Before designing an inverter control system, it is essential to thoroughly understand the system configuration, operational mode, environmental conditions, control strategy, customer-specific requirements, and whether the project involves a new design or the retrofit of existing equipment. This foundational knowledge ensures that the system will be both functional and efficient.
When dealing with the retrofit of old equipment, electrical engineers must gather detailed technical specifications and requirements. These include:
1. **Motor parameters**: such as factory date, manufacturer (domestic or imported), rated voltage, current, and number of phases.
2. **Load characteristics** of the motor.
3. **Work system** details.
4. **Starting method** of the motor.
5. **Environmental factors**, including site temperature, protection level, electromagnetic radiation, explosion-proof rating, and power distribution specifics.
6. **Distance** between the inverter cabinet and the motor location — this is a critical parameter affecting performance.
7. **Number and method** of motors connected to the inverter.
8. **Switching relationships** between the frequency conversion cabinet and the existing electrical system, often involving Δ-Y start and frequency conversion switching.
9. **Sensor and transmitter** selection and placement for peripheral devices.
10. **Control mode** of the inverter, such as manual/automatic, local/remote, signal range, and communication network compatibility.
11. **Isolation** between high-voltage and low-voltage circuits, as well as signal acquisition and control.
12. **Power quality** at the site, including lightning protection, surge suppression, and electromagnetic interference levels.
For new inverter systems, close collaboration with mechanical engineers is crucial to analyze the mechanical load characteristics and determine the appropriate motor type and capacity. Based on these, the right inverter type and size can be selected.
There are three common types of mechanical loads:
1. **Constant torque load** (e.g., conveyors, elevators). The formula P = T × N / 975 applies. Design considerations include using inverter-specific motors, dedicated cooling fans, and increasing motor and inverter capacities by 1.1–1.5 times the motor rating.
2. **Square torque load** (e.g., fans, pumps). The relationship is T = K1 × N² and P = K2 × N³. Inverter applications for these loads typically save 40–50% energy and account for 30–40% of all inverter uses. Key design points include using standard AC motors, selecting appropriate protection levels, and ensuring proper ventilation for larger systems.
3. **Constant power load** (e.g., winches, machine tool spindles). Here, torque decreases as speed increases. These systems require more complex control strategies and careful matching of motor and inverter capacities.
Each inverter control cabinet represents the core of the system and reflects the quality of the product. Design aspects should include:
1. **Schematic design** of the control system.
2. **Main circuit design**, involving components like transformers, fuses, contactors, and reactors.
3. **Control circuit design**, including conventional control systems and PLC interfaces.
4. **Inverter networking** capabilities.
5. **Process design**, covering electrical layout, cable routing, grounding, and anti-interference measures.
6. **Sheet metal design** of the cabinet, considering environmental factors, weight, transportability, and safety.
In main circuit design, component selection follows a logical sequence: first determining load characteristics, then motor specs, and finally selecting components based on system needs. For example, fuses should be chosen at 2.5–4 times the inverter’s rated current, while contactors must match the inverter’s current rating.
The control circuit design must isolate weak signals from PLCs, sensors, and instruments to prevent interference. Surge protectors and isolation transformers are also necessary for reliable operation.
Cabinet process design includes wire sizing, grounding, shielded cabling, and proper cable routing to reduce noise. Sheet metal design should consider environmental conditions, structural integrity, and ease of installation.
In conclusion, a high-quality inverter control cabinet requires attention to every stage — from initial design and manufacturing to transportation and packaging. Each step must meet strict standards to ensure optimal performance and reliability. By following these guidelines, engineers can create robust, efficient, and long-lasting inverter control systems.
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