An electrical transformer is a static device that transfers electrical energy between circuits through electromagnetic induction. Its principal function is to increase or decrease voltage, with a corresponding change in current, although transformers also provide electrical isolation and impedance matching. Typically consisting of coupled wire windings and a magnetic core, transformers are fundamental components of electrical grids and many electronic systems. (siemens-energy.com)
Operating principle
A transformer’s input winding is called the primary, and its output winding the secondary. An alternating voltage applied to the primary establishes a changing magnetic field. The resulting magnetic flux links the windings and induces an electromotive force in the secondary. A connected load then draws electric current. Ordinary transformers therefore operate with alternating current rather than a steady electrical input. (energy.gov)
According to Faraday’s law of induction, the induced voltage in a winding depends on its number of turns and the rate of change of flux:
[ e=-N\frac{d\Phi}{dt}, ]
where (N) is the number of turns and (\Phi) is the flux through each turn. The negative sign expresses the opposition of the induced electromotive force to the flux change. In an ideal transformer, both windings link the same flux, giving
[ \frac{V_s}{V_p}=\frac{N_s}{N_p}. ]
Subscripts (p) and (s) denote primary and secondary quantities. These voltage ratios apply to instantaneous values and to root-mean-square values. (openstax.org)
A step-up transformer has more secondary than primary turns; a step-down transformer has fewer. For an ideal transformer supplying a resistive load, input and output power are equal:
[ V_pI_p=V_sI_s,\qquad \frac{I_s}{I_p}=\frac{N_p}{N_s}. ]
Increasing voltage consequently decreases current in the corresponding ratio; a transformer does not create energy. The current actually delivered depends on the connected load, not on the turns ratio alone. (openstax.org)
A steady direct current produces no continuing change in flux and therefore no sustained transformer output. Electronic converters can nevertheless use transformers with a DC supply by switching that supply to create a time-varying waveform. (openstax.org)
Construction and materials
The basic construction places insulated windings around a common magnetic core. The core provides a path linking the windings, while their insulation prevents unwanted electrical contact. Close magnetic coupling is important: flux linking only one winding, rather than both, is termed leakage flux. (energy.gov)
Core design depends on operating frequency and application. Laminated iron construction was already a feature of nineteenth-century commercial transformers. High-frequency electronic transformers commonly use ferrite cores, for which manufacturers offer materials with different magnetic and loss characteristics. (nationalmaglab.org)
Power and distribution transformers are manufactured in liquid-filled and dry-type forms. Liquid-filled designs can use mineral oil or natural or synthetic ester fluids. Dry-type designs include open-wound and vacuum-cast-resin construction. These choices affect the arrangement of insulation, cooling, and installation. (hitachienergy.com)
Losses and practical performance
Real transformers depart from the ideal model because their windings and cores dissipate energy. Winding resistance produces heat, while eddy currents and magnetic hysteresis contribute to core losses. Eddy currents are induced currents within conducting core material; hysteresis losses accompany repeated changes in magnetization. (openstax.org)
Losses are commonly separated into no-load losses, occurring while the transformer is energized without supplying a load, and load losses, associated with supplying current. No-load losses arise primarily in the core. Efficiency therefore depends on loading as well as construction: a fixed core loss represents a greater fraction of useful output at light load. Core and winding dimensions and material choices influence the balance between manufacturing cost and operating losses. (energy.gov)
Transformers can also change the impedance presented to a source. In the ideal model, a secondary load impedance (Z_s) appears at the primary as
[ Z_{\mathrm{in}}=\left(\frac{N_p}{N_s}\right)^2Z_s. ]
This relationship supports impedance matching between circuits whose preferred voltage and current levels differ. (energy.gov)
Types and applications
In power systems, transformers raise generator voltage for transmission and lower it for distribution and use. For a given transmitted power, a higher voltage permits a lower current. Because resistive line losses scale with current squared, this reduces energy dissipated in transmission conductors. Transformers thereby connect equipment operating at different voltage levels within the same electricity network. (openstax.org)
Large power transformers may incorporate a tap changer, which selects different winding connections to adjust the effective turns ratio. An on-load tap changer permits voltage adjustment during operation and helps maintain suitable network voltages as conditions change. (siemens-energy.com)
Electronic applications include transformers in switched-mode power supplies, current-sensing devices, and signal circuits. Pulse transformers are used in communication and semiconductor gate-drive circuits; their specifications include turns ratio, inductance, resistance, capacitance, and isolation test voltage. Such devices may operate at frequencies far above those of utility power systems. (tdk.com)
Historical development
On August 29, 1831, Michael Faraday demonstrated induction between coils wound around an iron ring. Connecting a battery to one coil produced a transient current in another, establishing the principle underlying transformer operation. (aps.org)
Commercial development followed decades later. Lucien Gaulard and John Dixon Gibbs demonstrated an early transformer system, and William Stanley subsequently developed an improved design for George Westinghouse. On March 20, 1886, Stanley demonstrated a parallel-connected AC lighting system in Great Barrington, Massachusetts. His transformers used cores assembled from E-shaped iron plates, and the demonstration helped establish practical transformer-based electricity distribution in the United States. (nationalmaglab.org)