The inverter circuit structure mainly consists of a rectifier circuit, a current limiting circuit, a filter circuit, a braking circuit, an inverter circuit, and a detection and sampling circuit.
1) Drive circuit
The drive circuit takes the six PWM signals generated by the CPU in the main control circuit, and after opto-isolation and amplification, provides drive signals for the commutation device (inverter module) of the inverter circuit.
The requirements for drive circuits vary depending on the converter devices. Meanwhile, some developers have created dedicated drive modules suitable for various converter devices. Some brands and models of frequency converters directly use dedicated drive modules. However, most frequency converters use drive circuits. From a repair perspective, a typical drive circuit is introduced here. Figure 1 shows a common drive circuit (the drive circuit power supply is shown in Figure 2). The drive circuit consists of an isolation amplifier circuit, a drive amplifier circuit, and a drive circuit power supply. The three upper bridge arm drive circuits are three independent drive power supply circuits, while the three lower bridge arm drive circuits share a common drive power supply circuit.
2) Protection circuit
When a frequency converter malfunctions, in order to minimize, or even reduce to zero, the losses caused by the malfunction, every brand of frequency converter attaches great importance to protection functions and strives to increase the number and effectiveness of these protection functions.
In the field of inverter protection functions, manufacturers have gone to great lengths to improve them. This has resulted in the diversity and complexity of inverter protection circuits. There are conventional detection and protection circuits, as well as integrated software protection functions. Some inverters even have built-in protection functions in their drive circuit modules, intelligent power modules, and rectifier-inverter combination modules.
The circuit shown in Figure 3 is a typical overcurrent detection and protection circuit. It consists of three parts: current sampling, signal isolation and amplification, and signal amplification and output.
3) Switching power supply circuit
The switching power supply circuit provides low-voltage power to the operation panel, main control board, drive circuit, and fan, among other circuits. Figure 4 shows the structural diagram of the Fuji G11 switching power supply circuit.
A high-voltage DC voltage (P terminal) is applied to the primary winding of a high-frequency pulse transformer. A switching transistor is connected in series to the other primary winding of the pulse transformer, and then connected to the high-voltage DC voltage (N terminal). The switching transistor periodically turns on and off, converting the primary DC voltage into a rectangular wave. This wave is coupled to the secondary winding by the pulse transformer, and after rectification and filtering, the corresponding DC output voltage is obtained. The output voltage is then sampled and compared to control the pulse width modulation circuit, thereby stabilizing the output voltage by changing the pulse width.
4) Communication circuit on the main control board
When frequency converters are controlled by programmable logic controllers (PLCs), host computers, human-machine interfaces, etc., signals must be exchanged through a communication interface. Figure 5 shows the communication interface circuit of an LG frequency converter.
Inverter communication typically uses a two-wire RS485 interface. Siemens inverters are no exception. The two wires are used for transmitting and receiving signals, respectively. Before transmitting a signal after receiving it, both signals pass through integrated circuits such as buffers A1701 and 75176B to ensure good communication. Therefore, the communication interface circuit on the inverter's main control board mainly refers to this part of the circuit, along with the signal anti-interference circuit.
5) External control circuit
The external control circuit of a frequency converter mainly refers to the frequency setting voltage input, frequency setting current input, forward, reverse, jogging, and stop operation control, as well as multi-speed control. The frequency setting voltage (current) input signal enters the CPU through the A/D conversion circuit inside the frequency converter. Other control signals are transmitted to the CPU through optocoupler isolation in the input circuit inside the frequency converter.
