AUTO TRANSFORMER

             Generally we know about two winding transformer but there is another special type of transformer , namely autotransformer which is also used in several applications. Auto transformer is a single winding transformer in which a part of the winding is common to both high voltage and low voltage sides. That is only one winding is shared by the input and output side. The part of the  winding which is common to both high voltage and low voltage side is termed as ‘common winding’ and the remaining part of the main winding is called ‘series winding’. This series winding is connected in series with the common winding.
             Auto transformer is kind of electrical transformer where primary and secondary shares same common single winding.

Types of autotransformer:

According to the operation, the autotransformers are classified in two general categories.

•  Step up autotransformer –  In this type of autotransformer, the source voltage is connected to the common winding  and the load is connected across the main single winding .

• Step down autotransformer – In this type of autotransformer, the source voltage is applied to the main winding and common winding is connected across the load.

• Working principle of autotransformer

  The figure shows the circuit diagram of a stepup autotransformer. Now we are going to discuss the working principle of autotransformer. Here we can see that, the alternating voltage source is connected to the winding section ‘bc’ and the load is connected across the full winding section, that is ‘abc’. So, the single winding is shared by low voltage and high voltage side. Assume, TH= Number of turns of full winding abc, TL= Number of turns of the common winding bc, Tab= Number of turns of the series winding ab, VH= Voltage of high voltage side, VL= voltage of low voltage side, IH = Current in high voltage side, IL= Current in the low voltage side.
  So, balancing the part winding emf , IHTab= ITbc ………………..(1)
  Consider, the transformation is y= Tab / Tbc = VH / VL
  So, from the equation (1); I / IH = Tab / Tbc
  But, from the circuit diagram,  Tab = Tac - Tbc
  ∴ I / IH = (Tac - Tbc) / Tbc
  ∴ I / IH  = (Tac / Tbc )H
  ∴ I / IH = y-1.
  So from this equation we can say that, (I+IH) / IH = y………………(2)
  But, I + IH = IL
  Therefore, from  eqation (2),  IL / IH = y.
  And, I / IL= (y-1) / y.
  Applying KVL at high voltage side,
  Eab+Ebc = Eac
  ∴ Eab = Eac - Ebc  = yEbc - Ebc  =yEL-EL.
  Thus, Eab = (y-1)EL
  We can see from the equtaion that, Eab, that is the output voltage is nothing but the product of input voltage EL and (y-1). So, the output voltage is greater than the input volatge. Hence, the voltage level is increased after the transformation. In this way, by using a common winding, the transformer action is accompolished.

Copper Savings in Auto Transformer

Now we will discuss the savings of copper in auto transformer compared to conventional two winding electrical power 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

(N1 − N2)I1

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

N2( I2 − I1)

Hence, total weight of copper in the winding of Auto Transformer proportional to

(N1 − N2)I1 + N2( I2 − I1)

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,

Auto transformer employs only single winding per phase as against two distinctly separate windings in a conventional power transformer. Advantages of using auto transformer. 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 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.

Important features of an autotransformer

• The transformation ratio ( we considered y) of an autotransformer is always greater than unity.

• VA rating of an autotransformer is greater than VA rating of two winding transformer.
  That is, VA autotransformer > VA two winding transformer.

• Weight of conductor material in a winding is proportional to the product of current and number of turns. Hence there is a saving of the conductor material in the autotransformer as it consist of only a single winding. In more simple words, as we use only a single winding instead of two separtae winding, so, much less amount of core winding material (copper) is required and in this way, for the same purpose or application much less conductor is used. Saving of conductor material is more if the
  (1 / transformation ratio) approaches unity.

• Efficiency of autotransformer is more than 2 winding transformer. In the former one energy is transferred from primary to secondary side partly by induction and partly by direct electrical connection ie conduction . So losses are much less.

• Advantages of autotransformer

  • Autotransformers are smaller in size and cheaper than the two winding transformer of the same rating . Therefore it is little more economical than a convenient two winding transformer.
  •  In autotransformer, only a single winding is used. So, voltage regulation is superior because of the reduced voltage drop in resistance and reactance of the winding. Due to elimination of one winding, resistance and leakage flux for that is zero.
  • Autotransformer has more efficient than two winding transformer.

  • Disadvantages of Using Auto Transformer

    But auto transformer has the following disadvantages:
    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 to be same, except when using interconnected starring connections. This introduces complications due to changing primary and secondary phase angle particularly in the case-by-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 have to have their neutrality either earth or isolated.
    5. It is more difficult to preserve the electromagnetic balance of the winding when voltage adjustment tappings are provided. It should be known that the provision of adjusting 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 autotransformer

    •  Autotransformer is used widely in interconnection of power systems of different voltage levels. As an instance, autotransformer is used to make an interconnection of a 132 KV line and 230 Kv line.
    •  They are also used in boosting the supply voltage in distribution system to compensate voltage drop.
    •   Autotransformers with several number of tappings are used for starting of induction and synchronous motors. This starting methos is termed as autotransformer starting method.
    •  They are also used as variac in laboratories where continuous variable voltage is required and this variac is nothing but a variable AC voltage source.
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