Unit 3: Fundamentals of electrical machines - Practice Quiz

1 What does mutual inductance between two coils cause?

2 On which principle does a transformer operate?

3 For an ideal transformer, which expression gives the voltage ratio?

4 A transformer has more turns on the secondary winding than on the primary winding. What type of transformer is it?

5 What energy conversion occurs in a DC motor?

6 In which DC motor is the field winding connected in parallel with the armature?

7 Why is a starter used with a DC motor?

8 Which quantity is commonly varied for field control of a DC motor's speed?

9 Which DC motor is commonly used where high starting torque is required?

10 Why is a basic single-phase induction motor not self-starting?

11 What is the purpose of a capacitor in a capacitor-start induction motor?

12 What does a balanced three-phase supply produce in an induction motor stator?

13 Which motor is commonly used to drive industrial pumps and fans?

14 Fleming's left-hand rule is used to determine the direction of what quantity?

15 Fleming's right-hand rule is mainly associated with which device?

16 Which material is commonly used for a transformer core to improve magnetic coupling?

17 What happens when a transformer primary is connected directly to a steady DC supply?

18 What is a basic feature of an auto-transformer?

19 What is the main purpose of a current transformer?

20 Which instrument transformer is used to measure a high AC voltage safely?

21 Two coils have self-inductances and , with a coupling coefficient of . What is their mutual inductance?

22 A transformer transfers power between two circuits primarily through which mechanism?

23 An ideal transformer has primary turns and secondary turns. If the primary voltage is , what is the secondary voltage?

24 In a DC generator, the commutator performs which function?

25 Which DC motor is most suitable for applications requiring nearly constant speed despite changes in load?

26 Why is a starter required for a DC shunt motor?

27 In a DC shunt motor, reducing the field current while keeping the armature voltage constant generally causes the speed to:

28 Which motor is preferred for electric traction because it provides high starting torque?

29 A single-phase induction motor is not self-starting because its pulsating field at standstill produces:

30 A three-phase induction motor has a synchronous speed of and a rotor speed of . What is the slip?

31 Which AC motor is commonly used for constant-speed industrial drives because it can operate at a leading power factor?

32 According to Fleming's left-hand rule, if the magnetic field direction and current direction are both reversed, the force on a conductor will:

33 Fleming's right-hand rule is used to determine the direction of induced current in a:

34 If the dot convention indicates that currents enter the dotted terminals of both coupled coils, the mutual-induced voltages at the dotted terminals have:

35 What is the most likely result of connecting the primary of a transformer directly to a DC supply?

36 Compared with a two-winding transformer of the same rating, an auto-transformer generally uses:

37 The secondary of a current transformer should not normally be left open-circuited while its primary is energized because the secondary may develop:

38 A potential transformer has a primary-to-secondary turns ratio of . When the primary voltage is , the secondary voltage is approximately:

39 An ideal transformer supplies at on its secondary. If its primary voltage is , the primary current is:

40 A three-phase induction motor operates from a supply and has four poles. Its synchronous speed is:

41 Two magnetically coupled coils have self-inductances and , with coupling coefficient . What is their equivalent inductance when connected in series opposition?

42 A transformer designed for , has 500 primary turns. If it is supplied with at , what approximate maximum core flux results, assuming a sinusoidal supply?

43 An ideal transformer supplies a load from its secondary. What impedance is seen at the primary terminals?

44 A 4-pole DC generator has 600 armature conductors, flux per pole of , and a simplex lap winding. If it runs at , what generated emf is produced?

45 A long-shunt cumulative compound DC motor operates normally. The series-field connections alone are reversed while the shunt-field and armature connections remain unchanged. How is the machine then classified?

46 A DC shunt motor has an armature resistance of . Neglecting brush drop, what external starting resistance limits the initial armature current to ?

47 A DC shunt motor initially takes with armature resistance . Its flux is reduced to of the initial value, and the new armature current is . What external armature resistance maintains the original speed?

48 Which DC motor is most appropriate for a rolling mill requiring high starting torque but substantially better speed regulation than a series motor under sudden load changes?

49 A single-phase induction motor is manually accelerated in the forward direction to of synchronous speed. According to double-revolving-field theory, what are the forward and backward slips?

50 A wound-rotor induction motor has standstill rotor resistance and standstill reactance per phase. Neglecting stator impedance, what external resistance per phase makes maximum torque occur at slip ?

51 A factory requires a motor for a large constant-speed compressor, and the motor should also supply leading reactive power to improve the plant power factor. Which choice best satisfies both requirements?

52 A straight conductor carries conventional current vertically upward in a magnetic field directed geographically north. According to Fleming's left-hand rule, in which direction does the conductor experience force?

53 A conductor moves horizontally east through a magnetic field directed north. According to Fleming's right-hand rule, what is the direction of the induced conventional current in the conductor?

54 Two coupled coils have , , and . Their measured series equivalent inductance is . What does this measurement indicate?

55 A transformer primary rated at has a winding resistance of . It is accidentally connected to an ideal source. Which steady-state description is most accurate?

56 A , two-winding transformer is connected additively as a autotransformer. Assuming the original winding-current limits remain unchanged, what is its autotransformer rating?

57 An ideal current transformer carries in its primary. Which statement correctly gives the secondary current and the consequence of opening the secondary circuit?

58 A wattmeter is connected through an ideal CT and an ideal PT. If the wattmeter reads , what is the actual primary power?

59 A transformer has a secondary and equivalent secondary parameters and . What is its approximate full-load voltage regulation at power factor lagging?

60 An ideal transformer with no residual flux is energized from a sinusoidal source. Which switching condition produces the greatest first-cycle core-flux excursion?