Chapter 7 : Effect of Temperature Upon Resistance s1

CHAPTER 23: INTRODUCTION TO THREE-PHASE SYSTEMS

1. Three 4.85 Ω resistors are connected in star (wye) across a 420-V, three-phase, supply. Calculate (a) the phase current, (b) the line current, and (c) total power. [Answer: (a) 50 A; (b) 50 A; (c) 36.38 kW]

2. Three 4.85 Ω resistors are connected in delta across a 420-V, three-phase, supply. Calculate (a) the phase current, (b) the line current, and (c) total power. [Answer: (a) 86.6 A; (b) 150 A; (c) 109.13 kW]

3. Three identical coils are connected in star (wye) across a 400-V, 50-Hz, supply and develop a total power of 1.5kW at a power factor of 0.2 lagging. Find (a) the resistance, and (b) the inductive reactance of each coil. [Answer: (a) 4.26 Ω; (b) 0.665H]

4. Three identical inductors, of inductance 19 mH and resistance 10 Ω are connected in delta across a 415-V, 50-Hz, supply. Calculate the resulting (a) line current, (b) power factor, and (c) total power of the circuit. [Answer: (a) 61.7 A; (b) 0.858 lagging; (c) 38 kW]

5. The current flowing through each of the three identical loads in a 400-V delta-connected circuit is 20 A with a phase angle of 40° lagging. What is (a) the line current drawn from the supply, and (b) the total power? [Answer: (a) 34.64 A; (b) 18.39 kW]

6. A balanced star (wye) connected load is connected to a 440-V; 50-Hz., three-phase supply. If the current in each phase is 30 A, at a phase angle of 30°lagging, calculate (a) the phase voltage; (b) the total power of the load. [Answer: (a) 254.1 V; (b) 19.8 kW]

7. Each phase of a 440-V, 50-Hz, delta-connected load comprises a 50-Ω resistor in series with a 50-μF capacitor. Calculate (a) the impedance of each phase, (b) the phase currents, (c) the line currents, (d) the total power, and (e) the total apparent power. [Answer: (a) 80.6 Ω; (b) 5.46 A; (c) 9.46 A; (d) 4480 W; (e) 7240 V·A]

8. The line current of a three-phase 440-V induction motor is found to be 41.2 A. What is the motor’s output power when it is operating at a power factor of 0.88 lagging and at an efficiency of 0.81? [Answer: 22381 W]