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Why does the starting current of the three-phase asynchronous motor start relatively large? Why does it turn up after starting up normally? Here, the editor of Dalan Motor needs to understand it from the perspective of the starting principle of the motor and the Rotation principle of the motor:
1. When the induction motor is in a stopped state, from the electromagnetic point of view, like a transformer, the stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed rotor winding is equivalent to the secondary coil of the transformer being short-circuited; There is no electrical connection between the stator winding and the rotor winding, only the magnetic connection, and the magnetic flux forms a closed circuit through the stator, the air gap, and the rotor core. At the moment of closing, the rotor has not turned up due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed-synchronous speed, so that the rotor winding induces the highest potential that can be reached. Therefore, a large amount of current flows in the rotor conductor Electric current. This current generates magnetic energy that cancels the magnetic field of the stator, just as the secondary magnetic flux of the transformer should cancel the effect of the primary magnetic flux.
Second, in order to maintain the original magnetic flux compatible with the power supply voltage at that time, the stator automatically increases the current. Because the rotor current is large at this time, the stator current also increases greatly, even as high as 4 to 7 times the rated current, which is the reason for the large starting current. Why the current is small after starting: As the motor speed increases, the speed of the stator magnetic field cutting the rotor conductor decreases, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases, so the stator current is used to offset the rotor current. The part of the current affected by the magnetic flux is also reduced, so the stator current is from large to small until normal.
3. The starting current of three-phase squirrel-cage asynchronous motors is generally 4 to 7 times, but not absolute. However, it is generally required that the starting current of the motor cannot exceed 2 to 5 times its rated current. Motors with a motor power of more than 30kw are not suitable for frequent starting, because the starting current of motors above 30kw is generally 6-7 times the rated current. Frequent starting will increase the temperature rise of the motor and cause the possibility of burning the motor.
Fourth, the direct starting current of the general three-phase asynchronous motor is about 4-7 times the rated current, the motor is 7 times smaller, and the motor is 5-4 times larger. Because the motor has impedance. At light load, the motor starts faster. When the pointer of the meter has not risen to the maximum, the motor has started and the current starts to drop, so it seems that the current is small. When the load is heavy, the motor starts slowly, and the meter can basically keep up with the change in current. It seems that the current is larger. It's actually the same.
----- Responsible Editor: Dalan Oil Pump Motor 02-Procurement Consultant
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