Transformer operation at no-load: When no load is connected to the secondary winding (open-circuit secondary), what happens to the primary current and the secondary voltage of an ideal power transformer?

Introduction / Context:
A core concept in transformer theory is understanding behavior at no-load, i.e., when the secondary is open-circuited. Technicians frequently measure voltages and currents under this condition to verify turns ratio, excitation current, and core losses. Recognizing what stays the same and what changes prevents misdiagnosis of “faults” that are actually normal behaviors.

Given Data / Assumptions:

• The secondary winding is open (no external load is connected).
• The transformer is considered ideal for first-order reasoning (leakage drops and copper loss neglected).
• Rated primary voltage is applied at the primary terminals.

Concept / Approach:
With an open secondary, there is no load current in the secondary. However, an ideal transformer still requires a small primary current, called magnetizing current, to establish alternating flux in the core and to supply core (hysteresis and eddy) losses. Because the secondary is open, essentially no I*Z drops occur in the windings, so the secondary terminal voltage is very close to the ideal turns-ratio value (V2 ≈ V1 * N2/N1).

Step-by-Step Solution:
Secondary open → I_secondary ≈ 0 A.Primary still draws magnetizing current I_m to create core flux and supply core losses.Copper drops are negligible at no-load, so V_secondary ≈ induced emf per turn × N2 → near rated turns-ratio value.Therefore: primary current is small but non-zero; secondary voltage remains approximately nominal.

Verification / Alternative check:
Practical measurement with a voltmeter on an unloaded secondary shows close to nameplate secondary voltage. A clamp meter on the primary shows a small current (typically a few percent of full-load current) confirming non-zero magnetizing draw.

Why Other Options Are Wrong:
Primary current becomes zero and the secondary voltage becomes zero: incorrect; magnetizing current never vanishes and secondary voltage does not collapse at no-load.Primary current becomes inversely proportional to primary impedance and the secondary voltage collapses: this confuses general Ohm’s law with transformer excitation behavior; the secondary does not collapse at no-load.Secondary voltage proportional to primary impedance: secondary voltage is set primarily by turns ratio, not the primary impedance.

Common Pitfalls:
Assuming “no load” means “no current at all.” In reality, excitation current flows even with an open secondary. Also, small regulation effects can make the no-load secondary voltage slightly higher than the full-load value due to the absence of copper drops.

Final Answer:
Primary current is small but non-zero (magnetizing current) and the secondary voltage remains approximately the rated value set by the turns ratio