Transformer no-load current refers to the current flowing through the primary winding when the secondary winding of the transformer is open-circuited. It is mainly composed of magnetizing current and loss current, and its generation is related to the magnetic circuit and core material of the transformer. One of the main causes is the need to establish a magnetic field in the core. When the primary winding is connected to an alternating current power supply, an alternating magnetic field is generated in the core to induce an electromotive force in the secondary winding (according to electromagnetic induction principle). The magnetizing current is the current required to establish and maintain this alternating magnetic field, which is a reactive current and does not consume active power.

Another cause is the core loss in the transformer, which gives rise to the loss current. As mentioned earlier, core loss includes hysteresis loss and eddy current loss. Hysteresis loss is caused by the repeated magnetization and demagnetization of the core material under the action of alternating current, which requires energy input, resulting in a loss current component in the no-load current. Eddy current loss is caused by the induced eddy currents in the core, which also consume energy and contribute to the loss current. The loss current is an active current that converts electrical energy into heat energy.

The magnitude of the no-load current is also affected by factors such as core material, core structure, and manufacturing processes. Transformers using high-quality magnetic materials (such as oriented silicon steel sheets or amorphous alloys) have lower hysteresis and eddy current losses, thus reducing the no-load current. A well-designed core structure with a high magnetic permeability and low magnetic resistance can reduce the magnetizing current required to establish the magnetic field. Poor manufacturing processes, such as insufficient lamination of core sheets, uneven gaps, or excessive mechanical stress on the core, can increase the magnetic resistance of the core, leading to an increase in the no-load current. Additionally, the no-load current may increase due to aging of the core insulation or the presence of impurities in the core over time, which affect the magnetic properties of the core.

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