This is one of the most frequently repeated questions in job interviews and college exams, and the answer tests your true understanding of how a transformer works. The short answer: because direct current does not produce a changing flux. But let's understand the full story.
The Physical Reason
The voltage induced in the secondary winding is generated only when the magnetic flux is changing with time. After the steady-state moment, direct current produces a constant magnetic field, and a constant field does not produce a continuous induced voltage in the secondary. Therefore, the statement "the transformer works on AC" is a fundamental operating rule, not merely a theoretical fact.
What Happens If We Connect a Transformer to a DC Source?
- At the very moment of connection, a voltage pulse may appear on the secondary (due to the flux changing from zero to its value).
- After steady state is reached, the flux becomes constant and no voltage appears on the secondary.
- More seriously: the ohmic resistance of the primary winding is very small, and in the absence of inductive reactance (which appears with AC), a very large current flows that may burn out the winding.
Two reasons must be mentioned together: (1) no voltage is induced in the secondary because the flux is constant, and (2) the primary winding may be damaged due to the high current resulting from the absence of inductive reactance.
Sample answer: No voltage will be induced in the secondary winding because the magnetic flux will become constant after steady state, and more seriously, the current in the primary will rise to a very large value because the impedance drops to just the small ohmic resistance in the absence of inductive reactance, which may lead to the winding burning out.
Simply saying "it doesn't work" without mentioning the risk of the primary winding burning out. The complete answer covers both aspects: the absence of induced voltage, and the rise of damaging current.
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