Why does the current flow when the induction motor starts up and the current becomes smaller after starting?
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; the stator winding There is no connection between the rotor winding and the rotor. Only the magnetic connection is made. The magnetic flux is closed through the stator, air gap and rotor core. At the moment of closing, the rotor has not turned due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed-synchronous speed, so that the rotor winding senses the highest potential that can be reached, and thus, a large flow in the rotor conductor Current, this current produces magnetic energy that cancels the stator's magnetic field, just as the transformer's secondary flux cancels the action of a magnetic flux.
In order to maintain the original magnetic flux that is compatible with the power supply voltage, the stator automatically increases the current. Because the current of the rotor is very large at this time, the stator current also increases greatly, even up to 4 to 7 times the rated current, which is the reason for the large starting current.
Why is the current small after starting: as the motor speed increases, the stator magnetic field cuts the rotor conductor at a reduced speed, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases, so that the stator current is used to cancel the rotor current. The portion of the current affected by the flux is also reduced, so the stator current is from large to small until normal.
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