formers, used for substations, large electrical consumers, and for power plants and transmission.
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:
- the secondary has more turns than the primary
- the secondary has fewer turns than the primary
- 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).
- the primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer.
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.
An ideal transformer would have no loss, and would therefore be 100% efficient. However, the coils of a real transformer have resistance. When modeling a real transformer the resistance can be considered as existing in series with the winding of an ideal transformer.
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
- Eddy currents
- Induced currents circulating in the core causing resistive heating of the core.
- Stray losses
- Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects, such as the transformer's support structure, and be converted to heat.
- 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.
- The flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic 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 itself.
- 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.
High frequency operationEdit
The universal transformer emf equation indicates that at higher frequency, the core flux density will be lower for a given voltage. This implies that a core can have a smaller cross-sectional area and thus be physically more compact without reaching saturation. It is for this reason that the aircraft manufacturers and the military use 400 hertz supplies. They are less concerned with efficiency, which is lower at higher frequencies (mostly due to increased hysteresis losses), but are more concerned with saving weight. Similarly, flyback transformers which supply high voltage to cathode ray tubes operate at the frequency of the horizontal oscillator, many times higher than 50 or 60 hertz, which allows for a more compact component.
A transformer usually has:
- two or more insulated windings, to carry current
- a core, in which the mutual magnetic field couples the windings.
In transformers d