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Autotransformers
In cases where the ratio of transformation does not differ greatly from unity, and in cases that do not require the secondary winding to be isolated from the primary winding, a great saving in size can be effected by using autotransformers in place of the twocircuit transformers discussed previously in this chapter. An ordinary twocircuit transformer can be converted to an autotransformer by connecting its windings 1 and 2 in series with each other as shown in Fig. 618. In the autotransformer of Fig. 618, consider winding 1 to be between points a and b and winding 2 to be between points b and c. The voltage V_{H} on the high side of the autotransformer is the phasor sum of the terminal voltages of windings 1 and 2, i.e.
The phase angle between the primary and secondary voltage of a power transformer is small. In the case of the transformer in Example 62, the angle between the primary and secondary voltage phasors is smaller than 1°, although phase angles as high as 10° are possible. Nevertheless, it can be assumed for most practical purposes that the arithmetic sum of the voltages V_{1} and V_{2} equals the phasor sum, i.e.
In addition, the ratio of the terminal volts V_{2} and V_{1} is within a few percent of the turns ratio, as indicated in the transformer of Example 62. Hence
The exciting ampere turns required by the flux in the core is the phasor difference between the ampere turns of the two windings. Generally, in ironcore transformers the exciting current is small in comparison with the rated current. (In the transformer of Example 62 the ratio of exciting current to rated current is 1.70 / 62.5 or 2.72 percent.) Then, if the exciting current is neglected, the ampere turns of the two windings must be equal, and we have
The current I_{x}, which flows in the lowvoltage terminals, is the sum of the secondary current I_{L} and the primary current I_{1} as shown in Fig. 618, so that
A comparison of Eqs. 663 and 664 shows that
When the ratio of transformation is near unity, the transformer has a much higher rating when operating as an autotransformer than when operating as a 2circuit transformer. This is shown as follows
Thus, if the voltage rating as a 2circuit transformer is 2400/240, the voltampere rating as a 2640/2400v autotransformer is eleven times that of the voltampere rating as a 2winding transformer.


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