We know that the direct starting current of the motor is 4 to 7 times the rated current, so if the power of the motor is relatively small, then the impact on the power grid is not very large, we allow it to start directly, the general motor power below 7.5KW is allowed to start directly.
However, for large motors, the power is particularly large, then it is not allowed to start directly, if his rated current is 100A, then he can reach several hundred amperes at the start of the instant current, such a large current impact on the power grid is very large, will affect the normal operation of the same grid with other loads.

This situation we can often see, For example, a large load starts the moment the light bulb will darken, etc., we sometimes have this situation at home, for example, when the TV is turned on, the light bulb sometimes suddenly darkens, but also because the starting current of the load is too large, the insulation of the motor winding is also extremely unfavorable.

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