Transformers are widely used because of their ability to change the frequency of AC and voltage or current of DC. Simple transformers consist of two electrical conductors, known as primary coil and secondary coil. Electric current in the primary coil creates a varying magnetic field, in which the secondary coil is placed. A potential difference, developed by the secondary coil is known as electromotive force. The ends of the secondary coil may be connected together to form an electrical circuit. In this case the current is forced to flow in the secondary.
According to the power handled in the circuit transformers are classified into signal(up to 1 watt), small power transformers(1-1000 watts), power transformers(1 kilowatt - 1 megawatt), large power transformers(over 1 megawatt).
According to the number of turns in the coils and the type of isolation transformers are classified into step down, step up, isolation and variable transformers. The secondary coil has more turns than the primary in step up transformers and fewer turns than primary in step down transformers. Step up transformers allow an appliance, which requires a high voltage power supply to work from a low voltage source.
Step down transformers allow an appliance, which requires a low voltage power supply to work from a high voltage. Isolating transformers have the equal number of turns in the primary and secondary coils, they are used to transform one voltage into the same voltage. In variable transformers the number of turns of the primary and secondary coils is adjustable. There are also transformers with a single winding, which serves as both primary and secondary. These are known as autotransformers.
Power transformers are designed as either a single-phase or three-phase configuration. Single-phase configurations include single, dual, quad and 5-lead. Three-phase transformers are linked in delta or wye configurations, which include delta-delta, delta-wye, wye-wye, wye-delta, wye-single phase and international. The primary frequencies of voltage available for such transformers include 50Hz, which is common for Europe, 60Hz, used in North America and 400Hz, used in aerospace applications.
Power transformers may provide two or more secondary voltage values. We should differentiate between toroidal and laminated transformers.
Toroidal transformers have good coil efficiency, since the cooper wire is wrapped around a cylindrical core, so that the magnetic flux doesn't leak out. Laminated transformers, also known as E-I transformers have laminated steel cores, which are formed in such a way that reduce electrical losses. Other types of power transformers include control transformer, current transformer, general-purpose transformer, distribution transformer and voltage transformer.
A transformer is considered to be ideal in case it is 100% efficient. Large power transformers are usually 98% efficient. The 2% of energy is lost to winding resistance, stray magnetic coupling, mechanic losses, magnetostriction and cooling system.