Transformer Loss Explained: No-Load vs. Load Loss
No-load loss and load loss in a transformer are two primary forms of energy loss during its operation. The following provides a detailed explanation of how they are generated, distributed, and dissipated within the transformer.
No-load loss occurs when the transformer's primary winding carries only a small excitation current under no-load conditions. At this time, copper loss is negligible, and the majority of the loss occurs in the core, known as iron loss. Iron loss consists of hysteresis loss and eddy current loss. Hysteresis loss results from the continuous flipping of magnetic domains within the core under the influence of an alternating magnetic field, which consumes energy and generates heat. Eddy current loss occurs because the core itself is conductive. When subjected to a changing magnetic field, an electromotive force is induced in planes perpendicular to the magnetic field lines, forming closed loops that generate currents—known as eddy currents—which produce heat due to the resistance of the core material.
No-load loss is mainly concentrated in the core, as the core is the primary component of the magnetic circuit, and both hysteresis and eddy current losses occur within its material. Additionally, a small amount of copper loss is generated in the primary winding due to the no-load current, but this is typically negligible. The heat generated by no-load loss is dissipated primarily through heat conduction and radiation from the core to the surrounding environment. The core is in contact with transformer oil or air, transferring heat to the oil or air. This heat is then carried away through natural convection of the oil or airflow, ultimately dissipating into the surrounding environment.
Load loss includes basic copper loss and additional losses. Basic copper loss is the loss due to resistance in the windings when load current passes through them. According to Joule's law, heat is generated when current flows through a conductor, and the power loss is proportional to the square of the current. Additional losses include eddy current losses in the windings, circulating current losses, stray losses in structural components, etc., which are proportional to the square of the current and the square of the frequency.
Load loss is primarily distributed in the windings and the core. The load current in the windings generates copper loss, while the core also experiences some loss due to leakage flux caused by the load current, though this is relatively small compared to the copper loss in the windings. Additionally, leakage flux induced by the load current generates stray losses in metal structural components outside the windings.
The heat generated by load loss is partly transferred to the transformer oil through heat conduction from the windings and then carried away by natural convection or forced circulation of the oil. Another portion is dissipated into the surrounding environment through heat conduction and radiation from the core and structural components. In large transformers, forced oil circulation cooling or similar methods are often employed to enhance heat dissipation, improving the operational efficiency and reliability of the transformer.