1. Introduction Since the invention of AC asynchronous motors, people have continuously researched their starting methods. After seventy or eighty years of updates and iterations, dozens of starting methods have been invented today. Each technological update represents human exploration and progress in new science and technology, just like the metabolism of biological cells, constantly renewing themselves to ensure the health and vitality of their bodies. AC asynchronous motors are divided into squirrel-cage and wound-rotor types, and in terms of voltage levels, they are divided into high-voltage motors and low-voltage motors. Currently, many starting methods are popular in the market, and choosing an economical and reasonable starting method has become a thorny issue in the control of low-voltage AC motors. This paper classifies and studies the various starting methods of low-voltage squirrel-cage AC asynchronous motors currently popular and applied in the market, and explains their application value and their respective advantages and disadvantages. 2. Selection of starting method for low-voltage motors 2.1 Provisions in GB50055-93 The "Design Specification for General Electrical Equipment and Distribution Equipment" GB50055-93 stipulates: (1) When the motor starts, its terminal voltage should be able to guarantee the starting torque required by the machinery, and the voltage fluctuation caused in the power distribution system should not hinder the operation of other electrical equipment; (2) When the AC motor starts, the voltage on the distribution bus should meet the following requirements: Under normal circumstances, when the motor starts frequently, it should not be lower than 90% of the rated voltage; when the motor does not start frequently, it should not be lower than 85% of the rated voltage; when there is no lighting or other loads that are sensitive to voltage fluctuations connected to the distribution bus, and the motor does not start frequently, it should not be lower than 80% of the rated voltage; when there is no other electrical equipment connected to the distribution bus, it can be determined according to the condition of guaranteeing the starting torque of the motor; for low-voltage motors, it can also be guaranteed that the voltage of the contactor coil is not lower than the release voltage. (3) The selection of starting methods for squirrel-cage motors and synchronous motors shall comply with the following provisions: The motor shall be started at full voltage when the following conditions are met: The voltage of the distribution busbar meets the provisions of Section 2 when the motor starts; The mechanical capacity can withstand the impact torque when the motor starts at full voltage; The manufacturer has no special provisions on the starting method of the motor. When the conditions for full voltage starting are not met, the motor should be started at reduced voltage, or other appropriate starting methods should be selected. When there are speed regulation requirements, the starting method of the motor should be matched with the speed regulation method. 2.2 The provisions in JGJ/T 16-92 The industry standard of the People's Republic of China "Code for Electrical Design of Civil Buildings" JGJ/T 16-92 stipulates that in cases where the motor is directly powered by the urban public low-voltage network, the allowable full-voltage starting capacity of the motor shall be coordinated with the provisions of the local power supply department. If the local power supply department does not have a clear regulation on the permitted full-voltage starting capacity of squirrel-cage motors, it can be determined according to the following conditions: when powered by a public low-voltage network, motors with a capacity of 11kW or less can start at full voltage; when powered by a residential substation's low-voltage distribution equipment, motors with a capacity of 15kW or less can start at full voltage. The above are the national and industry standards for motor starting requirements. Designers are most likely to consider electrical parameters, but easily overlook the requirement that "the mechanical system can withstand the impact torque during full-voltage starting of the motor." This regulation means: firstly, the impact torque during full-voltage starting of the motor must not cause the mechanical equipment's reliability to fail to meet requirements; secondly, the losses caused by the reduced lifespan or increased failure rate of the mechanical equipment due to the impact torque during full-voltage starting must outweigh the investment in adding reduced-voltage starting equipment. For example, in the application of belt conveyors, the causes of belt damage are belt wear during operation, wear between the belt and the material at the moment of starting, and the loosening of the belt at the moment of starting. Since belt conveyors are more expensive than soft starters, soft starting is generally more economical for belt conveyors above 15kW, as it reduces belt damage compared to full-voltage starting. Similarly, soft starting is more economical for water pumps or fans above 45kW. Furthermore, rising non-ferrous metal prices have brought the cost of soft starters below that of autotransformers. With the advancements in soft starter technology, its superior starting performance and cost-effectiveness have made it a dominant force in reduced-voltage starting applications. The introduction of the TJNR1000 online low-power soft starter in 2005 revolutionized motor starting methods. It eliminates the need for a bypass contactor, significantly reducing system costs and offering higher reliability than contactors. The price of high-power models is now comparable to imported contactors, while energy consumption is over 60% lower, resulting in more economical operation and significantly improving motor starting performance. 3. Reduced-voltage starters For low-voltage squirrel-cage AC asynchronous motors with rated parameters of 3kW and below, the rated winding voltage is 220V; for series above 3kW, the rated winding voltage is 380V. Generally, full-voltage starting is preferred when selecting the starting method for low-voltage motors. However, in practical applications, if the electrical or mechanical requirements for full-voltage starting are not met, reduced-voltage starting, also known as soft starting, is required. The following types of starters are commonly used for reduced-voltage starting: 3.1 Autotransformer starters. Autotransformer starters utilize the voltage reduction principle of an autotransformer to reduce the voltage of the motor windings, as shown in Figure 1. This starting method has been widely used for decades. Standard autotransformers for starting motors have two taps: one for 80% of the rated voltage and the other for 65% of the rated voltage. The 65% rated voltage tap is commonly used. Its starting torque is 0.4225 times that under normal pressure, and it is larger than that of the "y-Δ" starter, so this type of reduced-voltage starting method is widely used. The disadvantages are voltage jumps, starting current jumps, and motor torque jumps. Figure 1 shows the principle diagram of an autotransformer starter. 3.2 "y/Δ" Starter Figure 2 shows the principle diagram of a "y/Δ" starter. The "y/Δ" starter utilizes the principle of "phase-to-line voltage change" to reduce the voltage of the motor windings, as shown in Figure 2. In the past few decades, a considerable number of motors have used this starting method. It uses three contactors to change the winding voltage from line voltage to phase voltage. Its starting torque is 1/3 times that under normal pressure, and it is smaller than that of an autotransformer starter, so this type of reduced-voltage starting method is less commonly used than an autotransformer starter. The disadvantages are voltage jumps, starting current jumps, and motor torque jumps. Compared to an autotransformer starter, its cost is exactly the same as that of an autotransformer. Since a "y/Δ" starter requires six power connections to the motor, the cost savings in system construction are not necessarily significant. It is mainly used in applications where starting torque requirements are not high and the power supply distance between the starter and the motor is short. 3.3 Magnetic-Controlled Soft Starter Figure 3 shows the schematic diagram of a magnetic-controlled soft starter. To solve the problems of stepped voltage and torque jumps during reduced-voltage starting, the magnetic-controlled soft starter was invented in the 1990s. It uses the principle of magnetic impedance to achieve stepless voltage reduction and increase, and is an improvement on the autotransformer starter, as shown in Figure 3. The magnetic-controlled soft starter can achieve continuously controllable voltage within a certain range. Magnetic-controlled soft starters are divided into wire-wound and foil-wound types. Theoretically, due to the capacitive reactance of the foil winding compensating for reactive current, the torque of the foil winding is greater than that of the wire-wound type under the same starting current. Because linear winding magnetic control soft starters were phased out by bypass-type soft starters at the end of the last century, foil winding magnetic control soft starters are a patented technology from the beginning of this century. Since the concept of magnetic control is considered outdated, they are rarely seen on the market. However, they have actually undergone a qualitative change, with starting torque higher than all other soft starters, making them ideal for applications where grid impact or reduced-voltage starting is difficult. Their advantages include low harmonics and the ability to achieve stepless voltage transformation within a certain range. 3.4 Electronic Soft Starters Figure 4 Online-running soft starter Figure 5 Bypass-type soft starter Figure 6 Built-in bypass-type soft starter Electronic soft starters were first introduced by AB Corporation in the 1970s, and were quickly adopted by major electrical companies worldwide. However, they all use thyristor online operation, as shown in Figure 4. Their advantages include intelligent starting control and protection of the motor, and a simple circuit. In terms of usage, soft starters suffer from drawbacks such as excessive size and weight (e.g., a 200kW soft starter weighs 56kg), high cost, excessive power consumption (equivalent to about 1.5% of the motor capacity), excessive heat generation, difficulty in heat dissipation, and high harmonic emissions, thus severely limiting their application. To overcome these shortcomings, domestic manufacturers began developing and producing bypass-type electronic soft starters at the end of the last century, as shown in Figure 5. In 1998, the national standard for soft starters, "Semiconductor Motor Controller Standard" GB14048.6-1998, was promulgated. In 2000, the author conducted nationwide lectures on soft starter applications, promoting the use of bypass-type soft starters. By 2001, design institutes began selecting bypass-type soft starters in their designs, and by 2002, 90% of engineering designs used bypass-type soft starters, with autotransformers or "Y/Δ" starters rarely chosen. However, bypass soft starters also have disadvantages, such as: (1) increased circuit complexity and reduced system reliability; (2) inability to fully utilize powerful intelligent controllers, and some cannot protect the motor; (3) increased size and cost of complete sets of equipment; (4) increased difficulty in maintenance and repair. In order to overcome the disadvantages of bypass soft starters, various electrical companies developed built-in bypass soft starters in 2003. Now, Tianjin Norhaden, American AB, and Siemens have launched their full range of built-in bypass soft starters, as shown in Figure 6, and many manufacturers are also launching built-in bypass soft starters in some series. Its advantage is that it avoids both the disadvantages of thyristor online operation and the disadvantages of bypass type. It is small in size, light in weight, low in cost, and powerful in control function. It is the most advanced soft starter at present and has the most practical application value. 3.5 Other soft starting methods There is another soft starting method for low-voltage squirrel-cage AC motors on the market, namely liquid resistance. There are two methods. One method relies solely on the thermal properties of the liquid resistor to change the motor's terminal voltage. This liquid is a conductive mixture of water and a suitable amount of baking soda (sodium bicarbonate, commonly known as "chewer") (the resistivity of this mixture is inversely proportional to its temperature; the lower the temperature, the higher the resistivity, and the higher the temperature, the lower the resistivity). At the start of startup, the liquid is cold, its resistivity is high, and the motor's terminal voltage is low. As the liquid resistor consumes power and heats up, its resistivity decreases, and the motor's terminal voltage increases. When the motor reaches a certain speed, a contactor bypasses the liquid resistor, achieving full-voltage power supply and enabling the motor to operate at full voltage. This principle can be implemented in two ways: one is to change the motor's starting voltage solely through the change in the liquid's electrical conductivity (see Figure 7); the other is to add a moving plate at the top of the liquid resistor and a stationary plate at the bottom. During startup, in addition to the temperature rise causing a decrease in resistance, the conductive distance of the liquid resistor is also reduced, accelerating the rate of resistance reduction (as shown in Figure 8). Figure 7. One type of liquid resistance voltage reduction starting scheme. Figure 8. Another type of liquid resistance voltage reduction starting scheme. Liquid resistance starters have many names, but their basic principle is the same: series resistance starting, suitable for starting wound-rotor AC motors. However, it is unsuitable for squirrel-cage motors, as the resistance is connected in series on the stator winding side. During starting, the resistance causes a large amount of energy waste, and the large amount of heat is difficult to dissipate. Furthermore, this product does not comply with national policies and is not included in national standards, so it cannot obtain "CCC" certification and is not suitable for widespread use. 4. Conclusion When selecting a starting method for low-voltage squirrel-cage AC motors, in application, one should first consider whether voltage reduction starting is needed, and then select a suitable starter. Soft starting technology has undergone revolutionary development from the era of autotransformers and "Y/Δ" starters to today's electronic soft starting era. Its excellent practicality and economy have given it greater practical application value. Among them, the built-in bypass type electronic soft starter has many advantages over other similar products, such as small size, light weight, low cost, and powerful control functions. It is the most advanced soft starter at present and has the most practical application value, playing the role of completely replacing other types of soft starters.