Transformer
A transformer is an electrical device that transfers energy from one electrical circuit to another by magnetic coupling without moving parts. It is often used to convert between high and low voltages and accordingly between low and high currents.
The transformer was an important element in the development of high-voltage power transmission and central generating stations.
Basic Concepts
A simple single phase (1φ) transformer consists of two electrical conductors called the primary coil and the secondary coil. The primary is fed with a varying (alternating or pulsed direct current) electric current which creates a varying magnetic field around the conductor. According to the principle of mutual inductance, 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.
In practical transformers, the primary and secondary conductors are coils of wire because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor.
Types of Transformers
Transformers can be classified into various types according to the ratio of the numbers of turns in the coils, as well as whether or not the primary and secondary are isolated:
Step-up
the secondary has more turns than the primary
Step-down
the secondary has fewer turns than the primary
Isolating
intended to transform from one voltage to the same voltage. The two coils have approximately equal numbers of turns, although often there is a slight difference in the number of turns, in order to compensate for losses (otherwise the output voltage would be a little less than, rather than the same as, the input voltage).
Variable
the primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer. The transformer may be an autotransformer used for regulation or adjustability. For example, a typical Variac ™ can transform 120 volts to an adjustable voltage that ranges from zero to 140 volts is an autoransformer with a sliding tap on the winding to allow adjustment.
In all cases the primary winding, or the secondary winding, or both, may have taps that allow selection of one of several different ratios of primary to secondary turns. A transformer with a single winding where part serves as both primary and secondary is known as an autotransformer.
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.
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