The relative movement of rotating magnetic field produced by the stator (whose speed is synchronous speed n1) and the rotor winding, the rotor winding line cut off magnetic induction generating inducing electromotive force, thus the induced current generating in the rotor windings. The induced current in the rotor windings interacts with magnetic field, generating an electromagnetic torque, making the rotor rotate. When the rotor speed gradually approaches synchronous speed, the induced current decrease gradually, the electromagnetic torque generated accordingly decreases.
Here's a little animation to summarize things and hopefully make it all clear:
1.2 pairs of electromagnet coil, as shown in picture, are made by an AC power supply electricity in turn (not shows, but lead to the right) in the future. Two red wiring series with the coil and wiring and electrify together, and it has the same wiring method as the two blue coil. Because it is alternating current, the current in each coil is not connected, and cut off suddenly, such as the picture shown), but rises and falls smoothly in the shape of the sine wave: when the red coils are at their most active, blue coil is completely in the inactive state, and vice versa. In other words, the current is out of step (90 ° phase difference).
2. When the coil is energized, the magnetic field they produce between them induces an electric current in the rotor. The current produces its own magnetic field, trying to oppose what caused it (magnetic field from the coil). The interaction between two fields to make the rotor rotate.
3. When the changes in the magnetic field between the red and blue coil, effectively make the motor around rotating magnetic field makes the rotor rotate in the same direction and (in theory) at almost the same speed.