Transformer

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

''Transformers
Typical electrical configurations
See standard symbols below''.

A transformer is an electrical device that transfers energy from one electrical circuit to another by magnetic coupling. It is often used to convert between high and low voltages and accordingly between low and high currentss.

Table of contents

1 Basic principles

1.1 Electrical laws

2 Practical transformers

2.2 Losses

3 Designs

3.3 Invention
3.4 Circuit symbols
3.5 Construction
3.6 Autotransformers

4 Uses of transformers
5 See also

Basic principles

A simple transformer consists of two electrical conductorss called the primary coil and the secondary coil. The primary is fed with a varying (alternating or pulsed continuous) electric current which creates a varying magnetic field of voltage around the conductor. According to the principle of mutual inductance, which is a special case of electromagnetic induction applied to two coupled conductors, the secondary, which is placed in this varying magnetic field, will develop a potential difference called an electromotive force or EMF. If the ends of the secondary are connected together to form an electrical circuit, this EMF will cause a current to flow in the secondary. Thus, some of the electrical power fed into the primary is delivered to the secondary.

Electrical laws

Consider the following two electrical laws:

According to the law of conservation of energy, the power delivered by a transformer cannot exceed the power fed into it.
The power dissipated in a load at any instant is equal to the product of the voltage across it and the current passing through it.

It follows from the above two laws that if the transformer is used to change power from one voltage to another, the magnitudes of the currents in the two windings must also be different, in inverse ratio to the voltages. 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.

Practical transformers

Transformers can be classified into three types according to the ratio of the numbers of turns in the coils:

Step-up

the secondary has more turns than the primary

Step-down

the secondary has fewer turns than the primary

Isolating

the two coils have equal numbers of turns

In most practical transformers, the primary and secondary conductors are coils of wire (usually copper), because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor. The EMF developed in the secondary is proportional to the ratio of the number of turns in the secondary coil to the number of turns in the primary coil. Hence, the Transformer Equation:

Where is the voltage in the primary coil, is the voltage in the secondary coil, is the number of turns of wire on the primary coil, and is the number of turns of wire on the secondary coil. This leads to the commonest use of the transformer: to convert power at one voltage to power at a different voltage.

Losses

The difference between the power output and the power input is called the loss. An ideal transformer would have no loss, and would therefore be 100% efficient. In a practical transformer, there are losses due to:

Eddy currents

Induced currents circulating in the core causing resistive heating of the core.

Winding resistance

The current flowing in the windings causes resistive heating of the conductors.

Stray magnetic coupling

Not all the magnetic field produced by the primary is intercepted by the secondary, the remainder being absorbed by other nearby objects and converted to heat.

Mechanical losses

The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations which consume power.

Magnetostriction

A minor effect that causes periodic stresses, and therefore losses due to frictional heating, in certain types of core.

Designs

Invention

Those credited with the invention of the transformer include:

Michael Faraday, who invented an 'induction ring' on August 29, 1831. This was the first transformer, although Faraday used it only to demonstrate the principle of electromagnetic induction and did not foresee the use to which it would eventually be put.
Lucien Gaulard and John Dixon Gibbs, who first exhibited a device called a 'secondary generator' in London in 1881 and then sold the idea to American company Westinghouse. This may have been the first practical power transformer, but was not the first transformer of any kind. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Their early devices used a linear iron core, which was later abandoned in favour of a more efficient circular core.
William Stanley, an engineer for Westinghouse, who built the first practical device in 1885 after George Westinghouse bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886.
Hungarian engineers Ottó Bláthy, Miksa Déri and Károly Zipernowsky at the Ganz company in Budapest in 1885, who created the efficient "ZBD" model based on the design by Gaulard and Gibbs.
Nikola Tesla, who is often incorrectly credited with its invention, although his true achievement was to develop and patent (in 1888) a complete polyphase AC system, including a polyphase transformer, for power distribution. He sold his patents to Westinghouse in the same year. In 1891 he invented the Tesla transfomer or Tesla coil, which is a high-voltage, air-core, self-regenerative resonant transformer for generating very high voltages at high frequency.

Circuit symbols

Standard symbols

Transformer with two windings and iron core.

Transformer with three windings.
The dots show the adjacent ends of the windings.

Step-down or step-up transformer.
The symbol shows which winding has more turns,
but does not usually show the exact ratio.

Transformer with electrostatic screen,
which prevents electrostatic coupling between the windings.

Construction

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 minimize resistive power dissipation (commonly called 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. In transformers designed to operate at low frequencies, the windings are usually formed around an iron core. This helps to confine the magnetic field within the transformer and increase its efficiency, although the presence of the core causes energy losses.

Transformers often have silicon steel cores to channel the magnetic field. This keeps the field more concentrated around the wires, so that the transformer is more efficient. The core also keeps the field from being wasted in nearby pieces of metal. The core of a power transformer must be designed so that it does not reach magnetic saturation. Carefully designed gaps are sometimes placed in the magnetic path to help prevent saturation.
Laminated cores are made of many stamped pieces of thin steel. The high resistance between layers reduces eddy currents in the cores that waste power by heating the core. These are common in power and audio circuits. In higher frequency circuits, powdered iron cores are sometimes used. These are common, for instance, in switching power supplies. At even higher frequencies (radio frequencies typically) other types of core made of nonconductive magnetic materials, such as various ceramic materials called ferrites are common.
High-frequency transformers in low-power circuits where moderate losses are acceptable may have air cores. These save weight and cost.
The voltage difference between parts of the primary and secondary windings can be quite large, and layers of insulation are sometimes required between windings to prevent arcing.
Although an ideal transformer is purely magnetic in operation, the close proximity of the primary and secondary windings can create a mutual capacitance between the windings that sometimes can not be ignored in analyzing the circuit behavior. Sometimes an electrostatic shield is placed between windings to minimize this effect. This is common, for instance, in transformers designed to achieve high electrical isolation between primary and secondary circuits.
Power transformers are usually more than 98% efficient which makes them the most efficient devices man can make. The higher-voltage transformers are bathed in nonconductive oil that is stable at high temperatures. This used to be polychlorinated biphenyl, the famous toxic waste, "PCB". Nowadays, nontoxic, very stable fluorinated hydrocarbons are preferred. The oil cools the transformer, and helps prevent short circuits. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire.
See how to make a transformer for instructions on how to make a very simple transformer suitable for demonstration of the principles in a school classroom setting.

Autotransformers

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.

Uses of transformers

If electrical power needs to be transmitted over long distances, the loss is usually lower if a higher voltage is used. This is because, for a given amount of power to be transmitted, the current decreases as the voltage increases, and the resistive power loss in the wires is proportional to the square of the current. But high voltage is dangerous in the home, so transformers are employed to step the voltage up at the power station and back down at the consumer's premises. (At the very highest voltages, typically 500 kV and above, transmission lines are sometimes DC instead of AC, and simple transformers can not be used to convert these voltages.)
Small transformers are often used to isolate and link different parts of radios. See electronics and impedance match.
Some transformers are designed so that one winding turns or slides, while the other remains stationary. These can pass power or radio signals from a stationary mounting to a turning mechanism, such as a machine tool head or radar antenna.
Some moving transformers are precisely constructed in order to measure distances. Most often, they have several primaries, and electronic circuits measure the shape of the wave in the different secondaries.
Impedance transformation. The transformer's ability to multiply voltages and currents also leads to an ability to transform impedances. This property is used in signal-processing circuits such as radio receivers and audio amplifiers. See impedance matching for details.
Balanced-to-unbalanced conversion. A special type of transformer called a balun is used in radio and audio circuits to convert between balanced circuits and unbalanced transmission lines such as antenna downleads. A balanced line is one in which the two conductors (signal and return) have the same impedance to ground: twisted pair and "balanced twin" are examples. Unbalanced lines include coaxial cables and strip-line traces on printed circuit boards.

	Transformer with two windings and iron core.
	Transformer with three windings. The dots show the adjacent ends of the windings.
	Step-down or step-up transformer. The symbol shows which winding has more turns, but does not usually show the exact ratio.
	Transformer with electrostatic screen, which prevents electrostatic coupling between the windings.