A transformer is an extended version of an inductor. The flux that is created inside the inductor is used here to induce voltages at other coil, which is termed as secondary coil. If the rate of change of flux can induce voltage across the primary coil, from which it is created, then it is also possible to induce voltage across secondary coil, provided that we can pull the flux to flow through the other coil. The rate of change of flux will induce voltage as many turn we use. If the turn is double the turn in primary then the voltage will also be double. If we increase the number of secondary coils, then voltage will be induced in all the secondary coils according to the number of turns present in each secondary coil.
We can increase or decrease the secondary voltage level according to our requirement. If the secondary voltage is increased then it is called step up transformer and for the decreasing case it is called step down transformer. Each secondary voltage will act as a separate voltage source. Here the other advantage we get from a transformer is that each secondary voltage source is an isolated voltage source. There is no electrical connection between the primary and the secondary. Whatever voltage level is that, the secondary is totally an isolated part.
An autotransformer has only a single winding with two end terminals, plus a third at an intermediate tap point. The primary voltage is applied across two of the terminals, and the secondary voltage taken from one of these and the third terminal. The primary and secondary circuits therefore have a number of windings turns in common. An adjustable autotransformer is made by the secondary connection through a sliding brush, giving a variable turns ratio.
For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. In this case, the magnetic circuits are connected together, the core thus containing a three-phase flow of flux. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Δ, Δ-Y, Δ-Δ and Y-Y. If a winding is connected to earth (grounded), the earth connection point is usually the center point of a Y winding.