Auto Transformer

An Auto Transformer is a transformer with only one winding wound on a laminated core. An auto transformer is similar to a two winding transformer but differ in the way the primary and secondary winding are interrelated. A part of the winding is common to both primary and secondary sides. On load condition, a part of the load current is obtained directly from the supply and the remaining part is obtained by transformer action. An Auto transformer works as a voltage regulator.


Explanation of Auto Transformer with Circuit Diagram

In an ordinary transformer, the primary and the secondary windings are electrically insulated from each other but connected magnetically as shown in the figure below and in auto transformer the primary and the secondary windings are connected magnetically as well as electrically. In fact, a part of the single continuous winding is common to both primary and secondary.


Figure A: Ordinary Two Winding Transformer

There are two types of auto transformer based on the construction. In one type of transformer, there is continuous winding with the taps brought out at convenient points determined by desired secondary voltage and in another type of auto transformer, there are two or more distinct coils which are electrically connected to form a continuous winding. The construction of Auto transformer is shown in the figure below.


Figure B: Auto – Transformer

The primary winding AB from which a tapping at C is taken, such that CB acts as a secondary winding. The supply voltage is applied across AB, and the load is connected across CB. The tapping may be fixed or variable.When an AC voltage V1 is applied across AB, an alternating flux is set up in the core, as a result, an emf E1 is induced in the winding AB. A part of this induced emf is taken in the secondary circuit.


  • V1 – primary applied voltage
  • V2 – secondary voltage across the load
  • I1 – primary current
  • I2 – load current
  • N1 – number of turns between A and B
  • N2 – number of turns between C and B

Neglecting no load current, leakage reactance and losses,

V1 = E1 and V2 = E2

Therefore the transformation ratio

As the secondary ampere-turns are opposite to primary ampere turns, so the current I2 is in phase opposition to I1. The secondary voltage is less than the primary. Therefore current I2 is more than the current I1. Therefore, the resulting current flowing through section BC is (I2 – I1)

The ampere-turns due to section BC = current x turns
auto-transformer-eq2Equation (1) and (2) shows that the ampere turns due to section BC and AC balance each other which is characteristic of the transformer action.

Saving of Copper in Auto Transformer as Compared to Ordinary Two Winding Transformer

The weight of the copper is proportional to the length and area of a cross section of the conductor.

The length of the conductor is proportional to the number of turns, and the cross section is proportional to the product of current and number of turns.

Now, from the above figure (B) shown of the auto transformer, the weight of copper required in an auto transformer is

Wa = weight of copper in section AC + weight of copper in section CB


If the same duty is performed with an ordinary two winding transformer shown above in the figure (A)

The total weight of the copper required in the ordinary transformer

W0 = weight of copper on its primary winding + weight of copper on its secondary winding


Now, the ratio of the weight of the copper in an auto transformer to the weight of copper in an ordinary transformer is given as

Saving of copper affected by using an auto transformer = weight of copper required in an ordinary transformer – weight of copper required in an auto transformer


Saving of copper = K x weight of copper required for two windings of the transformer

Hence, saving in copper increases as the transformation ratio approaches to unity. Hence the auto transformer is used when the value of K is nearly equal to unity.

Advantages of Auto transformer

  • Less costly
  • Better regulation
  • Low losses as compared to ordinary two winding transformer of the same rating.

Disadvantages of Auto transformer

There are various advantages of the auto transformer, but then also one major disadvantage, why auto transformer is not widely used, is that

  • The secondary winding is not insulated from the primary winding.
    If an auto transformer is used to supply low voltage from a high voltage and there is a break in the secondary winding, the full primary voltage comes across the secondary terminal which is dangerous to the operator and the equipment. So the auto transformer should not be used to for interconnecting high voltage and low voltage system.
  • Used only in the limited places where a slight variation of the output voltage from input voltage is required.

Applications of Auto transformer

  • It is used as a starter to give upto 50 to 60% of full voltage to the stator of a squirrel cage induction motor during starting.
  • It is used to give a small boost to a distribution cable, to correct the voltage drop.
  • It is also used as a voltage regulator
  • Used in power transmission and distribution system and also in the audio system and railways.

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