( 1 ) N refers to the number of turns V is voltage s and p refer to the secondary and primary coil. From the conservation of energy we get: V p I p = V s I s
( 2 ) From ( 1 ) : V s V p = 900 00 3 00 = 300 ∴ V s = 300 V p Substituting for V s into ( 2 ) ⇒ V p I p = 300 V p × 0.5 ∴ I p = 150 A Seems a big current.