Electrical machines power An autotransformer is a transformer having part of its

Question

Electrical machines power An autotransformer is a transformer having part of its winding included in both the input and output circuit. A simple autotransformer connection is shown in Fig. 1(b). The autotransformer can be constructed from a regular transformer. Fig 1(a) with its secondary winding connected to the primary as shown in Fig. 1(b). If the regular transformer is rated for 10 kVA and 120 V/24 V, what is the new output voltage? How much current can be drawn from the secondary terminals?

Explanation / Answer

Given rating of two winding transformer 10 kVA and 120 V / 24 V . Now it was reconnected as auto transformer as shown in Figure. b .

Voltage rating of auto transformer is 120 V / 144 V as dot are in adding.

We know transformation ratio of auto transformer is K/(1-K) where K = 120/144 of regular transformer or two winding transformer.   

We know power rating of auto tranformer is P' = P/(1-K) = 10/(1-0.833) = 60 kVA .

Current drawn by secondary terminal is I'2 = I2 = P/24 = 10000/24 = 416.667 A which is same as secondary winding current of regular transformer.

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