We know that the direct starting current of an electric motor is 4 to 7 times its rated current. If the motor's power is relatively small, the impact on the power grid is not significant, so we allow it to start directly. Generally, motors with power below 7.5 kW are allowed to start directly. However, for large motors with extremely high power, direct starting is not permitted. If its rated current is 100A, the instantaneous starting current can reach several hundred amps. Such a large current has a very large impact on the power grid and can affect the normal operation of other loads connected to the same grid. We often see this situation; for example, the bulb dims the instant a large load starts. We sometimes experience this at home, such as when the television bulb suddenly dims when turned on, also due to the starting current of the load. Too high a voltage is extremely detrimental to the insulation of the motor windings. Therefore, large motors generally require soft starting. Soft starting means applying a voltage lower than the rated voltage at the initial startup. Common soft starting methods include soft starters and autotransformer step-down starting. Autotransformer step-down starting initially applies a voltage of 60% to 80% of the rated voltage to get the motor moving. Once it's running, it switches to the rated voltage. This is a simple soft starting process. Soft starters are more advanced than autotransformer step-down starting, although the principle is the same. A soft starter gradually applies voltage to the motor from 0 to the rated voltage, resulting in a smoother startup process, better starting performance, and minimal impact on the power grid and damage to the windings—superior to autotransformer step-down starting! This is the advantage of soft starters in industrial applications!
Three-phase AC induction motors are the most commonly used in modern transmission engineering. In many cases, due to their starting characteristics, these motors are directly connected to the power supply system. Direct online starting will generate inrush currents up to six times the motor's rated current. This current can overload the power supply system and series-connected switching equipment. Direct starting also generates high peak torque, which can impact not only the drive motor but also mechanical components such as auxiliary power transmission parts (V-belts, gears). To reduce direct current, starting aids such as starting reactors or autotransformers should be used. However, conventional methods like starting with reactors or autotransformers only gradually reduce the voltage, while soft starters achieve shock-free starting by smoothly increasing the terminal voltage. Therefore, they provide optimal protection for both the power supply system and the motor.
A soft starter is a novel motor control device that integrates soft starting, soft stopping, light-load energy saving, and multiple protection functions. Its main components are three anti-parallel thyristors connected in series between the power supply and the controlled motor, along with their electronic control circuitry. By controlling the conduction angle of the three anti-parallel thyristors using different methods, the input voltage of the controlled motor can be varied according to different requirements, thus achieving different functions. (Source: Electrical Engineering Home)
In essence, a soft starter functions as a voltage regulator, changing only the voltage and not the frequency during motor starting. It is easy to install and adjust, with all control connections and parameter adjustments made on the front panel. Users can easily modify the soft starter on-site after installation, such as adding current limiting functions and internal/external connection selection. The soft starter can operate continuously online without a bypass. Separate control relays are provided for bypass and fault conditions. All parameters are set intuitively and accurately via three rotary potentiometers and one DIP switch on the front panel. It can be widely used in industries such as textiles, metallurgy, petrochemicals, water treatment, shipbuilding, transportation, pharmaceuticals, food processing, mining, and machinery.
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