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A transformer in which part of the winding is common to both the primary and secondary circuits is known as an auto-transformer. The primary is electrically connected to secondary, as well as magnetically coupled to it.

Refer Fig. 57. LM is primary winding having N1 turns and MS is secondary winding having N2 turns. If no-load current and iron losses are neglected.

Auto-transformerThe current in the section MS is vector difference of I2 and I1; but since the two currents are practically in phase opposition, the resultant is (I2 – I1) where I2 > I1.

Saving of copper (in comparison to conventional two winding transformer) :

The volume and hence weight of copper is proportional to the length and area of cross-section of the conducts. But the length of conductor is proportional to the number of turns and cross-section depends on current. Hence the weight of copper is proportional to the product of number of turns and current to be carried.




Advantages of auto-transformer. An auto-transformer entails the following advantages :

(i)                           Higher efficiency

(ii)                        Small size

(iii)                      Lower cost

(iv)                      Better voltage regulation when compared with a conventional two-winding transformer of the same rating.

Disadvantages. Following are the disadvantages / limitations of auto-transformers :

(i)                           The primary and secondary are conductively connected, rather than isolated as in the conventional (ordinary) transformer. Because of this, both sides are subject to any stresses set up by disturbances on either side. The low-voltage side is subject to high-voltage stress and should be insulated for the higher voltage. In the case of a step down transformer, the high voltage may still be impressed upon equipment connected to the low-voltage side. This is shown in Fig. 58 where the low-voltage coil has accidentally opened. The voltage on the load is nearly 2300 V, being less than that by the impedance volt drop in the primary coil.

(ii)                        As the voltage ration of an auto-transformer increases, the common coil is much smaller compared with the entire winding. This means that the economy gained is only a small part of the transformer and therefore this advantage is minimized.

Thus because of the above disadvantages, lack of isolation and decreased economy, auto-transformers are rarely used in rations greater than 4:1 except for low-power devices on low-voltage systems.


Uses. The auto-transformers find the following applications:

  1. The tie together transmission or distribution circuits of slightly different voltages (e.g. 11000 V system with a 13200 V system).
  2. To obtain partial line voltages for starting induction and synchronous motors with squirrel-cage windings.
  3. To give a small boost to a distribution cable to correct for the voltage drop.
  4. As furnace transformers for getting a convenient supply to suit the furnace winding from a 230 V supply.
  5. As regulation transformers.
  6. To obtain a neutral in a 3-wire A.C. distribution system in the same way as a balancer set is used in a 3-wire D.C. distribution system
  7. A continuously variable auto-transformer finds useful applications in electrical testing laboratory.