What is the voltage output of a transformer with turns ratio 3:1 and a 240V DC primary?

• A. 0VAC
• B. 80VAC
• C. 720VAC
• D. 120VAC

Answer: (A)

Transformers rely on changing magnetic flux to transfer energy. The secondary voltage is induced by the rate of change of that flux, which comes from an alternating current in the primary.

With a steady DC input, the flux in the core rises briefly when the connection is made, then becomes constant. Once the flux stops changing, Faraday’s law gives no induced voltage in the secondary, so there is no sustained secondary output. In practice, the transformer can also overheat from the large steady magnetizing current caused by DC, further confirming there isn’t a useful secondary voltage in steady state.

So the correct idea is that, under a constant DC supply, the transformer produces no ongoing secondary voltage. The other options assume a DC input somehow yields a specific secondary voltage, but that isn’t how transformers operate.