A transformer is a device that has two or more magnetically-coupled windings. The basic operation is shown in Figure. The action of a transformer is such that a time-varying (AC) voltage or current is transformed to a higher or lower value, as set by the transformer turns ratio.
The transformer does not add power, so it follows that the power (V X I) on either side must be constant.
That is the reason that the winding with more turns has higher voltage but lower current, while the winding with less turns has lower voltage but higher current. The dot on a transformer winding identifies its polarity with respect to another winding, and reversing the dot results in inverting the polarity Example of Transformer Operation:
An excellent example of how a transformer works can be found under the hood of your car, where a transformer is used to generate the 40 kV that fires car‘s spark plugs.
