First, the internal main circuit structure
The low-voltage inverter typically follows an "AC-DC-AC" configuration. Its internal main circuit is divided into two key sections: rectification and inversion. As illustrated in Figure 1, three-phase AC power enters through terminals R, S, and T. This power is first converted into DC using a three-phase bridge rectifier made up of diodes D1 to D6, resulting in a DC voltage labeled as UD. Capacitors C1 and C2 serve as filter components to smooth out the DC voltage. The inverter section consists of six IGBTs (Insulated Gate Bipolar Transistors), V1 to V6, forming a three-phase inverter bridge that converts the DC back into adjustable AC with variable 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 placed in parallel across the filter capacitors C1 and C2 to balance their voltages and prevent damage during operation. Although modern low-voltage (380V) inverters often eliminate the need for series electrolytic capacitors, this resistor remains important for voltage balancing. A resistor R and a pair of contactor contacts KM are connected between the rectifier bridge and the filter capacitor. When the inverter starts, the filter capacitor is initially at 0V, while the rectified voltage can reach up to 537V. This sudden voltage difference causes a large charging surge current that could damage the rectifier diodes or cause interference on the power network. To prevent this, the current limiting resistor R restricts the initial charging current. However, if R remains in the circuit continuously, it would lower the output voltage and reduce efficiency. Therefore, once the capacitor is charged, the contactor KM short-circuits R to remove it from the active circuit.Third, the external connection terminal of the main circuit
The main circuit’s external terminals are generally similar across different inverters, as shown in Figure 2. The R, S, and T terminals connect to the three-phase AC power supply. U, V, and W are the inverter’s output terminals, connected to the motor. P+ is the positive terminal of the rectifier bridge, and at the factory, it is connected to the P terminal via a copper plate. If a DC reactor DL is needed, the copper plate is removed, and the DL is connected between P+ and P. P and N represent the filtered DC bus, which can be connected to a brake unit and braking resistor. Lastly, the PE terminal is the grounding point.
Figure 2: Main circuit external connection terminal
Fourth, the common DC bus of the frequency conversion system
When a motor operates in braking mode, it generates regenerative energy that is stored in the DC link capacitors of the inverter, causing the DC bus voltage to rise. If the inverter has a brake unit and braking resistor, it can dissipate this energy as heat, known as dynamic braking. Alternatively, a regenerative energy feedback system can return the energy to the power grid, improving overall efficiency. Standard PWM inverters usually do not support this feature. However, when multiple inverters share a common DC bus, the regenerative energy from one motor can be absorbed by Others, reducing the need for additional braking resistors. In more advanced systems, a shared DC bus combined with an energy feedback unit allows excess energy to be returned to the grid, enhancing energy savings.Figure 3 illustrates a commonly used shared DC bus solution, which includes:
Figure 3: Common DC bus of the inverter
1. Three-phase AC power supply line
Each inverter's power input is connected in parallel to the same AC bus, ensuring consistent phase alignment. Circuit breakers QF protect each inverter, while LR (line reactors) help reduce interference and improve the input power factor when multiple inverters operate together.2. DC bus
KM controls the connection of the inverter’s DC link to the common bus. FU is a fast-blow fuse rated for 700V, with a current rating based on the maximum load of the drive motors. Typically, it should be sized at 125% of the motor’s rated current.3. Common brake unit and/or energy feedback device
If the DC bus accumulates unabsorbed regenerative energy, a common braking resistor can be used to dissipate it. With an energy feedback device, this energy can instead be sent back to the grid, significantly improving energy efficiency.4. Control unit
The control unit manages the connection and disconnection of each inverter to the common DC bus via KM. It also ensures quick disconnection in case of inverter failure, maintaining system stability and safety.4 states truck parts，big truck parts，truck body parts，toyota truck parts
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