An induction motor draws heavy current at starting because the internal impedance of the rotor, which is the resistance and reactance of the rotor winding, is lower at start-up compared to when the motor is running.
When the motor is first started, the rotor is at rest and the stator is producing a rotating magnetic field. However, since the rotor is not rotating, there is no induced current flow in the rotor, resulting in a low impedance in the rotor circuit. To overcome this and to bring the rotor up to speed, a high starting torque is required.
As the rotor begins to rotate, the induced current flow in the rotor windings increases, which increases the impedance of the rotor circuit. Since the rotor impedance is greater when the motor is running, the current drawn by the motor is less than the current drawn at start-up.
In simple terms, the internal impedance of the rotor is greater when the motor is running than when it is starting. Since the current is inversely proportional to the impedance, the current drawn by the motor is greater at start-up than when the motor is running.