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A single-phase, 22 kVA, 2200 V/220 V, 50 Hz distribution transformer is to be connected as an autotransformer to get an output voltage of 2420 V. Its maximum kVA rating as an autotransformer is

1. 22
2. 24.2
3. 242
4. 2420

Correct answer: 22

Solution

The transformer has a 2200 V winding and a 220 V winding. To obtain 2420 V, the 220 V winding is used in series aiding, so the autotransformer rating becomes the same as the original two-winding rating in this case. Hence the maximum kVA rating is 22 kVA.

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