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See the respective unrelated articles about the an animated television series from the 1980s, Transformers, and the glam rock album by Lou Reed, named Transformer, for those respective topics.

Electrical diagrams''

A transformer is an electrical device that transfers energy from one electrical circuit to another by means of magnetic coupling. An electrical transformer is the name given to any device for producing by means of one electric current another of a different character. It typically transforms between high and low voltages and accordingly between low and high currentss.

Table of contents
1 Machines
2 Principles
3 Invention
4 Transformer practice
5 See also


Electrical transformer are a simple machines in mechanics, both transforming power from one form into another with a certain energy-dissipation depending upon frictional losses, or something equivalent to them. The working of such an appliance is, of course, subject to the law of conservation of energy. It works only with current that changes, and most often it is used with alternating current.

Electrical transformers may be divided into classes, according to the nature of the transformation effected. They are:

[1] Transformers changing the form of the power, but keep the type of the current the same.
[2] Transformers changing the type of the current as well as the form of power.
A transformer which raises voltage is generally called a step-up transformer, and one which lowers voltage a step-down transformer.

An ideal transformer has perfect magnetic coupling, so that all the power fed into the device is available at its output, but in practice there are losses. The resulting current represents less power than the applied current, the difference being represented by the power dissipated in the translating process. The power given up electrically to any circuit is measured by the product of the effective value of the current, the effective value of the difference of potential between the ends of the circuit and a factor called the power factor. In dealing with periodic currents, the effective value is that called the root-mean-square value (RMS), that is to say, the square root of the mean of the squares of the time equidistant instantaneous values during one complete period (see Electrokentics).

In the case of continuous current, the power factor is unity, and the effective value of the current or voltage is the true mean value. As the electrical measure of a power is always a product involving current and voltage, we may transform the character of the power by increasing or diminishing the current with a corresponding decrease or increase of the voltage.


A transformer usually has two induction coils or windings. As the changing current flows through the powered or primary winding, it produces a changing magnetic field that grows through the unpowered or secondary windings. This changing magnetic field induces a current in the secondaries. (The changing magnetic field also causes magnetostriction which in turn results in mechanical forces that make parts of the transformer vibrate, which is why transformers hum. The hum consists of a fundamental note at twice the mains frequency, plus harmonics. The energy converted to motion is one form of core loss.)

The winding with fewer turns of wire has higher current, at a lower voltage. The winding with more turns of wire has less current, at a higher voltage. The ratio of voltages is proportional to the ratio of the numbers of turns of wire. For example, a 5-to-1 step-down transformer might take 1200-volt 60 Hz alternating current, and change it to the 240-volt 60 Hz alternating current distributed to houses. It might have 500 turns of wire on the primary, and 100 turns on the secondary. Many electronic appliances have several secondaries to produce different voltages needed in the appliance.

The high-current low-voltage windings have fewer turns of thicker wire. The thicker wire helps carry more current. The high-voltage, low-current windings have more turns of thinner wire. The thinner wire carries less current, but at a higher voltage. Transformer designers will optimize the wire sizes so that each winding will have the lowest resistance while keeping the winding size as small as possible, in an effort to reduce copper losses.

Some transformers have equal numbers of windings on both coils. These "isolation" transformers are used to prevent direct current flow between electric circuits, while transferring power.


An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed DC power is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. Autotransformers are commonly used as spark coils in automotive engines, and as high-voltage flyback transformers in television sets and computer monitors.

Variac was a trademark in the mid-20th century for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. A sliding contact determined what fraction of the winding was connected across the output; a common configuration provided for 120 V as input and percentages of that voltage as high as about 110%. More compact semiconductor light dimmers have displaced them in many applications, such as theatrical lighting.


Those credited with the invention of the transformer include:

Transformer practice

Uses of transformers


See also


General Electric power Electronics (List of electronics topics) Objects Standards