As we all know, frequency converters are widely used in various industries. However, how to design a frequency converter control system, and the specific requirements for the design and manufacture of frequency converter control cabinets in practical applications, are questions that many electrical engineers, manufacturers, and customers want to understand clearly. This chapter summarizes the key design points from actual design and application cases, offering my humble opinions for reference by colleagues. Design Based on Actual Requirements and Customer Needs Before designing a frequency converter control system, it is essential to understand the system configuration, operating mode, environment, control method, and specific customer requirements. The specific system may be a newly designed system or a system for upgrading existing equipment. For upgrading existing equipment, electrical engineers should know the following technical parameters and requirements precisely: 1. Specific parameters of the motor: manufacturing date, manufacturer (domestic, imported), rated voltage, rated current, and number of phases. 2. Load characteristics and operating mode of the motor. Motor starting method. 3. Working environment: such as on-site temperature, protection level, electromagnetic radiation level, and explosion-proof level. 4. Specific power distribution parameters: actual distance from the installation location of the frequency converter cabinet to the motor location. (The distance from the frequency converter cabinet to the motor is a very important parameter.) The number and method of motors driven by the frequency converter cabinet. The switching relationship between the frequency converter cabinet and the old electrical system. Generally, Δ-Y starting and frequency converter operation are mutually redundant, with switching protection. Selection parameters and sampling locations of the peripheral sensors and transmitters for the frequency converter cabinet. The control mode of the frequency converter control cabinet, such as manual/automatic, local/remote, and the range of the control signal. Whether communication networking is required. Isolation between high-voltage and low-voltage circuits. Isolation of acquisition and control signals. Power supply quality of the working environment, such as lightning protection, surge protection, and electromagnetic radiation. For new frequency converter systems, electrical engineers should work with mechanical engineers to thoroughly understand the characteristics of the transmission mechanical load to determine the motor type and capacity. Based on the motor's mechanical load characteristics and capacity, select the type and capacity of the frequency converter. Currently, there are many types of mechanical loads and motor torque characteristics; three are commonly used: 1. Constant torque load. For example, conveyor belts, elevators, etc. The formula is P=T＊N/975 P-motor power T-motor torque N-motor speed For constant torque, the system design should pay attention to: (1) The motor should be a dedicated motor for the frequency converter (2) The frequency converter cabinet should be equipped with a dedicated cooling fan (3) Increase the motor capacity (4) Reduce the load characteristics (5) Increase the capacity of the frequency converter (6) The relationship between the capacity of the frequency converter and the capacity of the motor should be based on the brand, generally 1.1～1.5 of the motor capacity. 2. Square torque load. For example, fans, water pumps The formula is T=K1＊N2, P=K2＊N3 P-motor power T-motor torque N-motor speed Generally, fans and water pumps use frequency conversion for energy saving. Theoretically and practically, the energy saving is about 40～50%. This type of application accounts for about 30～40% of the application of frequency converters. For square torque load, the system design should pay attention to: (1) The motor is usually an asynchronous AC motor. According to environmental requirements, select the motor protection level and method. (2) For frequency converter cabinets larger than 7.5KW, ventilation and heat dissipation facilities should be added. (3) Relationship between motor and frequency converter capacity. Relationship coefficient Foreign frequency converter capacity Domestic frequency converter capacity Foreign motor capacity Equal equivalent motor capacity 1.3-1.5 Motor capacity Domestic motor 1.2 Motor capacity 1.5-2 Motor capacity 3. Constant power load. Such as winch, machine tool spindle Formula: P=T＊N/975=CONT. Generally, when a specific speed range is reached, it operates with constant torque, and when the specific speed is exceeded, it operates with constant power. Constant power mechanical characteristics are more complex. For each frequency converter control cabinet, the design is the key point of the whole system and the key link that best reflects the product quality. For frequency converter control cabinets, electrical design engineers should start from the following design aspects. 1. Schematic design of frequency converter control system 2. Main circuit design. 3. Circuit control circuit design. Including conventional control circuit, PLC control interface circuit, frequency converter networking, etc. 4. Process design of frequency converter control cabinet. Including electrical process design and cabinet sheet metal process design. Schematic diagram design Based on the above-mentioned principle rules, refer to the schematic diagram of the frequency converter control cabinet, and draw the schematic diagram according to actual needs. Main circuit design. Select the main circuit electrical components in the following order. (1) Determine the mechanical load characteristics, power, torque, and speed. (2) Determine the motor characteristics, rated voltage, rated power, and rated current. (3) Based on the above conditions and the actual needs of the customer, select the following electrical components: TR-Transformer is optional, and is selected according to the voltage level standard. FU-Fuse, generally, should be selected as 2.5-4 times the rated frequency converter current. Note that the fuse should be a fast-acting type. QA - Circuit breaker, generally required, selected to be 1.2 times the rated inverter current. KM - Contactor, mandatory, selected to be the rated inverter current. LY - Surge protector, highly recommended, especially in areas prone to thunderstorms and AC power surges, to protect the inverter system from accidental damage. Generally, a 40KVA surge protector is used. DK - Reactor The function of the reactor is to suppress the adverse effects of high-order harmonic components in the inverter's input and output currents, while the filter's function is to suppress radio wave interference (radio noise) introduced by the inverter. Some inverters have built-in reactors, while in some cases, reactors are not required. Generally, the inverter manufacturer provides parameters specifying the appropriate reactor size for the inverter's power rating. The parameters of the reactor can be calculated by the following formula: L = (2%~5%)V/6.18*F*I V - Rated voltage V I - Rated current A F - Maximum frequency HZ LBI/LBO Input and output filters should generally be configured according to the frequency R - Braking resistor calculation is more complicated and should be configured under the guidance of the inverter cabinet manufacturer. Circuit control loop design should be designed according to the knowledge of electrical engineers and the requirements of inverters. However, attention should be paid to (1) The weak electrical signals of input/output must be isolated from PLC, instruments, and sensor transmitters, otherwise the control system signals will be chaotic and the system will not function properly. (2) Surge absorbers must be installed at the interface with PLC and conventional control systems. (3) The control power supply should use an isolation transformer for electrical isolation. Process design of inverter control cabinet. Electrical process design After the electrical engineer designs the inverter circuit, the next step is electrical process design, including: 1. What power inverter, what type of cable, what wire diameter, and how far apart. Generally, you can check the table or calculate. 2. Grounding wiring. 3. Anti-interference wiring. These are very important. Generally, shielded cables are used for high-voltage power cables. The cable and shielding layer are fixed to the base plate with metal clips. Some also have shielding metal rings added for anti-interference. 4. Configuration of cable conduit joints for incoming and outgoing lines. The cabinet sheet metal process design should be based on the following principles: 1. Ambient temperature of the frequency converter: The ambient temperature of the frequency converter is -10 degrees to 50 degrees. Ventilation and heat dissipation must be considered. Humidity: Vibration: Gas: Presence of explosive or corrosive gases. 2. Cabinet load-bearing weight. 3. Transportation convenience. Install lifting hooks for safe handling. 4. Nameplate of the cabinet, manufacturer's CI logo. Conclusion: A high-quality frequency converter control cabinet requires high standards in design, process, manufacturing, transportation, and packaging. Quality assurance at every stage is necessary to produce a high-quality control cabinet.