The principal of operation of a multiple winding transformer is no different from that of an ordinary transformer. Primary and secondary voltages, currents and turns ratios are all calculated the same, the difference this time is that we need to pay special attention to the voltage polarities of each coil winding, the dot convention marking the positive (or negative) polarity of the winding, when we connect them together.
Multiple winding transformers, also known as a multi-coil, or multi-winding transformer, contain more than one primary or more than one secondary coil, hence their name, on a common laminated core. They can be either a single-phase transformer or a three-phase transformer, (multi-winding, multi-phase transformer) the operation is the same.
Multiple Winding Transformers can also be used to provide either a step-up, a step-down, or a combination of both between the various windings. In fact a multiple winding transformers can have several secondary windings on the same core with each one providing a different voltage or current level output.
As transformers operate on the principal of mutual induction, each individual winding of a multiple winding transformer supports the same number of volts per turn, therefore the volt-ampere product in each winding is the same, that is NP/NS = VP/VS with any turns ratio between the individual coil winding being relative to the primary supply.
In electronic circuits, one transformer is often used to supply a variety of lower voltage levels for different components in the electronic circuitry. A typical application of multiple winding transformers is in power supplies and Tricia switching converters. So a transformer may have a number of different secondary winding, each of which is electrically isolated from the others, just as it is electrically isolated from the primary. Then each of the secondary coils will produce a voltage that is proportional to its number of coil turns for example.