Which statement is true for an ideal transformer regarding energy transfer?

• A. The voltages are equal in magnitude on both windings.
• B. The currents on the two windings are always equal.
• C. The energy transfer is lossy and depends on winding resistance.
• D. The voltages are related by turns ratio and power is conserved, so Vp Ip = Vs Is.

Answer: (D)

In an ideal transformer, energy transfer is lossless and power must be conserved. The voltages on the two windings are linked by the turns ratio: the secondary voltage equals the primary voltage times the ratio of the number of turns on the secondary to the primary. Because power in equals power out, the currents adjust inversely to the turns: the secondary current equals the primary current times the turns ratio inverted. In other words, Vp/ Vs = Np/Ns and Is/ Ip = Np/Ns, so Pp = Vp Ip = Vs Is.

This means the voltage can be stepped up or down depending on turns, and the current changes to keep power constant. The statement that energy transfer is lossy or depends on winding resistance is not true for an ideal transformer, where those losses are neglected. Likewise, equal voltages on both windings occur only in a 1:1 turns case, and currents are not generally equal unless the turns are equal.