Ideal transformer polarity — dot convention statement: In an ideal transformer, when the primary and secondary dotted terminals are taken as the positive reference ends, the secondary voltage is in phase with the primary voltage.

Introduction / Context:
Transformer polarity determines the phase relationship between primary and secondary voltages. The dot convention marks terminals that have the same instantaneous polarity, which is crucial when connecting multiple windings, building rectifiers, or summing secondary voltages without unintended cancellation.

Given Data / Assumptions:

• Ideal transformer behavior (no losses, no leakage, infinite magnetizing inductance assumed for simplicity).
• Dots identify the corresponding winding ends.
• Phase comparison is with respect to the dotted ends as reference polarity.

Concept / Approach:
In the dot convention, an instantaneous positive voltage applied to the dotted end of the primary produces an instantaneous positive voltage at the dotted end of the secondary (scaled by turns ratio). Therefore, referenced at the dotted terminals, the voltages are in phase regardless of the magnitude change set by the turns ratio.

Step-by-Step Solution:

Verification / Alternative check:
Reversing either winding (switching the dot to the opposite end physically) introduces a 180° phase reversal. Practical tests with low AC safely confirm the indicated polarity via oscilloscope observation.

Why Other Options Are Wrong:
Polarity is independent of step-up/step-down status, line frequency within typical operation, and load type; these factors change magnitudes and currents, not the dot-defined phase relationship.

Common Pitfalls:
Confusing the sign of induced voltage due to winding direction, or ignoring the dots when paralleling or series-connecting secondaries, leading to inadvertent cancellation or shorting.

Final Answer:
Correct