Ideal transformers and phasing: without specifying dot orientation or reference terminals, is it valid to state that “the primary and secondary are in phase”?

Introduction / Context:
Transformer voltage phasing is essential whenever windings are interconnected, rectified, or paralleled. The relative instantaneous polarity of primary and secondary voltages is not inherently “always in phase” or “always out of phase.” It is defined by the dot convention and how terminals are referenced. This question probes that concept.

Given Data / Assumptions:

• Ideal transformer model (no loss, infinite magnetizing inductance for analysis).
• No explicit dot markings or terminal reference given in the statement.
• Sinusoidal steady-state assumed.

Concept / Approach:
For an ideal transformer with dots on one end of each winding, the instantaneous polarities at the dotted ends are in phase: a rising voltage applied to the dotted primary end induces a voltage of the same instantaneous polarity at the dotted secondary end. If one observes opposite (dotted to undotted) terminals, the measured voltages appear 180 degrees out of phase. Therefore, the blanket statement “primary and secondary are in phase” lacks required context and is not universally valid.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer schematics always include dot marks or explicit polarity marks to avoid ambiguity when building multiwinding supplies or audio crossovers.

Why Other Options Are Wrong:
Correct: ignores the need to specify terminals/dots.
Depends only on turns ratio: ratio sets magnitude, not phase.
True only for step-down: phasing is not tied to whether N2 < N1 or N2 > N1.

Common Pitfalls:
Assuming “step-down equals in phase”; forgetting that scope probe reference selection can flip apparent phase; overlooking the dot convention entirely.

Final Answer:
Incorrect