AUTO TRANSFORMERS

An auto transformers has windings common to primary and secondary, so that the input and output circuit are electrically connected as one continuous winding per phase. In the common part of the winding, the input and output currents are superposed. The principal application of the auto transformer is in cases where separation of the primary and secondary circuits is not necessary, and the voltage ratio is not great. Such applications include boosters, the static balancers and induction – motor starters. The advantage gained is a considerable saving in conductor material, cores, and losses.
Auto transformer required no conductor material, for actually no transformer is needed. The savings in materials and cost is, of course, nearly as large as with the normal transformer, and the cost and efficiency of transformers are not directly proportional to their KV ratings.

Multi ratio auto transformer could be graded with a suitable conductor area for each section according to the kind of rating – constant output KV, or constant output current, or constant load impedance. Auto transformer are not generally employed when the voltage ratio exceeds 3:1, except for motors starting duty, as the disadvantages preponderate. The reactance is inherently low on account of (a) the “coalescence” of the primary and secondary windings and (b) the smaller currents, so that the conditions on short circuit are more severe .

The savings in materials and cost is, of course, nearly as large as with the normal transformer, and the cost and efficiency of transformers are not directly proportional to their KV ratings.In the common part of the winding, the input and output currents are superposed. The principal application of the auto transformer is in cases where separation of the primary and secondary circuits is not necessary, and the voltage ratio is not great. Such applications include boosters, the static balancers and induction – motor starters. The advantage gained is a considerable saving in conductor material, cores, and losses.