Open-circuit secondary — primary current behavior: If the secondary winding of a transformer is open-circuited, the primary current is nearly zero (only the small magnetizing current flows).

Introduction / Context:
Understanding primary current under different load conditions is crucial for safe testing and operation of transformers. With the secondary open, the transformer is unloaded; therefore, only the magnetizing current (to establish core flux and cover core losses) flows in the primary.

Given Data / Assumptions:

• Idealized view: no load on the secondary, so no real power transfer.
• Real transformers: a small magnetizing current and small core-loss current still exist.
• Frequency within normal operating range.

Concept / Approach:
When the secondary is open, there is no path for load current. The primary draws just enough current to magnetize the core and supply core losses (hysteresis and eddy currents). This current is typically a few percent of full-load current. Thus, compared to loaded operation, the primary current is “nearly zero.”

Step-by-Step Solution:

Verification / Alternative check:
Clamp-meter measurement on a small power transformer with open secondary typically shows 2–10% of rated primary current, verifying the “nearly zero” characterization for power transfer purposes.

Why Other Options Are Wrong:
Step-up/step-down status, exact frequency, or core lamination do not change the fundamental behavior; only the magnitude of magnetizing/core-loss current varies slightly.

Common Pitfalls:
Expecting literally zero current; in practice, some current is always required to sustain core flux and cover losses.

Final Answer:
Correct