What is auto transformer and how is it different from normal transformer

What is a transformer

A transformer is a power converter that can transfer energy between two electrical circuits by inductive coupling between two or more windings. The transformer plays a major role in the power system, the transformer has made it possible that the power generated at low voltages for transmission over a long distance.

What is the function of a transformer and what is the use of a transformer

A transformer changes voltage by means of mutual electromagnetic induction, the electrical current in the primary coil is used to create a magnetic field. Changes in current will result in changes in the magnetic field, and it will alter the voltage of the secondary coil or circuit. Mostly transformers are used to convert one voltage to others. The process of increasing the voltage is called stepping up while decreasing the voltage is called stepping down, the most electronic device needs a transformer to lower the main voltages to a usable level. Transformers can minimize energy loss during long-distance electricity distribution.

What is an autotransformer


An autotransformer consists of a single winding with one or more taps on it, autotransformers are not automatic. They are used for low power applications and they are dangerous because they don’t provide any electrical isolation. Autotransformers are used in places where variable voltage is needed, they are special types of the power transformer and they have only one winding. Different voltages can be obtained by tapping or connecting at certain points. Autotransformer is a kind of electrical transformer in which the primary and secondary shares same common secondary winding. So the autotransformers use one single winding as primary and secondary

What is the difference between autotransformer and normal transformer

An autotransformer has only a single winding per phase, while the conventional transformer has two windings. In an ordinary transformer, there are two windings called the primary and secondary and they are magnetically coupled and electrically isolated. But in an autotransformer part of windings are common to both the primary and secondary. The size of the autotransformer is less than a normal transformer because of the single winding so the autotransformer will be half the size of a two winding transformer. Because of the single winding, the autotransformer would be able to do some saving in the lamination weight also. The larger the ratio of voltages smaller is the savings. So autotransformer is an ideal choice for close ratio transformations. In two winding transformer, the primary voltage is associated with the primary winding, the secondary voltage is associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary voltage.

Autotransformer Two winding transformers
They have a single winding per phase A pair of winding are used per phase
The primary and secondary of autotransformer share the same winding in this primary and secondary has separate winding
Wide range of voltage variation is possible Small variation of output voltage for a given input voltage is possible
Energy transfer occurs by both conduction and induction Energy transfer occur only by induction
Required excitation current is very small Required excitation current is high when compared to autotransformer
The load connected to the autotransformer is electrically connected with the source Load connected to this transformer is electrically isolated from the source
Autotransformers are efficient Less efficient when compared to the autotransformer

How to construct an autotransformer and how does it work

An autotransformer can be used to convert a fixed AC voltage into a variable AC voltage. The primary and secondary of an autotransformer is electrically connected so that a part of the winding is common to both the windings. It can also be explained as the autotransformer is the only transformer with one winding and the same windings act as both the primary and secondary sides of a transformer.

Autotransformer consists of a single copper wire common to both the primary and secondary circuits. The copper wire is wound over a silicon steel core, mostly three taps are provided over the windings which will provide three levels of the output voltage. By providing a smooth sliding brush over the winding we could obtain a variable turn ratio. The primary and secondary windings are connected electrically as well as coupled magnetically and this would make the autotransformer cheaper, smaller, and more efficient

In an autotransformer, energy is transferred mainly through the conduction process and only a small part is transferred inductively. Voltage per turn is the same for the primary and secondary so the voltage can be easily varied by varying the number of turns. Load connection is done by connecting one of its terminals to one of the tappings and the neutral is connected to the other one. An autotransformer has a single winding with two end terminals and one or more terminals at immediate tap points. The primary voltage is applied across the two terminals and the secondary voltage will be taken from the two terminals, almost always having one terminal in common with a primary voltage. The primary and secondary circuits will have a number of windings that turn in common. The volts per turn are the same in both windings so each will develop a voltage which is in proportion to its number of turns. In this transformer part of the current flows directly from the input to the output, and an only small part is transferred inductively

What is a variable autotransformer

A variable autotransformer is used when we need to get a variable AC voltage from a fixed voltage source. The transformer is composed of a single layer winding wound uniformly on a toroidal iron core, a movable carbon brush in sliding contact with the winding serves as a variable tap. When the brush slides over the bared portion of the winding, the secondary voltage increases in proportion to the number of turns swept out. The input voltage is connected to the tap on the winding. Variable autotransformers are efficient and provide good voltage regulation under variable loads.

What are the advantages of autotransformer

  • Voltage can be varied smoothly as per our requirement
  • Efficiency is higher than a normal transformer
  • Less conducting material is required when compared to two winding transformer
  • Small in size and less expensive than the two winding
  • Resistance and leakage reactance is less
  • Copper loss is less
  • Voltage regulation is better than the two winding transformer

What are the disadvantages of autotransformer

  • Major disadvantage of this transformer is that the primary and secondary is not electrically isolated
  • There is no isolation, so the generated harmonics by the equipment connected to the supply will be passed to the supply
  • Autotransformers have low impedance so high short circuit currents on the secondary side
  • If the section common to both the primary and secondary is opened whole primary voltage will occur across the secondary which may lead to severe accidents

What are the applications of autotransformer

  • Autotransformer can be used as an auto starter for induction machines
  • It is used in testing laboratories
  • They are used in the interconnection of EHV systems
  • It can be used in places where electrical isolation is not a critical requirement