The current carrying rotor being placed in a magnetic field experiences a torque and hence begins to rotate in the direction of rotating magnetic field

. Thus we see that Induction Motor is self-starting.

Why induction motors are not self starting?

We can easily conclude that the single-phase induction motors are not self-starting because the produced stator flux is alternating in nature and at the starting, the two components of this flux cancel each other, and hence there is no net torque.

Why induction motor is self starting and synchronous motors are not?

Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. ... Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.