1, direct starting. However, the three-phase asynchronous motor can reach 6-7 times the rated current when it starts directly, and it will have a greater impact on the power grid, especially the high-power motor.

　　2, frequency sensitive resistance starting. Frequency sensitive resistance starting means that the frequency sensitive resistor is connected in the main path when the motor starts, thereby reducing the starting current. The frequency sensitive resistor can smoothly change the starting current and has less impact on the power grid, which is an ideal starting mode. But at present, high-power frequency sensitive resistors are all in the form of inductors, so when they are used, they will produce large eddy currents, which will reduce the power factor of the power grid.

　　3, depressurization starting. The step-down start is mainly based on the thermal self coupling starting and the star delta starting. The thermal self coupling starting is to reduce the motor voltage and reduce the starting current by starting the lotus root transformer. It is generally reduced to about 55%-75% of the rated voltage. The advantage is that the starting voltage can be easily changed by changing the number of taps of the self coupling transformer. The disadvantage is the need for self lotus transformer, the cost is large. The starter starting is a method of changing the starting voltage by changing the connection mode of the motor and reducing the starting current. It can only be applied to the motor with the normal connection mode as the triangle connection. When starting, the relay method is used to make the connection mode of the motor star. At this time, each phase voltage of the motor is reduced to 1/3 of the original root number, and the motor speed reaches about 80% of the rated speed. The control relay changes the motor connection mode to the triangle, and the motor begins to run normally. The advantages are that the transformer can be saved and the cost is reduced, and the wiring method is simple and the reliability is relatively large. The disadvantage is that the ratio of starting voltage can not be changed and the star connected motor can not be used.