Starting of three-phase asynchronous motors
Apr 03,2024
1、Direct starting
However, the current of three-phase asynchronous motor can reach 6-7 times of the rated current when starting directly, which has a big impact on the power grid, especially for the high-power motor.
2、Frequency sensitive resistance starting
Frequency-sensitive resistance starting refers to connecting frequency-sensitive resistance in series in the main circuit when the motor starts, so as to reduce the starting current. Frequency sensitive resistance can change the starting current smoothly, the impact on the power grid is small, it is a more ideal way to start. However, high-power frequency sensitive resistors are used in the form of inductance, so in use will produce a large electromagnetic eddy current, will reduce the power factor of the grid.
3, reduced voltage starting
Reduced-voltage starting mainly has hot auto-root voltage reduction starting and star-delta voltage reduction starting.
Hot self-coupled voltage reduction starting refers to the self-coupled transformer to reduce the motor voltage during starting, and at the same time reduce the starting current. Generally, it is 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 needs to use the autotransformer, the cost is larger.
Star-delta buck starting refers to a method of changing the starting voltage by changing the wiring of the motor, thus reducing the starting current, which can only be applied to motors whose normal wiring is delta connection. At the time of starting, use the relay method to make the motor wiring for star, at this time the motor voltage per phase is reduced to one-third of the original root, the motor speed reaches about 80% of the rated speed, the control relay to change the motor wiring for delta, the motor starts to run normally. Advantage is that it can save the autotransformer, reduce costs, while the wiring method is simple, greater reliability. The disadvantage is that it is not possible to change the ratio of the starting voltage, and at the same time, it is not possible to use the motor in the star connection.
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