Fig. : AC generator
The axle of an AC generator is rotated such that one arm AB moves up and other CD moves down. The change in the magnetic field linked to the coil induces current in the coil. From Fleming’s right hand rule, the current flows from AB to CD in the coil during half rotation and from B₂ to B₁ in the external circuit.
After half rotation, the arm CD starts moving upward and AB downwards. So, the current starts flowing from CD to AB by Fleming’s Right hand rule. As a result the current starts flowing from B₁ to B₂ in the external circuit after half rotation.
In this way, the direction of induced current changes after each half rotation in an AC generator.

Fig. : Fleming’s right hand rule.