The rotor works like a magnet. The slip rings supply a DC excitation current to the rotor windings and it then becomes a rotating electromagnet. As the rotor rotates, the magnetic field produced on the rotor cuts the stator windings. The resultant effect of this field on stator windings is an induced voltage which causes a current to flow through the stator.
An AC voltage is one that is always changing in magnitude and every 180° it crosses from above the x-axis to below the x-axis and thus there is a change in polarity.
Electricity leaves the Generator in Aghada at 21kV and passes through a step-up Transformer before it is then exported to the National Grid at 220V.
Diagram: The Periodic Nature of Alternating Current.