A transformer is an electrical device that transfers electrical energy from one electrical circuit to another by means of electromagnetic induction, normally without changing the signal frequency. It is central to modern alternating-current systems and appears in sizes ranging from tiny coupling units found inside a stage microphone to very large grid transformers rated for hundreds of megavolt-amperes. The basic purpose of a transformer is to convert power at one voltage level to another, allowing efficient long-distance transmission at high voltages and safe local use at lower voltages.

Basic principles and components

Most transformers consist of two or more wire windings (coils) wrapped around a magnetic core. The winding connected to the source is called the primary; the winding that delivers energy to the load is the secondary. For ideal windings, the ratio of voltages equals the ratio of turns: Vp/Vs = Np/Ns, and the currents are inversely related so that power is conserved (neglecting losses). Transformers operate only with alternating current or time-varying signals, because a changing magnetic flux is required to induce voltage in the secondary.

  • Core: laminated iron, ferrite or toroidal cores guide magnetic flux and reduce losses.
  • Windings: copper or aluminum conductor turns form the primary and secondary coils.
  • Insulation and cooling: paper, oil, or resin are common; cooling can be natural or forced to remove heat from losses.

Energy transfer relies on electromagnetism, not a direct electrical connection, so transformers can provide electrical isolation as well as voltage conversion. Because they do not change frequency, equipment connected through a transformer operates at the same cycle rate as the supply.

Types and common uses

Transformers are classified by function and construction. Typical categories include step-up and step-down units for changing distribution voltages; isolation transformers used for safety and noise reduction; autotransformers that use a shared winding to save materials; and instrument transformers (current and potential transformers) used for measurement and protection in power systems. In neighborhoods, pole-mounted and pad-mounted distribution transformers step down medium-voltage lines (for example, systems that drop from several kilovolts to domestic voltages such as 220–240 V or in some countries about 120 V) to feed homes and businesses, powering appliances like refrigerators.

Smaller transformers inside electronics provide required lower voltages, impedance matching, or isolation for different circuit blocks. Large power transformers in substations are vital components of transmission networks, enabling long-distance delivery of electric power with reduced losses by stepping voltage up and later down for local distribution.

Performance, losses and limitations

Real transformers are not lossless: they exhibit copper losses in windings, iron (core) losses from hysteresis and eddy currents, and additional losses from leakage flux and stray effects. Efficiency of large grid transformers can exceed 98–99%, while small or specialized units typically show lower efficiencies. Transformers cannot increase total delivered power; when voltage is raised, current falls proportionally and vice versa, consistent with conservation of energy, so the product of voltage and current (minus losses) remains the same. They also cannot be used with steady direct current because a constant magnetic flux does not induce a secondary voltage.

Historical notes and notable facts

The principles behind transformers follow from the discovery of electromagnetic induction in the early 19th century. Practical transformer designs and the adoption of alternating-current distribution developed in the late 19th century as part of expanding electrical networks; multiple inventors and companies contributed to their refinement. Today, transformers remain indispensable in power engineering, electronics, audio systems, industrial drives and measurement equipment, and are available in numerous specialized forms to meet voltage, power, safety and frequency requirements.

For further technical details, comparisons, or application notes, see introductory resources on voltage conversion, circuit design and power distribution: electrical energy, electrical circuit, electromagnetic induction, frequency, stage microphone, voltage, alternating current, electromagnetism, refrigerators, power, current, electronics.