Problem Statement 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 modifying existing equipment. For the modification of 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 external 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., use the formula 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 to 1.5 of the motor capacity. 2. Square torque load. For example, fans, water pumps, use the formula 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 to 50%. This type of application accounts for about 30 to 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 the environmental needs, select the motor protection level and method. (2) For frequency converter cabinets with a capacity greater 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; 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 entire system and best reflects the key link of 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 selected according to the voltage level standard. FU-Fuse, generally required, 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 as 1.2 times the rated frequency converter current. KM-Contactor, required, selected as the rated frequency converter current. LY-Surge protector, preferably required, especially in areas with frequent thunderstorms and AC power surges, to protect the frequency converter system from accidental damage. Generally equipped with a 40KVA surge protector. The function of the DK-reactor is to suppress the adverse effects of high-order harmonic components in the input and output current of the frequency converter, while the function of the filter is to suppress radio wave interference caused by the frequency converter, i.e., radio wave noise. Some frequency converters have built-in reactors, and in some cases, reactors are not required. Generally, the frequency converter manufacturer provides parameters for the reactor size required for the power of the frequency converter. The parameters for selecting the reactor can be calculated by the following formula: L=(2%～5%)V/6.18＊F＊I V-rated voltage VI-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 frequency converter cabinet manufacturer. The circuit control loop design should be based on the knowledge of electrical engineers and the requirements of the frequency converter. However, it should be noted that (1) weak input/output signals 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) When interfaced with PLC and conventional control systems, surge absorbers must be installed. (3) The control power supply should use an isolation transformer for electrical isolation. Variable frequency control cabinet process design. Electrical process design After the electrical engineer designs the variable frequency circuit, the next step is electrical process design, including: 1. What power of the variable frequency drive, what type of cable, what wire diameter, and how far apart. Generally, you can check tables or calculate. 2. Grounding wiring. 3. Anti-interference wiring. This is very important. Generally, shielded cables are used for high-voltage cables, and the cable and shielding layer are fixed on the base plate with metal clips. Some also add shielding metal rings to resist interference. 4. Configuration of cable conduit joints for incoming and outgoing lines. Cabinet sheet metal process design It should be designed according to the following principles: 1. Ambient temperature of the variable frequency drive: The ambient temperature of the variable frequency drive is -10 degrees to 50 degrees. Ventilation and heat dissipation must be considered. Humidity: Vibration: Gas: Whether there is explosive or corrosive gas 2. Cabinet load-bearing weight. 3. Convenience of transportation. 4. The cabinet should have a nameplate and the manufacturer's CI logo. Conclusion: A high-quality frequency converter control cabinet requires high standards in design, manufacturing, transportation, and packaging. Quality assurance at every stage is essential to producing a high-quality control cabinet.