Question

Problem 3: A single phase 60 Hz two-winding power transformer is rated at 2400V/240V and 100 kVA with
Zeq = 0.5 + j0.8 Ohm. If an autotransformer is built from this conventional transformer to supply a load at
2640V with supply voltage 2400V. Draw the autotransformer and calculate Zeq, the voltage, current and
apparent power rating of the auto transformer. Also calculate the apparent power rating advantage of
autotransformer over conventional transformer.

Answer 1

Advantages of using Auto Transformers

1. For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size of two winding transformer. For transformation ratio say 20 however the size would be 95 %. The saving in cost of the material is of course not in the same proportion. The saving of cost is appreciable when the ratio of transformer is low, that is lower than 2. Thus auto transformer is smaller in size and cheaper.
2. An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to reduction of transformer material.
3. Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less.