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EXAMPLE 21.3 For the iron-core transformer of Fig. 21.6:<br />

a. Find the magnitude of the current in the primary and the impressed<br />

voltage across the primary.<br />

b. Find the input resistance of the transformer.<br />

Solutions:<br />

I p<br />

N s<br />

REFLECTED IMPEDANCE AND POWER ⏐⏐⏐ 943<br />

a. � � �<br />

Is Np<br />

Np = 40 t<br />

N s<br />

5 t<br />

� �<br />

Np 40 t<br />

VL � IsZL � (0.1 A)(2 k�) � 200 V<br />

I p � I s � � � (0.1 A) � 12.5 mA<br />

Also, �<br />

b. Z p � a 2 Z L<br />

Np �<br />

Ns<br />

Vg �<br />

VL<br />

a � �8<br />

N p<br />

� Ns<br />

Np �<br />

Ns<br />

40 t<br />

�<br />

5 t<br />

V g � V L � � � (200 V) � 1600 V<br />

Z p � (8) 2 (2 k�) � R p � 128 k�<br />

EXAMPLE 21.4 For the residential supply appearing in Fig. 21.7,<br />

determine (assuming a totally resistive load) the following:<br />

+ R R<br />

V L<br />

–<br />

Main service<br />

+<br />

I p<br />

2400 V<br />

–<br />

N 1<br />

Residential service:<br />

120/240 V, 3-wire,<br />

single-phase<br />

N 2<br />

120 V<br />

240 V<br />

120 V<br />

Ten 60-W bulbs<br />

FIG. 21.7<br />

Single-phase residential supply.<br />

a. the value of R to ensure a balanced load<br />

b. the magnitude of I 1 and I 2<br />

c. the line voltage V L<br />

d. the total power delivered<br />

e. the turns ratio a � N p/N s<br />

I 1<br />

TV<br />

400 W<br />

I 2<br />

Air<br />

conditioner<br />

2000 W<br />

+<br />

V g<br />

–<br />

I p<br />

Z p<br />

Denotes iron-core<br />

Is = 100 mA<br />

N s = 5 t<br />

FIG. 21.6<br />

Example 21.3.<br />

R 2 k�<br />

+<br />

V L<br />

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