Losses
An ideal transformer would have no loss, and would therefore be 100% efficient. However, the coils of a real transformer have resistance, inductance and capacitance. When modeling a real transformer the resistance can be considered as existing in series with the winding of an ideal transformer, whilst the inductance can be considered to be in parallel. The bulk of the capacitance is between windings.
Large power transformers are often more than 98% efficient, in terms of energy supplied to the primary winding of the transformer and coupled to the secondary. The remaining 2% (or less) of the input energy is lost to:
Winding resistance
The current flowing in the windings causes resistive heating of the conductors. This is referred to as copper loss (to distinguish this from the rest of the losses below which are primarily attributable to the magnetic core and known as core losses)
Eddy currents
Induced currents circulating in the core causing resistive heating of the core.
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. Any magnetic field not coupled to the secondary circuit contributes to Leakage inductance
Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. Differing core materials will have different levels of hysteresis loss.
Mechanical losses
The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.
Magnetostriction
A minor effect that causes the core to expand and contract under the mechanical forces imposed by the alternating magnetic field. This in turn causes losses due to frictional heating in susceptible types of cores. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also due to magnetostriction vibration of the core.
Cooling system
Large power transformers may be equipped with cooling fans, oil pumps or water-cooled heat exchangers designed to remove the heat caused by copper losses and core losses. The power used to operate the cooling system is typically considered part of the losses of the transformer. Small transformers, such as a plug-in "wall wart"/"power brick" used to power small consumer electronics, often have high losses and may be less than 85% efficient.
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 in 1891 invented the Tesla coil, which is a high-voltage, air-core, dual-tuned resonant transformer for generating very high voltages at high frequency.
Solid cores
In higher frequency circuits such as switch-mode power supplies, powdered iron cores are sometimes used. These materials combine a high magnetic permeability with a high material resistivity. 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. Some transformers in radio-frequency circuits have adjustable cores which allow tuning of the coupling circuit.
Air cores
High-frequency transformers may also use air cores. These eliminate the loss due to hysteresis in the core material. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.
Toroidal cores
Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel wound into a coil. This construction ensures that all the grain boundaries are pointing in the optimum direction, making the transformer more efficient by reducing the core's reluctance, and eliminates the air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to prevent the core's magnetic field from generating electromagnetic interference.
Toroidal cores for use at frequencies up to a few tens of kilohertz may also be made of ferrite material to reduce losses. Such transformers are used in switch-mode power supplies.
Toroidal transformers are more efficient (around 95%) than the cheaper laminated EI types. Other advantages, compared to EI types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and more choice of shapes. This last point means that, for a given power output, either a wide, flat toroid or a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantage is higher cost.
When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core (e.g. by carelessly fitting a steel mounting bolt through the middle and fastening it to metalwork at both ends). This would cause a large current to flow through the bolt, converting all of the mains input power into heat, and blowing the input fuse. To avoid this, only one end of the mounting bolt must be fixed to the surrounding metalwork.
Windings
The winding material depends on the application. Small power and signal transformers are wound with insulated solid copper wire. Larger power transformers may be wound with wire, copper or aluminum rectangular conductors, or strip conductors for very heavy currents. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made of Litz wire, to minimize the skin effect losses in the conductors. Very large power transformers will also have multiple strands in the winding, for the same reason (see skin effect).
Windings on both primary and secondary of a power transformer may have taps to allow adjustment of the voltage ratio; taps may be connected to automatic on-load tapchanger switchgear for voltage regulation of distribution circuits.
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Insulation
The conductor material must have insulation to ensure the current travels around the core, and not through a turn-to-turn short-circuit.
In power transformers, the voltage difference between parts of the primary and secondary windings can be quite large. Layers of insulation are inserted between layers of windings to prevent arcing.
Shielding
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. Where transformers are intended for high electrical isolation between primary and secondary circuits, an electrostatic shield can be placed between windings to minimize this effect.
Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer or to prevent the transformer from affecting the operation of other devices (such as CRTs in close proximity to the transformer). Transformers may also be enclosed for reasons of safety, both to prevent contact with the transformer during normal operation and to contain possible fires that occur as a result of abnormal operation. The enclosure may also be part of the transformer's cooling system.
Coolant
Small transformers up to a few kilowatts in size usually are adequately cooled by air circulation. Larger "dry" type transformers may have cooling fans.
High-power or high-voltage transformers are bathed in highly-refined mineral oil that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl, "PCB" was used as it was not a fire hazard in indoor power transformers. Due to the stability of PCB and its environmental accumulation, it is no longer permitted in new equipment. Today, nontoxic, stable silicone-based or fluorinated hydrocarbons may be used, where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil.
The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. To improve cooling of large power transformers, the oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) may have cooling fans, oil pumps and even oil to water heat exchangers. Large and high-voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load.
Experimental power transformers in the 2000 kVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.
Terminals
Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must both provide electrical insulation, and contain oil within the transformer tank.
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 used to compensate for voltage drop in a distribution system or for matching two transmission voltages, for example 115,000 V and 138,000 V. For voltage ratios, not exceeding about 3:1, an autotransformer is less costly,lighter, smaller and more efficient than a two-winding transformer of a similar rating.
Variac is a trademark of General Radio (mid-20th century) for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. The term is often used to describe similar variable autotransformers made by other makers. A variable autotransformer is an efficient and quiet method for adjusting the voltage to incandescent lamps. While lightweight and compact semiconductor light dimmers have replaced variacs in many applications such as theatrical lighting, variable autotransformers are still used when an undistorted variable voltage sine wave is required.
