Why is the secondary voltage of a transformer lower at full-load than at no-load?

The secondary voltage of a transformer being lower at full-load compared to no-load is primarily due to internal volt drop caused by winding impedance.

When a transformer operates under load, current flows through its windings. This current flow generates a voltage drop across the transformer's internal impedance, which consists of both the resistance and reactance of the windings. The winding resistance contributes to a real voltage drop (I²R losses), while the inductive reactance can cause additional voltage drop as the reactance depends on the load current and frequency.

At no-load, the transformer has minimal current flowing through it, resulting in negligible voltage drop across the internal impedance. However, once the transformer is at full-load, the increased load current results in a significant voltage drop due to the impedance, leading to the observed decrease in secondary voltage.

This phenomenon is crucial for understanding the performance and efficiency of a transformer in practical applications, as it highlights how loading conditions impact voltage regulation.