Over the past two decades since its introduction, frequency converters have only achieved significant progress in widespread application. Understanding and mastering the various basic functions and control modes of frequency converters, as well as the methods for setting their parameters, is a fundamental skill that every electrical design, installation, commissioning, and maintenance personnel should possess. Variable frequency speed control technology offers a wide range of functions; this article provides a brief introduction to the basic knowledge of frequency converter application technology.
I. Main Circuit of Frequency Converter Application
The main circuit for the frequency converter application is shown in Figure 1. A circuit breaker QF and an AC contactor KM are connected between the power supply and the frequency converter. When the frequency converter is not used for a long time or during equipment maintenance, the circuit breaker acts as an isolating switch to cut off the power supply and ensure maintenance safety. The circuit breaker also has overcurrent and undervoltage protection functions, providing a certain degree of protection for the frequency converter's main circuit. The AC contactor serves two purposes: first, it allows for convenient connection or disconnection of the frequency converter's power supply via a button; second, when the frequency converter's alarm output terminal activates, the contactor can quickly disconnect the frequency converter from the power supply.
Generally, AC contactors are not connected between frequency converters and motors, but they are indispensable in the following situations: The first situation is when one frequency converter drives two or more motors, each motor should be equipped with a contactor; the second situation is when the motor needs to switch between frequency converter and mains frequency. Before the motor switches to mains frequency power, the connection between it and the frequency converter must be disconnected in advance through a contactor, so obviously a contactor must also be connected between the motor and the frequency converter.
If a frequency converter controls only one motor and does not need to switch between frequency and mains frequency, then a thermal relay is unnecessary because the frequency converter itself has comprehensive protection functions. However, if a frequency converter connects to multiple motors, or needs to switch between frequency and mains frequency, then each motor must have its own thermal relay for protection.
II. Control terminals of the frequency converter
In addition to the primary circuit terminals that connect to the three-phase power supply and the motor, frequency converters also have a large number of control terminals, including analog input terminals, contact input terminals, analog output terminals, pulse output terminals, transistor output terminals, contact output terminals, and communication interface terminals.
In the Fuji P11S series inverter control terminal arrangement, terminals 11, 12, and 13 can be connected to a potentiometer for frequency setting, which is one of several frequency setting methods available for the inverter. A 10V DC voltage provided by the circuit board is available between terminals 13 and 11. Alternatively, a potentiometer can be omitted, and a voltage signal of 0-5V or 0-10V can be input via terminals 12 and 11; therefore, these are analog voltage input terminals. C1 and 11 are analog current input terminals for frequency setting, with an input range of 4-20mA. Which of these frequency setting methods is effective depends on the setting and selection of the function parameters.
Terminals FMA and 11 can connect to a pointer-type analog frequency meter, while FMP and CM can connect to an external digital frequency meter. You can choose to install only one, or install one at the operating site and one remotely. A switch connects between terminals FWD and CM; when closed, the inverter starts running in forward direction; when open, it stops. REV is the reverse control terminal, with the same function. Through setting the function parameters, you can select to start, reverse, or stop the inverter using the aforementioned terminals or the buttons with the same names on the control panel. X1-X9 are multi-function terminals, which can be configured to provide control functions such as multi-speed operation.
The Fuji inverter has four transistor output terminals, all open-collector type, allowing connection to external loads such as relays and indicator lights. Terminals 30A, 30B, and 30C are internal alarm relay contacts, which can be used to connect external audible and visual alarm components or to control contactors in the main circuit to disconnect the inverter from the power supply. DX+, DX-, and SD are RS485 communication interfaces, allowing connection to a PLC or host computer.
When the frequency converter is operating at a high carrier frequency, shielded wires should be used for wiring the control terminals to prevent interference with adjacent equipment and to prevent interference sources in the power grid from affecting the operation of the frequency converter.
III. Parameter Settings of Frequency Converter
Inverters are high-tech, intelligent products. To enable them to perform designated control tasks, their parameters must be set; otherwise, they are like computers without applications—they cannot function properly. Parameter settings are typically done through the inverter's control panel. The control panels of various inverters are not only similar but also share many similarities and commonalities. These commonalities include:
1. All inverters have a switch key for switching between the operating interface and the parameter setting interface. For example, the "PRG" key on the Fuji P11S series inverter and the "Function/Data" key on the Senlan SB12 series inverter. After the inverter is powered on, the operating interface is displayed first. By pressing the switch key, you can switch to the parameter setting interface.
2. All inverters have an increment key "∧" and a decrement key "∨", and most also have a shift key "》". The "∧" and "∨" keys are used to increment or decrement the code number when selecting a function code (parameter number). They are used to increase or decrease the value when modifying the parameter value; the "》" key is used to select the "bit" to be modified when modifying the parameter value.
3. Each option has a confirmation button. After selecting the parameter code, press the confirmation button to enter the parameter value modification program. Once finished, press the confirmation button to save the changes.
4. All inverters have one or two LED displays, or one LED display plus one LCD display. Inverters with only one display first show the parameter code in parameter setting mode, and then switch to displaying the parameter value after pressing the confirmation key; inverters with two displays use one to display the parameter code and the other to display the parameter value.
5. Some inverters have a forward "FWD" key and a reverse "REV" key to start forward or reverse rotation; while some inverters only have a single "RUN" key, with the forward or reverse rotation determined by an external circuit.
6. There is a "STOP" button. Using these buttons, and referring to the inverter's instruction manual, you can set parameters or start the inverter.
Figure 3 shows the button layout related to parameter settings on the Fuji P11S series frequency converter. The "FUNC/DATA" button confirms and saves data; the "RESET" button is for resetting the frequency converter in case of an alarm. The reset button must be pressed after troubleshooting to restart operation.
Before the frequency converter is put into operation, the parameters in the attached table must be set. In addition, depending on the requirements of operation control, such as closed-loop control, PID-related parameters also need to be set. These parameters will be introduced in another article.
Various frequency converters have a parameter code for "restore factory settings" or "data initialization". Before starting parameter setting, you can use this parameter code to initialize the parameter values. In this way, many parameter values may be exactly what we need and do not need to be modified, thus simplifying the setting process.
After connecting the inverter to the power supply, motor, and necessary control terminals, and setting the parameters, you can start trial operation by pressing the forward or reverse buttons on the control panel.
