Abstract: This article discusses the development, benefits, and applications of soft-start technology for medium and high voltage motors, starting from the current challenges in power electronics technology. It focuses on the advantages of motor soft starting and its future development direction. Keywords: Thyristor , Silicon Controlled Rectifier (SCR) I. Introduction Power electronics technology is an electronic technology applied to the field of electronics. It is a discipline closely related to electronics, control, and power. The invention of the thyristor in 1957 brought about a revolution in power electronics, leading to its rapid development. A thyristor is short for crystal thyristor, also known as a silicon controlled rectifier (SCR). Its characteristic is that the turn-on time can be controlled, and it turns off when the current crosses zero, making it a semi-controlled device. At the beginning of its invention, the thyristor was widely welcomed because its performance significantly surpassed that of the previous mercury arc rectifier, thus ushering in a new era of rapid development in power electronics technology. Since the 1980s, some applications of thyristors have begun to be replaced by various fully controlled devices with better performance. However, because its voltage and current capacity is still the highest among current power devices, it still holds an important position in high-capacity applications. Its main features are as follows: (1) High withstand voltage: There are currently products with a voltage of 8500V. (2) Large current: There are currently products with a voltage of 5000A. (3) Low voltage drop: The voltage drop of the tube is <2V when it is conducting. (4) Low power loss and long life. (5) Strong ability to withstand instantaneous overcurrent impact. II. Common starting methods for medium voltage (6~10 kV) motors . AC motors are the most widely used motors in various fields. In order to solve the impact of AC motors on the power grid and machinery during the starting process, people have adopted many methods. Traditional methods include series resistance starting, series reactor starting, star-delta conversion starting, autotransformer starting, frequency conversion starting, etc. The soft starter device for motors controlled by thyristors appeared in the 19th century. At that time, an engineer named Frank from NASA obtained a patent called "Power Cycle Controller". It uses anti-parallel connection of thyristors and adjusts the conduction angle of the thyristors to achieve the purpose of AC voltage regulation. This technology was applied to solve the problem of low power factor of AC asynchronous motors under no-load and light-load conditions. Later, current feedback technology was introduced, which greatly improved the control level and was widely adopted. The United States named the device using this technology "energy saver". III. Advantages of Soft Starting of Motors (1) It can reduce the impact on the power grid and reduce the capacity of the transformer. When a common squirrel-cage motor is started directly under no-load full voltage, the starting current will reach 5 to 7 times the rated current. When the motor capacity is relatively large, this starting current will cause the grid voltage to drop sharply, which will disrupt the normal operation of other equipment on the same grid and may even cause the grid to lose stability and cause a bigger accident. Therefore, it is generally required that the voltage fluctuation caused by frequently started motors should not exceed 10%; the voltage fluctuation caused by occasionally started motors should not exceed 15%. After using soft starting, the starting current can be reduced to 1.5 to 3 times the rated current, which can greatly reduce the voltage fluctuation rate of the power grid. For motors powered by a separate transformer, when using direct full voltage starting, the capacity of the motor should not exceed 80% of the transformer capacity. When using soft starting, the capacity of the transformer can be the same as the apparent power of the motor (generally S=1.1PN). The equivalent circuit of the transformer-motor unit is shown in Figure 1. U0 is the infinite power grid. U1 is the motor terminal voltage. XT is the short-circuit impedance of the transformer. Under rated conditions, when the motor current equals the transformer's rated current, the voltage drop across XT is the transformer's short-circuit voltage value UK. If the maximum current of the motor during soft start is 3IN, then the voltage drop across XT is 3UK. For small and medium-sized power transformers, UK = (4~10.5)%UN; for large transformers, UK = (12.5~17.5)%UN. Under normal circumstances, as long as the terminal voltage of the motor at startup is not lower than 65% of the rated voltage (at maximum current), it can start successfully. [align=center] Figure 1[/align] Under rated operation, I = IN, U0 - U1 = UK, U1 = UN. If calculated according to the extreme case: UK = 17.5%; I = 3IN, then: U1 = U0 - 3UK = UN - 2 × 17.5%UN = 65%UN. This also meets the starting requirements. In other cases, U1 will definitely be greater than 65%. It is not a problem. For the case where the motor and other loads share the transformer, it is generally required that the capacity of the motor that is frequently started should not be greater than 20% of the transformer capacity; the capacity of the motor that is occasionally started should not be greater than 30% of the transformer capacity. If Imax=6IN when starting directly at full voltage and Imax=3IN when starting softly, then considering the equal current and line voltage drop, the capacity of the motor that is frequently started can reach 40% of the transformer capacity; the capacity of the motor that is occasionally started can reach 60% of the transformer capacity. (2) It can reduce the damage to the motor and extend the motor life. 1. The large current when the motor starts directly at full voltage generates a large impact force on the stator coil and the rotor squirrel cage bar, which increases the wear of the stator coil (especially the end) and the iron core, and will damage the winding insulation; the impact force on the squirrel cage bar is also easy to cause breakage, causing motor failure. The magnitude of the electric force is proportional to the square of the current. The electric force when starting directly at full voltage is 36 times the electric force when running normally at rated speed (according to Imax=6IN). 1. The electric force during soft starting is nine times the electric force during normal rated operation. The effect is clearly significant. 2. The large current during direct full-voltage starting of a motor generates a large amount of Joule heat in the stator and rotor windings. This burns the winding insulation and reduces the motor's lifespan. Soft starting can greatly reduce heat generation, thus increasing motor lifespan. High heat is generated at the maximum current. Joule heat is proportional to the square of the current, therefore the concentrated high heat is reduced by a factor of two. Generally, soft starting is done under no-load or light-load conditions, with the maximum current often around 2IN. At this point, the high heat is only generated during direct starting. This undoubtedly greatly benefits extending the motor's lifespan. 3. When a motor is started directly at full voltage, the rated voltage is instantaneously applied to the motor windings. This generates operational overvoltage, which, in the worst-case scenario, can reach five times the rated voltage, causing significant damage to the motor insulation. Many motor failures occur during closing due to operational overvoltage. (3) It can reduce damage to machinery and extend the service life of machinery. The maximum torque of the motor when starting directly at full voltage is about twice the rated torque. For gear transmission equipment, a large impact force will accelerate the wear of gears or even break them. For belt transmission equipment, it will increase the wear of belts or even break them. For water pump equipment, it will produce the "water hammer effect" and damage the pipes and impellers. When starting softly, the motor accelerates slowly and the torque gradually increases, completely eliminating the above-mentioned hazards. When starting softly, the motor speed rises slowly, which is conducive to the full delivery of lubricating grease and also eliminates the dry grinding phenomenon. All of these greatly reduce the damage to the motor and help improve the service life of machinery. IV. Problems of Power Electronics Technology (1) The single-machine capacity of electrical loads is getting larger and larger, and the capacity of power electronic devices is also getting larger and larger. The requirements for the voltage capacity and current capacity of power electronic devices are also getting larger and larger. This is a difficult problem in current power electronics technology. At present, people can only solve this problem by using series and parallel technology, but reliability has been a problem that hinders the large-scale application of series and parallel technology. If the voltage and current capacity of power electronic devices can be solved, a very bright prospect will be presented to people: the uncontrollable problem of AC power grid will be solved, freeing people from complicated calculations and panic about accidents; all electrical equipment will work in an orderly manner: no impact, no negative sequence, no harmonics... (2) Series and parallel application For larger power electronic devices, when the voltage or current rating of a single power electronic device cannot meet the requirements, it is often necessary to connect the power electronic devices in series or in parallel. [align=center] a) Difference in volt-ampere characteristics b) Difference in series voltage equalization Figure 2[/align] 2.1 Series connection of thyristors When the rated voltage of a thyristor is less than the actual requirement, two or more devices of the same type can be connected in series. Ideally, each device should bear the same voltage, but in reality, due to the differences in device characteristics, there will generally be a problem of uneven voltage distribution. The leakage current flowing through the devices in series is always the same, but due to the dispersion of static volt-ampere characteristics, the voltage borne by each device is not equal. Figure 2a shows that when two thyristors are connected in series, the forward voltages they withstand under the same leakage current IR are different. If the applied voltage continues to increase, the device with the higher voltage will reach its break-through voltage first and conduct, causing the other device to bear the full voltage and also conduct, resulting in both devices losing their control function. Similarly, in reverse, due to the different volt-ampere characteristics, the voltage is uneven, which may cause one device to break down in reverse first, followed by the other. This voltage equalization problem caused by the difference in the static characteristics of the devices is called the static voltage imbalance problem. To achieve static voltage equalization, devices with parameters and characteristics that are as consistent as possible should be selected. In addition, resistor voltage equalization can be used, as shown by RP in Figure 2b. The resistance of RP should be much smaller than the forward and reverse resistance when either device is blocked, so that the voltage shared by each thyristor depends on the voltage division of the resistor. Similarly, the voltage imbalance problem caused by the difference in the dynamic parameters and characteristics of the devices is called the dynamic voltage imbalance problem. To achieve dynamic voltage equalization, devices with dynamic parameters and characteristics that are as consistent as possible should also be selected. In addition, RC parallel branches can be used for dynamic voltage equalization, as shown in Figure 2b. For thyristors, using gate strong pulse triggering can significantly reduce the difference in device turn-on time. 2.2 Parallel Connection of Thyristors In high-power thyristor devices, multiple devices are often connected in parallel to handle large currents. When thyristors are connected in parallel, uneven current distribution can occur due to differences in static and dynamic characteristic parameters. Poor current sharing leads to insufficient current in some devices and overload in others, hindering the improvement of the overall device output and even causing damage to the devices and the device. The primary measure for current sharing is to select devices with characteristic parameters that are as consistent as possible. In addition, current-sharing reactors can be used. Similarly, using gate strong pulse triggering also helps with dynamic current sharing. When it is necessary to connect thyristors in series and parallel simultaneously, the method of connecting in series first and then in parallel is usually adopted. Comparatively, even with redundant design, series connection is riskier than parallel connection. Therefore, when designing circuits, if both methods can achieve the requirements through circuit transformation, parallel connection should be preferred, as current sharing is relatively easier to achieve. V. Comparison of the performance of soft starters Currently, there are two main types of medium and high voltage soft starter products at home and abroad: one is soft starter using high voltage frequency converters and the other is soft starter using thyristors. Here, I will briefly introduce the performance of the two devices. (1) Soft starter using high voltage frequency converters Frequency converter devices are mainly used for speed regulation of AC motors and have obvious energy-saving effects. If it is used for soft starting of motors, its speed regulation performance should not be compared with our company's products. When the frequency converter is used for soft starting, the motor will not have overcurrent during the entire starting process, the starting torque is large, and it has good starting performance. However, for loads such as fans and pumps with small starting torque, this advantage of the frequency converter is not apparent. At the same time, there are also the following shortcomings: 1. Frequency conversion technology is still in the development stage. Due to the relatively large switching losses, the reliability is relatively low and the failure rate is relatively high. It is a maintainable device. 1. Various units often experience prolonged downtime due to inadequate maintenance technology. For example, a steel company once experienced a 35,000 KW blast furnace blower that took over a week to start. There are also cases where the equipment is left unused. 2. The high harmonic content in the output voltage of frequency converters can cause localized overheating in the motor's gear slots, burning insulation and affecting the motor's lifespan. This is why specially designed frequency converter motors are required for speed regulation applications. 3. When using frequency converters for soft starting, a good synchronization function is essential when switching from the frequency converter power supply to the mains frequency power supply to reach sub-synchronous speed (some frequency converters lack this function). Otherwise, mechanical shock will occur, damaging the machinery. 4. Frequency converters are expensive. 5. The circuit principles of frequency converters are complex, requiring a high level of maintenance expertise and resulting in long maintenance times. (2) Thyristor soft starter: There are two topologies for this method. One is to directly connect the thyristors in series, and the other is to use switching transformer technology. The main circuit forms of these two methods are as follows: [align=center] (a) (b) Figure 3[/align] The circuit in Figure 3a is clear and simple, but there are problems with component parameter consistency and output harmonics. Although it has the advantage of smaller footprint, its safety and stability are questionable. The circuit in Figure 3b is also clear. Compared with a, it has a slightly larger footprint, but there are no requirements for parameter consistency or output harmonic pollution. It is safe and stable. In my opinion, given the current state of power electronic devices, b has a greater technical advantage. However, with the continuous upgrading and improvement of power electronic technology, b will eventually be replaced by a in the field of soft starter. VI. Conclusion Soft starters for medium-voltage motors are now widely used in metallurgy, petrochemicals, pharmaceuticals, municipal engineering and other fields. There are many defects in the traditional starting scheme, and the problems are even more prominent for large-capacity equipment. With the continuous application of soft starters in production practice, choosing soft starter motors will be an inevitable trend in the future.