An Auto Transformer is a transformer with only one winding wound on a laminated core. An autotransformer is similar to a two winding transformer but differs in the way the primary and secondary winding are interrelated. A part of the winding is common to both primary and secondary sides. On load condition, a part of the load current is obtained directly from the supply and the remaining part is obtained by transformer action. An Autotransformer works as a voltage regulator.
Autotransformers can be used as a method of soft starting induction motors.
The autotransformer winding has at least three taps where electrical connections are made. Since part of the winding does "double duty", autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased VA rating for a given size and mass.
Applications of Autotransformer
It is used as a starter to give up to 50 to 60% of full voltage to the stator of a squirrel cage induction motor during starting.
It is used to give a small boost to a distribution cable, to correct the voltage drop.
It is also used as a voltage regulator
Used in power transmission and distribution system and also in the audio system and railways.
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