An auto transformer is a special type of electrical transformer with only one winding on an iron core. In auto transformer, one single winding is used as primary winding as well as secondary winding.
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 that the number of turns in between points B and C is N2. If V1 voltage is applied across the primary winding i.e. in between A and B. So voltage per turn in this winding is V1 / N1
Hence, the voltage across the portion bc that is the secondary, will be V1N2 /N1 and this voltage is V2.
Hence, V1N2 / N1 = V2
⇒ V2 / V1 = N2 / N1 = Constant = k
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 the voltage ratio of that autotransformer.
Uses of an autotransformer
Power distribution : Auto transformers are frequently used in power applications to interconnect systems operating at different voltage levels, such as to 66 kV to 138 kV transmission line.
Another use of auto transformer is in industry to adapt machinery built for 480 V supplies to operate on a 600 V supply.
Auto transformers are also used for providing conversions in between the two common main voltage bands in the area
(suppose 100 to 130 and 200 to 250).
In long power distribution lines, special auto transformers equipped with automatic tap-changer are inserted as voltage regulators. Thus, customers at the far voltage source.
A special type of auto transformer is used to provide grounding on three-phase systems.
Tapped auto transformers are frequently used to match impedance, eg: they are inserted in between a low-impedance microphone and a high-impedance amplifier input.
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