Direct start is hard start. The starting current of hard start (direct start) is 3-7 times of the rated current of the motor. In the case of direct start (hard start), the situation of exceeding the rated current of the motor brings many restricted problems to the manufacturing process and structure of the motor itself.
The reason why the shaft of the motor is very thick is actually because there was no soft start in the past, and the sudden overload of 5-6 times the starting impact torque of the motor brought by the hard start will break the motor shaft. This is one of the reasons why the motor shaft is designed to be thick. For low-power motors, although the current is large when starting directly, the shock torque at startup is very large for the motor, but the mechanical strength and shock resistance can still be tolerated. For high-power motors, there is a problem. The overload shock caused by the startup, the strength and capacity design of the machine and electricity are all very difficult. And cause a lot of additional costs.
Because of this, people started brainstorming to solve this problem and invented soft start. Soft start, as the name suggests, is not to start directly, but to start slowly and little by little. For example, at the input end of the motor, the voltage is gradually increased from 0 to the rated voltage, and the frequency is gradually changed from 0 to the rated frequency, so that the starting current of the motor during the starting process is changed from the uncontrollable overload impulse current in the past. It becomes a controllable starting current that can be adjusted as needed. There is no impact torque in the whole process of starting the motor, but it starts and runs smoothly. This is the so-called soft start of the electric motor.
There are two ways of soft start now, one is to use a special soft starter to achieve soft start; the other is to use inverter control to achieve soft start. And the old method of traditional soft start is rarely used. Therefore, variable frequency start can also be said to be a kind of soft start, but the application of variable frequency is too wide, and the frequency can be adjusted when the motor is running normally, so it can be divided.
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