116 Chapter 8 • Transformers


11. A single-phase 220-V/15-V transformer has 1 000 primary turns. Neglecting losses, calculate; 11.1 the number of secondary turns,


11.2 the primary current when a current of 10 A is drawn from the secondary, and


11.3 the rating of the transformer if the maximum allowable secondary current is 15 A.


(69 turns) (0,68 A) (225 VA)


12. An 11-kV/220-V single-phase step down transformer has 3 600 primary turns. Calculate; 12.1 the transformer ratio,


12.2 the number of secondary turns,


12.3 the primary current if 200 A is drawn from the secondary, and


12.4 the power output if the secondary power factor is 0,8 when supplying 200 amperes.


8.2 Three-phase transformers


Tree-phase transformers may be regarded as three single-phase transformers with their primary and secondary windings connected in either star or delta. As mentioned previously, when making use of the transformer equation in calculations involving three-phase transformers, it is of utmost importance to make use of phase values, i.e.:


In Star IL In Delta VL = Iph = Vph and VL and IL = 3 Vph = 3 Iph . .


Te primary and secondary windings may be connected in either of the following combinations, fig. 8.1:


(50:1)


(72 turns) (4 A)


( 35,2 kW)


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