There are two main reasons:
1. Mainly from the rotor aspect: when the induction motor is in a stopped state, from the electromagnetic point of view, just like the transformer, the stator winding of the motor connected to the power supply side is equivalent to the primary winding of the transformer, and the rotor winding in a closed circuit is equivalent to the secondary winding of the transformer that is short-circuited. There is no electrical connection between the stator winding and the rotor winding, but only magnetic connection. Magnetic flux forms a closed loop through the stator, air gap and rotor core. When the rotor is turned on due to inertia, the rotating magnetic field cuts the rotor winding at the maximum cutting speed (synchronous speed), causing the rotor winding to induce the highest possible electromotive force. Therefore, a large current flows in the rotor conductor, which generates magnetic energy to offset the stator magnetic field, just as the secondary magnetic flux of a transformer will offset the primary magnetic flux.
In order to maintain the original magnetic flux which is suitable for the power supply voltage at that time, the stator automatically increases the current. At this time, the rotor current is very large, so the stator current also increases greatly, even up to 4~7 times of the rated current, which is the reason for the large starting current.
As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductor decreases, the induced electromotive force in the rotor conductor decreases, and the current in the rotor conductor also decreases. Therefore, the part of the stator current used to counteract the influence of the magnetic flux generated by the rotor current also decreases, so the stator current changes from large to small until it is normal.
2. Mainly from the stator aspect: According to Ohm’s law, when the voltages are equal, the smaller the impedance value, the greater the current. At the moment of motor start-up, the impedance in the current loop is only the resistance of the stator winding, which is generally made of copper conductor, so the resistance value is very small, otherwise the current will be very large.
During the starting process, due to the effect of magnetic induction, the reactance value in the loop gradually increases, so that the current value naturally decreases slowly until it becomes stable.
Post time: Oct-28-2022