Generator can be modified to an a.c generator by replacing its commutators with two (separate) slip rings. The two slip rings rotate in tandem with the armature. Carbon brushes connect the armature to the external circuit. The armature is initially at the vertical position. No magnetic flux is cut and hence no induced current exists.
When the armature rotates, the change in magnetic flux increases and the induced current increases until its maximum value at the horizontal position. The direction of the induced current can be determined from Fleming's Right Hand rule.
As the armature continues on its rotation, the change in magnetic flux decreases until at the vertical position, no induced current exists.
Subsequently upon reaching the horizontal position again, the induced current is maximum, but the direction of the induced current flowing through the external circuit is reversed.
The direction of the induced current (which flows through the external circuit) keeps changing depending on the orientation of the armature. This induced current is also known as alternating current. The current is positive (+) in one direction and negative in the other (-). The smooth rings play a critical role in the generation of alternating current.
AC and DC Generator - Walter Fendt
AC Generator - Molecular Expression