In case the two windings in an autotransformer are directly activated one after the other, they can be properly controlled by means of series winding or by a parallel winding. The reduced voltage winding is basically a parallel winding, which can be found parallel to the load in order to decrease the voltage, and in order to increase the voltage, it is found in parallel with the grid. The highest voltage winding is the serial connection of the series and parallel windings. Because the autotransformer is considered to be a real transformer, all principles applied to the normal multi-winding transformer can be applied.
It can be stated that the basic principles of application of the autotransformer are exactly the same as the principles of the multi-winding transformer air vents bell. However, it is important to note that one winding is partially contained in the other. This is another factor that contributes to arouse so much interest in the economic autotransformer. The terminals in all of the autotransformer windings are indicated by a number or by a combination of numbers and letters in the same way as in multi-winding transformers.