Autotransformer - Construction, Working, Advantages, Disadvantages and Applications

What is an Autotransformer?

The normal power transformer has different primary windings and secondary windings. But the autotransformer is a special type of transformer in which a part of the winding is common for the primary and secondary windings. In ordinary power transformers. primary and secondary windings are insulated with proper insulation and connected magnetically from each other. Auto transformer is single winding transformer.

In an autotransformer, primary and secondary windings are connected electrically as well magnetically to each other. In autotransformer transformation ratio is unity. In auto transformer more saving in copper because only one winding is used. 

The construction of the autotransformer is shown in below fig.

The autotransformer consists of one full copper winding wound on a laminated magnetic core, with a rotary movable contact with different voltage tappings. Thus from the autotransformer, three terminals are brought out for connection. The autotransformer can operate as a step up or step down transformer. 

Autotransformer as a step-down transformer 

The connection of the autotransformer as a step-down transformer is shown in fig. 

It shows that two fixed terminals A and B are connected to the single-phase AC supply V₁. thus the winding AB acts as the primary winding.

A part of complete transformer winding i.e CB which is full windings that act as the secondary winding across the load is connected. The operating principle of an autotransformer is the same as a normal transformer.

Therefore load voltage for step down autotransformer is given by 

N_{2 = Number of turns corresponding to secondary windings i.e CB}

N_{1  =} Number of turns corresponding to primary windings i.e AB

As the number of turns corresponding to the winding CB i.e N_{2 is less than that of winding AB i.e} N_{1, this configuration operates as a step-down transformer.}

If we neglect the losses, the magnetising current and the leakage reactance, then the transformer ratio is defined as 

I_{1 = Primary current and} I_{2 = Load current} 

Autotransformer a step-up transformer 

Fig shows the connection of an autotransformer for operating as a step-up transformer.

Note that part CB of the complete winding acts as the primary winding. The ac input voltage V1 is applied between terminals C and B. The full winding AB acts as a secondary winding of autotransformer and the load is connected between the terminals of winding AB.

As the number of turns of winding AB now N2 is higher than the number of turns of winding CB now N1, the autotransformer now acts as a step-up transformer.

Neglecting the losses, the magnetizing current and the leakage reactance, the load voltage V2 is given by 

Advantages of Autotransformer

1. In autotransformer, only one winding is used as well as primary and secondary windings, the copper required for making the windings is very little. In a transformer copper saving is more if transformation ratio is near to unity. In case of autotransformer transformer transformation ratio is unity so there are more copper saving in autotransformer. 
2. The only winding is used in autotransformer as primary as well as secondary so the Size of the transformer is reduced and also cost is reduced in autotransformer as compared to the conventional power transformers.
3. The losses taking place in the winding are reduced hence the efficiency is higher than the conventional transformer.
4. Due to reduced resistance, the voltage regulation is better than the conventional transformer. Due to one winding in autotransformer, resistance is less so voltage drop is less in autotransformer.
5. The cost of an autotransformer is very less, voltage regulation is better and less ohmic losses in autotransformers because only one winding is used. The varying voltage can be obtained in case of autotransformer by varying the different tapping. 

Disadvantages of an autotransformer

1. There is no electrical isolation between the primary and secondary windings which is very dangerous for high voltage applications. 
2. If the common part of winding i.e CB breaks means open circuited then the transformer action is lost and full primary voltage appears across the secondary winding as shown in fig.So Autotransformer is not used for interconnecting the high-voltage and low voltage systems.

3. It possesses a low impedance, hence if the secondary circuit is short-circuited, then a large current will flow on the secondary side.

Applications of an Autotransformer

1. It can be used as a variac i. e variable ac supply to vary the ac voltage applied to the load smoothly from 0V to about 270 V.
2. Autotransformer is used to start the ac machines such as induction motors and synchronous motors.
3. Autotransformer is used to vary the supply voltage as a requirement of induction furnaces.
4. Autotransformer is used in dimmer stat. When the variac autotransformer is used to control the intensity of light of lamps in the cinema halls etc. it is called dimmer stat.
5. By using an autotransformer, compensate the voltage drop by boosting the supply voltage in the various distribution system.
6. The autotransformer is used as a voltage regulator.
7. The autotransformer is used as a stater to give 50 to 60 % voltages to the stator of the squirrel cage induction motor.
8. Autotransformer is used in transmission and distribution systems as well as audio systems and railways. The autotransformer is used as booster transformer for raising the voltage in AC feeders.