A transformer is an electrical device that transfers energy between circuits through electromagnetic induction, normally using two or more coils wound around a magnetic core. The coil connected to the input supply is called the primary winding, while the output coil is called the secondary winding. When an alternating voltage is applied to the primary, alternating current produces a changing magnetic flux in the core. This changing flux links the secondary winding and induces an emf according to Faraday's law.
The primary and secondary circuits are therefore magnetically coupled but can be electrically isolated from one another. For an ideal transformer, the voltage ratio is approximately equal to the turns ratio, so V secondary divided by V primary equals N secondary divided by N primary. A transformer with more secondary turns than primary turns is a step up transformer, while one with fewer secondary turns is a step down transformer. Transformers are fundamental to electrical power systems because they allow voltage to be changed efficiently without changing the AC frequency.
The magnetic core provides a low reluctance path for magnetic flux and improves coupling between the windings. Transformer cores are commonly made from laminated electrical steel in power frequency applications. Lamination reduces eddy currents because thin electrically insulated layers interrupt large circulating current paths within the conductive core. At higher frequencies, ferrites and other materials may be used because their magnetic and electrical properties can reduce particular losses.
The windings are arranged so that changing magnetic flux generated by one winding passes effectively through the other. In an ideal transformer, essentially all primary flux links the secondary. Real transformers have leakage flux, meaning some magnetic flux does not link both windings. This produces leakage inductance and reduces the ideal coupling. Core geometry, winding placement, insulation and material properties are therefore important design factors. The transformer works because magnetic coupling transfers energy between circuits rather than because electrons physically travel through the insulating space between primary and secondary.
Transformers normally operate with alternating current because continuous induction requires changing magnetic flux. When an AC voltage is applied to the primary winding, the resulting current and magnetic flux vary with time. Faraday's law then induces a voltage in the secondary. If direct current is applied to an ordinary transformer, there is only a transient change in flux when the current is switched on or off. Once the magnetic flux becomes approximately steady, the continuous induced secondary voltage disappears.
A sustained DC input can also cause excessive primary current because the winding's resistance may be the main limitation once inductive opposition has disappeared, potentially overheating the transformer. Transformers are therefore fundamentally AC devices in their conventional form. Electronic circuits can use high frequency switching to create alternating currents from DC, allowing transformer based voltage conversion in switched mode power supplies.
The ideal transformer also illustrates conservation of power. If losses are neglected, input power approximately equals output power, so increasing voltage requires a corresponding decrease in current, while decreasing voltage permits a corresponding increase in current. The approximate relationship is V primary I primary equals V secondary I secondary. Thus a step up transformer does not produce more total electrical power simply by increasing voltage. It trades current for voltage.
Real transformers have losses, so output power is lower than input power. Copper losses arise from resistance in the windings, while core losses include hysteresis and eddy currents. There are also leakage and stray losses, dielectric losses and cooling related limitations. Transformer efficiency is therefore high but not exactly 100 percent. The power relationship is particularly important in electrical transmission because high voltage allows a given power level to be transmitted with lower current, reducing resistive losses in transmission conductors.
The ability to increase transmission voltage is one of the most important practical uses of transformers. For a fixed transmitted power, raising the voltage reduces the current. Resistive heating in a conductor is proportional to I squared R, so lower current substantially reduces power lost as heat in transmission lines. Power stations therefore commonly use step up transformers to raise voltage before long distance transmission.
Near consumers, substations use step down transformers to reduce voltage to appropriate distribution and utilisation levels. Multiple transformation stages may be used throughout a power network. Transformers also provide electrical isolation, allowing two circuits to exchange energy magnetically while remaining galvanically separated. Isolation transformers can reduce some forms of unwanted direct connection and are used in specialised power and safety applications. The exact voltage levels depend on the electrical system and engineering requirements.
Transformers are not restricted to large power grid equipment. Small transformers appear in chargers, audio equipment, instrumentation, control systems and electronic power supplies. High frequency transformers in switched mode supplies can be much smaller than traditional low frequency transformers because increasing frequency allows the required magnetic energy transfer to occur with smaller magnetic components, subject to material and loss constraints. Audio transformers can transfer AC signals while providing impedance transformation and electrical isolation.
Current transformers use electromagnetic induction to produce a measurable secondary current related to a large primary current, allowing electrical measurement and protection systems to monitor high current conductors without directly carrying the full current through the measuring instrument. Potential transformers and voltage transformers similarly provide scaled voltages for measurement. These specialised transformers demonstrate how magnetic coupling can be used for both energy transfer and signal measurement.
Transformer behaviour becomes more complex under load. When the secondary circuit is open, the primary draws a magnetising current needed to establish the core flux and overcome losses. When a load is connected, secondary current creates a magnetic effect that tends to oppose the primary flux change. The primary then draws additional current from the supply so that the magnetic flux remains approximately at the required level. This response is consistent with conservation of energy and electromagnetic induction.
The transformer's regulation, meaning the change in secondary voltage between no load and load conditions, depends on winding resistance, leakage reactance, core characteristics and load power factor. Large transformers therefore require careful design for voltage regulation, thermal management, insulation coordination, mechanical strength and fault withstand capability. High voltage systems also require robust insulation and appropriate clearances because electrical stress can become substantial.
The transformer is a direct practical consequence of Faraday's law and the unified relationship between electricity and magnetism. Its historical development followed nineteenth century discoveries of electromagnetic induction and alternating current systems. Transformers made practical long distance electrical distribution possible because voltage could be changed efficiently. The device does not alter the fundamental frequency of the ideal AC supply, although nonidealities and switching arrangements can introduce harmonics. Its core role is magnetic coupling.
An alternating current in one winding creates a changing magnetic field, that field establishes changing magnetic flux in the core, and the changing flux induces an emf in another winding. The turns ratio determines the ideal voltage ratio, while energy conservation determines the corresponding current relationship. Real performance is governed by losses, magnetic saturation, leakage flux, winding resistance, insulation and thermal constraints.
