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It is a one winding transformer. It works on the principle of self induction.
Physical arrangement of auto transformer is shown below.
Total number of turnsbettween A and C are T1.
At point 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 g,
V1: V2 = T1: T2
For simplifying analysis, the magnetizing current of the transformer is neglected.
When the secondary winding delivers a load current of I2 ampere the demagnetizing ampere turns is I2T2.
This will be countered by a current I1 owing from the source through the
T1 turns such that, I1T1 = I2T2
A current of I1 ampere shows through the winding between B and C . The current in the winding between A and B is (I2 -I1) ampere.
The cross section of the wire to be selected for AB is proportional to this current assuming a constant current density for the whole winding.
Thus some 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 former as,
This 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 copper.
The equivalent circuit an d phasor diagram of autotransformer are shown in figures 2.14 and 2.15 respectively.
Auto transformers are u sed in applications where electrical isolation is not a critical requirement.
The wide spread application of auto transformer type of arrangement is in obtaining a variable a.c. voltage from a fixed a.c. voltage supply.
The secondary voltage is tapped by a brush whose position and hence t he output voltage is variable.
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