1. Introduction In hydropower plants, with the increasing size of production equipment, the capacity of electric motors (such as oil pumps, fire pumps, and drainage pumps) is growing larger. AC motors, due to their simple structure, reliable operation, convenient maintenance, and low price, are widely used. However, during direct starting, AC motors generate a starting current 5 to 8 times their rated current, causing significant damage to power supply equipment, the motor, and the driven machinery. Simultaneously, the starting torque is only 0.4 to 1.2 times the rated torque (the larger the motor capacity, the smaller the starting torque ratio). For loads with high starting torque (such as ball mills and oil pumps), the motor capacity must be increased to start the motor normally, resulting in a situation where a large motor is used for a small load. Furthermore, when the motor power is large, it causes a significant voltage drop, affecting the safe operation of plant power, especially in the event of an accident, potentially escalating the accident. Therefore, larger capacity motors should use reduced-voltage starting. Traditional reduced-voltage starting methods include YIΔ conversion and autotransformer reduced-voltage starting. The starting control process relies on time relays, resulting in unsatisfactory starting characteristics, large inrush currents during starting transitions, complex starting control equipment, and high costs. Furthermore, the starting torque is directly proportional to the square of the starting current; when the current decreases by 50%, the starting torque decreases by 75%. Moreover, the momentary switching between voltage levels and full voltage application after the reduced-voltage starting process still generates peak currents several times the rated current, affecting the safety of the plant power supply and the lifespan of electrical and mechanical equipment. The advent of power electronic soft starters solves these problems. As a reduced-voltage starting element, it can change the starting characteristics of the motor, protect the driven mechanical equipment, reduce the starting current, protect electrical equipment, and reduce the impact of motor starting on the power supply system, making it an ideal choice for reduced-voltage starting of motors. 2. Working Principle and Structural Characteristics of Magnetic-Controlled High-Voltage Motor Soft Starters In Figure 1, QS is a high-voltage disconnect switch, QF is a vacuum circuit breaker, SR is a magnetically saturated reactor, and M is the motor. Most high-voltage, high-capacity motors have 6 stator outputs. Magnetic-controlled soft starters are derived from reactor-based soft starters. The reactance value in a reactor-based soft starter is fixed. During soft starting, the motor current decreases as the speed increases. If the reactance value is too small, the soft-start inrush current is too large; if it is too large, the motor current is too small during soft starting (especially in the latter half), resulting in insufficient "power" and a prolonged process. Furthermore, the short circuit of the reactor at the end of the soft start can cause a "secondary current surge." Magnetic-controlled soft starters overcome these shortcomings. The main difference between magnetic-controlled soft starters and reactor-based soft starters is that their equivalent reactance value is controllable. The role of the SR (Satellite Reactor) in soft starting: After QS and QF1 begin closing sequentially, the SR has a large reactance value. Subsequently, it is gradually reduced through feedback adjustment, and finally bypassed by QF2 after completion. In a soft starter, the SR is not a linear device; it is essentially a power electronic switch. The difference lies in the fact that the switching on and off of the SR is indirectly controlled by the excitation current of its DC winding. The SR has three pairs of AC windings (two per phase, connected in series with two wires) used for current limiting, connected in series in the stator circuit of the motor. Each AC winding is wound around a U-shaped iron core. The purpose of using two AC windings per phase is twofold: ① to ensure the AC current waveform of the motor does not contain even-order harmonic components. ② to reduce the harmonic potential induced in the DC winding by the AC winding current. Harmonic induced potentials may endanger the safe operation of the excitation circuit. The SR has only one excitation winding, which encases six iron cores, controlling their saturation. The control of the SR is power electronic. Magnetic soft starters can achieve soft stops and possess almost all the functions of thyristor soft starters. The control center of the magnetic soft starter is a PLC with strong anti-interference capabilities, featuring PID regulation, signal generator, integrated protection, and remote communication functions. The PLC receives signals from the current transformer and voltage transformer, performs PID calculations on the deviation, and then controls the DC excitation current and core saturation through a three-phase bridge thyristor rectifier circuit. The R has only one excitation winding, which encloses six cores. The DC excitation current changes the saturation of all six cores. The high-voltage magnetic saturation reactor is not fundamentally different from the low-voltage (380 V) magnetic saturation reactor in principle and structure. However, when the number of turns in the DC control winding is several times that of the AC winding, it is necessary to prevent electromagnetic induction of the DC control winding by the high AC voltage. Therefore, some targeted treatments must be made in terms of withstand voltage and anti-interference. Figure 2 is a top-view cross-sectional view of the saturation reactor. 3 Design of Magnetic Control Soft Starter/Magnetic Saturation Reactor SR In the development of the magnetic control soft starter, the design of the magnetic saturation reactor SR is one of the key and difficult points. The design includes circuit, magnetic circuit, and SR (Regenerative Starter) structure design. The circuit/magnetic circuit design task is to determine the core cross-sectional area and window area of ​​the SR, as well as the number of turns and cross-sectional area of ​​the AC and DC winding conductors, based on the field conditions and process requirements of soft starting. The SR structure design task is to implement the circuit/magnetic circuit design structurally and ensure that the SR meets requirements for withstand voltage, size, and environment. In the SR circuit/magnetic circuit design, the following parameters are crucial: maximum core magnetic flux density BM, current density j, and the ampere-turns ratio of the DC/AC windings, which need to be selected according to the actual situation before design. The mathematical model of the SR needs to be clearly defined in the design. Equating the SR to a controllable switch and a linear inductor series branch, and performing time-domain analysis based on this, is a feasible design method. Therefore, in the design, it is necessary to make relatively accurate predictions of the voltage and current waveforms of the power grid, reactor, and motor at the start of soft starting based on the existing mathematical model. The production of the magnetically controlled soft starter is customized according to user requirements. Before design, the soft starting process is predicted through offline simulation. Successful soft starting is ensured through current closed-loop control. The "microelectronics" in the developed magnetically controlled soft starter uses Siemens' S7 PLC as its core component. The thyristors in the magnetically controlled soft starter operate at low voltage (below 500V), and high- and low-voltage isolation is achieved by the SR (Self-Regulating Thyristor). The SR has strong resistance to harsh environments (temperature, humidity, and altitude tolerance), thus exhibiting high reliability. 4. Practical Results The main powerhouse and No. 12 dam seepage pumps at a hydropower plant operate at a pressure of approximately 0.4 MPa, running several times a day for about 30 minutes each time during the high-water season, under automatic control. During full-pressure (380V) startup, the starting inrush current exceeds 700A, resulting in excessively fast startup and significant noise. After installing the magnetically controlled soft starter, the starting current is limited to twice the rated current, the startup process is smooth, noise is greatly reduced, and equipment vibration is significantly decreased. When the pump stops freely, the sound of water hammer against the check valve is clearly audible, and the check valve and foot valve are frequently damaged. After installing the magnetically controlled soft starter, and setting the motor's braking time, the water pump decelerates slowly and smoothly to zero upon shutdown, eliminating water hammer noise and significantly reducing backflow. This substantially extends the service life of the check valve and foot valve. Although the output waveform of the magnetically controlled soft starter is not ideal during braking, the infrequent start-stop cycles have no adverse impact on equipment safety. 5. Conclusion Using a magnetically controlled soft starter to address electrical, mechanical, and water hammer phenomena during water pump start-up and shutdown offers advantages such as high effectiveness, low cost, and ease of implementation, and it also shows promise for widespread application in other fields.