An autotransformer is a special variation of a transformer in which the primary and secondary windings are combined into one winding, affording no galvanic separation of circuits.
The ratio of turns counted on the primary side to that on the secondary side determines the primary to secondary voltage ratio. This is the same as a conventional transformer, except the winding forms both part of primary and secondary. The formula for turns and voltage ratio is given below, together with diagrams of both conventional and auto transformers. 

Diagrams of transformer and autotransformer

The amount of iron and copper used for autotransformer construction is considerably smaller than a conventional transformer due to the galvanic combination of both windings. The auto transformer offers reduced losses (increase in efficiency) as well as reduction in weight, size and price.
Comparing a transformer and an autotransformer with the same transitive (output) power  SPRZECH , the autotransformer has lower electromagnetic power (core power factor) SWT :

and therefore it is lighter:

where: SPRZECH – autotransformer transitive (output) power, SWT – transformer electromagnetic power, mA, mT – autotransformer and transformer respective weights.

Example :

  • if power output SPRZECH = 100 kVA;
  • primary and secondary voltages  U1 = 230 V, U2 = 115 V
  • then – autotransformer electromagnetic power 
  • and respectively the relation of autotransformater and transformer weights is as follows

It results from above that for transformation of 100 kVA it is enough to apply a step-down autotransformer with a core corresponding to the power of 50 kVA transformer, and the weight of autotransformer will be about 60% of that of the 100 kVA transformer.

Reduction of autotransformer weight and dimensions is its advantage in relation to a transformer, however an autotransformer has the following disadvantages:

  • galvanic combination primary and secondary circuits meaning disturbances and over-voltage are transmitted directly to the secondary circuit.
  • earth faults in the power network are transmitted by the autotransformer which can result in risk to the user.
  • short-circuit voltage is lower as given by the following formula.:

where: UZA, UZT – short-circuit voltage of autotransformer and transformer, respectively

Therefore the application of an autotransformer is limited. They are applied for example in electromagnetic systems for connecting networks with different voltage levels, in start-up systems of large squirrel-cage induction motors, in laboratories, and everywhere where the lack of galvanic separation of primary and secondary circuits is allowed and where the advantages resulting from lower weight and losses exceed the expenditures associated with reduction of short-circuit current. Autotransformers are not allowed in mining, medical or nautical applications.

Autotransformer ELHAND, type EA3M-8/30kVA