Various applications of autotransformer are :-
Autotransformers are used to compensate for voltage drops in transmission lines. When used for this application, these are known as booster transformers.
Autotransformers are used to obtain a variable supply.
It is also used as a reduced voltage starter for the Induction motor.
They offer variable voltage control in the laboratory setup: as we move the sliding contact, virtually all of the coil can become the series coil. Therefore, the entire coil must be sized for maximum current.
They are generally used to connect transmission lines of slightly different voltages ( e.g., 115 kV and 138 kV or 138 kV and 161 kV)
They are used to adjust the transformer output voltage in order to keep the system voltage constant with varying load.