1 Introduction Soft starter has been used more and more because of its small size, adjustable torque, smooth start, small impact and soft stop function, and has the trend of replacing traditional autotransformer and star angle conversion step-down start. Because soft starter is a newly developed starting equipment in recent years, it lacks corresponding design and use experience. Cage asynchronous motor is the most widely used electrical equipment. Because the impulse current when the motor is directly started is very large, especially when the large-capacity motor is directly started, it will interfere with the power grid and other loads, and even endanger the safe operation of the power grid. Therefore, according to different working conditions, a variety of decompression start-up methods are adopted. The early methods include series reactance or resistance, series autotransformer, star century triangle conversion and so on. Since 1970s, the soft starter made of thyristor AC voltage regulation technology has been popularized in engineering. This kind of soft starter is a new motor control device which integrates motor soft start, soft start, light load energy saving and various protection functions, and is called soft starter abroad. The soft starter is mainly composed of three anti-parallel thyristors connected in series between the power supply and the controlled motor and its electronic control circuit. The working principle of the soft starter is that the control circuit controls the conduction angle of the three-phase parallel thyristor in different ways, so that the input voltage of the motor gradually rises from zero to the end of the start, and the motor is fully charged to realize soft start. In the process of soft start, the starting torque of the motor increases gradually, and the rotating speed also increases gradually. On the above basis, the early soft starter combined with power factor control technology, replaced the analog control circuit with microprocessor, and developed into an intelligent soft starter. This lecture mainly discusses the principle of current soft start technology, its application achievements in various fields of industry and existing problems. 2 Introduction to Soft-start Technology 2. 1 Necessity of Application of Soft-start Technology Motor start-up generally adopts auto-coupling decompression and star-angle conversion decompression, among which star-angle conversion decompression start-up method is the most commonly used, in which the torque of the motor is proportional to the square of the voltage applied to the motor stator. Step-down starting means that the motor reduces the voltage applied to the stator winding of the motor during starting. Assuming that the starting voltage U =0.5Ue and the torque when the motor starts is 0.25Mm, if the voltage U is raised to the rated voltage Ue of the motor at this time, the torque of the motor will jump to Mm from 1/4 at once, so the starting process is jumping and uneven, so it is also called hard starting. Generally, the control technology of step-down start-up has poor reliability and instability, which will impact the power grid and mechanical equipment every time it starts and stops, causing a series of technical problems. For example, in this control mode, the outlet valve of the water pump motor must be closed when it is started, and it can only be started when the load is low, otherwise the switch will trip, which will affect the normal start of the motor. Generally speaking, the traditional starting mode has the following problems: (1) has a great influence on the power grid, which affects the power supply quality of the power grid and requires a large transformer margin; (2) The impact on mechanical equipment is great, which reduces the service life of the equipment; (3) Y？ The switching time of Δ start-up is generally set according to experience, which is not suitable for occasions requiring stable start-up of production process. Soft start is to use voltage regulator to automatically and continuously raise the starting voltage within the specified starting time until it reaches the rated voltage. At this time, the torque of the motor will increase steadily until the torque reaches the maximum value of Mm, and the starting process is over. The soft starter can make the motor start and stop freely, reduce idling and save energy. Soft starter also has the following advantages: ① reducing impact force and prolonging equipment life; ② Choose different starting modes according to different loads to improve acceleration and deceleration characteristics; ③ The protection function is comprehensive; ④ improve the reliability; ⑤ Matching different load objects by modifying parameters; ⑥ Intelligent and can communicate with PLC. 2.2 Classification of soft start technology There are several different ways of soft start technology for motors, such as magnetic control soft start, silicon controlled soft start and hydraulic resistance soft start, among which silicon controlled soft start is the most widely used. Its starting types are: (1) infinite current soft start. At start-up, the start-up current increases with a certain slope until the start-up is completed, during which the start-up current is not limited. This starting mode is generally not used because it does not limit the starting current and has great influence on the power grid, so it is suitable for heavy load starting occasions. (2) Slope constant current soft start. The starting mode is that the starting current of the motor increases gradually at the initial stage of starting, and remains constant when the current reaches the preset value until the starting is completed. During the starting process, the rate of current rise and change can be adjusted and set according to the motor load. When the current rise rate is large, the starting torque is large and the starting time is short. This startup mode is most used, especially the startup of fans and pumps. (3) Pulse impact start. At the beginning of start-up, the thyristor is nearly completely turned on in a short time, and then returns to a smaller turn-on angle for normal constant current soft start. It is suitable for occasions with large static friction moment when starting. (4) Start step by step. Step-by-step starting mode is to make the starting current reach the set value quickly in the shortest time when starting. By adjusting the set value of starting current, the effect of fast starting can be achieved. 2.3 Silicon controlled soft start technology principle Soft start combines power electronics technology, automatic control technology and single chip microcomputer technology, which is a fully digital intelligent starting device specially designed for three-phase asynchronous motors. Its basic principle is to control the start and stop of the motor by controlling the power device, namely the thyristor. Using the working principle of voltage slope, the voltage output to the motor is controlled to rise from the adjustable initial value to the full power supply voltage after the adjustable slope time. Thus, the capacity requirement for the motor power supply is reduced, and the impact on the power supply network and mechanical transmission is reduced. The soft starter adopts three thyristors connected in parallel as voltage regulator, which is connected between the power supply and the motor stator. This circuit is similar to a three-phase fully controlled bridge rectifier circuit. The trigger angle is changed by changing the trigger time of the trigger pulse through the internal single chip microcomputer, and then the terminal voltage applied to the stator winding is adjusted. The starting performance of asynchronous motor mainly has two indexes: starting current multiple and starting torque multiple. The soft starter can reduce the starting current and starting torque by changing the power supply voltage applied to the motor during starting. The traditional starting methods of motor include auto-coupling decompression, Y/△ decompression and so on. Its characteristics are simple control circuit, non-adjustable starting torque, non-adjustable secondary impact current and load impact torque. Soft start can be 50% of standard motor hard start current and 20% of high efficiency motor hard start current. The current-limiting characteristic of soft start can effectively limit the surge current, avoid the impact of unnecessary impact torque and current on the distribution network, and effectively reduce the false triggering action of circuit breakers and contactors; For the motor that starts and stops frequently, it can effectively control the temperature rise of the motor and prolong its life. At present, the widely used and common soft starter in engineering is SCR soft starter. Principle of SCR soft start: Three-phase thyristor is connected in series between three-phase power supply and motor, and the trigger angle is changed by using the principle of SCR phase shift control. At start-up, the terminal voltage of the motor gradually increases with the conduction angle of the thyristor from zero, so that the output voltage can be adjusted and the motor speed gradually increases until the starting torque is met. The output of soft starter is a smooth step-up process (and can have current limiting function) until SCR is completely turned on and the motor works at rated voltage; At this time, the bypass contactor is turned on (to avoid harmonic pollution to the power grid during the operation of the motor and prolong the life of the thyristor), and the motor enters a stable operation state; When stopping, first cut off the bypass contactor, and then gradually reduce the conduction angle of SCR in the soft starter, so that the three-phase power supply voltage is gradually reduced, and the motor speed is gradually reduced from large to zero to complete the stopping process. The design of SCR soft starter adopts the dynamic monitoring technology of current and voltage vector sensing, which does not change the original operating characteristics of the motor; Based on the logic relationship of voltage-controlled oscillator locking three-phase synchronous signal, a SCR trigger system is designed by using phase-locked loop technology and single chip microcomputer, which controls the phase shift of output pulse, and controls the output voltage to be linearly superimposed on the full voltage by detecting the current, thus limiting the starting current and realizing the soft start of the motor. 3 Performance characteristics of soft starter Soft starter uses software control method to start the motor smoothly, and the control method is to control the strength (electricity) with software (components). The control results changed the starting characteristics of the motor from? Work hard? Smooth as? Soft? Smooth, so it's called? Soft start? . There are two kinds of soft start: one is variable frequency constant torque current limiting start; The other is to start with thyristor voltage regulation, also known as intelligent soft start. 3. 1 Comparison of two soft starters ① Technical performance. Using variable frequency speed regulation to start, it has good static and dynamic performance. Even at low speed, the motor torque can be adjusted at will, and the motor can be started with constant torque, and the starting current can be limited below the rated current. With intelligent soft start, the starting torque is very small, because the torque is reduced by the secondary voltage ratio. The soft starter can be started with current feedback or constant current, that is, the starting current remains constant during the starting process until the motor approaches the synchronous speed. Considering the technical performance, the variable frequency speed regulation start-up is suitable for the load with large starting torque, which is generally greater than the occasion, such as reciprocating air compressor, centrifugal separator, loaded conveyor, crusher, screw or rotary air compressor, centrifugal fan, centrifugal pump, conveyor with no-load start-up and various equipment with no-load start-up. 2 economy. The investment cost of starting with frequency converter is twice or even three times that of intelligent soft start. Based on the above technical performance and economy, the latter is a more practical start-up mode for industrial and mining enterprises. 3.2 Intelligent soft starter Intelligent soft starter is mainly composed of three pairs of anti-parallel thyristors connected in series between the power supply and the controlled motor, with the microprocessor as the control core, and the whole starting process is automatically carried out under the control of digital program software. The intelligent soft starter uses the electronic switching characteristics of three pairs of thyristors, and the microprocessor in the starter controls the trigger pulse in the morning and evening to change the trigger delay angle, thus changing the conduction time of the thyristors and finally changing the magnitude of the three-phase voltage applied to the stator winding. The characteristic of stator voltage regulation of asynchronous motor is that the motor torque is approximately proportional to the quadratic square of stator voltage, and the motor current is proportional to stator voltage. Therefore, the limitation of motor starting torque and initial current can be realized by controlling stator voltage. The stator voltage of the motor is controlled by the conduction angle of the thyristor, so different initial phase angles can realize different terminal voltages to meet different load starting characteristics. In the process of starting the motor, the conduction angle of the thyristor increases gradually, and the output voltage of the thyristor also increases gradually. The motor accelerates from zero until the thyristor is completely turned on and the start is completed, thus realizing the stepless smooth start of the motor. The maximum starting torque and starting current of the motor can be set according to the load. 3.3 Technical features and functions of intelligent soft starter 3.3. 1 Basic features of intelligent soft starter ① Microprocessor full digital automatic control and monitoring is adopted. When starting, the starting current rises steadily with a constant slope, and there is no impact current on the power grid, which will not cause a large voltage drop and ensure the stability of the power grid voltage. The starting torque, current, voltage and time can be set according to different loads, and the best current impact and torque control characteristics can be realized, which greatly reduces the impact of motor torque on the load, meets different requirements of different working objects for starting torque, and protects the driven mechanism. (2) The starting of the motor is not affected by the fluctuation of the grid voltage. Due to the introduction of current feedback in the thyristor phase-shifting circuit, the motor keeps a constant current during the starting process and starts smoothly. At the same time, because the starting current is set to a constant value, when the grid voltage fluctuates up and down, the control circuit automatically increases or decreases the thyristor conduction angle to maintain the original set value, which can protect the starting current from changing. Some soft starters also use dual power supply isolation to ensure that the control part is not disturbed by various strong electricity. (3) According to different working objects, the motor can choose a variety of start-stop modes, and different start-up modes have different starting torques. Generally, the initial torque range of motor soft start is selected. Starting from the initial torque, it can be started at the level of starting torque according to the requirements of users. The stator voltage of the motor increases steplessly in the ramp acceleration time, and the ramp acceleration time is set by the user. The motor can be freely soft-stopped, and the soft-stop time can be adjusted. The soft stop function greatly prolongs the life of electrical contacts. ④ Simple structure, light weight, no noise and small floor space. As a contactless control, the service life of soft starter is much longer than that of traditional contactor. If used properly, it can be used for decades, free of maintenance and easy to install, operate and use. The smooth and gradual starting process of soft starter can reduce the vibration and noise of equipment, prolong the life of motor and driven machinery and improve the working environment of workers. ⑤ Various protections such as overcurrent, overload and power failure can be selected to ensure the safety of equipment and motors. Soft start can provide rapid fault diagnosis information for equipment monitoring and protection, such as current limiting, overload, phase failure, rotor stall and so on. The protection setting value can be specified by the user, and the protection performance is reliable. Some soft starters also have the functions of automatic phase sequence identification and phase sequence protection. 6 with standard RS-232C interface and communication function. Intelligent soft starter transmits data through standard interface, which can be integrated into the network to realize decentralized control and centralized management. Its all-digital setting and external control function are very convenient for users and cost-effective. The intelligent soft starter has a friendly man-machine interface, which displays the working voltage, working current and maximum current when working, and displays the fault category when a fault occurs. It has convenient external control interface, digital delay start control, soft stop control input, start time delay relay output, fault relay output and other functions. 3.3.2 Starting characteristics of intelligent soft starter ① Current limiting type: limiting starting current, reducing starting voltage drop, arbitrary adjustment and keyboard setting. ② Voltage control type: set the percentage of allowable voltage drop, automatically measure and limit the voltage drop, adaptively control the starting current by measuring the voltage drop, the debugging data is automatically memorized by the microprocessor, and the operation is automatically monitored by the intelligent program during operation. (3) Torque control type: control the running characteristics of the motor when starting and stopping, protect the motor and starter from overload, protect the transmission machinery, eliminate surge torque and reduce surge current, control the acceleration torque within a given interval and adjust the motor torque according to the application requirements. ④ Torque control plus jump type: If the starting time of torque control is long, the static torque can be overcome by torque jump to speed up the starting cycle. 3.3.3 Intelligent soft starter shutdown features ① Free shutdown: free power outage shutdown, external fault shutdown and programmable self-reset. ② Soft stop: Soft stop (0 ~ 200 s is free stop) is set by itself. The traditional way to stop the motor is to stop it freely, that is, by instantaneous power failure, but many devices such as belt conveyors and elevators are not suitable for sudden stop, and the soft stop function can just meet this requirement. After the thyristor receives the soft stop signal, the conduction angle gradually decreases, and it takes a certain time to transition to full shutdown, that is, the terminal voltage of the motor gradually decreases to zero. Parking time can be set according to actual needs. ③ Brake parking: 0 ~ 60s for self-determination and forced parking.