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Vg � Ip(Rp � j XLp ) �� �Xm�90° Thus, the input impedance is<br />

Zi � �Rp�j XLp �<br />

or, defining<br />

Zp � Rp � j XLp Zs � Rs � j XLs and Xm �90° ��j qM<br />

we have<br />

(�j qM)<br />

Zi � Zp �<br />

2<br />

(Xm �90°)<br />

�<br />

Zs � ZL 2<br />

�IpXm �90°<br />

��<br />

Rs � j XLs � ZL Vg � ��<br />

Ip<br />

Rs � j XLs � ZL ( qM)<br />

and Zi � Zp � �� (21.36)<br />

Z � Z<br />

The term (qM) 2 /(Z s � Z L) is called the coupled impedance, and it is<br />

independent of the sign of M since it is squared in the equation. Consider<br />

also that since (qM) 2 is a constant with 0° phase angle, if the load<br />

Z L is resistive, the resulting coupled impedance term will appear capacitive<br />

due to division of (Z s � R L) into (qM) 2 . This resulting capacitive<br />

reactance will oppose the series primary inductance L p, causing a<br />

reduction in Z i. Including the effect of the mutual term, the input<br />

impedance to the network will appear as shown in Fig. 21.32.<br />

EXAMPLE 21.10 Determine the input impedance to the air-core<br />

transformer in Fig. 21.33.<br />

Z i<br />

Solution:<br />

q = 400<br />

R p<br />

3 �<br />

L p = 6 H<br />

M = 0.9 H<br />

s<br />

2<br />

L s = 1 H<br />

FIG. 21.33<br />

Example 21.10.<br />

L<br />

R s<br />

0.5 �<br />

R L<br />

40 �<br />

Zi � Zp �<br />

� Rp � j XLp �<br />

� 3 ��j 2.4 k��<br />

� j 2.4 k�� 129.6 � 103 ((400 rad/s)(0.9 H))<br />

�<br />

��<br />

40.5 � j 400<br />

2<br />

(qM)<br />

���<br />

0.5 ��j 400 ��40 �<br />

2<br />

(qM)<br />

��<br />

Rs � j XLs � RL 2<br />

�<br />

Zs � ZL AIR-CORE TRANSFORMER ⏐⏐⏐ 957<br />

Z i<br />

R p<br />

L p<br />

Coupled<br />

impedence<br />

2M2 –––––– ω<br />

Zs + ZL FIG. 21.32<br />

Input characteristics for the air-core<br />

transformer.

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