What is an autotransformer?

The primary and the secondary of the autotransformer will be connected magnetically and physically, due to this the transfer of power will be done by both induction and conduction. In an autotransformer, the primary and the secondary would use the same winding. So this transformer can be considered as a one winding transformer and that’s the major difference between an autotransformer and a normal transformer. This transformer has only one winding which is a common winding. The voltage will be varied in the winding by varying the secondary tapping on the body of the coil.

What are the uses of autotransformers?

  • These transformers are used for power transmission due to their ability to provide better KVA.
  • These transformers are used in radial distribution feeder circuits because of their ability to regulate the voltage.
  • It is used to provide the starting voltage for an induction motor
  • These transformers can be seen in certain control equipment for the electrical locomotives.

What is the difference between an autotransformer and a two-winding transformer?

The major difference between these two transformers is that the autotransformer is a one winding transformer and the normal transformer has two windings. The energy transfer of the autotransformer is mostly done by conduction and a small portion of the energy transfer is done inductively. The windings of the two winding transformers are isolated electrically, but in the case of the autotransformer, the windings are directly connected. So if there is a fault in one part then it would affect the other part too. An autotransformer has higher efficiency than the two winding transformers because it has very small core loss and ohmic loss. The voltage regulation capability of the autotransformer is really high when compared to a two-winding transformer due to the less reactance and resistance for the single winding.

The autotransformer is used in a high voltage substation instead of the two winding transformers. The size of the normal transformer is really high and due to this weight, copper, and cost would increase too. The territory winding in the autotransformer can balance the single-phase unbalanced loads which are connected to the secondary and these unbalanced currents won’t reach the primary. So if there is no unbalanced current then there will be no harmonics and voltage imbalance. The amp turns would be balanced by the territory windings, so this transformer could acquire the magnetic separation like a two winding transformer. In a two-winding transformer, the output voltage can be varied only by changing the input voltage or by tap changing but in the case of the autotransformer, the output voltage can be varied by changing the number of the secondary turns. In an autotransformer wide range of voltage, variations can be achieved but in the case of two wining transformers, only a small variation of the voltage is possible. The excitation current required for a two-winding transformer is really high but for an autotransformer very small excitation current is used. The load which is connected to the autotransformer is connected with the source electrically, but in a two-winding transformer, the load will be electrically isolated from the source.

Why is the autotransformer not used as a distribution transformer?

The ratio difference in the distribution transformer is really high when compared to the autotransformer. For example, in a distribution transformer, the voltage level would be 11000v/400v, whereas in an autotransformer it would be 220Kv/132Kv.

What is the purpose of a tap changer in a transformer?

We can vary a transformer’s output voltage by tap changing. The above image shows the transformer tapping and this image shows multiple tapping. The transformer must provide the full load output on all taps and the transformer must be capable of working on all taps without any problems. 

How is an autotransformer constructed?

This transformer consists of a single copper winding and this will be common for both the primary and the secondary circuit. The copper wire would be wound on a laminated silicon core and it would have three tappings. The secondary and the primary would have the same neutral point. The variable turn ratio can be achieved by the winding tappings. The primary terminals of the autotransformer are fixed.

How does an autotransformer work?

As we discussed above there is only one winding for this transformer and both the primary and the secondary part shares it. The number of turns which is for the secondary will be variable and this is done to get the required output. The output voltage of this transformer can be determined by combining the voltage of both the first and second winding. The common winding is the winding in which the primary and the secondary voltage would appear. The winding which is small would be the series winding and it would be connected to the common winding.

The part of the winding which is connected to the power source is the primary winding and the secondary winding is the part of the winding which is connected to the output or the load. There will be a common neutral point for the secondary and the primary, the secondary voltage can be obtained from any of the tapping and neutral points.

What is a variable autotransformer?

Certain autotransformers have sliding taps and they can be used to change the turn ratio over the full range of the transformer and they are the variable autotransformer. In this transformer, a single primary winding would be wrapped around a magnetic core and in this, there won’t be any fixed tapping point. A carbon brush is used to tap the secondary voltage, the carbon brush would be allowed to move along a section of the primary winding. Due to the carbon brush sliding contacts are made to the primary winding section and thus required voltage level will be supplied. 

What are the types of autotransformers?

The output voltage of an autotransformer can be stepped up or stepped down.

Step down autotransformer

In the step-down autotransformer, the whole winding would act as the primary and a part of the winding would be used commonly to both the primary and the secondary. In this transformer, the high voltage AC will be converted to a low voltage AC.

Step up autotransformer

In this type, the whole winding would act as the secondary and a part of the winding would be used commonly to both the primary and the secondary. This transformer would convert the low alternating voltage to high. The output voltage is dependent on the turn ratio of the transformer.

What are the advantages of autotransformers?

  • Small in size
  • Less weight
  • Low cost
  • Better voltage regulation
  • Losses are reduced for the given KVA capacity
  • The KVA rating is increased for the given core size
  • The KVA transfer capability of the autotransformer is really better than the conventional transformer
  • A small amount of the excitation current is used
  • The amount of copper which is used for the construction of the autotransformer is really low
  • According to the requirement, the transformer output can be varied
  • The leakage reactance is low because the leakage flux between the secondary and the primary flux is low and due to this low impedance
  • These transformers are affected by atmospheric overvoltages
  • High insulation is required in this when compared to the normal transformer
  • Surge arrester is inevitable due to the chances of faults
  • Better efficiency when compared to the normal transformer

What are the disadvantages of autotransformers?

  • There is no electrical isolation, so in case of any noise on one side then it would affect the other part too. So if we need to use these transformers in electronic circuits then filtering circuits are needed.
  • If there is any failure in the winding insulation then the whole input voltage will be transferred to the output
  • Autotransformer connection has a limitation that they are not suitable for all the types of three-phase connections
  • Due to the lower series impedance, a higher short circuit current can be formed, and this could create a lot of damage if there are no preventive measures. 
  • These transformers are not suitable to step down a high voltage to a low voltage due to the safety issues   

What are the applications of the autotransformer?

  • Autotransformer is used for voltage regulation, so these transformers are used in the transmission and the distribution lines to handle the voltage drop problems
  • These transformers are used as a continuous variable supply
  • It is used to decrease the voltage supplied to an AC motor during the starting period
  • The autotransformer can be used to interconnect two systems that have different voltages
  • These transformers are used in high voltage substation


post-graduate in Electronics & communication.

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