It is a
one winding transformer. It works on the principle of self induction.
arrangement of auto transformer is shown below.
number of turnsbettween A and C are T1.
B a connection is taken. Section AB has T2 turns. As the volts p er turn, which
is proportional to the flux inn the machine, is the same for the whole windin
V2 = T1: T2
simplifying analysis, the magnetizing current of the transformer is neglected.
secondary winding delivers a load current of I2 ampere the
demagnetizing ampere turns is I2T2.
be countered by a current I1 owing from the source through the
turns such that, I1T1 = I2T2
of I1 ampere shows through the winding between B and C . The current
in the winding between A and B is (I2 -I1) ampere.
section of the wire to be selected for AB is proportional to this current
assuming a constant current density for the whole winding.
amount of material saving can be achieved compared to a two winding
transformer. To quantify the saving the total quantity of copper used in an
auto transformer is expressed as a fraction of that used in a two winding trans
means that an auto transformer requires the use of lesser quantity of copper
given by the ratio of turns. This rattio therefore denotes the savings in
equivalent circuit an d phasor diagram of autotransformer are shown in figures
2.14 and 2.15 respectively.
transformers are u sed in applications where electrical isolation is not a
spread application of auto transformer type of arrangement is in obtaining a
variable a.c. voltage from a fixed a.c. voltage supply.
secondary voltage is tapped by a brush whose position and hence t he output
voltage is variable.