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N A<br />

To obtain the relationships necessary to convert from a Y to a D, first<br />

divide Eq. (8.6a) by Eq. (8.6b):<br />

R3 (RA RB)/(RA � RB � RC) RA<br />

� ���� � �� R1 (RB RC)/(RA � RB � RC) R<br />

RC R3 or RA ��<br />

R1<br />

Then divide Eq. (8.6a) by Eq. (8.6c):<br />

R3 (RARB)/(RA � RB � RC) RB<br />

� ���� � �� R2 (RARC)/(RA � RB � RC) RC<br />

R3 RC or RB ��<br />

R2<br />

Substituting for RA and RB in Eq. (8.6c) yields<br />

(RC R3 /R1)RC R2 �����<br />

(R3 RC /R2) � (RC R3/R1) � RC (R3 /R1)RC ����<br />

(R3 /R2) � (R3 /R1) � 1<br />

Placing these over a common denominator, we obtain<br />

R 2 �<br />

�<br />

(R 3R C /R 1)<br />

����<br />

(R 1R 2 � R 1R 3 � R 2R 3)/(R 1R 2)<br />

R 2R 3R C<br />

���<br />

R 1R 2 � R 1R 3 � R 2R 3<br />

R1R2 � R1R3 � R2R3 and RC � ���<br />

(8.7a)<br />

We follow the same procedure for R B and R A:<br />

R1R2 � R1R3 � R2R3 RA � ���<br />

(8.7b)<br />

R1R2 � R1R3 � R2R3 and RB � ���<br />

(8.7c)<br />

Note that the value of each resistor of the D is equal to the sum of the<br />

possible product combinations of the resistances of the Y divided by<br />

the resistance of the Y farthest from the resistor to be determined.<br />

Let us consider what would occur if all the values of a D or Y<br />

were the same. If R A � R B � R C, Equation (8.6a) would become<br />

(using R A only) the following:<br />

2<br />

R<br />

R3 ��<br />

ARB<br />

R<br />

� ��<br />

ARA<br />

RA<br />

RA � � �� � �<br />

RA � RB<br />

� RC RA � RA<br />

� RA 3RA<br />

3<br />

and, following the same procedure,<br />

R A<br />

R3<br />

R1<br />

R2<br />

R1 � R2 � RA � �3<br />

3<br />

C<br />

Y-D (T-p) AND D-Y (p-T) CONVERSIONS ⏐⏐⏐ 297

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