When the induction motor is in the stop state, from the electromagnetic point of view, just like a transformer, the stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed-circuit rotor winding is equivalent to the short-circuit secondary coil of the transformer; there is no electrical connection between the stator winding and the rotor winding, only the magnetic connection, and the magnetic flux is closed through the stator, air gap and rotor core. At the moment of closing, the rotor has not yet turned due to inertia. The rotating magnetic field cuts the rotor winding at the maximum cutting speed synchronous speed, so that the rotor winding can induce the highest possible potential. Therefore, a large current flows through the rotor conductor, which generates magnetic energy to counteract the stator magnetic field, just as the secondary magnetic flux of a transformer counteracts the primary magnetic flux.

In order to maintain the original flux corresponding to 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 increases greatly, even up to 4 ~ 7 times of the rated current, which is the reason for the large starting current.

Why is the current small after starting: with the increase of the motor speed, 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. Therefore, the part of the current in the stator current used to counteract the influence of the magnetic flux generated by the rotor current also decreases, so the stator current goes from large to small until it is normal.