+2 votes
in Mathematics by kratos

For the circuit shown in Fig. , find the values of R and C so that Vb = 3Va, and Vb and Va are in phase quadrature. Find also the phase relationships between Vs and Vb, and Vb and I.

1 Answer

+3 votes
by kratos
 
Best answer

∠ COA = φ = 53.13°

∠ BOE = 90° − 53.13° = 36.87°

∠ DOA = 34.7° Angle between V and I

Angle between Vs and Vb = 18.43°

XL = 314 × 0.0255 = 8 ohms

Zb = 6 + j 8 = 10 ∠ 53.13° ohms

Vb = 10 I = 3 Va, and hence Va = 3.33 I

In phasor diagram, I has been taken as reference. Vb is in first quadrant. Hence Va must be in the fourth quadrant, since Za consists of R and Xc. Angle between Va and I is then 36.87°. Since Za and Zb are in series, V is represented by the phasor OD which is at angle of 34.7°.

| V |= √10 Va = 10.53 I

Thus, the circuit has a total effective impedance of 10.53 ohms.

In the phasor diagram, OA = 6 I , AC = 8 I, OC = 10 I = Vb = 3 Va

Hence, Va = OE = 3.33 I,

Since BOE = 36.87°, OB −RI = OE × cos 36.87° = 3.33 × 0.8 × I = 2.66 I.

Hence, R = 2.66 And BE = OE sin 36.87° = 3.33 × 0.6 × I = 2 I

Hence Xc = 2 ohms. For Xc = 2 ohms, C = 1/(314 × 2) = 1592 μF

Horizontal component of OD = OB + OA = 8.66 I

Vertical component of OD = AC − BE = 6 I

OD = 10.54 I = Vs

Hence, the total impedance = 10.54 ohms = 8.66 + j 6 ohms

Angle between Vs and I = ∠ DOA = tan−1 (6/866) = 34.7°

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