变压器

A transformer transfers electrical energy through electromagnetic induction using two separate windings wrapped around a soft iron core. The alternating current in the primary winding generates an alternating magnetic flux in the core, which induces an emf in the secondary winding. This induced emf delivers energy to the connected load resistor without direct electrical connection between the windings.

The transformer equation states that the ratio of secondary emf to primary emf equals the ratio of secondary to primary turns. This relationship arises because the same magnetic flux passes through both windings. When windings have zero resistance, induced emfs equal terminal voltages, giving the fundamental transformer equation used to calculate voltage transformation ratios.

The soft iron core has substantial relative permeability, confining magnetic field lines almost entirely within the core. This ensures the same magnetic flux permeates each turn of both primary and secondary windings, maximizing mutual inductance between them. High mutual inductance is critical for efficient energy transfer and accurate voltage transformation in ideal transformers.

An ideal transformer maintains frequency because all currents and emfs have the same frequency as the AC source. The transformer only changes voltage magnitude through the turns ratio, not the frequency of oscillation. This frequency preservation is fundamental to transformer operation in alternating-current circuits.

An ideal transformer assumes zero energy loss, zero winding resistance, and no magnetic hysteresis or eddy current heating in the core. These assumptions mean all magnetic flux generated in the primary winding passes through the secondary winding, and induced emfs equal terminal voltages. Real transformers approximate this ideal behavior but experience energy losses in transformers.

The turns ratio, defined as the ratio of secondary turns to primary turns, directly determines the voltage transformation. A step-up transformer has more secondary turns than primary turns, increasing voltage. A step-down transformer has fewer secondary turns, decreasing voltage. The transformer equation shows output voltage equals input voltage multiplied by this turns ratio.

The two windings are electrically insulated from each other, preventing direct current flow between them while allowing magnetic coupling through the iron core. This insulation enables voltage transformation without electrical connection, allowing the secondary winding to deliver power to a load resistor independently while maintaining the same frequency as the primary AC source.