotor current changes by almost 180°, making the motor in the power generation state, also known as the regenerative braking state.

The electric energy regenerated by the motor is fed back to the DC circuit after full-wave rectification by the freewheeling diode (VD7~VD12 in the figure above). Since the electric energy of the DC circuit cannot be fed back to the power grid, it can only be absorbed by CF1 and CF2, which increases the DC voltage to a "pump-up voltage". Too high a DC voltage will damage the converter. Therefore, when the DC voltage exceeds a certain value, a discharge circuit is required to consume the regenerated electric energy. This discharge circuit is the energy consumption braking circuit.

2. Composition of energy-consuming braking circuit

The energy-consuming braking circuit consists of a braking resistor RB and a braking unit BV, as shown in the figure below.

The brake resistor RB is used to consume the excess electric energy in the DC circuit to keep the DC voltage stable. The function of the brake unit BV is to control the operation of the discharge circuit. Specifically, when the voltage UD of the DC circuit exceeds the specified limit, VB is turned on, so that the DC circuit releases energy through RB and reduces the DC voltage; and when UD is within the normal range, VB will be reliably cut off to avoid unnecessary energy loss.

4. Main circuit

The above-mentioned circuit parts are combined into the main circuit, as shown in the figure below.

1. AC-AC inverter is also called direct inverter. The output voltage of AC-AC inverter is achieved by adjusting the trigger phase of the anti-parallel rectifier bridge thyristor, and the output frequency depends on the commutation frequency between the output terminals A, B, and C. According to the different output voltage waveforms, AC-AC inverters can be divided into 120° conduction type square wave current source inverters and 180° conduction type sine wave voltage source inverters.

2. The vector control variable frequency speed regulation system can make the speed regulation of asynchronous motors have high precision and fast response capabilities comparable to those of DC motors. The vector control variable frequency speed regulation system can adapt to harsh working environments and can be applied to working machinery requiring high-speed response and high-precision electric traction devices.