What is Auto Transformer? Its types, working, advantages, disadvantages & Applications

This is a type of transformer that is mostly seen in our electrical laboratories. This auto transformer is an improved version of the original transformer. A single winding is taken to which both sides are connected to power and the ground. Another variable tapping is made by whose movement secondary of the transformer is formed.

The following figure shows the circuit of an auto-transformer.

Auto Transformer

As shown in the figure, a single winding provides both primary and secondary in a transformer. Various tapping of secondary winding are drawn to select various voltage levels at the secondary side.

The primary winding as shown above is from A to C and the secondary winding is from B to C whereas the variable arm B is varied to get the required voltage levels. A practical auto transformer looks like the figure below.

Practical Auto Transformer

By rotating the shaft above, the secondary voltage is adjusted to different voltage levels. If the voltage is applied across the points A and C is V1, then the voltage per turn in this winding will be


Working of Autotransformer:

A standard autotransformer, as shown in the figure below, has one single winding around a laminated core. This single winding is used for both primary & secondary circuit.


Their winding consists of at least three terminals i.e. A, B & C as shown in the figure. The terminals A & B are fixed terminals while the terminal C is a variable tap point. The AC supply is applied to fixed terminals A & B while the load is connected between variable Tap point C & B.

Types of Autotransformers:

Autotransformer is divided into two types, i.e., step up transformer & step down transformer.

Advantages of Auto Transformer:

  • The most prominent feature of an autotransformer is that it saves copper. The amount of copper used in an autotransformer is less than a two winding transformer of the same ratings. Thus, it reduces the capital required for its construction.
  • The single winding used in the autotransformer significantly reduces its size & weight.
  • Having small size & weight of the autotransformer, it enables it to have a high VA rating rather than an ordinary two winding transformer for the same amount of material.

Disadvantages of Auto Transformer:

  • There is no electrical isolation between the windings. So grounding the primary of an autotransformer won’t eliminate the risk of an electrical shock as both windings are electrically connected. The circuit will still complete through the ground.
  • Due to the electrical isolation present between the winding, two winding transformers block the transfer of harmonics between the load & supply, while autotransformer actually can’t.


  • They are used for the compensation of the voltage drop in the distribution transformers.
  • For starting induction & synchronous motors, several methods are used. One of the methods is using an autotransformer.

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