The autotransformer has only one winding. In this special kind of transformer, part of the winding is common to both the primary and the secondary one.
The principle of operation of the transformer is the same as the one of the common transformer, then the relation between input and output voltages and input and output currents and the ratio of number of turns between the primary and the secondary winding is the same.
The currents of the primary and secondary windings are flowing on the opposite directions, so the total current flowing through the common part of the winding is equal to the difference between the current on the low-voltage winding and the current on the high-voltage winding. In order for an autotransformer to work properly, the two windings should have the same winding sense.
If we apply an AC voltage between points A and B, and we measure a lower output voltage between points C and D, we have an step down autotransformer. Turn ratio = Ns / Np < 1.
If we apply an AC voltage between points C and D, and we measure a bigger output voltage between points A and B, we have a step up autotransformer. Turn ratio = Ns / Np > 1.
The autotransformers have some advantages over the common transformer, its weight and its lower cost.
Instead of N1 turns on the high voltage winding, it shall be given for the low voltage winding (with N2 turns) a number of additional turns (N1 – N2). We must also take into account that the wire of the common winding section should have a cross-section that depends on the difference between the currents of the low voltage and high voltage windings.
Another advantage of the autotransformer is that it does not require an isolation between primary and secondary windings. However, this brings the disadvantage that the primary winding is not independent of the secondary one.
This is dangerous for a person, because between the common point of secondary and primary windings and ground, there is the primary winding voltage. See the step down autotransformer diagram.