1. Overview Reduced-voltage starting utilizes a starting device to appropriately lower the power supply voltage before applying it to the stator windings of a motor (cage type) for starting. Once the motor is running, the voltage is then restored to its rated value for normal operation. However, since motor torque is proportional to the square of the voltage, reduced-voltage starting significantly reduces the motor's starting torque, requiring the motor to start under no-load or light-load conditions. Furthermore, when the terminal voltage drops to 65% or even lower than normal, the starting time is excessively long. Additionally, when the motor is short-circuited or disconnected from the starting device to apply full voltage, the sudden voltage change generates a current surge—a large current secondary impact! This is a disadvantage of reduced-voltage starting and one of the reasons we are undertaking technical modifications. Soft starting is essentially also reduced-voltage starting (except for frequency converters), changing the originally immutable impedance to a controllable one. In simple terms, by smoothly changing the series impedance (resistance) value, the motor terminal voltage changes smoothly, achieving smooth motor starting, further reducing the impact of starting current on the equipment, basically eliminating current jumps, softening the motor's starting characteristics, and protecting the motor and mechanical equipment. Currently, our institute's 221 gas source station operates under light load reduced voltage starting, followed by full voltage operation, requiring no speed adjustment during operation. 2. Problems with Current Starting Methods Common reduced voltage starting methods include Y-Δ, resistor, reactor, and autotransformer, which are used at different technological levels and have their own advantages and disadvantages, all achieving the goal of reducing starting current. Currently, the 2500kW motor at the 221 gas source station uses a stator winding series reactor for reduced voltage starting, with a starting current of 4Ie (starting current 1150A) and a starting time of t=18s. The starting current is still relatively large, which has a certain impact on the reliable operation of the motor itself, the power grid, and mechanical equipment, mainly in the following aspects. 2.1 The 2500kW motors at the 221 gas source station have been in use for nearly 30 years. The overall insulation level of the motors has deteriorated, and the excessively high starting current causes a large temperature rise, accelerating motor aging and increasing the possibility of motor failure. In recent years, our unit has experienced several short-circuit accidents at the ends of the motor stator windings, all of which occurred during startup. 2.2 The series reactor reduces the voltage for starting. During startup, the system power factor is low, and the voltage drop across the busbar is still relatively large, which can easily affect the operation of other equipment in the power grid, potentially causing them to trip out of sync. Our institute is supplied by a dedicated line, so this phenomenon has not yet had a significant impact. 2.3 There is a possibility of bearing burnout. On the surface, motor starting and compressor bearing burnout seem unrelated, but in fact, they are related. The compressor bearings at the 221 gas source station use hydrodynamic bearings, which are lubricated by an oil film generated by their own rotation. It generally takes a certain amount of time for a complete lubricating oil film to form. When a motor starts, the starting current is large, and the corresponding starting time is also fast. If it starts rotating at high speed before the bearing has a chance to form a lubricating oil film, this situation can easily cause the bearing to break. To better solve the above problems, we introduced a soft starter device and used soft starter to technically upgrade six 2500kW motors at the 221 gas source station. Currently, the high-voltage soft starter methods we are familiar with mainly include frequency conversion soft starter, series variable resistor (thermal resistance, liquid resistance), and series adjustable reactor (magnetic control). 3 Introduction and Comparison of Frequency Conversion, Resistance, and Reactor Soft Starters 3.1 High-Voltage Frequency Conversion Soft Starter The principle of high-voltage frequency conversion starting and speed regulation has actually been mature for a long time, but due to manufacturing bottlenecks, it has only gradually been applied in the last twenty years, enabling soft starting and speed regulation. High-voltage frequency conversion represents the development direction of soft starter technology for large motors and has achieved great success in recent years. Compared with magnetic control soft starter, thermal resistance, and other starting schemes, it has obvious technological progress, which is undeniable. 3.2 High-Voltage Resistor Soft Starters The earliest step-down starting methods used solid resistors. However, due to the unavoidable drawbacks of solid resistors (such as low thermal capacitance), their use in step-down starting of high-voltage power equipment was limited. With technological advancements, frequency-sensitive solid resistors were applied in both low-voltage and high-voltage power equipment. In the early 1980s, liquid resistors and thermocouples appeared in high-voltage power equipment. Liquid resistors possess higher thermal capacitance. Liquid soft starters control the transmission mechanism through a current closed-loop automatic control unit, with the motor dragging the plates to change the inter-electrode resistance value to achieve soft starting. Thermocouple starting devices utilize an electrolytic liquid with negative temperature characteristics, changing the resistance value with temperature variations to achieve soft starting. Liquid soft starters have good control functions, similar to magnetic soft starters. However, their disadvantages include complex control and transmission mechanisms, multiple failure points, the need for regular inspection of the liquid resistor, alternating primary and secondary power supplies, and high insulation requirements. Compared to liquid resistors, thermocouples have a simpler structure, better starting characteristics (manufacturer-provided characteristic curves for comparison), lower maintenance requirements, long-term safety and reliability, and are suitable for larger motor capacities. Therefore, the following section will only use thermal resistance starting devices as an example. 3.3 High-voltage reactor-type soft starters In the early high-voltage step-down starting, my country mostly used reactor step-down starting. Traditional reactors have disadvantages such as non-adjustable impedance, poor starting characteristics, and low power factor, and are now rarely used in China. Magnetic control is an improvement on the above. By adding a control winding to the reactor, and using electro-magnetic control technology, an external automatic control unit adjusts the current in the control winding, and controls the permeability to regulate the voltage, thereby changing the excitation to achieve soft starting of the motor. During the starting process, the voltage (current) across the reactor is automatically adjusted according to the starting current, changing steplessly from large to small, so that the motor terminal voltage smoothly rises to the rated value. Magnetic control can theoretically adjust speed, but in practical applications, this is reflected in the starting process. Once the optimal starting is achieved, the starting current is not adjusted. 3.4 Comparison of Characteristics of Three Types of Soft Starters 3.4.1 Technical Comparison From a purely technical perspective, high-voltage frequency converters have unparalleled advantages: good starting characteristics, capable of multiple consecutive starts, starting current controllable below rated current, high grid power factor (0.9–0.95) during startup, low grid voltage drop, and speed regulation, reducing power consumption of equipment and saving energy. The disadvantage is the generation of high magnetic harmonics, polluting the grid and affecting the power quality of other equipment in the system. Addressing harmonic pollution requires additional equipment investment. Reactor-based (magnetically controlled) soft starters offer more flexible control and easier setting of starting current. The disadvantage is that the already low power factor of the motor during startup is further reduced by the series reactor, offering little benefit to the power system, resulting in a large bus voltage drop and some harmonic pollution. High-voltage thermal resistor soft starters have less impact on motors and mechanical equipment. Compared with reactors, it has better performance, a higher power factor (above 0.7) during startup, smaller voltage drop across the starting grid, no harmonic pollution, and its service life can be extended by replacing the electrolyte. The disadvantage is that the resistance is slightly affected by temperature. See Table 1 for a detailed comparison. [align=center]Table 1: Performance Comparison of Variable Frequency Soft Starter, High Voltage Thermocouple Soft Starter, and Magnetic Control Soft Starter[/align] Notes: 1. Data in this table is referenced from Xiangfan Dali, Robicon (USA), Wuhan Kerui, and some other sources. 2. Items marked with "*" require further confirmation. 3.4.2 Economic Comparison From a practical economic perspective, high voltage variable frequency starter is an overly extravagant technical solution. Although variable frequency starter can reduce the starting current below the rated current, for large power equipment that does not start frequently and does not require speed regulation, investing huge sums just for starting is too uneconomical. The price of high-voltage thermal resistor soft starters and magnetically controlled soft starters is 1/8 to 1/10 that of high-voltage frequency converter starters. For our institute, using one of these two can save several million yuan in investment. 3.4.3 Reliability Comparison When the equipment operating conditions are low and several devices meet the requirements, reliability becomes relatively important, excluding price. Choosing equipment with poor quality and low reliability will inevitably have a negative impact on our institute's future scientific research and production. We know that, without considering the influence of product quality and other factors, from a purely technical perspective, equipment with simple structure, easy use, and convenient operation is easier to maintain in case of failure. Compared with frequency converters and magnetically controlled soft starters, high-voltage thermal resistor soft starters have a simpler structure and fewer bypass systems, and should have certain advantages in terms of reliability. However, it is, after all, a new product (like magnetically controlled starters), and its reliability should be understood from the user's perspective. High-voltage frequency converter soft starters have high technical content, complex equipment, and greater technical difficulty. Their use, maintenance, and fault handling require high technical skills from technicians. When a fault occurs, the technical difficulty of solving the problem is high, and the accident handling cycle is long. The biggest advantage of magnetically controlled soft starters is that they can still be used as ordinary reactors even if the external circuit fails; this is a consideration in fault situations. 4. Conclusion When selecting a starting method, under current technological levels, the principle is to protect the motor, extend its service life, protect the power grid and mechanical equipment, reduce equipment maintenance and management workload, and ensure the reliability of the selected equipment. The selection of a product should be a systematic project, combining various factors, considering both the equipment's advanced features and price; both price and product quality. Similarly, the starting method of large motors is closely related to equipment management. An inappropriate selection of the motor starting method can negatively impact the power system, the motor itself, and even the mechanical equipment, increasing the difficulty of future equipment management and maintenance. This article strives for rigor, but due to limitations in technical expertise and perspective, some arguments may be biased or even erroneous; your corrections are welcome.