Principle − Based on the phenomenon of electromagnetic induction
Construction
M ain parts of an ac generator:

Armature − Rectangular coil ABCD

Filed Magnets − Two pole pieces of a strong electromagnet

Slip Rings − The ends of coil ABCD are connected to two hollow metallic rings R _{ 1 } and R _{ 2 } .

Brushes − B _{ 1 } and B _{ 2 } are two flexible metal plates or carbon rods. They are fixed and are kept in tight contact with R _{ 1 } and R _{ 2 } respectively.
Theory and Working − As the armature coil is rotated in the magnetic field, angle θ between the field and normal to the coil changes continuously. Therefore, magnetic flux linked with the coil changes. An emf is induced in the coil. According to Fleming’s right hand rule, current induced in AB is from A to B and it is from C to D in CD. In the external circuit, current flows from B _{ 2 } to B _{ 1 } .
To calculate the magnitude of emf induced:
Suppose
A → Area of each turn of the coil
N → Number of turns in the coil
→ Strength of magnetic field
θ → Angle which normal to the coil makes with at any instant t
∴ Magnetic flux linked with the coil in this position:
= NBA cos θ = NBA cos ω t …(i)
Where, ‘ ω ’ is angular velocity of the coil
As the coil rotates, angle θ changes. Therefore, magnetic flux Φ linked with the coil changes and hence, an emf is induced in the coil. At this instant t , if e is the emf induced in the coil, then
∴ e = NAB ω sin ω t