Transformers and Transformer Technology

A transformer is a passive electrical device that transfers energy between two or more circuits through electromagnetic induction, usually changing the voltage in the process. It consists of two or more coils of wire, the primary and secondary windings, wound on a common magnetic core. Because it relies on a changing magnetic field, a transformer works only with alternating current; a steady direct current produces no changing flux and no transformed output. Transformers are among the most important components in power and electronics, making the efficient transmission and distribution of electricity possible.

The operating principle is straightforward. Alternating current in the primary winding sets up an alternating magnetic flux in the core, and this flux links the secondary winding and induces a voltage in it. The ratio of the secondary voltage to the primary voltage equals the ratio of the number of turns in each winding. A secondary with more turns than the primary steps the voltage up; fewer turns steps it down. In an ideal transformer power is conserved, so a step-up in voltage is accompanied by a proportional step-down in available current, and the reverse. The laminated iron core concentrates the magnetic flux and links the windings tightly.

Real transformers come in many forms and have small losses. Power transformers handle the high powers of the grid, distribution transformers feed local networks, and small transformers supply electronic equipment, while isolation transformers separate circuits for safety. Autotransformers use a single tapped winding and are more compact for modest voltage changes. Losses arise from the resistance of the windings (copper losses) and from hysteresis and eddy currents in the core (iron losses); the core is laminated and made of special alloys to keep these low. Cooling, often by oil in large units, removes the heat that these losses produce.

Frequently asked questions

Why do transformers only work with AC?

A transformer depends on a changing magnetic flux to induce voltage in the secondary. Direct current produces a steady flux that does not change, so no voltage is induced.

What determines whether a transformer steps voltage up or down?

The turns ratio. More turns on the secondary than the primary steps voltage up; fewer turns steps it down, with current changing inversely.

What causes losses in a transformer?

Copper losses from winding resistance and iron losses from hysteresis and eddy currents in the core. Laminated cores and special alloys reduce the iron losses.