I. Internal Main Circuit Structure
The low-voltage frequency converter adopts an "AC-DC-AC" structure. Its internal main circuit consists of two main parts: rectification and inversion, as shown in Figure 1. The three-phase AC power input from the R, S, and T terminals is rectified into DC power (DC) by a three-phase rectifier bridge (composed of diodes D1 to D6), with a voltage of UD. Capacitors C1 and C2 are filter capacitors. Six IGBT (Insulated Gate Bipolar Transistor) transistors V1 to V6 form a three-phase inverter bridge, inverting the DC power into three-phase AC power with adjustable frequency and voltage.
Figure 1 Internal main circuit of the frequency converter
II. Voltage equalizing resistor and current limiting resistor
In Figure 1, a resistor is connected in parallel across each of the filter capacitors C1 and C2 to ensure that the voltages across the two capacitors are approximately equal, preventing damage during operation (currently, due to technological advancements, most electrolytic capacitors in low-voltage (380V) frequency converters no longer require series connection). A resistor R and a pair of contactor contacts KM are connected between the rectifier bridge and the filter capacitors. This is because when the frequency converter is first powered on, the voltage across the filter capacitors is 0V, while the peak rectified voltage at a power supply voltage of 380V is 537V. This results in a large charging inrush current at the moment of power-on, potentially damaging the rectifier diodes. Furthermore, the 0V voltage across the filter capacitors causes the rectified voltage to drop instantaneously to 0V, interfering with the power network. To address these issues, a current-limiting resistor R is connected between the rectifier bridge and the filter capacitors, limiting the charging current of the filter capacitors to an acceptable range. However, if the current-limiting resistor R is always connected in the circuit, its voltage drop will affect the output voltage of the frequency converter and reduce the power conversion efficiency of the frequency converter. Therefore, after the filter capacitor is fully charged, the contactor KM will short-circuit the current-limiting resistor R to take it out of operation.
III. External Connection Terminals of the Main Circuit
The external connection terminals of the main circuit of various frequency converters are roughly the same, as shown in Figure 2. R, S, and T are the power supply terminals of the frequency converter, connected to a three-phase AC power supply; U, V, and W are the output terminals of the frequency converter, connected to the motor; P+ is the positive terminal of the rectifier bridge output. At the factory, P+ and P terminals are shorted by a copper strip with a sufficiently large cross-sectional area. When a DC reactor DL needs to be connected, the copper strip is removed, and DL is connected between P+ and P; P and N are the positive and negative terminals of the filtered DC circuit, which can be used to connect the braking unit and braking resistor; PE is the grounding terminal.
Figure 2 External connection terminals of the main circuit
IV. Common DC bus of the frequency conversion system
When a motor is in braking (generating) mode, the energy absorbed by the inverter from the motor is stored in the electrolytic capacitors of the inverter's DC link, causing the DC bus voltage in the inverter to rise. If the inverter is equipped with a braking unit and a braking resistor (these two components are optional), the inverter can briefly switch on the resistor to dissipate the regenerative energy as heat, a process known as regenerative braking. Of course, adopting a regenerative energy feedback scheme can also solve the regenerative energy problem in variable frequency speed control systems and achieve energy savings. However, standard general-purpose PWM inverters are not designed to feed regenerative energy back to the three-phase power supply. If the DC links of multiple inverters are interconnected through a shared DC bus, the regenerative energy generated by one or more motors can be absorbed and consumed by other motors in an electric manner. Alternatively, a set of braking units and braking resistors of a certain capacity can be installed on the DC bus to absorb regenerative energy that cannot be absorbed by motors in electric mode. If the shared DC bus is combined with an energy feedback unit, excess energy on the DC bus can be directly fed back to the power grid, thereby improving the system's energy-saving effect. In conclusion, in a variable frequency speed control system with multiple motors, using a shared DC bus scheme and configuring a set of braking units, braking resistors, and energy feedback units is a better solution to improve system performance and save investment.
Figure 3 shows a widely used shared DC bus scheme, which includes the following parts.
Figure 3 Common DC bus of the frequency converter
1. Three-phase AC power input line
The power input terminals of each frequency converter are connected in parallel to the same AC bus, ensuring that the power phase of the input terminals of each frequency converter is consistent. In Figure 3, circuit breaker QF is the incoming line protection device for each frequency converter. LR is the incoming line reactor. When multiple frequency converters operate in the same environment, adjacent frequency converters will interfere with each other. In order to eliminate or mitigate this interference, and to improve the power factor on the input side of the frequency converters, it is necessary to connect LR.
2. DC bus
KM is the control switch connecting the DC link of the frequency converter to the common DC bus. FU is a semiconductor fast-acting fuse with a rated voltage of 700V. The rated current must take into account the maximum current of the drive motor during motoring or braking. Generally, 125% of the rated load current can be selected.
3. Common braking unit and/or energy feedback device
Regenerative energy fed back to the common DC bus, if not fully absorbed, can be dissipated through a shared braking resistor. If an energy feedback device is used, this portion of regenerative energy will be fed back into the power grid, thereby improving energy efficiency.
4. Control Unit
Each frequency converter, according to the instructions of the control unit, connects its DC link to the common DC bus via KM, or quickly disconnects from the common DC bus after a frequency converter failure.
