Correct Answer - Option 4 : core flux density is increased
Core flux of Transformer:
RMS voltage of the transformer is given by
V = 4.44fϕ N
Where f =supply frequency
ϕ = core flux = B × A
N = number of turns
B = core flux density
\(ϕ \propto {\rm{B}} \propto \frac{{\rm{V}}}{{\rm{f}}}\)
Core flux of transformer mainly depends on:
→ Supply voltage
→ Supply frequency
So the core flux of a transformer is strictly constant with load changes
\( {\rm{B}} \propto \frac{{\rm{V}}}{{\rm{f}}}\)
\( {\rm{B}} \propto \frac{{\rm{1}}}{{\rm{f}}}\)
∴ When a given transformer is run at its rated voltage but with reduced frequency, its core flux density is increased.