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 refers to the starting of the motor in series frequency sensitive resistor in the main circuit, thus reducing the starting current. The frequency - sensitive resistor can change the starting current smoothly and has little impact on the power grid. However, at present, high-power frequency-sensitive resistors are all in the form of inductors, so there will be a large electromagnetic eddy current when they are used, which will reduce the power factor of the power grid.
3. Step-down starting. Step-down starting mainly consists of hot-lotus root step-down starting and star-delta step-down starting. Step down starting with the lotus root is to reduce the motor voltage and the starting current when starting with the lotus root transformer. Generally reduced 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. Disadvantage is the need to use the lotus root transformer, the cost is large. Star-delta step-down starting refers to a method that changes the starting voltage and thus reduces the starting current by changing the connection mode of the motor. It can only be applied to the motor whose normal connection mode is triangle connection. At this time, the voltage of each phase of the motor is reduced to one third of the original square root. The motor speed reaches about 80% of the rated speed. When the control relay changes the motor connection mode to triangle, the motor starts to run normally. The advantage is can save the lotus root transformer, reduce the cost, at the same time the wiring method is simple, the reliability is bigger. The disadvantage is that the ratio of starting voltage cannot be changed and the motor cannot be used in star connection.