Skip to main content

Auto Transformer: What is it? (Definition, Theory & Diagram)

 


What is an Autotransformer?

An autotransformer (or auto transformer) is a type of electrical transformer with only one winding. The “auto” prefix refers to the single coil acting alone (Greek for “self”) – not to any automatic mechanism. An auto transformer is similar to a two winding transformer but varies in the way the primary and secondary winding of the transformer are interrelated.

Autotransformer Theory

In an auto transformer, one single winding is used as primary winding as well as secondary winding. But in two windings transformer two different windings are used for primary and secondary purpose. A circuit diagram of auto transformer is shown below.

auto transformer

The winding AB of total turns N1 is considered as primary winding. This winding is tapped from point ′C′ and the portion BC is considered as secondary. Let’s assume the number of turns in between points ′B′ and ′C′ is N2.

If V1 voltage is applied across the winding i.e. in between ′A′ and ′C′.

Hence, the voltage across the portion BC of the winding, will be,

As BC portion of the winding is considered as secondary, it can easily be understood that value of constant ′k′ is nothing but turns ratio or voltage ratio of that auto transformer. When load is connected between secondary terminals i.e.between ′B′ and ′C′, load current I2 starts flowing. The current in the secondary winding or common winding is the difference of I2 and I1.

Copper Savings in Auto Transformer

Now we will discuss the savings of copper in auto transformer compared to conventional two winding transformer.
We know that weight of copper of any winding depends upon its length and cross-sectional area. Again length of conductor in winding is proportional to its number of turns and cross-sectional area varies with rated current.
So weight of copper in winding is directly proportional to product of number of turns and rated current of the winding.

Therefore, weight of copper in the section AC proportional to,

and similarly, weight of copper in the section BC proportional to,

Hence, total weight of copper in the winding of auto transformer proportional to,

In similar way it can be proved, the weight of copper in two winding transformer is proportional to,

N1I1 + N2I2

⇒ 2N1I1 (Since, in a transformer N1I1 = N2I2)

Let’s assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer respectively,

∴ Saving of copper in auto transformer compared to two winding transformer,

what is auto transformer
Auto transformer employs only single winding per phase as against two distinctly separate windings in a conventional transformer.

Advantages of using Auto Transformers

The advantages of an auto transformer include:

  1. For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size of two winding transformer. For transformation ratio say 20 however the size would be 95 %. The saving in cost of the material is of course not in the same proportion. The saving of cost is appreciable when the ratio of transformer is low, that is lower than 2. Thus auto transformer is smaller in size and cheaper.
  2. An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to reduction of transformer material.
  3. Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less.

Disadvantages of Using Auto Transformer

The disadvantages of an auto transformer include:

  1. Because of electrical conductivity of the primary and secondary windings the lower voltage circuit is liable to be impressed upon by higher voltage. To avoid breakdown in the lower voltage circuit, it becomes necessary to design the low voltage circuit to withstand higher voltage.
  2. The leakage flux between the primary and secondary windings is small and hence the impedance is low. This results into severer short circuit currents under fault conditions.
  3. The connections on primary and secondary sides have necessarily needs to be same, except when using interconnected starring connections. This introduces complications due to changing primary and secondary phase angle particularly in the case of delta/delta connection.
  4. Because of common neutral in a star/star connected auto transformer it is not possible to earth neutral of one side only. Both their sides should have their neutrality either earth or isolated.
  5. It is more difficult to maintain the electromagnetic balance of the winding when voltage adjustment tappings are provided. It should be known that the provision of tapping on an auto transformer increases considerably the frame size of the transformer. If the range of tapping is very large, the advantages gained in initial cost is lost to a great event.

Applications of Auto Transformers

he applications of an auto transformer include:

  1. Compensating voltage drops by boosting supply voltage in distribution systems.
  2. Auto transformers with a number of tapping are used for starting induction and synchronous motors.
  3. Auto transformer is used as variac in laboratory or where continuous variable over broad ranges are required.

Comments

Post a Comment

Popular posts from this blog

Hysteresis Eddy Current Iron or Core Losses and Copper Loss in Transformer

Losses in Transformer As the electrical transformer is a static device, mechanical loss in transformer normally does not come into picture. We generally consider only electrical losses in transformer . Loss in any machine is broadly defined as difference between input power and output power. When input power is supplied to the primary of transformer , some portion of that power is used to compensate core losses in transformer i.e. Hysteresis loss in transformer and Eddy current loss in transformer core and some portion of the input power is lost as I 2 R loss and dissipated as heat in the primary and secondary windings, because these windings have some internal resistance in them. The first one is called core loss or iron loss in transformer and the later is known as ohmic loss or copper loss in transformer . Another loss occurs in transformer, known as Stray Loss, due to Stray fluxes link with the mechanical structure and winding conductors. Copper Loss in Transformer Copper loss...
Post a Comment
Read more

Shell Type Transformer: What is it? And it's Applications

D epending on the construction of its magnetic core, we can categorize transformers into two types: Shell Type Transformer Core Type Transformer Core of Shell Type Transformer We use ‘E’s and ‘L’s shape laminations to make the core of the shell-type transformer . Limb of Shell Type Transformer The core of a single phase shell type transformer is constructed with of three limbs (legs). This design increases the mechanical strength of the core. It also improves the protection of windings from external mechanical shocks. The HV and LV windings are wound around the central limb. The central limb carries the entire flux (φ), whereas the side limbs carry half of the flux (φ/2). Hence, to accommodate the flux the cross-section of the central limb is twice than that of the side limbs. The magnetic flux flows through two closed magnetic paths which decrease the core losses and hence increase the efficiency of transformer . So, shell type transformer gives more output compared to similar c...
Post a Comment
Read more