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. Frequency sensitive resistance starting

Frequency-sensitive resistor starting means that the frequency-sensitive resistor is connected in series in the main circuit when the motor is started, thereby reducing the starting current. Frequency-sensitive resistor can smoothly change the starting current, the impact on the grid is small, is an ideal way to start. However, high-power frequency-sensitive resistors are all in the form of inductors, so large electromagnetic eddy currents will be generated during use, which will reduce the power factor of the power grid.

3. Step-down starting

The step-down starting mainly includes hot self-coupling step-down starting and star-delta step-down starting.

Hot self-coupling step-down starting refers to reducing the motor voltage and starting current through the auto-coupling transformer during starting. 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 autotransformer. The disadvantage is that it is necessary to use the autotransformer, the cost is larger.

Star-delta step-down starting refers to a method that changes the starting voltage by changing the wiring mode of the motor, thereby reducing the starting current. It can only be applied to motors with normal wiring mode as triangle connection. When starting, the relay method is used to make the motor connection mode star. At this time, the voltage of each phase of the motor is reduced to the original root 1/3, and the motor speed reaches about 80% of the rated speed. The control relay changes the motor connection mode to triangle, and the motor starts Normal operation. The advantage is that it can save the auto-transformer, reduce the cost, and the wiring method is simple and the reliability is large. The disadvantage is that the ratio of the starting voltage cannot be changed, and the motor cannot be used in the star connection.