1. Overview With the development of the national economy, production equipment is becoming increasingly larger. The capacity of motors in industrial equipment such as ball mills, crushers, fans, pumps, pulpers, and rolling mills is increasing. Direct starting of a squirrel-cage three-phase asynchronous motor results in a starting current 5-8 times the motor's rated current, and a starting torque only 0.6-1.2 times the rated torque (the starting torque decreases with increasing motor capacity), leading to a starting ratio of less than 0.2. Using reduced-voltage starting not only complicates the starting equipment and increases costs, but also, because the starting torque is proportional to the square of the starting current, the starting ratio decreases proportionally. Therefore, squirrel-cage three-phase asynchronous motors with starting devices can only be used in motors with low starting torque, such as those for fans and pumps, and not in equipment like ball mills, crushers, and rolling mills. Three-phase wound-rotor AC asynchronous motors, on the other hand, can have their rotors connected to external resistors, frequency-sensitive rheostats, and back EMF. It not only limits the starting current of the motor but also increases the starting torque and power factor during startup, making the starting ratio close to 1 (the starting ratio is close to 1 when the rotor is connected in series with resistance and back EMF; the starting ratio can reach up to 0.55 when a frequency-sensitive resistor is connected in series). Therefore, three-phase wound-rotor AC asynchronous motors are widely used in large-capacity, high-torque mechanical equipment. However, three-phase wound-rotor AC asynchronous motors have disadvantages such as complex starting equipment, high failure rate, and large workload for operation and maintenance because the rotor current needs to be led to the ground starter cabinet through brushes, slip rings, cables, etc. The brushless self-controlled motor soft starter was invented to overcome the shortcomings of wound-rotor motors and starting devices by utilizing the fact that the starting ratio of wound-rotor motors can approach 1. 2. Common Starting Methods and Main Problems of Wound-Rolled Motors Common starting methods for wound-rotor motors include: A. Frequency-Sensitive Resistor Starter: This method involves introducing the rotor circuit of the wound-rotor motor into a frequency-sensitive resistor starter cabinet via slip rings and carbon brushes. During motor starting, a frequency-sensitive resistor is connected in series. As the motor speed increases and the rotor current frequency decreases, the impedance of the frequency-sensitive resistor gradually decreases, achieving the purpose of continuously limiting the motor's starting current. After the motor starts, the frequency-sensitive resistor needs to be short-circuited via a contactor or short-circuit ring (some small and medium-sized motors have short-circuit rings installed on the rotor). Series frequency-sensitive resistor starting, because the frequency-sensitive resistor is actually an inductive element, reduces the motor's power factor, resulting in a starting ratio of less than 0.55. Therefore, it is generally used in motors of fans, water pumps, air compressors, and ball mills where the starting torque requirement is not very high. A variation is the brushless ringless starter, which directly mounts the frequency-sensitive resistor on the motor rotor to achieve brushless operation. The brushless ringless starter has the following problems: (1) The starting current cannot be adjusted once it leaves the factory. (2) The weight is more than 1.8 times that of the brushless self-controlled motor soft starter, and the maximum power of the starter cannot exceed 500KW. (3) When the motor is running normally, the frequency-sensitive resistor is not short-circuited, which will generate a certain power loss. B. The multi-stage starter cabinet for wound-rotor asynchronous motors with series resistance also introduces the motor rotor current into the starter cabinet and changes the resistance through time, current or cam controller to increase the motor starting torque and reduce the starting current. Its biggest advantage is that since the series circuit is pure resistance, the power factor is high and the starting ratio is close to 1. However, the starting control device is complicated and the starting resistor needs to be switched frequently during the starting process, causing multiple impact currents. Therefore, it is only used on cranes, rolling mills and other equipment with particularly high starting torque requirements. Its variant is the wound-rotor asynchronous motor liquid resistance starter cabinet. It connects a liquid resistor in series with the motor's rotor circuit, and a servo motor changes the distance between the plates, thereby altering the liquid resistance and achieving stepless continuous adjustment of the motor's starting torque and starting current. Compared to a cam controller that changes resistance, its biggest advantages are: 1) stepless continuous adjustment of the motor current; 2) the ability to easily change the starting resistance by altering the electrolyte concentration, thus changing the starting motor current. Its disadvantages are: 1) common wound-rotor asynchronous motor liquid resistance starter cabinets do not consider the corrosive effect of the liquid resistor on the plates and transmission equipment; 2) they do not consider the sealing and explosion-proof properties of the device, nor the influence of ambient temperature on the starting device, resulting in a short lifespan; 3) the motor starting process is entirely controlled by the servo motor and cannot automatically adapt to the motor's starting process; 4) limited application range, unable to be installed in vibrating environments (such as on cranes) or outdoors in northern regions; 5) large size, high workload for operation and maintenance, and high cost. Cabinet-type starting equipment requires the rotor current to be led to the ground control cabinet through brushes, slip rings, cables, etc. Carbon powder worn between the brushes and slip rings can reduce the insulation of the motor and cause motor failure. Corrosion of slip rings and brush pressure springs can cause arcing between the brushes and slip rings. This places higher demands on motor maintenance. At the same time, the motor starting process is manually set and cannot adapt to different loads; the starting device has many components, complex secondary circuits, high failure rate, high maintenance technical requirements, and high overall cost of the drive system, which are common problems. 3 Working principle and structural characteristics of brushless self-controlled motor soft starter 3.1 Introduction to brushless self-controlled motor soft starter The brushless self-controlled motor soft starter is designed to combine the advantages of brushless ringless starters and liquid resistance starter cabinets. Its control principle is similar to that of a wound-rotor motor liquid resistance starter cabinet. The difference is that the starting resistor is directly installed on the motor shaft, using the centrifugal force generated during motor rotation as power to control the magnitude of the starting resistor. This reduces the motor's starting current and increases the starting torque, enabling the wound-rotor asynchronous motor to achieve brushless self-controlled operation. It mainly consists of a housing, electrolyte, moving electrode plate, Schmidt contact device, spring, terminals, safety valve, and exhaust valve. A simplified internal structure diagram is shown in Figure 1. When the motor starts, on the one hand, as the motor speed increases, the moving electrode plate gradually approaches the housing under the action of centrifugal force, gradually reducing the resistance in series with the motor rotor. At the rated speed, it short-circuits with the housing, and the resistance drops to zero. On the other hand, the water resistance between the moving electrode plate and the housing heats up due to the current flowing through it, and under the negative temperature characteristic of water resistance, the resistance also gradually decreases. Under the combined effect of these two aspects, the motor starts with approximately constant current and constant torque. 3. The working principle of a brushless self-controlled motor soft starter is shown in Figure 2. After the starter leaves the factory, users can change the concentration of the electrolyte to alter the starting resistance, thus easily adjusting the motor's starting (locked) current to meet the starting current requirements of different loads. During motor starting, the water resistance is adjusted using the following methods: a) Changing the distance between the plates: As the motor speed increases, the centrifugal force increases, and the distance between the water resistance plates gradually decreases. After a delay when the motor speed reaches its rated speed, the distance between the water resistance plates is reduced to zero. b) Changing the temperature of the water resistance: The water resistance automatically heats up after current flows through it. Due to the negative temperature characteristic of the water resistance, the resistance gradually decreases. 3.3 Structural Features of Brushless Self-Controlled Motor Soft Starter To ensure long-term maintenance-free operation of the water resistor on the motor rotor, the brushless self-controlled motor soft starter incorporates the following technologies in its overall design: 1. The water resistor is selected with the following characteristics: A. Rust prevention for metals (copper and steel); B. No chemical reaction causing electrolyte deterioration, electrode corrosion, or gas generation after passing a large current; C. The electrolyte has a freezing point of -25℃ and a boiling point of 120℃ to meet the requirements for safe operation in different environments. 2. The starter adopts a fully sealed metal structure, minimizing the number of sealing surfaces. 3. A safety exhaust valve is installed. After the motor reaches its rated speed, the exhaust valve automatically opens to ensure that the pressure inside the starter is consistent with atmospheric pressure during normal operation. Simultaneously, to prevent the electrolyte from boiling and the internal pressure of the starter from increasing due to prolonged motor stall, a safety valve is installed to ensure the safety of the motor and starter. 4. To prevent the plates from being in intermittent contact near the rated speed, and to increase contact stress and reduce contact resistance, a Schmidt contact device is installed to improve the service life of the short contact head and reduce losses and heat generation of the starter during normal motor operation. 4. Achieved Effects The brushless self-controlled motor soft starter enables brushless self-controlled operation of wound-rotor asynchronous motors, avoiding the disadvantages of high maintenance workload and high failure rate caused by slip rings and carbon brushes in wound-rotor asynchronous motors. It achieves the following effects: a) The brushless self-controlled motor soft starter can start the motor with a starting ratio of 1. This avoids the direct starting of squirrel-cage asynchronous motors, where the starting ratio is less than 0.2. The installation of liquid resistors (thermal variable resistors), magnetically controlled reactors, and solid-state (thyristor) soft starter devices, where the starting torque is proportional to the square of the starting current, results in a 75% reduction in starting torque when the current decreases by 50%, leading to a starting ratio less than 0.1. b) After the brushless automatic motor soft starter leaves the factory, users can adjust the starting (stalled) current of the motor by changing the concentration of the electrolyte and the magnitude of the starting resistance, thus meeting the requirements of different loads for starting current and starting torque. c) The starting process of the brushless automatic motor soft starter is automatically completed by the motor according to its own speed, without human intervention. Other starting devices generally rely on indirect control such as time relays and current relays; after the motor starts, the starting device still needs to be manually deactivated (especially for high-voltage motors). If the deactivation is not done at the right time, it will threaten the safe operation of the motor, machinery, power supply, and starting equipment. d) When the mains voltage is low or the load is heavy, causing insufficient motor output torque, the temperature of the water resistor will rise due to the current flowing through it, and the resistance will automatically decrease, thereby gradually increasing the motor current and the motor starting torque, ensuring a successful start on the first attempt. e) The brushless automatic motor soft starter limits the motor's starting current to near the rated current, avoiding various damages to the motor and power supply equipment caused by excessive starting current. f) If the motor stalls, the motor will automatically heat the electrolyte. Because the active power consumed in the electrolyte is much greater than that consumed in the motor windings, and the thermal inertia of the electrolyte is less than that of the motor windings, when the electrolyte dries up, the motor rotor has no energized circuit, which is equivalent to the starting resistance becoming infinite. The motor rotor current will automatically drop to zero, preventing the motor from burning out due to prolonged stall. g) During motor operation, if a sudden load is applied (such as a stall in a rolling mill), as the motor speed decreases, the starting resistance will automatically be connected in series with the rotor circuit to increase torque, reduce current, and protect the motor. h) The market price of a 1400KW brushless automatic control motor soft starter is only 20,000 yuan. A conventional soft starter of the same capacity costs more than 100,000 yuan. A 1400KW brushless automatic control motor soft starter is only a Φ600×320 cylinder, which is directly installed on the motor. The liquid resistance (thermal resistance) starter for a high-voltage squirrel-cage asynchronous motor of the same capacity has dimensions of (1700+1200)×1200×2800, requiring infrastructure investment. i) The motor and starter are integrated into a brushless self-controlled motor. For the user, it is a "squirrel-cage asynchronous motor" with excellent starting performance (no need to configure a starter control cabinet and corresponding power cables and control cables, no need to consider the installation and commissioning of the starter equipment, the motor can start at a starting ratio of 1). After the starter equipment is installed, no operation or maintenance is required for the starter. 5 Typical Applications The Jinghai Water Conservancy Bureau Irrigation and Drainage Center Station in Tianjin has dozens of water pump motors, all of which were originally started using frequency-sensitive rheostat starter cabinets. Because it required the use of carbon brushes, slip rings, starter control cabinets, etc., the failure rate was high. To improve the reliability of its drainage system, the company decided to adopt brushless self-controlled motor soft starters for all its vehicles. This reduced the starting current from four times the rated motor current to twice the rated current, meeting the company's requirements for starting devices. Hunan Shizhuyuan Nonferrous Metals Co., Ltd. has two 380V, 280KW ball mill motors. Previously, both used frequency-sensitive rheostat starter cabinets. During motor startup, the starting current exceeded four times the rated motor current, and the voltage drop during startup for a single motor exceeded 10% of the rated power supply voltage. Furthermore, due to the company's distance from the power supply center and the low power supply voltage, motor starting failures were frequent, sometimes resulting in damage to the starting devices. After adopting brushless self-controlled motor soft starters, the starting current of the motors dropped to approximately twice the rated motor current, the starting voltage drop decreased to 6% of the rated voltage, and there have been no motor starting failures. This has reduced the workload of motor operation and maintenance and lowered the failure rate, making it popular with on-site operation and maintenance personnel. On the 3000-meter ore conveyor belt of the Shougang Water Plant's iron ore mine in Beijing, all eight 6KV, 480KW conveyor belt motors have been replaced with brushless self-controlled soft starters. The results were significant, and the company published an academic paper titled "Application of Brushless Starters in Wound-Rotor Asynchronous Motors" in the 12th issue of the 2004 magazine *Mining Machinery*, which was widely welcomed by readers. At the Ningguo Cement Plant of Conch Group, all ball mill motor starters have been replaced with brushless self-controlled soft starters. Due to their high reliability and minimal operation and maintenance workload, this has not only reduced motor operation and maintenance costs but also increased cement plant output. This has attracted the attention of Conch Group, which has issued a document requiring all its subordinate cement plants to retrofit their existing ball mill motor starters with brushless self-controlled operation. 6. Conclusion The brushless self-controlled motor soft starter organically combines electrical control, mechanical design, and electrochemical technology, enabling AC motors to achieve soft starting with a starting ratio of 1 (squirrel-cage motors, direct starting, starting ratio less than 0.2; even lower with reduced voltage starting; wound-rotor motors, starting with a frequency-sensitive resistor, starting ratio less than 0.55; resistor starting, starting ratio can reach 1), and easily stepless adjustment of starting current and starting torque, all with a relatively simple structure and low cost. The brushless self-controlled motor soft starter provides an effective solution for users requiring high starting torque and low starting current. With its excellent starting performance, low price, adaptability to power supply and load, small size, energy saving, and convenient operation and maintenance, the brushless self-controlled motor soft starter will undoubtedly gain wider application. The brushless self-controlled motor soft starter was jointly developed and researched with Wenzhou Shuguang Starting Equipment Co., Ltd., based on the utility model patent for "Brushless Self-Controlled Wound-Rotor Asynchronous Motor" (Patent No.: 012774022), and obtained a utility model patent for "Brushless Self-Controlled Motor Soft Starter" (Patent No.: 032197888). The brushless self-controlled motor soft starter passed performance tests twice, in December 2002 and March 2004, by the Jiangxi Provincial Motor Product Testing Station, with all indicators meeting the requirements of the enterprise standard "WZR Brushless Self-Controlled Motor Soft Starter" (Q/SG001-2003). The brushless self-controlled motor soft starter was included in the first batch of provincial new product trial production plan of Zhejiang Province in 2004 (Zhejiang Development and Planning [2004] No. 140, Project No.: 2004D60SA390030), and passed the provincial new product appraisal in December 2004 (Zhejiang Science and Technology Appraisal No. [2004] 130). The appraisal committee believed that the brushless self-controlled motor soft starter is at the leading level in China in terms of starting wound-rotor asynchronous motors.