ü  Transformer is a Device that transfer electric
power
ü  Transformer operates on ac supply
ü  It can increase the voltage or current
ü  If voltage is increasing the current is
decreasing 
ü  And if current is increasing the voltage is
decreasing 
ü  Without change of frequency and increasing power
ü  It has two coil or more (primary and
secondary) 
This
video shows you 
1-     What is Transformer ?
2- Calculation
of Transformer :
3- how to build any Transformer ?