D B1D009S Pages: 4

(a) Evaluate the capacitive reactance XC and express it in polar form. (1)

(0) Evaluate the total impedance across the source when the switch ऽ 6 in position |. 2 and 3
(൧. 1. and 25) respectively. ١

) For each of the position of 5. calculate the currents taken from the source. Express them

in polar form.

PART 0
Answer THO full questions. Each full question carries 10 marks.

7. (a) Define complex power. How is it related to “Apparent 17٤7 (4)

(b) A resistance of 400. an inductance of 318 H are connected in parallel. Find the
capacitance of a capacitor which. when connected in parallel with the combination. will
produce resonance with a supply frequency of 1 MHz. If a second capacitor of capacitance
23.5 pF is connected in parallel with the first capacitor. find the frequency at which resonance

will occur. What 15 the Q-factor in each case?
OR

. (a) Define quality factor. Give the expression for Q-factor in a series R-L-C circuit.(2)

(b) See Figure 9. The resistance is 20 Q and the capacitive and inductive reactances are -j20
and j20 respectively. If an average power of 500 W is dissipated in the 20 Q resistor, find (a)
|Vrms| of the voltage across the resistor, (0) |IL:rms|, (0) [Isrms|. (ध) the power factor seen by
the source, and (€) |Vs;rms|.

۷٦ = ]۴ 7

Figure 9: A series-parallel ac circuit

. (2) A 600 V rms three-phase Y-connected source has two balanced A loads connected to the

lines. The load impedances are 401 30° and 50L_- 60° respectively.

Determine the rms line current and the total average power.

(b) For power measurement in three-phase circuits, under which circumstances will you use
(i) Three wattmeter method. (ii) Single wattmeter method and (iii) Two wattmeter method?
Write answers in not more than two sentences for each case.

OR

20. The two-wattmeter method is used to determine the power drawn by a three-phase 440 V rms

motor that is a Y-connected balanced load. The motor draws a power of 20 kW. The
magnitude of the line current is 52.5 A rms. The wattmeters are connected in the A and C
lines. Find the reading of each wattmeter. The motor has a lagging power factor. Draw the
phasor diagram showing the voltages and currents measured by the two wattmeters.

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