Auto transformer:-

Generally transformer has two winding one for input and other for output but in autotransformer we have single winding which is tapped in order to connect load.A part of winding is common to both primary and secondary winding is kuown as autotransformer.

In two winding transformer both winding are electrically isolated therefore the power is transferred only due to induction.while is case of an autotransformer both winding are electrically connected therefore power is transferred due to induction as well as conduction.

In variable autotransformer the tap is movable by which the number of turn can be varied to change to output voltage.

If number of turn are higher on load side then it is step up autotransformer and if number of turn are less an load side then it is step down autotransformer.

The kva rating of autotransformer is much high as compare to two winding transformer which is an advantage.

If an autotransformer is construed using Two winding transformer then voltage and current remains same so core and copper loss are also same and hence efficiency is increase.

                           Autotransformer 

Types of magnetic coupling:-
  1. Positive couplig
                   

2. Negative coupling

Transformer is a negative coupling device.

Advantage of autotransformer :-
  1. Saving of copper.
  2. As amount of conducting material is reduced therefore copper loss also reduces.
  3. Volume of core reduces therefore core loss also reduces.
  4. Efficiency increase.
  5. Leakage flux is less.
  6. Leakage reactance is less therefore voltage regulation improve.

Disadvantage:-
  1. Since there is no electrical isolation therefore there is risk of appearing a dangerously high voltage acrossed load side in step down mode when common winding develops open circuited.
  2. Short circuit current of autotranformer is more than of two winding transformer.
Application:-
  1. Auto transformer is used to interconnect two different voltage levels.
  2. It is used to start three phase induction motor.
  3. It is used as a booster for line drop compensation in traction system.
  4. Used in laboratories.