What is measured to determine the performance of a transformer under load?

To determine the performance of a transformer under load, the measurement that is crucial is the load loss or copper loss, typically assessed through the current flow. When a transformer is under load, the current flowing through the windings generates heat due to the resistance of the winding materials. This measurement helps evaluate how efficiently the transformer operates under actual working conditions, as opposed to no-load conditions.

While the insulation resistance is important for understanding the quality of the insulation in a transformer, it primarily assesses potential leakage currents and the safety of the transformer rather than its operational performance under load. In contexts where performance under load is the focus, the current flow provides direct insight into the transformer's efficiency and heating, which are critical factors in assessing its operational effectiveness during normal functioning. Thus, measuring current flow reveals how well the transformer handles the electrical load and informs you about its overall performance characteristics.