Abstract: With the widespread application of intelligent systems, the requirements for the control methods of electric motors are becoming increasingly stringent. This paper makes a detailed comparison of the selection and starting methods of electric motors, and proves through a design scheme that starting asynchronous motors using electronic soft starters is currently the most reliable starting method. Keywords : soft starter; electric motor; intelligent system [b][align=center]Application of Electrical Motor by Means of Soft Starter FAN Jing[/align][/b] (Department of Mechatronic Engineering, Xi'an University of Arts and Science, Xi'an 710065, China) As a crucial driving actuator, the control method of electric motors is highly valued by technical personnel. It's essential to lay a solid foundation for intelligent control while minimizing the impact on the power grid during motor startup. The advent of electronic soft starters for squirrel-cage induction motors has solved this problem. It alters the motor's starting characteristics, protects the drive system, ensures reliable motor starting, reduces starting shock, and features a computer communication interface for intelligent control. 1. Selection of Motor Starting Method As the most widely used squirrel-cage induction motor, its reduced-voltage starting is used under the following conditions: firstly, the motor cannot withstand the impact torque of full-voltage starting; secondly, the terminal voltage cannot meet the specifications; and thirdly, starting the motor affects the normal operation of other loads. Traditional reduced-voltage starting methods for squirrel-cage motors include Y-Δ starting, autotransformer starting, and reactor starting. These methods all belong to stepped reduced-voltage starting, which has a significant drawback: secondary inrush current occurs during startup. Currently, the most advanced and popular method is the electronic soft starter. The difference between soft starting and traditional reduced-voltage starting methods is: (1) No inrush current. When starting a motor, the soft starter gradually increases the thyristor conduction angle, causing the motor starting current to rise linearly from zero to the set value. (2) Constant current starting. The soft starter can introduce current closed-loop control to keep the motor at a constant current during the starting process, ensuring a smooth start. (3) The starting current can be freely and steplessly adjusted to the optimal value based on the load conditions and the characteristics of the power grid relay protection. So what issues should be considered when using an electronic soft starter? As a soft starter, its starting performance and stopping performance should be considered first. There are currently 5 starting methods for soft starters, as shown in Figure 1. Current-limiting starting limits the starting current of the motor. It is mainly used for light-load starting to reduce the starting voltage drop. However, it is difficult to know the starting voltage drop during startup, so the voltage drop space cannot be fully utilized, resulting in a loss of starting torque, which is detrimental to the motor. Ramp voltage starting involves a linear increase in voltage from low to high. It changes the traditional stepped starting to stepless starting and is mainly used for heavy-load starting. Its disadvantages are low initial torque, a parabolic torque increase which is detrimental to the drive system, and a long starting time which is harmful to the motor. Torque control starting is used for heavy-load starting. It linearly increases the starting torque of the motor from low to high. Its advantages are smooth starting, good flexibility, and better protection of the drive system. Its purpose is to protect the drive system and extend its service life. It also reduces the impact on the power grid during motor startup and is the optimal heavy-load starting method. Its disadvantage is a relatively long starting time. Torque plus jump control starting is similar to torque control starting, but the difference is that a jump torque is used at the moment of startup to overcome the static torque of the motor, and then the torque increases smoothly, shortening the starting time. However, sudden jumps can send spikes to the power grid, interfering with other loads. Voltage-controlled starting is used in light-load starting situations, maximizing the motor's starting torque while ensuring the starting voltage drop, and minimizing starting time, making it the optimal light-load soft-start method. There are three stopping methods: free stop, soft stop, and braking stop. The biggest advantages of electronic soft start are soft stop and braking stop. Soft stop eliminates the anti-inertia shock to the drive system caused by free stop. Braking stop replaces reverse braking stop in certain situations. 2. Working Principle and Main Circuit Diagram of Soft Starter The soft starter uses three anti-parallel thyristors as voltage regulators, connected between the power supply and the motor stator. This circuit is similar to a three-phase fully controlled bridge rectifier circuit, and the main circuit diagram is shown in Figure 2. When starting the motor using a soft starter, the output voltage of the thyristors gradually increases, and the motor gradually accelerates until the thyristors are fully conducting. The motor operates at its rated voltage mechanical characteristics, achieving smooth starting, reducing starting current, and avoiding overcurrent tripping during starting. When the motor reaches the rated speed, the starting process ends. The soft starter automatically replaces the thyristor that has completed its task with the bypass contactor to provide the rated voltage for the normal operation of the motor, thereby reducing the heat loss of the thyristor, extending the service life of the soft starter, improving its working efficiency, and avoiding harmonic pollution of the power grid. The soft starter also provides a soft stop function. The soft stop process is the opposite of the soft start process. The voltage gradually decreases and the speed gradually drops to zero, avoiding the torque impact caused by free stop. The voltage curves of soft start and soft stop are shown in Figures 3 and 4. 3 Application of the soft starter The design adopts a one-to-two scheme, as shown in Figure 5, that is, one soft starter drives two water pumps, which can start and stop the two water pumps in sequence. The main feature of the one-to-two scheme is that it saves one soft starter, reduces investment, and fully reflects the economy and practicality of the scheme. (1) Starting process: First, select one motor and gradually increase the output voltage under the drive of the soft starter according to the selected starting method. After reaching the power frequency voltage, the bypass contactor is connected. Then, the soft starter is disconnected from the circuit to start the next motor. (2) Stopping process: First, start the soft starter and the bypass contactor to run in parallel, then disconnect the bypass, and finally the soft starter gradually reduces the output voltage according to the selected stopping method until it stops. 4 Conclusion Through discussion, the reduced voltage starting method of motors has gone through the “Y-Δ” starter and autotransformer reduced voltage starter to magnetic control soft starter, and now it has developed into electronic soft starter. Therefore, in engineering applications, when the motor cannot meet the requirements of direct starting, the first thing to consider is electronic soft starter. This is a historical stage of technological development and an inevitable stage to lay a good foundation for the systematization of intelligent control systems in the future. [References] [1] Mo Zhengkang. Semiconductor converter technology (2nd edition) [M]. Beijing: Machinery Industry Press, 2005. [2] Shao Quntao. Fundamentals of motors and drives (5th edition) [M]. Beijing: Machinery Industry Press, 2004.