Auto Transformers: A Complete Guide for Engineers

 

What is an Auto Transformer?

 

An auto transformer “commonly known as Variac” is a type of transformer that has only one winding which acts as both primary winding as well as secondary winding. Unlike a conventional two winding transformer, which has two separate windings primary and secondary that are electrically isolated, in an auto transformer a part of primary winding is used as a secondary winding by the help of tapping. 

 

The schematic diagram of a single-phase auto transformer is shown in the given figure, where we can clearly see that there is only one winding and a section of it is tapped to serve as the secondary winding. In fact, it is nothing but a conventional two winding transformer that is connected in a special way.

 

This design reduces the amount of copper required, as separate secondary winding is not present, making autotransformers more compact, efficient and more cost effective as compared to traditional two winding transformers. 

Due to the presence of this electrical connection between primary and secondary winding in the auto-transformer the power is transferred by the help of induction as well as conduction process. 

Actually, an auto transformer is nothing but a conventional two winding transformer that is connected in a special way as shown in the given figure.

 

Working of an Auto Transformer

 

Like a conventional two winding transformer, an auto transformer also works on the principle of Faraday's law of mutual induction, according to which the power is transferred from one voltage level to another voltage level through mutual induction.

However, unlike a conventional transformer, where the primary and secondary windings are electrically isolated, an autotransformer has a direct electrical connection between the primary and secondary sides. Because of this connection, in autotransformers the power is transferred through both induction and conduction processes.

 

Now, let us discuss this in details with the help of optimum schematic diagram and mathematical equations

 

Consider an auto transformer having N₁ turns on the primary side and N₂ turns are tapped for the lower voltage secondary side as shown in the given figure.

In the given figure above, the branch AC is referred to as primary winding having N₁ turns and the winding section BC having N₂ turns is common for both primary and secondary, and the section AB is referred to as series winding having and (N₁ - N₂) turns. 

 

MMF Balance of Autotransformer

 

(N₁ - N₂) I₁   -    N₂.(I₂ - I₁)  =  0

 

N₁. I₁  -  N₂.I₁  -  N₂.I₂  +  N₂.I₁   =   0

 

N₁. I₁  -  N₂.I₂   =   0

 

N₁. I₁   =   N₂.I₂ 

The two winding counterpart of the given auto transformer can be drawn as shown in the given figure. (As we stated above an auto transformer is nothing but a convention to winding transformer connected in a special way)

With reference of the above figure the transformation ratio of the two winding transformer is 

And the transformation ratio of its counterpart autotransformer is 

By observing both of the above equation we can conclude that,

a_(auto)  =  a_(TW) +  1 

 

 

Now, let's compare the VA rating of the two winding Transformer and its counterpart auto transformer as shown in the above figure.

 

The VA Rating of the Autotransformer shown in the above figure   =  V₁.I₁   =  V₂.I₂

 

And VA Rating of its counterpart Two Winding Transformer   =   (V₁ - V₂).I₁

By carefully observing the above equation then we conclude that a two winding transformer of a given VA rating when connected as an auto transformer can handle a high power (VA) rating. This is because in an auto transformer connection a part of VA (power) is transferred through the conduction process as discussed previously in this article. 

Copper saving in Autotransformer as compared to its two winding equivalent

 

As discussed in the previous sections, the main advantage of an auto transformer as compared to its equivalent two winding transformer is copper saving. Now in this section let us understand how much copper is saved when a transformer is reconnected as an auto transformer. 

 

Let us take the example of an auto transformer which we have discussed in the previous section.

 

Since, the copper is primarily used in the windings of the transformers and as the secondary winding is absent in an auto transformer, the copper that would have been used for the secondary winding is saved. 

 

The weight of the copper used in the winding mainly depends on its cross-sectional area and length. The length of the copper used in windings depends on the number of turns of the winding while the cross-sectional area of the copper conductor is directly proportional to the current flow through it. 

 

Therefore, the weight of the copper used in the transformer is directly proportional to the number of turns and the current flows through it, which is equal to MMF.

Where G stands for weight of the copper winding material. After rearranging the above equation we get,

Above equation represents the saving of copper (winding material) in an auto transformer as compared to its two winding equivalents.

 

Using this equation, we can conclude, if a_(auto) = 10 then the copper saving is only 10% and if a_(auto) = 1.1 then the copper saving is 90%. Hence the use of auto transformers is more economical when the turns ratio is close to unity like in interconnecting two high voltage systems at different voltage levels. This will result in considerable saving of bulk copper and cost as compared to conventional to winding transformers. 

Advantages of autotransformer 

 

One of the major advantages of an auto transformer compared to its equivalent two winding transformer is copper saving. This is because, unlike a two winding transformer an auto transformer does not have a separate secondary winding, as discussed above.

 

Auto transformers are more economical as compared to two winding transformers due to the low amount of copper used. However, they only prefer where the voltage ratio is less than 2 and in which case electrical isolation to winding is not essential.

Due to the reduced use of copper, autotransformers also have lower copper losses.

Unlike a two winding transformer where the power is transferred only inductive in nature, in autotransformers power is transferred through inductive as well as conductive means. This conductive transfer enables autotransformers to handle a higher VA (volt-ampere) rating compared to their two-winding counterparts.

 

The auto transformer has lower reactance, lower losses, smaller exciting current and better voltage regulation compared to its two winding counterpart. All this is on account of the fact that in an auto transformer a part of energy is transferred through the conduction process.

 

 

 

Disadvantages of Auto transformers

 

Unlike a conventional two winding transformer, an auto transformer does not provide electrical isolation between two sides, therefore any disturbance on one side quickly spreads to the other side.

 

During the step-down operation, if the common winding develops an open circuit, then the load is subjected to full high voltage of the source. Therefore, to prevent the damage, the insulation of its low voltage winding must be strengthened in order to withstand the higher voltage during this operation.

 

Applications of Autotransformer

Auto transformers are generally used in following areas,

• To interconnect the two power-system at different voltage level where the voltage ratio is limited to 2:1

• As a booster for line drop compensation in the electrical traction supply system.

• To start a large three phase induction motor usually of the squirrel cage type or in the speed control of three phase induction motor, specifically in voltage reduction method

• Auto transformers are also used as continuously variable transformer Variac in lab experiments.

• Auto transformers are also used in manual, automatic and servo voltage stabilizers for domestic commercial and industrial use.