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990 ⏐⏐⏐ POLYPHASE SYSTEMS<br />

A<br />

3-phase<br />

∆-connected ac generator<br />

Phase sequence: ACB<br />

B<br />

C<br />

I Aa<br />

E AB = 120 V ∠ 0°<br />

E CA = 120 V ∠ v 3<br />

I Bb<br />

I Cc<br />

E BC = 120 V ∠ v 2<br />

FIG. 22.20<br />

Example 22.3: D-D system.<br />

c<br />

V ca<br />

+<br />

–<br />

I ca<br />

–<br />

5 �<br />

Solutions:<br />

a. For an ACB phase sequence,<br />

v2 � 120°<br />

b. Vf � EL. Therefore,<br />

and v3 � �120°<br />

Vab � EAB The phase currents are<br />

Vca � ECA Vbc � EBC V 120 V �0°<br />

ab 120 V �0°<br />

Iab ���——— �–––<br />

Zab (5 � �0°)(5 � ��90°) 25<br />

� ��90°<br />

�� 5 ��j 5 � 7.<br />

071<br />

��45°<br />

120 V �0°<br />

��� �33.9 A �45°<br />

3.54 � ��45°<br />

Vbc 120 V �120°<br />

Ibc ����� �33.9 A �165°<br />

Zbc 3.54 � ��45°<br />

Vca 120 V ��120°<br />

Ica ����� �33.9 A ��75°<br />

Zca 3.54 � ��45°<br />

c. IL � �3�If � (1.73)(34 A) � 58.82 A. Therefore,<br />

IAa � IBb � ICc � 58.82 A<br />

EXAMPLE 22.4 For the D-Y system shown in Fig. 22.21:<br />

a. Find the voltage across each phase of the load.<br />

b. Find the magnitude of the line voltages.<br />

Solutions:<br />

a. IfL � IL. Therefore,<br />

Ian � IAa � 2 A �0°<br />

Ibn � IBb � 2 A ��120°<br />

Icn � ICc � 2 A �120°<br />

V bc<br />

a<br />

I ab<br />

5 � 5 �<br />

5 �<br />

5 �<br />

+<br />

Vab 5 �<br />

I bc<br />

+<br />

–<br />

b

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