Autotransformer working

In this articale we will disscuss about the autotransformer working and application. we will discuss in detail that what is autotransformer.

1. Autotransformer working

autotransformer working, application of autotrasformer,what is autotransformer


Basically autotransformer working is same simple transformer but some difference in this type transformer only one winding are available but in case of simple transformer which have two winding.

2. Autotransformer working and construction

The primary and secondary windings of a two winding transformer have induced EMF in them due to a common mutual flux. The currents drawn by these two winding are out of phase by 180 dgree. This prompted the use of a part of the primary as primary as secondary. This is equivalent to fusing the secondary turn into primary turns. The fused section need to have a cross sectional area of the conductor to carry (secondary current – primary current) ampere. This ingenious thought led to the invention of an auto transformer.

for more learning of autotransformer working please s show the diagram the physical arrangement of an auto transformer. Total number of turns between A and C are primary turns. At point B a connection is taken. Section AB has secondary turns, As the volts per turn, which is proportional to the flux in the transformer, is the same for the whole winding,

Primary voltage:  secondary voltage = Primary turns:  secondary turns


For simplifying analysis, the magnetizing current of the transformer is neglected. When the secondary winding delivers a load current which is the magnetizing ampere turns. This will be countered by a primary current flowing from the source through the primary turns such that


 Primary current x secondary turns = Secondary current x primary turns

3. Advantage of an autotransformer

1. for the same capacity and voltage ratio, an autotransformer requires less winding material than a two-winding transformer. Hence, there is saving in copper.

2. An auto transformer is smaller in size and cheaper than a two winding transformer of same output.

3. An auto transformer has higher efficiency since, core loss and ohmic  losses are smaller.

4. Voltage regulation of an autotransformer is better because of reduced voltage drops in the resistance and reactance.

5. An auto transformer has variable output voltage when a sliding contact is used for the secondary.

4. Disadvantage of an autotransformer

1. There is direct connection between the high voltage and low voltage side. If there is an open circuit in the winding BC. The full primary voltage would be applied to the secondary. This high voltage may cause serious damage to the equipment’s connected on the secondary side.

2. The short circuit current is larger for an autotransformer due to reduced internal impedance.

5. Application of autotransformer

1. Autotransformers are used for obtaining continuously variable ac voltage.

2. They are used fro interconnection of power systems of different voltage levels.

 3. They are applied for boosting of a.c. mains voltage by a small amount .

4.  Autotransformers are used for starting the induction motors and synchronous motors.


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