A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. An alternating current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.
Preface
The transformer is one of the simplest of electrical devices. Its basic design, materials, and principles have changed little over the last one hundred years, yet transformer designs and materials continue to be improved. Transformers are essential for high voltage power transmission, providing an economical means of transmitting power over large distances. The simplicity, reliability, and economy of conversion of voltages by transformers was the principal factor in the selection of alternating current power transmission in the "War of Current" in the late 1880s.
Audio-frequency transformers, then referred to as repeating coils, were used by the earliest experimenters in the development of the telephone. While some early electronics applications of the transformer have been replaced by alternative techniques, transformers are still found in many electronic devices.
Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. All operate with the same basic principles and with many similarities in their parts.
Transformers alone cannot do the following:
1.Convert DC to AC or vice versa.
2.Change the voltage or current of DC.
3.Change the AC supply frequency.
However, transformers are components of the systems that perform all these functions.
Source:wikipedia.org
This treats the windings as a pair of mutually coupled coils with both primary and secondary windings passing currents. From the transformer equation, the primary MMF must equal the secondary MMF, and since these are in opposite directions, in an ideal transformer they cancel so that there is no overall resultant flux in the core. That this is so can be seen by realising that any unopposed primary emf would create a large primary current and therefore a large flux in the core due to the primary winding. However, this large flux would necessarily cause a large current to flow in the secondary circuit and this current must create an opposing flux that effectively cancels the initiating primary flux. In a non-ideal transformer, the resultant flux in the core is that needed to magnetise the core. This is called the magnetising flux. In a practical transformer, the higher-voltage winding will have more turns, of smaller conductor cross-section, than the lower-voltage windings.
