Three-Phase Systems: Exercises and Problems, Slides of Electrical Engineering

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Prepared By Engr. Jeffrey P. Landicho

1. In a balanced 3 phase wye connected system,

the line voltage and the corresponding phase

voltage are displaced from each other by______.

a. 0 b. 30 c. 90 d.

2. A 3 phase load is balanced if all three phases

have the same______.

a. Z b. p.f c. Z & p.f d. P

3. The phase sequence of a three phase system

is BCA. The other possible phase sequence can

be___________.

a. CBA b. CAB c. ACB d.BAC

7. Three heater units each taking 1500 W are connected to a 120 V three phase line. What is the resistance of each unit in ohms? a. 9.6 b. 5.4 c. 8.6 d. 7.
8. The resistance between any two terminals of a balanced delta connected load is 12 ohms. The resistance of each phase is ________? a. 12 b. 18 c. 4 d. 36
9. It is required to increase the power factor of a 750 kW 3-phase balanced load from 70% to 90% lagging. The line voltage is 6900 V. Specify the capacitor required to increase the power factor of this wye connected load in microfarad per phase. a. 20.45 b. 22.39 c. 18.58 d. 17.

10. Two balanced load are connected in parallel to a three phase 460 V source. Load A is 90 kVA at a p.f. of 0. lagging and load B is 25 kVA at unity p.f. What is the new p.f of the system? a. 0.74 lag b. 0.74 lead c. 0.80 lead d. 0.80 lag
11. Given the following voltages and load impedances. Vab = 220 cis 0 V Zab = 10 cis 36.87 ohm Vbc = 220 cis 240 V Zbc = 10 cis 0 ohm V ca = 222 cis 120 V Z ca = 10 cis 45 ohm Determine the reading of a kW meter connected with its current coil connected in the line “b” while the potential coil connected across lines “b” and “c” a. 5.45 kW b. 4.08 kW c. 5.12 kW d. 4.26 kW

15. Two watt-metr method is applied to a 3-phase motor running at full-load. The two wattmeters indicate 85.5 kW and 34.7 kW. What is the operating p.f. of the motor? a. 87.45% b. 80.69% c. 89.49% d. 94.76%
16. The power drawn by a balanced 3-phase load is 3000 W at 80% p.f. Two wattmeters Wa & Wc are used, what is the reading of Wc? a. 800 b. 950 c. 900 d. 850
17. A balanced delta connected load draws 10 A of line current and 3000 W at 220 V. The reactance of each phase of the load is_____? a. 38.1 ohm b. 30 ohm c. 23.5 ohm d. 22 ohm
18. A 480 V 3-phase system supplies 85 A at a power factor of 0.8. What is the power factor angle? a. 60 b. 45 c. 37 d. 15

19. The sequence components of phase A current are: I 0 = 0.47 + j1.49, I 1 = 18.4 cis (- 31.6), I 2 = 3.23 cis 168. Determine the phase C current. a. 17.5 cis 91 c. 18 cis 215 b. 22.5 cis 82 d. 15 cis 100
20. The three unbalance currents are: I a = 10 cis -30, I b

= 0, I

c = 10 cis 150. Find the negative sequence of phase A current a. 8.66 cis 30 b. 5.77 cis - c. -5.77 d. 5.