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ƒ r<br />

1 1 1<br />

and YT � �� � j<br />

R ��� � �� X X �<br />

(20.27)<br />

For unity power factor, the reactive component must be zero as<br />

defined by<br />

and XLp � XC (20.28)<br />

Substituting for X Lp yields<br />

� R2l � X 2 L<br />

� � XC X L<br />

(20.29)<br />

The resonant frequency, f p, can now be determined from Eq. (20.29) as<br />

follows:<br />

R 2 l � X 2 L � X C X L � � � qL �<br />

or X 2 L � �R 2 L<br />

� l<br />

C<br />

L 2<br />

with 2pfpL � � � R ��C ��l� and fp � �� � � R � 2 1 L<br />

� �<br />

C l�<br />

2pL<br />

Multiplying the top and bottom of the factor within the square-root<br />

sign by C/L produces<br />

fp � �� � R<br />

�1� ����<br />

2 1 � R 1<br />

lC<br />

�� �<br />

2pL�C�/L� L<br />

2 1<br />

l(C/L)<br />

� ��<br />

2pL C/L<br />

1<br />

and fp �� 2p�L�C� �<br />

or f p � f s�<br />

1 1<br />

� ���0 XC XLp<br />

1 1<br />

Therefore, � �<br />

XLp<br />

� XC<br />

1<br />

�<br />

qC<br />

1� �� �R<br />

1� �� �R<br />

�L<br />

�L<br />

2<br />

l C ��<br />

2 l C ��<br />

(20.30)<br />

(20.31)<br />

where f p is the resonant frequency of a parallel resonant circuit (for<br />

F p � 1) and f s is the resonant frequency as determined by X L � X C for<br />

series resonance. Note that unlike a series resonant circuit, the resonant<br />

C<br />

L p<br />

L<br />

� C<br />

PARALLEL RESONANT CIRCUIT ⏐⏐⏐ 903

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