I. Overview Electric actuators are the most important and fundamental tools for converting electrical energy into mechanical energy and driving various production machines. Due to the requirements of production processes and the need for energy conservation, adjustable speed electric actuators have always been a research subject for scientific and technological personnel, and their applications are becoming increasingly widespread. In the development of adjustable speed electric actuators, various speed regulation methods have emerged, including DC speed regulation, voltage regulation, electromagnetic speed regulation, electro-hydraulic coupling speed regulation, and slip speed regulation. However, all of these speed regulation methods have limitations and drawbacks in practical applications. With the rapid development of power electronics technology, automatic control technology, and computer technology, a new type of intelligent adjustable speed electric actuator integrating various high-precision technologies has emerged: the variable frequency electric actuator. With the increasing maturity of variable frequency technology, variable frequency electric actuators are gaining wider and wider application due to their unique performance. Compared with other adjustable speed electric actuators, they have the following advantages: a. Low starting current, no impact on production machinery, which can greatly extend the service life of equipment. b. High degree of intelligence, complete self-diagnostic and protection functions, improving the safety and reliability of the equipment. c. Simple installation, convenient use, and significantly reduced maintenance costs. d. Good mechanical characteristics, fast dynamic response speed, and improved control system regulation quality. e. High-power-consuming equipment such as fans and pumps can reduce energy consumption, save costs, and improve efficiency by adopting frequency conversion speed regulation. II. Principle of Frequency Conversion Device The frequency conversion electric actuator is a speed-adjustable drive mechanism composed of a frequency conversion device (frequency converter) and a motor. The speed and direction of the motor are controlled by the frequency conversion device, and it works according to the requirements of the production process. Its theoretical basis is the following formula: n＝no（1-s）no=60f／p Where: n——motor speed no——synchronous speed s——slip f——input voltage frequency D——number of pole pairs of the motor From the above formula, it can be seen that there are three ways to change the speed of the motor: one is to change the slip; the second is to change the number of pole pairs of the motor; and the third is to change the input voltage frequency. The theoretical basis for the operation of the frequency conversion electric actuator is to convert the fixed grid frequency into the frequency required by the motor by introducing a frequency conversion device. Variable frequency drives (VFDs) are generally implemented in two ways: AC-AC and AC-DC-AC. AC-AC VFDs have a small adjustment range, low power factor, and high harmonic pollution. Most modern VFDs use the AC-DC-AC implementation, which mainly consists of four parts: a rectifier, a filter, an inverter, and a control and protection section with a CPU as its core. The rectifier uses controllable components connected in a three-phase fully controlled bridge to convert the input three-phase AC power into adjustable DC power. The filter is mainly used to obtain relatively flat DC power and absorb energy fed back from the motor to achieve emergency braking. The inverter uses components such as gate turn-off thyristors (GTOs), high-power transistors (GTRs), and insulated-gate bipolar transistors (IGBTs), employing pulse width modulation (PWM) modulation or vector control to invert DC power into adjustable-frequency AC power. The CPU-centric control and protection section is the command center of the entire frequency converter. It performs various functions such as control and protection through software programs input into the CPU, and interfaces with DCS, PLC, and other control systems. Frequency converters are also classified by their operating voltage: high-voltage and low-voltage. High-voltage frequency converters can be further divided into direct high-voltage type and high-low-high type. Different types of frequency converters have different applications, and even frequency converters of the same type have different qualities and prices due to technical differences. Therefore, in the use of frequency converter electric actuators, it is necessary to scientifically select the frequency converter and motor to strive for the best performance-price ratio. The motor in a frequency converter electric actuator is an asynchronous motor. However, due to differences in the application and the technology used in the frequency converter, some can use ordinary asynchronous motors, while others require specialized motors. Therefore, when retrofitting an actuator that originally used an asynchronous motor for constant speed operation, careful consideration is needed: can the original motor be retained? If so, a scientific selection of the frequency converter is still required. III. Application of Variable Frequency Electric Actuators in Thermal Power Plants Variable frequency electric actuators were first successfully developed abroad. In China, they were initially used in industries such as petrochemicals and mining machinery. The power industry initially adopted variable frequency speed control starting with pulverized coal feeders. This is because pulverized coal feeders use low-voltage motors, and low-voltage frequency converters were mature earlier, simpler to install, and easier to retrofit the motors. Adopting variable frequency speed control for pulverized coal feeders not only yielded significant economic benefits but also offered many advantages compared to traditional mechanical speed control devices: more uniform pulverized coal feeding, reduced coal pulverization, and more stable boiler combustion; a wider adjustment range for the fuel control system, and enhanced protection functions and accident handling capabilities. For these reasons, most power plants subsequently retrofitted their pulverized coal feeder motors and coal feeders in direct-fired combustion systems with variable frequency technology, achieving excellent operational results. With the increasing maturity of high-voltage variable frequency technology, high-voltage variable frequency electric actuators are gradually being used in the power industry. Considering energy-saving effects, the equipment using high-voltage variable frequency devices in China is mainly concentrated on forced draft fans, induced draft fans, slurry pumps, low-pressure heater drain pumps, booster pumps, and condensate pumps. There are already examples abroad of using high-voltage variable frequency devices on feedwater pumps. For example, before the advent of variable frequency electric actuators, thermal power units used electric motors running at a constant speed, controlling the airflow in the boiler furnace by adjusting the opening of the guide vanes or dampers of the forced and induced draft fans. However, the forced and induced draft fans of units above 200MW use high-voltage, high-power electric motors, resulting in high power consumption. This leads to significant waste of plant power due to throttling losses, which is very uneconomical. Moreover, the nonlinearity and slow response speed affect the quality of automatic activation and regulation. Therefore, some older power plants have already adopted or are considering upgrading their airflow control methods using variable frequency electric actuators. While using integrated variable frequency electric actuators offers many advantages, the upgrade cost is high. Since the existing forced draft and induced draft fan motors meet the requirements for variable frequency speed control in terms of capacity and protection level, the existing equipment is retained with the addition of a variable frequency drive. There are two methods: One method is to connect the added variable frequency drive in parallel with the original electrical control circuit, using a switch to select the path controlling the motor's operation, as shown in the figure. Using this method, when the variable frequency drive fails or is under maintenance, the motor can revert to its original control mode without affecting the operation of the entire unit. However, an interlock is required between the two control paths to prevent short circuits. The second method is to remove the original electrical control circuit and directly use the variable frequency drive to control the motor's operation. Upgrading older, high-energy-consuming high-voltage motor equipment using these methods not only improves the regulation quality of the relevant control system and enhances its stability and reliability, but more importantly, it saves on plant power consumption. Therefore, it is receiving increasing attention from power plants. For example, Daqing Huaneng Xinhua Power Plant carried out frequency conversion retrofits on its induced draft fans and slurry pumps; Anhui Tongling Power Plant carried out frequency conversion retrofits on its low-pressure heater condensate pumps; and Qinghe Power Plant carried out frequency conversion retrofits on its induced draft fans. All of these retrofits have yielded good economic and social benefits. IV. Conclusion Clearly, replacing many traditional electric actuators with frequency conversion electric actuators is an effective means of saving energy, improving process flow, and enhancing control system quality. However, most frequency conversion devices currently rely on imports, are expensive, and require high initial investment. Widespread use of frequency conversion electric actuators is not yet realistic. Retrofitting actuators in older units that have shown significant energy-saving effects after adopting frequency conversion speed regulation and seeking high-performance-price ratio solutions should be the focus of current research. While using frequency conversion speed regulation devices in the retrofitting of older units, frequency conversion electric actuators should also be used as much as possible in the design of new units to avoid the waste of secondary investment. At the same time, dedicated frequency conversion motors designed for frequency conversion electric actuators have advantages such as good heat dissipation, low electromagnetic loss, and higher energy efficiency than ordinary motors, which can make the overall efficiency of the frequency conversion electric actuator higher.