Abstract: This paper first briefly introduces the problems of commonly used wound-rotor motors and starting devices, and then introduces the principle, structural characteristics, main performance, innovation, advancement, technological maturity, typical application effects, and main problems of brushless self-controlled motor soft starters. Keywords: soft starter, starting current, starting torque, starting ratio, brushless self-controlled wound-rotor motor . Background: Direct starting of squirrel-cage three-phase asynchronous motors results in a starting current that is 5-8 times the rated current, and a starting torque that is only 0.6-1.2 times the rated torque (the starting torque decreases as the motor capacity increases), with a starting ratio of less than 0.2. If reduced-voltage starting is used, not only is the starting equipment complex and costly, but the starting ratio also decreases proportionally because the starting torque is proportional to the square of the starting current. Therefore, squirrel-cage three-phase asynchronous motors with starting devices can only be used in motors with low starting torque, such as fans and pumps, and cannot be used in equipment such as ball mills, crushers, and rolling mills. Three-phase wound-rotor AC asynchronous motors can be externally connected to resistors, frequency-sensitive rheostats, and back EMF on the rotor. This not only limits the starting current but also increases the starting torque and power factor during startup, bringing the starting ratio close to 1 (the starting ratio is close to 1 when the rotor is connected to resistors and back EMF; with a frequency-sensitive rheostat, the starting ratio can reach up to 0.55). 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 require the rotor current to be led to the ground through brushes, slip rings, cables, etc., and commonly used starting equipment has the following shortcomings: A. Frequency-sensitive rheostat starter cabinet: This cabinet leads the rotor circuit of the wound-rotor motor to the ground through slip rings and carbon brushes. During motor startup, a frequency-sensitive rheostat is connected in series. As the motor speed increases and the rotor current frequency decreases, the impedance of the frequency-sensitive rheostat 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 by a contactor or short-circuit ring (some small and medium-sized motors have a short-circuit ring installed on the rotor). When starting with a series frequency-sensitive resistor, since the frequency-sensitive resistor is actually an inductive element, it reduces the power factor of the motor, and the starting ratio of the motor is less than 0.55. Therefore, it is generally used in fans, water pumps, air compressors and ball mill motors where the starting torque requirement is not high. Its variant product: the brushless ringless starter is to install the frequency-sensitive resistor directly on the motor rotor to achieve brushless operation of the motor. 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 twice 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 consumption and reduce the efficiency of the drive system. B. A multi-stage starter cabinet for wound-rotor asynchronous motors with series resistance also introduces the motor rotor current into the starter cabinet. The resistance is changed by time, current, or a cam controller to increase the motor's starting torque and reduce the starting current. Its biggest advantage is that, because the circuit in series with the rotor is purely resistive, the power factor is high, and the starting ratio is close to 1. However, the starting control device is complex, and frequent switching of the starting resistor during startup causes multiple inrush currents. Therefore, it is only used in equipment such as cranes and rolling mills that require extremely high starting torque. A variant is the liquid resistance starter cabinet for wound-rotor asynchronous motors. This cabinet inserts a liquid resistor into the motor's rotor circuit, and a servo motor changes the distance between the plates, thereby changing the liquid resistance to achieve stepless continuous adjustment of the motor's starting torque and starting current. Compared to cam controllers that change resistance, its biggest advantages are: 1) stepless continuous adjustment of motor current; 2) the ability to easily change the starting resistance by altering the electrolyte concentration, thus changing the starting motor current. Disadvantages include: 1) common wound-rotor asynchronous motor liquid resistance starter cabinets do not consider the corrosive effect of the liquid resistance 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 a servo motor and cannot automatically adapt to the motor's starting process; 4) limited application range, unsuitable for installation in vibrating environments (such as on cranes) and outdoors in northern regions; 5) large size, high workload for operation and maintenance, and high cost. Cabinet-type starter equipment requires the rotor current to be led to the ground control cabinet through brushes, slip rings, and cables. Carbon dust worn between the brushes and slip rings can reduce motor insulation, leading to 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. Meanwhile, the motor starting process is manually set, making it unsuitable for different loads; the starting device has many components, complex secondary circuits, high failure rate, high maintenance and technical requirements, and a high overall cost of the drive system – these are other common problems. 2. Technical Principles The brushless self-controlled motor soft starter is designed to combine the advantages of brushless ringless starters and liquid resistance starter cabinets while overcoming their shortcomings. 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 by the motor's rotation as power to control the magnitude of the starting resistor, thereby reducing the motor's starting current and increasing 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, Schmitt contact device, spring, terminals, safety valve, and exhaust valve. Its internal structural diagram is shown in Figure 1. When the motor starts, on the one hand, as the motor speed increases, the moving plate gradually approaches the casing under the action of centrifugal force, and the resistance in series with the motor rotor decreases proportionally. When the rated speed is reached, it is short-circuited with the casing, and the resistance drops to zero. On the other hand, the water resistance between the moving plate and the casing heats up due to the current passing through it, and the resistance also gradually decreases under the influence of the negative temperature characteristic of water resistance. Under the combined effects of motor speed and temperature, the motor starts with approximately constant current and constant torque. The control principle of the brushless self-controlled motor soft starter is shown in Figure 2. After the starter leaves the factory, the user can change the starting resistance by changing the concentration of the electrolyte to achieve convenient stepless adjustment of the motor's starting (locked rotor) current, meeting the starting current requirements of different loads. 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: A. The water resistor is selected using an electrolyte 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. B. The starter adopts a fully sealed metal structure, minimizing the number of sealing surfaces. C. 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. D. 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. 3. Performance Indicators The brushless self-controlled motor soft starter achieves 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 means that the starting torque is proportional to the square of the starting current. When the current decreases by 50%, the starting torque decreases by 75%, resulting in a starting ratio less than 0.1. b) After the brushless automatic motor soft starter leaves the factory, users can adjust the starting (locked rotor) current of the motor by changing the concentration of the electrolyte and the magnitude of the starting resistance. This meets the requirements of different loads for starting current and starting torque, avoiding the disadvantage of traditional starting devices where the starting current cannot be infinitely adjusted. c) The starting process of the brushless automatic motor soft starter is automatically completed by the motor based on 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 grid 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 experiences prolonged stall, it will automatically heat the electrolyte. Since the active power consumed by the electrolyte is far greater than that consumed by the motor windings, the electrolyte will quickly boil. Once the electrolyte has dried up, the motor rotor has no energized circuit, effectively making the starting resistance infinite. The motor rotor current will automatically drop to zero, thus protecting the motor. 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, increasing torque, reducing current, and protecting 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 over 100,000 yuan. A 1400KW brushless automatic control motor soft starter is simply a Φ600×320 cylinder, directly mounted 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. 4. Innovation and Advancement of the Technology The main innovation and advancement of this technology is that it is the first time in China that the following has been adopted: A. The starting resistor is directly installed on the motor shaft, using the centrifugal force generated when the motor rotates as power to control the size of the starting resistor, thereby reducing the motor starting current and increasing the starting torque, enabling the wound-rotor asynchronous motor to achieve brushless self-controlled operation. B. The electrolyte selected for the water resistance resistor has the following characteristics: A. It has rust-preventing properties for metals (copper and steel); B. It does not undergo chemical reactions such as 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 and extended service life under different environments. A safety vent valve is installed. After the motor reaches its rated speed, the vent 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 stalling, a safety valve is installed to ensure the safe and reliable operation of the motor and starter. 5. Technological Maturity, Applicability, and Safety The brushless self-controlled motor soft starter was jointly developed with Wenzhou Shuguang Starting Equipment Co., Ltd., based on the utility model patent for "Brushless Self-Controlled Wound-Roller 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 plans 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 concluded that the brushless self-controlled motor soft starter is at the leading level in China in the starting of wound-rotor asynchronous motors. The product is now in mass production. The brushless self-controlled motor soft starter can be widely used in the starting of wound-rotor asynchronous motors in ball mills, crushers, fans, pumps, pulpers, and other motor drive equipment in all industrial fields such as metallurgy, machinery, chemical industry, mining, and building materials. It is an ideal replacement for frequency-sensitive rheostat starting devices. The brushless self-controlled motor soft starter employs dual safety vent valves, with the maximum operating pressure of the safety vent valves being less than 0.2 MPa and the starter housing withstanding a pressure greater than 2 MPa. The electrolyte composition is comparable to automotive antifreeze, ensuring the safe and reliable operation of this device. Furthermore, if the motor stalls, it automatically heats the electrolyte. Since the active power consumed in the electrolyte is far greater than that consumed in the motor windings, the electrolyte boils rapidly. Once the electrolyte boils dry, there is no current path to the motor rotor, and the rotor current automatically drops to zero, thus protecting the motor. 6. Application and Existing Problems Since its market launch in March 2003, the brushless self-controlled motor soft starter has received widespread praise from users for its simple structure, excellent starting performance, low maintenance workload, automatic adaptation to different loads and power supply voltage fluctuations, motor protection, long service life, and price that is only about 30% of similar starting devices, creating significant social and economic benefits. Within two years, the products were sold throughout all provinces, municipalities, and autonomous regions of mainland China except Tibet, with an annual output value of 3 million yuan. Below are some typical application examples: ■ Tianjin Jinghai Water Conservancy Bureau Irrigation and Drainage Center Station has dozens of water pump motors, all originally started using frequency-sensitive rheostat starter cabinets. Because these required carbon brushes, slip rings, and starter control cabinets, the failure rate was high. To improve the reliability of the drainage system, the company decided to adopt "brushless self-controlled motor soft starters" for all motors, reducing the starting current from four times the motor's rated current to twice the rated current, thus meeting the company's requirements for starting devices. ◆ Hunan Shizhuyuan Nonferrous Metals Co., Ltd. has two 380V, 280KW ball mill motors, both originally started using frequency-sensitive rheostat starter cabinets. During motor startup, the starting current reached more than four times the motor's rated current, and the voltage drop during startup of a single motor exceeded 10% of the power supply's rated voltage. Meanwhile, due to the company's distance from the power supply center and the low power voltage, motor starting failures frequently occurred, sometimes even resulting in damage to the starting device. After adopting the "brushless self-controlled motor soft starter," the motor starting current dropped to approximately twice the rated current, the starting voltage drop decreased to 6% of the rated voltage, and there were no motor starting failures. This reduced the workload of motor operation and maintenance and 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 iron ore mine in Beijing, all eight 6KV, 480KW conveyor belt motors have been replaced with brushless self-controlled motor soft starters. Due to the significant effect, the company published an academic paper entitled "Application of Brushless Starters in Wound-Rotor Asynchronous Motors" in the 12th issue of the 2004 "Mining Machinery" magazine, which was widely welcomed by readers. ◆ The starting devices of the ball mill motors at the Ningguo Cement Water Plant of Conch Group have all been replaced with brushless self-controlled motor soft starters. Due to its high reliability and minimal operation and maintenance workload, the brushless automatic control soft starter not only reduces motor operation and maintenance costs but also increases 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 starting devices with brushless automatic control operation. The main problems with brushless automatic control soft starters are: ● However, due to limitations in heat dissipation and size, brushless automatic control soft starters are only suitable for wound-rotor asynchronous motors with a starting frequency of no more than six times per hour, continuous starts of no more than three times, and a power range of 30-2000kW. For example, the product cannot be used on crane-type loads with very frequent start-stop (forward and reverse rotation). ● Because it uses a water resistor, the water resistor needs to be replaced approximately every two to three years during normal motor operation, and it cannot achieve the goal of completely maintenance-free operation of the motor starting device. Note 1: The starting ratio is the ratio of the starting torque multiple to the starting current multiple, abbreviated as starting ratio. If a squirrel-cage induction motor is directly started, with a starting current of 6 times the rated current and a starting torque of 1.2 times the rated torque, then the starting ratio of the motor is 0.2. About the Author : Zhai Youhua: Senior Engineer, Electrical Expert Committee Member of the Electrical Professional Committee of the China Petroleum and Chemical Equipment Association, has applied for six patents and published more than ten papers.