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When a given transformer is run at its rated voltage but reduced frequency, its
1. flux density remains unaffected
2. iron losses are reduced
3. core flux density is reduced
4. core flux density is increased

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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.

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