Unit 3: Fundamentals of electrical machines - Practice Quiz

1 Mutual inductance between two coils is the property by which a changing current in one coil induces an EMF in the:

2 The SI unit of mutual inductance is the:

3 In a transformer, magnetic coupling between the two windings is provided mainly by the:

4 A transformer works on the principle of:

5 A transformer can operate only with:

6 A transformer that increases the output voltage is called a:

7 The turns ratio of a transformer is defined as the ratio of:

8 For an ideal transformer, the voltage ratio equals the turns ratio as

9 In an ideal transformer, if the voltage is stepped up, the current in that winding is:

10 The device used to convert the alternating EMF in a DC generator armature into DC at the output is the:

11 Based on the field connection, DC machines are commonly classified as series, shunt and:

12 A DC motor converts electrical energy into:

13 A starter is used with a DC motor mainly to limit the:

14 Which DC motor is best suited for applications requiring very high starting torque, such as traction?

15 The rotating magnetic field in a three-phase induction motor is produced by the:

16 A single-phase induction motor is not self-starting because its stator produces a:

17 Three-phase induction motors are widely used in industry mainly because they are:

18 Fleming's left-hand rule is used to find the direction of force in a:

19 In Fleming's right-hand rule, the thumb represents the direction of:

20 If a DC voltage is applied to a transformer primary, the output voltage at the secondary will be:

21 A single-phase transformer has 400 primary turns and 100 secondary turns. If the primary voltage is , what is the secondary voltage (assuming ideal transformer)?

22 An ideal step-up transformer increases voltage by a factor of 5. If the primary current is , the secondary current is:

23 What happens when a DC supply is connected to the primary of a transformer?

24 Two coils have a mutual inductance of . If the current in one coil changes at a rate of , the EMF induced in the other coil is:

25 Two coils with self-inductances and are perfectly coupled (). The mutual inductance is:

26 The primary function of the laminated iron core in a transformer is to:

27 The EMF equation of a transformer is . If the supply frequency is doubled while keeping voltage and turns constant, the maximum flux will:

28 Compared to a two-winding transformer of the same rating, an auto-transformer requires:

29 A current transformer (CT) is used to measure large currents. During operation, its secondary should:

30 A potential transformer (PT) with ratio shows on its voltmeter. The actual line voltage is:

31 Fleming's left-hand rule is used to determine the direction of:

32 In Fleming's right-hand rule, the forefinger, thumb, and middle finger respectively represent:

33 In a DC motor, the back EMF () at the instant of starting is:

34 A DC shunt motor runs at . If the flux is reduced to of its original value (armature EMF roughly constant), the speed becomes approximately:

35 Which DC motor is best suited for applications requiring high starting torque, such as electric traction and cranes?

36 A DC series motor should never be started without a mechanical load because:

37 A 4-pole, three-phase induction motor is connected to a supply. Its synchronous speed is:

38 A three-phase induction motor has a synchronous speed of and runs at . The percentage slip is:

39 A single-phase induction motor is not self-starting because:

40 Which AC motor is most commonly used in fans, pumps, and household appliances due to its ruggedness, low cost, and low maintenance?

41 Two coils have self-inductances and . When connected in series aiding, the total inductance is . What is the coefficient of coupling ?

42 A transformer rated for AC is accidentally connected to a DC supply. Which outcome best describes what happens and why?

43 A step-down transformer has turns ratio . The secondary is loaded with . What impedance is reflected to the primary?

44 In a practical transformer at no-load, the no-load current lags the applied voltage by nearly . What does this large angle primarily indicate?

45 A , step-down transformer is reconnected as an auto-transformer to give . What is the new kVA rating as an auto-transformer?

46 Compared to an equivalent two-winding transformer of the same output, an auto-transformer transfers part of the power conductively. For a turns ratio where (step-down), what fraction of the total power is transferred inductively (transformed)?

47 Why should the secondary of a current transformer (CT) never be left open-circuited while the primary carries current?

48 A potential transformer (PT) is used to measure a line voltage with a standard voltmeter. If the measured ratio error causes the voltmeter to read for an actual , what is the approximate percentage ratio error?

49 In a DC generator, the function of the commutator is best described as:

50 A DC shunt motor runs at drawing an armature current of from a supply with armature resistance . If flux is held constant and the load torque is unchanged, what series armature resistance must be added to reduce the speed to ?

51 A DC shunt motor has armature resistance . To limit the starting current to twice the full-load current of , what starter resistance is required at the instant of starting?

52 In a DC series motor, if the load torque suddenly drops to nearly zero (no-load), what happens and why?

53 A -pole, three-phase induction motor runs at . What is the frequency of the rotor-induced EMF?

54 Why is a single-phase induction motor not self-starting, according to the double-field revolving theory?

55 A conductor carries current in the direction and lies in a magnetic field pointing in the direction. Using Fleming's left-hand rule, in which direction is the force on the conductor?

56 Which statement correctly distinguishes the two Fleming's rules in the context of electrical machines?

57 For a constant-speed drive such as a large industrial fan or pump where power factor correction is also desired, which AC motor is most suitable?

58 A transformer has iron loss and full-load copper loss . At what fraction of full load does maximum efficiency occur, and how does the copper loss there compare with the iron loss?

59 In field (flux) control of a DC shunt motor above base speed, the output power is approximately constant. If flux is reduced to times its original value with armature voltage and current held at rated, the torque capability becomes:

60 Two magnetically coupled coils connected in series carry the same current. When connected series-aiding the total inductance is and when series-opposing it is . What is the mutual inductance ?