1. Introduction With the rapid development of the national economy and technological progress, soft starters for electric motors have been widely used. At the end of the last century, the proportion of motors starting in non-direct starting locations using soft starters was less than 5%, but today it accounts for over 90%. Only in rural areas and extremely underdeveloped regions are step-down starters still used in new construction projects. Therefore, soft starters have formed a large market. Soft starters have significantly reduced the impact of motor starting on the power grid and transmission machinery, extending the lifespan of transmission machinery and making a due contribution to national economic development. Soft starter technology itself is also developing rapidly and flourishing. This article focuses on introducing the advantages and disadvantages of various soft starters, correct application methods, and common fault analysis. 2. Classification of Soft Starters 2.1 Online Operation Soft Starters In the last century, soft starter products were mainly foreign brands sold in the Chinese market, such as AB, ABB, Schneider Electric, and Siemens, but they were all online operation models. During application, the following disadvantages were found in online operation. (1) The power consumption of thyristors during long-term online operation is too high, resulting in energy waste; (2) The heat dissipation of thyristors is too high, requiring mechanical air cooling, which brings great difficulties to the complete set; (3) Long-term online operation of thyristors brings high-order harmonic pollution to the power grid; (4) The reliability of thyristors as main switching elements during long-term operation is far lower than that of mechanical switches; (5) The cost is high and difficult for users to accept; (6) Due to the large size of the thyristor selection and heat dissipation considerations, the volume is large. (7) Advantages of soft starters: They integrate the starting, protection and control of motors, and the powerful intelligent controller plays a full role; They are suitable for frequent starting places due to the use of mechanical air cooling; The circuit is simple and easy to maintain and repair (see Figure 1). Figure 1 Circuit diagram of online soft starter 2.2 Bypass operation soft starter At the end of the last century and the beginning of this century, considering the disadvantages and technical difficulties of online operation, domestic manufacturers directly developed bypass type soft starters, that is, after the motor starts, it bypasses to the contactor to run. Its advantage is that it avoids the disadvantages of thyristor online operation, especially since it does not require mechanical air cooling. However, it also brings disadvantages. (1) The circuit becomes more complex, and the system reliability is reduced; (2) The powerful intelligent controller cannot be fully utilized, and some cannot protect the motor; (3) It increases the size and cost of the complete set of equipment; (4) It increases the difficulty of maintenance and repair. After comprehensive comparison, most of the market still adopts the bypass operation mode. Even if the soft starter with the online operation mode is selected, the designers still add a bypass operation contactor to make the soft starter bypass operation. It avoids the defects of thyristor online operation, as shown in Figure 2. Figure 2 Circuit diagram of bypass operation soft starter 2.3 Built-in thyristor bypass type online operation soft starter At present, many electrical companies at home and abroad are developing built-in thyristor bypass type online operation soft starters. An in-line soft starter with built-in thyristor bypass (referred to as an in-line bypass soft starter) incorporates a set of mechanical contacts connected in parallel with a thyristor within the starter. During soft starting and soft stopping of the motor, the thyristor operates, and the mechanical contacts open. When the motor is running normally, the thyristor closes, and the mechanical contacts close. This entire process is automatically completed by an internal controller, and from the perspective of external wiring, it is a single device, hence the term "in-line operation." It can also be called a bypass soft starter, integrating the external contactor into the soft starter while maintaining a compact size. Its advantages are that it possesses all the advantages of the above two types while avoiding their respective disadvantages: (1) Simple circuit; (2) Natural air cooling; (3) The thyristor only handles starting and stopping, avoiding the power consumption and heat dissipation caused by the online operation of the thyristor; (4) Small size (general size of the bypass type); (5) The powerful intelligent controller can be fully utilized, and can start, stop, protect and control the motor; (6) Saves space; (7) Due to the integrated design of the thyristor and mechanical contact, the intelligent controller realizes that the mechanical contact is arc-free, so that the electrical life of the mechanical contact is equal to the mechanical life, solving the problem that the contactor has been difficult to solve for a long time, and greatly improving the system reliability compared with the bypass type; (8) Energy saving refers to the soft starter itself. Compared to the thyristor-based online type, the energy saving is negligible, and compared to the bypass type, it reduces energy consumption by 60%. This is because the mechanical contacts of the built-in bypass type use arc-free control, which greatly reduces the hardness of the silver points and the contact resistance of the contacts, thus greatly reducing the closing pressure of the mechanical contacts. The magnetic force mechanism of the mechanical contacts is reduced by half, reducing energy consumption by half. The contact energy consumption of the mechanical contacts is also reduced by half. Overall, the energy consumption of the mechanical contacts and magnetic mechanism is reduced by half compared to the bypass contact. In addition, the energy consumption of the thermal relay is saved, so the overall energy saving is 60% compared to the bypass type, as shown in Figure 3. Figure 3 Built-in thyristor bypass type online soft starter 3. Selection of soft starters Based on the above analysis of the principle and advantages and disadvantages of soft starters, the built-in bypass type should be the most applicable, with the most advantages and the fewest disadvantages. In actual work, the specific product brand's functions and technical parameters, as well as the product's reliability, should also be considered, and the selection should be based on the actual usage. It's necessary to distinguish between frequent and infrequent starts. For soft starters, generally, if the starting interval doesn't exceed 2 minutes and 30 times/hour, it can be considered infrequent starting. Less than this should be considered frequent starting. Fans and pumps are generally considered infrequent starts. Mechanical drives can have both frequent and infrequent starting capabilities. Belt conveyors and ball mills can be considered infrequent starting, while starters or motors in large machinery requiring reversible functions are often considered frequent starters. In environments with frequent operation, the soft starter should be selected based on the motor's starting current. Soft starter manufacturers generally select a thyristor current rating that is 2.5 times the motor's rated current. The maximum motor starting current is limited to 4.5 times the rated current. To fully utilize the short-time overload capacity of the thyristor under infrequent operation, the soft starter capacity should be increased under frequent starting conditions, typically between 1.2 and 1.5 times the rated current, depending on the frequency. Furthermore, due to the frequent operation of the thyristor, the soft starter must be equipped with mechanical cooling to dissipate the large amount of heat generated. For mechanically air-cooled soft starters, it is best to place one soft starter per switch cabinet, and the switch cabinet should also be equipped with mechanical ventilation. 4. Main wiring scheme for soft starter circuit 4.1 All controllers of online soft starters have motor overload protection. When the soft starter is running online, the controller of the soft starter can provide overload protection for the motor. Do not install thermal overload relays. Because the current harmonics after passing through the thyristor are very large, electronic thermal overload relays cannot be installed. Otherwise, the malfunction of the thermal relay will prevent the system from working properly. Because thyristors are relatively expensive and difficult to replace, fast-acting fuses should be used to protect the thyristors from short circuits at the lower end of the soft starter and burn out the thyristors. Figure 4(a) shows a place where the soft starter is used frequently and no contactor is added at the upper end. Figure 4(b) shows a place where the soft starter is not used frequently and the power supply to the soft starter is disconnected after stopping. See Figures 4(a) and 4(b). Figure 4(a) Commonly used soft starters Figure 4(b) Infrequently used soft starters 4.2 Bypass type Bypass-operated soft starters cannot operate without the bypass contactor, so they are present in both main wiring schemes. The function of the contactor at the upper end of the soft starter is the same as in the online operation mode, and will not be repeated here. The thermal relay should be emphasized; it is best to install it at the lower end of the bypass contactor without passing the starting current, especially for electronic thermal relays, as the current harmonics after passing through the soft starter are very large and can interfere with the electronic thermal relay, causing it to malfunction and stop the motor. Furthermore, because the short-time operation of the thyristor does not require the installation of a fast-acting fuse, a fast-acting fuse is not installed in the main wiring scheme, as shown in Figures 5(a) and 5(b). Figure 5(a) Commonly used soft starters Figure 5(b) Infrequently used soft starters 4.3 Built-in bypass type Its main wiring is roughly the same as the online type, the only advantage being that the short-time operation of the thyristor does not require the installation of a fast-acting fuse. Overload protection for motors is achieved through the controller of a soft starter. It not only surpasses electronic thermal relays in function and performance but also avoids malfunction due to harmonic currents in the main circuit and external interference, as shown in Figures 6(a) and 6(b). Figure 6(a) Commonly used soft starters. Figure 6(b) Less frequently used soft starters. 5. Starting Characteristics of Soft Starters What issues should be considered when using electronic soft starters? First, the starting and stopping performance of a soft starter must be considered. Currently, soft starters have the following three starting methods. 5.1 Current-limiting start (see Figure 7) Figure 7 Current-limiting soft start 5.2 Ramp voltage start (see Figure 8) Figure 8 Ramp voltage start 5.3 Torque plus jump control start (see Figure 9) Figure 9 Torque plus jump control start Current-limiting start, as the name suggests, limits the starting current of the motor. It is mainly used for light-load starting to reduce the line voltage drop caused by the starting current and allows for a direct view of the starting current. The disadvantage is that it is difficult to know the starting voltage drop during startup, and the voltage drop space cannot be fully utilized, resulting in a loss of starting torque, which is detrimental to the motor. Ramp voltage start, as the name suggests, involves a linear ramp-up of voltage from low to high (also known as torque control). It changes the traditional stepped reduction starting from stepped to stepless, and is mainly used for heavy-load starting. The advantages are that the starting torque characteristic rises parabolically, which is beneficial to the drive system, the start is smooth and flexible, and it provides better protection for the drive system, extending the service life of the drive system. Its disadvantages are a large starting current and a voltage ramp (also known as torque ramp control) starting. This involves adding a voltage ramp (also known as torque ramp) to the voltage ramp control starting. It is used in situations where voltage ramp starting is difficult due to large static inertia, such as fan loads. At the moment of starting, the ramp torque overcomes the motor's static resistance torque, and then the torque increases parabolically, shortening the starting time. However, the ramp will send a 100ms surge current to the power grid, so caution is advised during application. There are two stopping methods: free stop and soft stop. The biggest advantage of electronic soft starters is soft stop. Soft stop eliminates the anti-inertia impact on the drive system caused by free stop. 6. Soft Starter Control Wiring Precautions 6.1 Influence of Control Cables The control input contacts of soft starters all use DC 24V (most domestic ones are 12V DC). If AC 220V or 380V control lines and the soft starter's control input lines are laid with the same cable, problems are likely to occur. It is essential to use an intermediate relay to isolate the control input line of the soft starter and convert the external line to the same voltage line. Otherwise, the high voltage will interfere with the normal operation of the soft starter or burn out the controller of the soft starter. 6.2 Control Distance If the control cable is too long, the soft starter will not operate reliably due to the low control voltage of the soft starter and the excessive line loss. Generally, when the control distance exceeds 200m, an intermediate relay should be used to increase the control voltage before remote control. 7. Common Faults of Soft Starters Currently, there are many soft starter manufacturers in China, most of which are bypass type. The reliability of their products is getting smaller and smaller compared with world-renowned brands, and their market share has exceeded that of foreign brands. The faults of soft starters can be roughly divided into the following categories. (1) The motor cannot start The reasons why the motor cannot start can be roughly divided into two situations: one is that one of the six thyristors is unreliable or does not conduct. At this time, one phase circuit passes through half-wave DC, and the DC passing through the two phase windings of the motor plays a braking role on the motor. Not only will the motor not start, but it will also burn out the motor and thyristors in severe cases. Second, the motor cannot start due to unsuitable starting parameters or starting curves, which is a common fault. (2) Thyristor burnout: Thyristor breakdown or explosion. This type of fault is not limited to domestic or foreign brands and varies from manufacturer to manufacturer, but the failure rate is lower than that of contactors. The main problem lies in the installation process of the disc thyristor. (3) Controller burnout: Compared with soft starters, controller burnout is the most serious fault. Some manufacturers have a return rate of more than 30% due to this type of fault. Imported or joint venture manufacturers do not have this problem. The main problem is that the power supply, trigger circuit and input circuit of the controller are easy to burn out. (4) Soft starter malfunction: The motor stops due to interference with the soft starter while running, and starts due to interference with the soft starter while stopped. This happens frequently. The former is more common, while the latter has only occurred in two brands. The reasons are, firstly, product quality issues, and secondly, related to the circuit layout. (5) Poor Contact of Internal Connectors in Soft Starters The selection of internal connectors in soft starters is not inherently a problem, but this is an issue that domestic manufacturers often overlook, frequently leading to malfunctions. 8. Conclusion Through discussion, the reduced-voltage starting methods for electric motors have evolved from "y-δ" starters and autotransformer reduced-voltage starters to magnetically controlled soft starters, and now to electronic soft starters. Due to the power consumption issues caused by thyristor junction voltage drop, soft starters have transitioned from online operation to bypass operation, and now have developed into online operation with built-in bypass. Generally speaking, the reliability of soft starters is sometimes higher than that of contactors, and sometimes lower. Science is developing and technology is progressing. Because the online operation soft starter with built-in thyristor bypass uses arc-free technology, it has laid the foundation for the birth of ultra-intelligent, arc-free, high-reliability contactors.