Abstract : This paper introduces a 1600T/h commercial coal blending project and discusses the design principle of a commercial coal blending system based on variable frequency drive technology. The focus is on the application design of Emerson CT SK6402 variable frequency drive technology in the low-frequency, high-torque drive system of the coal blending system. The project uses CTsoft monitoring software to comprehensively monitor the status of the variable frequency drive to ensure optimal system commissioning. Keywords : Power generation, coal blending, coal bunker, low-speed, high-torque, variable frequency drive 1 Introduction Coal blending is a large-scale industrial production process that artificially creates ideal and economical special-purpose coal that is not found in natural raw coal, according to different uses. Blended coal is a mixture of several different types and properties of coal in a certain proportion. Although it retains some characteristics of its component coals, its overall performance has been altered; it is essentially a new "coal type" artificially processed. The basic principle of power coal blending is to utilize the differences in properties of various coals, to "complement each other's strengths and weaknesses," and to maximize the advantages of each coal type, ultimately achieving "optimal performance" in the blended coal to meet user requirements. For coal used in industrial power generation boilers, excessively high calorific values ​​can damage equipment; conversely, insufficient calorific values ​​will reduce equipment efficiency. Therefore, "power coal blending" involves mixing low-quality coal (not meeting power plant design requirements) with high-quality coal (exceeding design requirements) to achieve optimal combustion results and closely approximate design specifications. The goal of power coal blending is to combine low-quality and high-quality coal to achieve a configuration close to the required specifications. For example, in the southeastern coastal region, where industrial development leads to high power plant demand, imported Indonesian coal is often low in ash, sulfur, and calorific value. Therefore, blending it with high-sulfur, high-calorific-value coal from inland areas can improve calorific value while reducing sulfur content. Imported coal is cheaper and more readily available, making it more competitive in price. Furthermore, from a coal utilization perspective, blending projects align with clean coal development policies. Employing sophisticated, fully automated control and identification systems ensures more stable and controllable product quality. 2. Coal Blending System Design The concept of coal blending for power generation has been around for a long time, but the technology is still relatively rudimentary. With the marketization of coal prices, coal producers and end-users have begun to interact directly, and many companies have started to engage in coal blending. However, most domestic companies still use extensive blending methods, which are not refined enough. The low technological content results in poor quality and stability of the blended coal. The unevenness of the coal makes sampling difficult, leading to significant blending errors. 2.1 Process Principle Industrial coal blending typically uses a multi-bin blending system. The system controls the mixing according to a set ratio, ensuring the coal in each bin is mixed together. This system consists of six silos, each holding 9000 tons of coal. Appropriate blending schemes are proposed based on user requirements and different coal sources. PLC automation technology is used to achieve precise and stable control of the blending ratio, meeting user requirements. This equipment can mix the coal in each silo according to a set ratio, ensuring the coal quality meets industrial requirements. The key to automated coal blending is the control of the coal output of each coal bunker. The automated coal blending system based on Emerson CT's Commander SK series frequency converter is shown in Figure 1. (1) System requirements: The coal output of the silo is 160T/h to 1600T/h, and the corresponding motor speed is 3 to 50HZ. (2) Driver parameters: SK6402 is a vector control driver with a rated voltage of 380 to 480V (±10%), a rated output current of 210A, and can operate for 60S under 150% overload current. The rated power is 110KW, and the output frequency is 0 to 1500HZ. (3) Motor parameters: Rated power 90KW; 3P; rated voltage 400V; rated current 95.3A; rated speed 1475RPM; power factor 0.89. 2.2 Electrical control system The electrical control system is realized by integrating frequency converter and PLC system. At the bottom of the silo is a coal scraper, driven by two symmetrically rotating reduction gears. These two motors are driven by a 110kW Commander SK series frequency converter SK6402, which is controlled by a PLC. The Emerson CT frequency converter drives the scraper motor at the bottom of the silo. The rotating scraper discharges coal from the silo, and the coal output is controlled by adjusting the scraper speed. Each silo contains coal with different requirements. When the scraper rotates, the coal in the silo is discharged from the outlet, falls onto the same conveyor belt for mixing, and then the finished coal is directly delivered to the cargo ship for shipment. [align=center] Figure 1 Schematic diagram of the coal mixer[/align] 2.3 Application design of SK6402 frequency converter Emerson CT, a world-renowned manufacturer of DC/AC drives, belongs to the American Emerson Electric Group. The SK6402 110/132kW AC400V 3Phase inverter drive belongs to Emerson CT's latest high-performance AC vector control drive product, the Commander SK series. The SK6402, with appropriate options and integrated use, completes inverter control, saving design and operating costs. Its features include: high-performance vector control drive; 150% continuous overload for 60 seconds; self-tuning capability for a complete motor model with the motor shaft not rotating; IP54 LED keypad for remote control; built-in filter that can be disconnected and re-established; closed-loop control capability; built-in boost macro; optional components are compatible with the SP series inverter drives; SMARTSTICK provides greater flexibility for commissioning; voltage options are 220V and 380V; power range covers 0.25kW-110kW. The electrical principle of the Commander SK series inverter drive SK6402 is shown in Figure 2. Because the frequency converter drives two motors simultaneously, when setting parameters, it is necessary to ensure that the motor current is the sum of the currents of the two motors, i.e., 190.6A. Vector control should not be selected, but V/F control mode should be selected instead. [align=center] Figure 2 SK6402 principle wiring diagram[/align] 2.4 Debugging of low-speed high-torque frequency converter system (1) Stall fault phenomenon. When the coal mixing machine system is running, the scraper removes coal from the silo. Due to uneven coal block size or dampness caused by weather conditions, the scraper is subjected to uneven force, which leads to unstable motor load and current fluctuation in the range of 90~120A. When the motor runs at low speed (below 3HZ), if the load is too large, the scraper will get stuck, and the driver output current will be too large. The driver will automatically limit the current and reduce the current size. The current status is detected by the software CTscope. The stall current monitoring waveform is shown in Figure 3. At this time, when the scraper is stuck and the motor cannot rotate, the maximum output current of the driver can reach 250A. Afterwards, the driver will limit the current output, and when it drops to 0A, it will increase the current output again. If this situation is allowed to continue, the driver will display "OVL.d", integrate and accumulate the overload current until the "It.AC" overcurrent alarm fault occurs, causing the system to stop production. [align=center] Figure 3 Waveform monitored when the motor is stalled[/align] (2) Analysis of stall cause. Since we use V/F control mode to drive the motor, when the motor runs at low frequency, the voltage output of the driver is low. Affected by the voltage drop caused by the resistance of the motor stator winding, it cannot provide enough torque output to the motor. [align=center] Figure 4 VF curve output by the driver[/align] (3) Solution to stall problem. In this case, we need to increase the voltage boost to compensate for the starting torque. The specific parameter settings can be determined by monitoring the current and motor speed waveforms to select the best curve. The relationship between the driver output voltage, frequency and voltage boost is shown in Figure 4. After adjusting the voltage boost value, the system can run normally at low frequency (2HZ). Figure 5 shows the monitored current and motor speed curves. The current stabilized between 90 and 120 A, and the speed curve remained stable, achieving the expected results. [align=center]Figure 5 Waveforms monitored during normal operation[/align] 3 Conclusion This system requires the motor to operate at low frequencies for extended periods. This requirement leverages the low-frequency output rated torque capability of the SK6402. The SK series drive can drive the motor to its maximum torque within a range from 1 Hz to the motor's rated frequency. We can also comprehensively monitor the drive status using the CTsoft monitoring software to ensure optimal system debugging. The debugged coal mixer significantly improved production efficiency, achieving a coal output rate of 1600 T/h during normal operation; it saved manpower, as all operations can be controlled from the central control room; the finished coal is uniformly mixed and of excellent quality.