‫بسم هللا الرحمن الرحيم‬

Omdurrman islamic university

Department of electrical and electronic engineering
19/03/2025
Open book quiz
By: Ahmed Ali
For all students male and female (A&B):
Section 1: Numerical Problems :
(Each question requires deep understanding and detailed calculations.)
A. Synchronous Machines (25 Questions)
1. A 100 MVA, 11 kV, 4-pole, 50 Hz synchronous generator has a synchronous
reactance of 1.1 pu and negligible resistance. Compute the excitation voltage for 0.85
lagging power factor.
2. A 3-phase alternator has an open-circuit voltage of 450 V at a field current of 5 A.
The short-circuit current at the same field current is 150 A. Determine the
synchronous reactance.
3. A synchronous motor running at 4000 RPM draws 300 kW at 0.8 power factor
(lagging) from a 6.6 kV supply. Calculate the armature current and excitation
voltage.
4. A 6-pole, 50 Hz synchronous machine operates at 400 V and supplies 500 kVA at
0.9 power factor lagging. Compute the torque angle and excitation voltage.
5. A turbo-alternator supplies 200 MW at 0.9 lagging power factor with an internal
excitation of 1.3 pu and reactance of 1.1 pu. Determine the power angle and system
stability.
6. A 3-phase synchronous motor with synchronous reactance 0.6 pu operates at 0.9
leading power factor with an input power of 200 kW. Compute the internal voltage
and torque angle.
7. A 3-phase, 60 Hz, 10-pole synchronous generator develops 50 MW at 0.8 power
factor lagging. Compute the developed torque.
8. A synchronous condenser supplies 2000 kVAR at 3.3 kV. Determine the required
excitation voltage and field current.
 9. A wound-field synchronous machine with a field current of 5 A produces an open-
circuit voltage of 600 V and a short-circuit current of 80 A. Determine the short-
circuit ratio (SCR).
10. A 6-pole alternator connected to a 50 Hz infinite bus has an inertia constant of 6
MJ/MVA. Compute the rotor acceleration when subjected to a 10 MW sudden load
change.
(Additional 15 questions covering transient stability, voltage regulation, V-curves, excitation
control, synchronizing power, and reactive power control.)

B. Induction Machines (25 Questions)

11. A 3-phase, 50 Hz, 4-pole induction motor has a full-load slip of 2%. Compute the
rotor frequency and full-load speed.
12. A 30 HP, 6-pole, 60 Hz induction motor has a rotor resistance of 0.1 Ω per phase
and reactance 0.8 Ω per phase. Compute the slip for maximum torque.
13. A 3-phase, 400 V, 50 Hz squirrel-cage induction motor delivers 40 kW at 0.85
power factor with an efficiency of 92%. Compute the stator current and input power.
14. A deep-bar induction motor operates at 4% slip and draws 60 A. Determine the
rotor copper loss and equivalent rotor resistance.
15. A wound rotor induction motor has a rotor resistance of 0.15 Ω per phase and a
reactance of 2.5 Ω per phase. Compute the external resistance needed to achieve
maximum starting torque.
16. A double-cage induction motor has an outer cage resistance of 0.25 Ω and an inner
cage resistance of 0.05 Ω. Determine the torque characteristics at start and full load.
17. A 3-phase, 400 V, 50 Hz induction motor has a rotor copper loss of 1.5 kW at full
load. Compute the air-gap power.
18. A three-phase slip-ring induction motor has a rotor impedance of (0.1 + j1.2) Ω per
phase. Compute the starting torque in terms of full-load torque.
19. A 6-pole, 50 Hz induction motor has a rotor EMF frequency of 3 Hz at full load.
Compute the slip and full-load speed.
20. A 2-pole, 50 Hz induction motor runs at 2900 RPM. Compute the slip and rotor
copper loss.
(Additional 15 questions covering equivalent circuits, circle diagrams, harmonics, induction
generator performance, and energy efficiency improvements.)

Section 2: Conceptual Questions (50 Questions)

21. Explain the impact of increasing excitation on synchronous motor power factor.
22. How does synchronous reactance affect the voltage regulation of an alternator?
23. Why does an induction motor draw high current at starting?
24. Compare squirrel-cage and wound-rotor induction motors in terms of efficiency.
25. Explain the significance of V-curves for a synchronous motor.
26. Why do synchronous motors require an external source for starting?
27. Discuss the methods of reducing hunting in synchronous machines.
28. How does the insertion of resistance in rotor windings affect induction motor
performance?
29. Explain why slip in an induction motor cannot be zero under load.
30. Why are induction generators not self-excited in a standalone system?
31. Discuss the concept of frequency control in synchronous machines.
32. What is the effect of armature reaction in alternators?
33. Why are induction motors preferred over synchronous motors for industrial
applications?
34. Explain the working of a synchronous condenser in power factor correction.
35. Why does an induction motor operate at a lagging power factor?
36. Discuss the role of short-circuit ratio (SCR) in synchronous machine design.
37. What are the effects of negative sequence currents on induction motors?
38. Explain why deep-bar rotors are used in some squirrel-cage induction motors.
39. Discuss the harmonics generated in synchronous machines and their impact.
40. What are the causes and effects of crawling and cogging in induction motors?
41. Explain how regenerative braking works in induction motors.
42. Why do wound rotor induction motors allow external resistance control?
43. Discuss the impact of rotor resistance on the efficiency of an induction motor.
44. Why do synchronous generators require parallel operation techniques?
45. Explain the effects of damper windings in synchronous machines.
46. How does saturation affect the performance of synchronous generators?
47. What are the different types of rotor windings used in synchronous machines?
48. How does field flux control affect the reactive power of synchronous machines?
49. Compare the starting performance of induction motors using DOL, star-delta, and
auto-transformer methods.
50. What are the conditions required for self-excitation in an induction generator?