AUTO-TRANSFORMERS – A transformer, in which a part of the winding is common to both the primary and secondary circuits, is called an auto-transformer. In a two winding transformer, primary and secondary windings are electrically isolated, but in an auto-transformer the two windings are not electrically isolated.
Advantages of auto-transformer over two winding transformer.
(1) The weight of conductor (copper or aluminium) for any winding depends upon the cross-sectional area and
length of the conductor. Now the conductor area is proportional to the current carried by it,whereas the length of conductor in a winding is proportional to the number of turns in the winding. Thus the weight of conductor in a winding is proportional to the current and number of turns in the winding.
(2) Owing to the reduction in conductor and core materials, the ohmic losses in conductor and the core loss are lowered. Therefore, an auto-transtormer has higher efficiency than a two-winding transformer of the same output.
(3) Reduction in the conductor material means lower value of ohmic resistance. A part of the winding being common, leakage flux and, therefore, leakage reactance is less. In other words,an Auto-Transformer has lower value of leakage impedandce and has Superior voltage regulation than a two-winding transformer of the same output.
Disadvantage of Auto-Transformer over two Winding transformer.
(1) If the ratio of transformation k differs far from unity, the economic
advantages of auto-transformer over two-winding transformer decrease.
(2) The main disadvantage of an auto-transformer is due to the direct electrical connection between the low-tension and high-tension sides. If primary is supplied at high voltage,
then an open circuit in the common winding BC, would result in the appearance of dangerously high voltage on the L.V. side. This high voltage may be detrimental to the load and the persons working there. Thus a suitable protection must be provided against such an occurrence.
(3) The short-circuit current in an auto-transformer is higher than that in a corresponding two-winding transformer.
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