Autotransformer Advantages and Disadvantages

In an autotransformer, the common portion of the single winding acts as part of both the “primary” and “secondary” windings. The remaining portion of the winding is called the “series winding” and is the one that provides the voltage difference between both circuits, by adding in series with the voltage of the common winding


The transfer of power between two circuits connected to an autotransformer occurs through two phenomena: the magnetic coupling (as in a common transformer) and the galvanic connection (through the common socket) between the two circuits. Likewise, a transformer increases its capacity to transfer power when connected as an autotransformer


Types of autotransformer


A. Reducing autotransformer

If an alternating voltage is applied between points A and B, and the output voltage between


points C and D, it is said that the autotransformer is voltage reducer.



Fig. 1 Reducing autotransformer


Ratio of turns Ns / Np <1


B. Autotransformer elevator

If an alternating voltage is applied between points C and D, and the output voltage is measured between points A and B, the autotransformer is said to be a voltage booster.


Elevator autotransformer

Fig. 2 Elevator Autotransformer


Ratio of turns Ns / Np> 1




The operating principle is the same as that of the common transformer, so the transformation ratio between voltages and currents and the number returned is maintained.


The primary and secondary currents are in opposition and the total current flowing through the coils in common is equal to the difference of the current of the low voltage winding and the high voltage winding.


For an autotransformer to work Properly the two windings must have the same sense of winding



It has a single coil wound on the core, but has four terminals, two for each circuit, and therefore has points in common with the transformer


It consists of a winding of ends A and D, which has been made a derivation in the intermediate point B. For now we will call primary to the complete section AD and secondary to the portion BD, but in practice it can be the other way round, when you want to raise the primary tension.



Fig. 3 Autotransformer operation


The voltage of the primary network, to which the autotransformer will be connected, is V1, applied to points A and D. shown in fig. 3 Like every coil with an iron core, as soon as that voltage is applied, a current flows that we have called vacuum in the previous theory. We also know that this vacuum current is formed by two components; one part is the magnetizing current, which is 90 ° behind the voltage, and another part that is in phase, and is the one that covers the losses in the iron, whose amount is found by multiplying that part of the vacuum current, by the applied voltage. We call the total current of vacuum I0, as we have done in other opportunities.


Types of construction

There are autotransformers with several sockets in the secondary and therefore, with several transformation ratios. In the same way as transformers, autotransformers can also be equipped with automatic tap changers and used in transmission and distribution systems to regulate the voltage of the electric network.


There are also autotransformers in which the secondary intake is achieved through a sliding brush, allowing a continuous range of secondary voltages ranging from zero to the voltage of the source. This last design was commercialized in the United States under the generic name of Variac and in practice it works as a source of alternating current in tension. In this way we have a more effective AC machine.



Fig. 4 Type of Autotransformer “Variac”



Within the autotransformers there are operating limitations that will be announced below


a) A fault in the insulation of the windings of an autotransformer can cause the load to be exposed to receive full voltage (that of the source). This situation must be taken into account when deciding to use an autotransformer for a certain application.

b) The advantages in saving material have a physical limitation, which in practice is a ratio of voltages of 3: 1. For higher voltage ratios, either the conventional two-winding transformer is more compact and economical, or it is impossible to build the autotransformer.

c) In electric power transmission systems, autotransformers have the disadvantage of not filtering the harmonic content of currents and acting as another source of ground fault currents.

d) There is a special connection -called “connection in” ZIG-ZAG “that is used in three-phase systems to open a return path to the ground current that otherwise would not be possible to achieve, maintaining the system ground reference.



Fig. 5 Zigzag connection (Z)


Advantages and disadvantages

The advantages and disadvantages of autotransformers will be explained below. In short, we say that there is a wide range of advantages of autotransformers




  • Only a percentage of the energy is transmitted by induction
  • The autotransformer due to its characteristics becomes smaller, so it would occupy less space
  • There is less field flow and smaller iron core size.
    Lighter autotransformers are obtained.
  • The autotransformer carries a single winding
  • Lower voltage drops
  • Lower vacuum intensity
  • It is easier to build and requires less copper.
  • Consequently it is more economical.
  • Part of the energy of the autotransformer is transmitted electrically.
  • The electrical losses are always less than the magnetic losses
  • The autotransformer has higher performance
  • The autotransformer generates more power than a normal transformer of similar specifications
  • It has a small short-circuit voltage which poses the disadvantage that the current in case of short circuit is high
  • Transfers more power than a normal transformer



Fig. 6 Autotransformer of a central


  • The main disadvantage of autotransformers is that unlike ordinary transformers there is a direct physical connection between the primary and secondary circuits, so the electrical insulation on both sides is lost.
  • Danger of cutting a turn, which would cause the secondary is subject to the voltage of the primary
  • Galvanic conduction between the primary and secondary
  • Low voltage regulation due to its low equivalent impedance
  • Due to the electrical construction of the devices, the input impedance of the autotransformer is lower than that of a common transformer. This is not a problem during the normal operation of the machine, but if for some reason there is a short circuit at the output
  • The output of the transformer is not isolated with the input, this becomes unsafe for the person who operates it.
  • They do not have isolations in the primary and secondary


There are very important applications for the operation of other machines that require autotransformers

  • Autotransformers are often used in electrical power systems, to interconnect circuits that operate at different voltages, but in a ratio close to 2: 1 (for example, 400 kV / 230 kV or 138 kV / 66 kV).



Fig. 7 Autotransformer


  • In industry, they are used to connect machinery manufactured for nominal voltages different from that of the power supply (for example, 480 V motors connected to a 600 V supply).
    They are also used to connect appliances, appliances and smaller loads in any of the two most common feeds worldwide (100-130 V to 200-250 V). In rural distribution systems, where distances are long, you can use autotransformers specials with ratios around 1: 1, taking advantage of the multiplicity of sockets to vary the supply voltage and thus compensate for the appreciable voltage drops at the ends of the line.



Fig. 8 Distribution autotransformer


  • Autotransformers are also used as a soft start method for squirrel-cage induction motors, which are characterized by demanding a high current during start-up.



Fig. 9 Motor soft start autotransformer
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  • In high-speed rail systems there are dual feeding methods such as the one known as 2 × 25 kV. In this, the transformers of the substations feed at +25 kV to the catenary, at -25 kV to the feeder or negative feeder and with the intermediate or neutral outlet placed on the rail. From time to time, typically 10 km, autotransformers are connected with 50 kV in the primary and 25 kV in the secondary. In this way, the load (trains) is fed at 25 kV between catenary and lane but the energy is transported to 50 kV , reducing losses.



Fig. 10 Autotransformer for railway systems



In this investigation we conclude with the points mentioned above in which we find a wide range of advantages of the autotransformer and disadvantages explain the operation and how the autotransformer operates if we say that the autotransformer is characterized by the others by having a single winding


Elsewhere we say that autotransformers offer applications such as railway systems, squirrel-cage motor starters and the rural distribution system


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