1. Direct starting. However, when the three-phase asynchronous motor starts directly, the current can reach 6-7 times of the rated current, which has a great impact on the power grid, especially for high-power motors.

　　2. Start with frequency sensitive resistor. Frequency sensitive resistor starting means that when the motor starts, the frequency sensitive resistor is connected in series in the main circuit, and then the starting current drops. The frequency sensitive resistor can change the starting current smoothly and has less impact on the power grid. However, at present, high-power frequency-sensitive resistors are in the form of inductors, so large electromagnetic vortexes will occur when they are used, which will reduce the power factor of the power grid.

　　3. Step-down starting. Step-down starting mainly includes hot-lotus root step-down starting and star-delta step-down starting. Step-down starting with the lotus root is to drop the motor voltage and the starting current together when starting with the lotus root transformer. Generally, the voltage drops to about 55%-75% of the rated voltage. The advantage is that the starting voltage can be easily changed by changing the tap winding number of the lotus root transformer. The defect is the need to use the lotus root transformer, the cost is large. Star-delta step-down starting is a method that changes the starting voltage and then drops the starting current by changing the wiring method of the motor. It can only be applied to the motor whose normal wiring method is triangle connection. At the time of starting, the relay method is used to make the motor wiring method star, at this time, the voltage of each phase of the motor drops to one third of the original square root, the motor speed reaches about 80% of the rated speed, the control relay changes the motor wiring method to triangle, and the motor starts normal operation. Advantage is to be able to save the lotus root transformer, lower costs, a simple wiring method, greater reliability. The defect is that the ratio of starting voltage cannot be changed and cannot be applied to a star connected motor together.