There are three most common starting modes of asynchronous motor: star triangle step-down start, auto-coupling transformer step-down start, and soft start. (a frequency converter is the best protection of the motor with additional cost)

Compare the advantages and disadvantages of these three methods

1. Star triangle step-down startup

Advantages: low cost, simple circuit, considering the length of the main line cable, the main circuit of the motor is six lines, more than three cable lines cost.

Disadvantages: not suitable for big power motor, generally lower than 55KW motor, and must be triangular connection motor, if the operation is a star connection motor, coil rated voltage 220V, connected to the triangle withstand 380V voltage, coil will burn. starting torque is small; limited to light load start.

2. Step-down start of the auto-coupling transformer

Advantages: star or triangular wiring, can meet load start, big power motor can also be applied.

Disadvantages: additional an independent transformer, equipment volume is big size and cost more.

3. Soft start

Advantages: Output torque can generally meet the requirements of starting torque, smooth acceleration, smooth transition, avoid the impact on the motor and power grid; set soft start, soft parking, phase deficiency protection function, overheating protection, etc. The motor with internal contracting lock can set the soft start parameters to achieve the perfect coordination with the lock.

Disadvantages: more cost. And in particular attention is that, some built-in bypass soft start, only two bypass contacts, the middle of a phase remain connect through, such as one of Siemens soft start. In this case, if the power switch is not pulled down, even if the shutdown state, the motor and cable are always charged. Therefore, it is strictly prohibited to open the motor junction box.

Therefore, to choose which way to start, it depends on your focus on the consideration, such as: cost, equipment operation stability, protection performance and so on.