MOD.3 (PARTE 4) - FUNDAMENTOS ELÉTRICOS

1) Two inductive coils are placed in close proximity with each other at 90 degrees. The number of flux linkages is. 0. maximum negative. maximum positive.

2) What is the mutual inductance if two coils of 10mH and 500mH have 90% of the flux transferred from one to the other?. 459mH. 4.5mH. 63mH.

3) An inductor has 1,000 turns of wire and a cross sectional area of 0.001m2. If the core has a permeability of 0.000,001 and the coil is 0.1m. What is the value of the inductor?. 100microH. 100mH. 10mH.

4) Three inductors 10 mH, 5 mH and 20 mH are connected in parallel. What is the total inductance?. Without knowing the coupling factor the total inductance cannot be found. 2.86mH. 35mH.

5) If the current reaches a maximum through an inductor of 2A in 15 seconds, what is the current after 3 seconds?. 1.26A. 0.63A. 1A.

6) The time constant of an inductor is. L/R. L^2 /R. LR.

7) If the rate of change of current is halved, mutual inductance will. stay the same. halve. double.

8) Why are the iron cores of most induction coils laminated?. To reduce the effects of eddy currents. To reduce the core reluctance. To increase the core permeability.

9) When more than two inductors of different inductance are connected in parallel in a circuit, the total inductance is. equal to the sum of the individual inductance. equal to the inductance of the highest rated inductor. less than the inductance of the lowest rated inductor.

10) The time constant in an inductive circuit is the time required for the current to reach. 70.7% of maximum value. 63.2% of maximum value. 63.7% of maximum value.

11) An AC inductive circuit has an inductance 0f 10 mH and a frequency input of 1000Hz. The opposition to current flow is approximately. 63 ohms. 63,000 ohms. 630 ohms.

12) Rotating coil B through 90° will. increase the EMF induced into B. increase the EMF induced into A. decrease the EMF induced into B.

13) An inductor of resistance 5 kΩ and inductance 2mH is connected to a DC circuit. The time constant is. 400 ns. 10 seconds. 40 ms.

14) When switching off the supply, the back EMF in a collapsing field in an inductor. can be multiple times bigger than forward EMF. never exceeds forward EMF. is equal to forward EMF.

15) An induced current in a coil. opposes the EMF producing it. does not affect the EMF producing it. aids the EMF producing it.

16) 1 Volt is produced when a current of 1 Amp per second is varied in a coil'. This is a definition of. 1 Ampere per metre. 1 Henry. 1 Coulomb per second.

17) Two inductor coils in very close proximity; if one of the coils is rotated 90 degrees slowly away from the other the mutual inductance. decreases. remains the same. increases.

18) An inductor is said to be saturated when. the current is at a maximum. the current through it is zero. it will no longer accept lines of flux.

19) In a DC motor, the stator's magnetic field, due to armature reaction, moves. in the same direction of rotation of the armature. in the opposite direction of rotation of the armature. into alignment with the GNA.

20) A small air gap between magnetic poles results. in a weaker field than a large air gap, for the same magnetising force. in a stronger field than a large air gap, for the same magnetising force. in the same field as a large air gap, for the same magnetising force.

21) The electromagnetic brake coil in an actuator would be energised. only at the instant of starting and stopping. all the time in flight. only when the actuator is running.

22) In a shunt wound direct current motor with a constant voltage field supply, the torque developed by the motor is. independent of load. directly proportional to armature current. inversely proportional to the armature current.

23) Decreasing the field current in a shunt motor will. decrease speed and increase torque. increase speed and increase torque. increase speed and decrease torque.

24) To calculate generator output you need to know the. armature speed and number of series conductors. armature speed and field strength. armature speed and number of parallel conductors.

25) In a shunt motor, if you reverse both field current and the armature current, the motor will. stop. change direction. continue to run in the same direction.

26) A starter generator has a. low resistance series field and a low resistance shunt field. low resistance series field and a high resistance shunt field. high resistance series field and a low resistance shunt field.

27) If the brake coil on an actuator motor goes open circuit, the actuator will. run slower. stop. over-ride its mechanical stops.

28) The diagram shows a 200 V long shunt generator. What is the voltage dropped across the series resistor?. 0.12 V. 0.2 V. 200 V.

29) If a generator speed is halved and the field strength is doubled, the voltage output will be. unchanged. doubled. halved.

30) How can the direction of rotation of a DC electric motor be changed?. reverse the electrical connections to either the field or armature windings. rotate the positive brush one commutator segment. interchange the wires which connect the motor to the external power source.

31) The only practical method of maintaining a constant voltage output from an aircraft generator under varying conditions of speed and load is to vary the. strength of the magnetic field. strength of the magnetic field. number of conductors in the armature.

32) Electric motors are often classified according to the method of connecting the field coils and armature. Aircraft engine starter motors are generally of which type?. Compound. Shunt (parallel). Series.

33) If a heavy mechanical load is removed from a series motor. the speed will increase and the armature current will increase. the speed will increase and the armature current will decrease. the speed will decrease and the armature current will decrease.

34) If the correct supply were connected to a DC shunt motor which had lost its residual magnetism it would. run in the reverse direction. fail to run. run normally.

35) One purpose of a growler test is to determine the presence of: a shorted armature. a broken field lead. an out-of-round commutator.

36) The purpose of an end-travel microswitch in a linear actuator is to. remain open and closes at end travel only. remain open during normal operation and only close if the actuator overruns its stops. remain closed and opens at end travel only.

37) Linear actuators used in aircraft are of. split field series wound type. compound wound type. split field shunt wound type.

38) If a generator sparks, a possible reason is. the brush springs are loose. magnetic flux deflecting the EMF. the brushes have been placed around the magnetic coil.

39) If the field strength of a generator is doubled, and the speed is doubled, the output EMF will. stay the same. double. quadruple.

40) In a DC motor the pole pairs are. connected to the brush gear. part of the armature. embedded within the field coils.

41) A series wound DC motor, the field has. few turns of thin wire. many turns of thin wire. few turns of thick wire.

42) Doubling the running time of an electrical machine would. double the current used. double the joules used. double the watts used.

43) On a linear actuator, the field cutoff coil will be energized when. only as the actuator commences movement. the actuator is not running. the actuator is running.

44) An electric motor produces a force of 5 N at a distance of 0.2m from the centre of rotation and rotates at a speed of 100 revs/sec. The motors output is. 100 horsepower. 628.4 watts. 314.2 watts.

45) Starter motors are usually. series wound. compound wound. shunt wound.

46) Generator brushes are normally made of. steel. carbon. brass.

47) As the generator load is increased (within its rated capacity), the voltage will. remain constant and the amperage output will decrease. decrease and the amperage output will increase. remain constant and the amperage output will increase.

48) The voltage output of a generator is controlled by. varying the current of the output. varying the resistance of the output. varying the current of the field.

49) The current flowing in the armature of a DC motor is equal to. (applied volts-generated volts) / armature resistance. (applied volts-generated volts) * armature resistance. applied volts + generated volts / armature resistance.

50) What device is used to convert alternating current, which has been induced into the loops of the rotating armature of a DC generator into direct current as it leaves the generator?. An inverter. A commutator. A rectifier.

51) Which of the following is not one of the purposes of interpoles in a generator?. Overcome armature reaction. Reduce arcing at the brushes. Reduce field strength.

52) To reverse the direction of a series motor. the direction of the current through the field and the armature is reversed. the direction of the current through the field is reversed. a second series field is fitted.

53) The current consumed by a DC starter motor will. remain relatively constant over the starting speed range. decrease as the engine speed increases. increase as the engine speed increases.

54) How are generators rated?. Impedance at rated voltage. Amperes at rated voltage. Watts at rated voltage.

55) Due to armature reaction in a DC motor. the leading pole tips are magnetically weakened. the trailing pole tips are magnetically weakened. the magnetic flux is restored by moving the MNA towards the GNA.

56) Some electric motors have two sets of field winding wound in opposite directions so that the. speed of the motor can be more closely controlled. motor can be operated in either direction. power output of the motor can be more closely controlled.

57) What polarity do interpoles take in a DC motor?. Either, provided they lay on the MNA. The same as the next main pole ahead in the direction of rotation. The same as the next main pole behind in the direction of rotation.

58) What is the principal advantage of the series-wound DC motor?. Suitable for constant speed use. High starting torque. Low starting torque.

59) Interpole windings fitted to DC series wound generators are. in series with the armature. in series with the field. in parallel with the armature.

60) What polarity do interpoles take in a DC generator?. the same as the next main pole behind in the direction of rotation. either, providing they lay on M.N.A. the same as the next main pole ahead in the direction of rotation.

61) The method most often used in overcoming the effect of armature reaction is through the use of. shaded poles. interpoles. drum-wound armatures in combination with a negatively connected series field.

62) The starting current of a series-wound DC motor, in passing through both the field and armature windings produces a. speed slightly higher when unloaded. high starting torque. low starting torque.

63) To increase the speed of a shunt motor a resistance is placed. in parallel with the field. in series with the field. in series with the armature.

64) A 200 volt motor is taking 10 amperes armature current, the armature resistance is 0.1 ohm. The Back-EMF under these conditions will be. 201 volts. 199 volts. 1 volt.

65) What is a method used for restoring generator field residual magnetism?. Energize the armature. Flash the fields. Reseat the brushes.

66) A high surge of current is required when a DC electric motor is first started. As the speed of the motor increase. the counter emf decreases proportionally. the applied emf increases proportionally. the counter emf builds up and opposes the applied emf, thus reducing the current flow through the armature.

67) Aircraft generators are cooled by. fuel cooling radiators. oil cooling radiators around the main body. ram air.

68) On a combined DC starter - generator system. the voltage regulator is connected to the shunt field after the start cycle is completed. the voltage regulator controls the start sequence during engine starting. the series coil is open circuit during the engine start sequence.

69) If the field current to a shunt wound DC motor is decreased when operating at a constant load, the motor speed will. reduce. not change. increase.

70) If a motor is spinning at 50 cycles per second, how long is 1 cycle of the output?. 50 seconds. 0.83 seconds. 0.02 seconds.

71) In a an DC motor, the rotation of the MNA. is opposite to the rotation of the armature. is slightly less speed than the rotation of the armature. is the same as the rotation of the armature.

72) In a wave wound generator with 4 poles, what would be the minimum number of brushes?. 4. 2. 8.

73) A wire is rotated through a magnetic field. To give DC it must be connected to. a commutator. slip rings. a rectifier.

74) If the supply to a DC shunt motor was reversed. it would rotate in the same direction. it would fail to run. it would rotate in the opposite direction.

75) A series motor draws a high current on start, and then the current reduces. This is due to. the resistance of the field and the armature in series. the resistance of the field and armature in parallel. the back EMF.

76) Interpoles in a DC generator are connected. in series with the armature. in series with the field. in parallel with the armature.

77) In a DC generator, the effect of armature reaction may be reduced by moving the brush gear. in the direction of armature rotation towards the MNA. from MNA to GNA. against the direction of armature rotation towards the MNA.

78) The windings embedded in the pole faces of a DC generator are. commutating coils. interpoles. compensating windings.

79) A 6 pole wavewound generator has. 3 brushes. 2 brushes. 6 brushes.

80) Armature reaction in a DC generator causes. MNA to move in the opposite direction of rotation. MNA to move in the direction of rotation. GNA to move in the direction of rotation.

81) In a DC motor, back-EMF rises if the motor. speed falls. speed faload rises. speed rises.

82) A DC generator armature has 6 poles and is lap wound. The number of brushes required are. 2. 3. 6.

83) Reactive sparking is reduced by. interpoles. compensating windings. auxiliary windings.

84) The back-EMF in a DC motor is. equal to the applied EMF. less than the applied EMF. greater than the applied EMF.

85) The flux density of a magnetic field is 1.5T. The length of the conductor in the field is 2 m and the current flowing through the onductor is 5 amps. The force on the conductor is. 0.6 Newtons. 15 Newtons. 3.75 Newtons.

86) A DC shunt motor has 28 volts applied. The current taken from the armature of 2 ohms resistance is 1 amp. The BackEMF is. 26 V. 30 V. 27 V.

87) In a motor, armature reaction causes the MNA to move. against the direction of rotation. in the direction of rotation. to the GNA.

88) The diagram shown is a. shunt wound machine. short shunt compound wound machine. long shunt compound wound machine.

89) In a generator system, a stabilizing winding is used. in series with the field to prevent oscillations. to control output current. to prevent voltage overshoot.

90) What part of a DC motor would you find the poles?. brushes. rotor. stator.

91) The main advantage of a starter-generator over conventional units is. it can produce a greater power output. for the same power output there is a weight reduction and the starter drive remains engaged. for the same power output there is a weight reduction and the drive is constantly engaged.

92) An A.C. shunt wound motor sometimes uses a volts dropping resistor. The resistor would be. in series with the field. in series with the supply. in series with the armature.

93) Commutator ripple can be reduced by. increasing generator frequency. the use of a ripple filter. a resistor in series with the armature.

94) In a DC motor the interpoles are part of the. rotor assembly. stator assembly. field.

95) Armature reaction is. the MMF opposing rotation. due to dirty or worn commutator. reactive sparking.

96) A shunt motor. is constant speed. has high starting torque. gives constant torque with variations in speed.

97) In a DC shunt motor, if the field resistance is increased what happens to the back EMF?. decreases. remains the same. increases.

98) To increase the voltage output of a generator you can. decrease speed. It is not speed dependant. increase speed.

99) Generator voltage is. dependant on speed of rotation. independent of speed of rotation. not dependant on speed of rotation.

100) The output from a generator to a commutator is. pulsed DC. DC. AC.

101) What is the main disadvantage of a starter generator?. Heavier than like for like. Will not supply output at low rpm. Can only be used on piston engines.

102) Back-EMF is the greatest when a motor is. at operating speed. under heavy load. just starting to rotate.

103) When a conductor carrying a current and placed in a magnetic field it experiences. an electrostatic force. an electromagnetic force. a magnetic force.

104) The type of motor used in a linear actuator is. parallel (shunt wound). compound. series.

105) If the speed of a coil in a magnetic field is doubled and the field strength is doubled the output will. quadruple. stay the same. halve.