Why do some compressors have to soft start

Why do some compressors have to soft start

We know that the motor's direct starting current is 4 to 7 times the rated current, so if the power of the motor is relatively small, then the impact on the grid is not great, we allow it to start directly, generally the motor power is allowed below 7.5KM Started directly.

Why does the motor soft start?

But for a large motor, the power is particularly large, so it is not allowed to start directly. If its rated current is 100A, then the instantaneous current can reach hundreds of amperes when it starts. Such a large current has a very large impact on the grid. Larger ones will affect the normal operation of other loads on the same grid. We often see this situation. For example, the light bulb will dim when a large load starts. This situation sometimes occurs in our homes. For example, when the TV is turned on, the light bulb may suddenly dim, and the starting current of the load is too large, which is extremely unfavorable to the insulation of the motor winding.

　　 Therefore, soft start is generally required for large motors. The so-called soft start means that the voltage we add to him at the beginning of the start is lower than the rated voltage.

Common soft starters include soft starter start and autotransformer step-down start. The autotransformer step-down starter adds a voltage of 60% to 80% of the rated voltage at the beginning of the motor startup, and then lets it move. When it is turned up, switch him to work at the rated voltage. This is a simple soft start process. The soft starter is more advanced than the autotransformer step-down start. The principle is the same, soft start When the voltage supplied to the motor by the converter gradually changes from 0 to the rated voltage, its starting process is smoother, the starting effect is better, the impact on the grid and the damage to the windings are also minimal, which is better than the 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 today's transmission engineering. In many occasions, these motors are directly connected to the power system due to their starting characteristics. If it is started directly online, an inrush current up to 6 times the rated current of the motor will be generated. This current will overload the power supply system and series-connected switchgear. If it is started directly, it will also produce a higher peak torque. This impact will not only impact the drive motor, but also damage the mechanical device. For example: auxiliary power transmission part (V-belt, gear). In order to reduce the direct current, starting auxiliary devices should be used, such as starting reactors or autotransformers. However, conventional methods such as using reactors or autotransformers for starting can only gradually reduce the voltage, while soft starters use smoothly raising terminals Voltage, can achieve no impact start.

　　 Therefore, the power supply system and the motor can be protected optimally.

　　 1. Reduce the starting current of the motor, reduce the power distribution capacity, and avoid the investment in capacity expansion;

　　 2. Reduce starting stress and extend the service life of motors and related equipment;

　　3. Smooth start and soft stop avoid the surge problem and water hammer effect of traditional starting equipment;

　　4. Various starting modes and wide range of current and voltage settings can be adapted to various load conditions;

　　6. ​​It can start/stop the motor smoothly without a step, avoiding mechanical and electrical shocks caused by traditional starting methods such as direct starting, star/delta starting, auto-decompression starting, etc.;

　　7. It can effectively reduce the starting current and power distribution capacity, avoiding investment in capacity expansion.