WAZIPOINT Engineering Science & Technology: Autotransformer Specialty Difference from Conventional Transformer

Monday, June 13, 2022

Autotransformer Specialty Difference from Conventional Transformer


Auto Transformer

What is an autotransformer?

Auto Transformer is not any automatic machine or equipment. Actually, the word auto adds before the Transformer because the autotransformer is single core transformer rather than conventional two cores.

Autotransformer is a type of electrical transformer with only one core that is its specialty. It is a transformer with one winding only, part of this being common to both primary and secondary. Obviously, in this transformer, the primary and secondary are not electrically isolated from each other as is the case with a 2-winding transformer. However, its theory and operation are similar to those of a two-winding transformer. Because of one winding, it uses less copper and hence is cheaper.

Circuit Diagram of Auto Transformer

The figure above shows the autotransformer circuit diagram. It shows both the step-down and step-up



As shown in the above figure (a), A B is primary winding having N1 turns, BC is secondary winding having N2 turns, and BC is secondary winding having N2 turns.  

Neglecting iron losses and no-load current.


The current in section CB is the vector difference between I2 and I1. But, as the two currents are practically in phase opposition, the resultant current is (I2 − I1) where I2 is greater than I1.


As compared to an ordinary 2-winding transformer of the same output, an autotransformer has higher efficiency but a smaller size. Moreover, its voltage regulation is also superior.


Types of Auto Transformers

There are three types of autotransformers:

1.      Step-up autotransformer;

2.      Step-down autotransformer; &

3.      Variable autotransformer.

Step-Down autotransformer
Fig-step-down autotransformer

You might understand the use of step-down and step-up autotransformers to step-down and step-up the voltages accordingly.

Step-Up autotransformer
Fig-Step-up autotransformer

The variable autotransformer actually uses to provide a wide range of AC voltages in industry and laboratory.


Why is Auto Transformer Used?

Autotransformers used industrial and power distribution networks for many purposes, such as- the starting of induction motors, used to regulate the voltage of transmission lines, and can use to transform voltages when the primary-to-secondary ratio is close to unity.

Difference between autotransformer and conventional transformer

Due to having a single core for primary and secondary winding in an autotransformer, the electrical power transfer from primary to secondary partly by the process of transformation and partly by the direct current where the Copper loss is negligible

The conventional transformer transfers the electrical power through the electric transformation due to which power loss occurs.


Working Principle of Autotransformer

Though the autotransformer works in a single core system, it uses the principle of Faraday’s Law of electromagnetic induction.

“Whenever a conductor is placed in a varying magnetic field, an electromotive force is induced”


The advantages of autotransformer

Less loss and better efficiency make the autotransformer different from conventional two-winding transformer. An autotransformer used less materials than 2-winding transformer that causes the Ohmic losses and core losses;


When does autotransformer use in transmission and distribution network?

When the transformation ratio for voltage is small, the autotransformer uses in the transmission and distribution systems to get the maximum transformation ratio;

In transmission line where the step-down voltage level is low like 230kV to 132kV, then autotransformer is used, but in distribution, system where the step-down level is too high like 11kV to 0.4kV, then autotransformer is avoid in distribution system;

The voltage control of an auto transformer is better because the voltage drop in resistance and reactance of the single winding is lower;

Less expensive;

Improved regulation;

When compared to a standard two-winding transformer of the same rating, there are fewer losses.

Disadvantages of an Autotransformer

The main disadvantage of an autotransformer is that it does not have the primary to secondary winding isolation of a conventional double-wound transformer. Then an autotransformer can not safely be used for stepping down higher voltages too much lower voltages are suitable for smaller loads.

If the secondary side winding becomes open-circuited, the load current stops flowing through the primary winding stopping the transformer action and resulting in the full primary voltage being applied to the secondary terminals.

If the secondary circuit suffers a short-circuit condition, the resulting primary current would be much larger than an equivalent double-wound transformer due to the increased flux linkage damaging the autotransformer.

Since the neutral connection is common to both the primary and secondary windings, earthing of the secondary winding automatically Earth’s primary as there is no isolation between the two windings. Double-wound transformers are sometimes used to isolate equipment from the earth.

You may know the details about the electrical transformer from the following articles:

  1. Working Principle of Transformer;
  2. Transformer Construction;
  3. Core-type Transformers;
  4. Shell-type Transformers;
  5. Elementary Theory of an Ideal Transformer;
  6. E.M.F. Equation of Transformer;
  7. Voltage Transformation Ratio;
  8. Transformer with losses but no Magnetic Leakage;
  9. Transformer on No-load;
  10. Transformer on Load;
  11. Transformer with Winding Resistance but no Magnetic Leakage;
  12. Equivalent Resistance;
  13. Magnetic Leakage;
  14. Transformer with Resistance and Leakage Reactance;
  15. Simplified Diagram;
  16. Total Approximate Voltage Drop in Transformer;
  17. Exact Voltage Drop;
  18. Equivalent Circuit Transformer Tests;
  19. Open-circuit or No-load Test;
  20. Separation of Core Losses;
  21. Short-Circuit or Impedance Test;
  22. Why Transformer Rating in KVA?;
  23. Regulation of a Transformer;
  24. Percentage Resistance, Reactance, and Impedance;
  25. Kapp Regulation Diagram;
  26. Sumpner or Back-to-back-Test;
  27. The efficiency of a Transformer;
  28. Condition for Maximum Efficiency;
  29. Variation of Efficiency with Power Factor;
  30. All-day Efficiency;
  31. Auto-transformer;
  32. Conversion of 2-Winding Transformer into Auto-transformer;
  33. Parallel Operation of Single-phase Transformers;
  34. Questions and Answers on Transformers;
  35. Three-phase Transformers;
  36. Three-phase Transformer Connections;
  37. Star/Star or Y/Y Connection;
  38. Delta-Delta or ∆/∆ Connection;
  39. Wye/Delta or Y/ Connection;
  40. Delta/Wye or ∆/Y Connection;
  41. Open-Delta or V-V Connection;
  42. Power Supplied by V-V Bank;
  43. Scott Connection or T-T Connection;
  44. Three-phase to Two-Phase Conversion and vice-versa;
  45. Parallel Operation of 3-phase Transformers;
  46. Instrument Transformers;
  47. Current Transformers;
  48. Potential or Voltage Transformers.


  1. By changing the AC voltage that is being applied to AC motors, their speed can be controlled. So, an autotransformer can be used to change AC voltage and control speed of motors which drive fans.

  2. Three phase auto transformer
    The three-phase auto-transformer has a single coil that acts both as the primary and secondary coil. They are used for starting induction motors, in audio systems, in power transmission and distribution, and even in railways.

  3. The speed of AC motors can be adjusted by adjusting the AC voltage provided to them. As a result, an autotransformer can be used to control the speed of motors that drive fans and modify the AC voltage. By the way, let me tell you that help with dissertation writing paperis something I'm telling a buddy about. I came across this post regarding the Autotransformer Specialty Difference from a Conventional Transformer at the moment. Thank you for sharing it with us.


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