In recent years, variable frequency control systems have been widely used, but their application and system configuration vary. This paper introduces the configuration of a variable frequency control system and its application in boiler coal feeding and grate motor speed control. 1. Overview: With the development of China's thermal power industry, the speed regulation of boiler coal feeding systems has received increasing attention from thermal power plants. Currently, slip speed regulation is the most commonly used speed regulation system. Its control method is relatively typical and reliable, but it has disadvantages such as poor speed regulation accuracy, narrow range, poor linearity, and high energy consumption. Variable frequency speed regulation systems overcome the shortcomings of traditional speed regulation systems, featuring high efficiency, no slip loss, wide speed regulation range, stiff characteristics, high accuracy, convenient and flexible starting and braking, and low energy consumption. It combines the advantages of AC induction motors with the speed regulation performance of DC motors, resulting in significant energy-saving effects. Based on the above comparison, Nantong Xinxing Thermal Power Co., Ltd. selected a variable frequency system for the coal feeding and grate speed regulation of its three 75T/H double-grate chain grate boilers. Simultaneously, through an intelligent interface with the DCS, automatic control of the boiler combustion system is achieved. 2. Process Requirements and Overall System Structure The combustion process regulation system of the boiler at Nantong Xinxing Thermal Power Co., Ltd. requires maintaining a constant main steam pressure under different load conditions. This is achieved by adjusting the amount of fuel and air entering the furnace to balance the boiler's steam output with the turbine's steam consumption and heat supply. The adjustment of the amount of fuel entering the furnace is achieved by changing the speed of the coal feeder. For this purpose, a frequency converter was selected to adjust the current and frequency of the three-phase AC power supply to the coal feeder motor, thereby controlling the feeder speed. Simultaneously, the grate speed adjustment and its operating status are closely related to the coal feeding; therefore, a frequency converter was also selected for its speed adjustment. Since each boiler has two grates (sides A and B) and two coal feeders, a total of four frequency converters are used per boiler. To save control panel space and reduce terminal wiring, a centralized operation control display was selected. It simultaneously controls the operation of four frequency converters, thereby controlling the operating speed of the coal feeding and grate motors, improving the precision, timeliness, and accuracy of control, and ensuring the safe and economical operation of the boiler. Simultaneously, the operation display can also be connected to the company's DCS system, receiving 4-20mA signals from the DCS to jointly complete the automatic adjustment of the combustion system. In addition, this system also incorporates a small programmable controller to ensure that if one motor in the system fails, it interlocks with other equipment, stopping them or placing them in a specific state. 3 System Implementation 3.1 System Configuration 3.1.1 Frequency Converter This is the core component of the system. A SANKEN general-purpose SAMCO-IF series frequency converter is selected, suitable for speed control of induction motors. It is equipped with a high-performance 32-bit microprocessor, making the device fully functional and easy to operate. 3.1.2 Operation Display This device uses an HCS-51 series microcontroller as the core component, forming an intelligent operation display. The main internal components are selected as follows: An ATM89C51 is used as the CPU, saving the need for external program storage; a TCL2543 multi-channel A/D converter chip is used, leveraging its short A/D conversion time to ensure real-time control. An 8253 timer, supplemented by simple peripheral circuitry, uses pulse width modulation to achieve multi-channel D/A output. The entire handheld controller has a compact hardware structure, is easy to operate, powerful in function, and has good expandability. 3.1.3 Programmable Controller (PLC) This device uses the ultra-compact NB2 Fuji Electric programmable controller. Its greatest advantages are its small size, low price, powerful functions, and arbitrary expandability. 3.2 System Functions 3.2.1 IF Series Inverters There are 151 function options in total, and each option has multiple options for different users. Therefore, to make good use of the inverter and give full play to its function, it is necessary to understand its functions and make reasonable selections according to the specific application and operation. In particular, for some special function settings, it is necessary to find a more suitable setting parameter so that the mutual matching between the devices in this system can reach the best state. It is necessary to make multiple adjustments. For example, CD083 was originally set to 10. After trial operation, it was found that the signal jitter was very large and the motor speed changed frequently. After repeated corrections, CD083 was set to 210. At this time, the operation was better, which ensured both the stability of the signal and the real-time data acquisition. 3.2.2 Operation Display For the operation display that can connect 4 inverters at the same time, it has the following functions: (1) Control mode ① Coal feeder: can realize automatic/manual, simultaneous/single operation control. Automatic control: accepts the automatic adjustment signal of DCS and controls the coal feeding speed. Manual Control: If the control system experiences significant disturbances or malfunctions, operators control the coal feeder speed via the keyboard on the control panel. Simultaneous Operation: Under normal circumstances, the speeds of the two coal feeders on both sides of the boiler increase or decrease simultaneously based on their respective speed ratios. Individual Operation: The speeds of the coal feeders on sides A and B are operated separately according to the combustion conditions on both sides of the boiler. Manual control allows for both individual and simultaneous operation. When the load remains constant, operators often need to switch the system to manual mode and adjust the speed of the corresponding coal feeder in individual operation mode to achieve optimal boiler operation. However, when the load changes, adjusting the speed of each coal feeder individually is not ideal. In this case, the control mode is switched to simultaneous operation, allowing the coal feeders on sides A and B to accelerate or decelerate simultaneously based on their original speeds to maximize boiler performance. Once the combustion conditions stabilize, the system is switched to automatic mode. ② Grate: Grate operation is not involved in automatic combustion control; only manual control is available. The operator sets the control panel according to the boiler's operating conditions to ensure complete and economical combustion of the boiler. It also has simultaneous/single operation functions, and the control method is the same as the simultaneous/single operation control of coal feeding. (2) Display ① The speed of each controlled object is displayed digitally. ② The operation mode of each controlled object is indicated by indicator lights. ③ The operating status of the frequency converter (overload, fault, etc.) is indicated by indicator lights. (3) Adjustment signal The adjustment signal is 4~20mA, and the corresponding speed adjustment range is 40~1500r/min. (4) Braking device is provided In case of accident or emergency shutdown, the coal feeding or grate motor can be quickly stopped by operating the switch on the display panel. 3.2.3 Programmable Controller (PLC) The interlocking protection circuit logic of the boiler combustion system of Nantong Xinxing Thermal Power Co., Ltd. is relatively complex. The PLC receives the switch input signals from the electrical system and the system itself, and outputs a switch signal after logic operation to control the start and stop of the corresponding motor. In this way, not only are a lot of relays saved, but internal wiring and space are also saved, and the reliability of the system is greatly improved. The specific operational logic control requirements are as follows: (1) If the blower fails: the secondary blower and the coal feeder will be stopped by interlocking. (2) If any secondary blower on side A or B trips: the coal feeder on the corresponding side will be stopped by interlocking. (3) If any grate motor on side A or B trips: the coal feeder on the corresponding side will be stopped by interlocking. (4) If any main coal feeder on side A or B trips: the coal feeder on the corresponding side will be stopped by interlocking. (5) If the coal feeders on sides A and B stop: the secondary blower, the blower, and the induced draft fan will be stopped by interlocking. The above functions can be easily implemented by PLC programming. 4 Usage This variable frequency speed control system was officially commissioned and put into operation in September 1998. It has been operating well since then. Compared with the traditional slip differential motor, it requires less maintenance, has a smaller starting current, more complete system functions, significant energy-saving effect, and provides convenience for operators.