Multiple choice question for engineering
Set 1
1. When excitation given to a 3-phase alternator is equal to terminal voltage then
a) the absorbed reactive power is zero
b) the absorbed real power is zero
c) the delivered real power is zero
d) any of the mentioned
Answer
Answer: a [Reason:] When excitation is equal to the terminal voltage there will not be any net flow of energy across the system.
2. The generator(3 phase alternator) absorbs reactive power from the infinite bus at leading power factor.
a) True
b) False
Answer
Answer: a [Reason:] Infinite bus bar provides the flux needed to set up the reactive power to produce the real power in the generator.
3. When terminal voltage exceeds the excitation, then operating power factor is
a) leading
b) lagging
c) uPF
d) zpf
Answer
Answer: a [Reason:] As the reactive power is negative the power factor based on the standard convention should be lagging.
4. Under the conditions of maximum reactive power output, the reactive power flowing out of an alternator is
a) positive
b) negative
c) zero
d) equal to power
Answer
Answer: b [Reason:] The alternator will always absorb the reactive power to meet its flux requirements to generate the real power.
5. Under the condition of maximum power delivered, reactive power is
a) negative
b) positive
c) zero
d) same as previous condition
Answer
Answer: a [Reason:] The alternator will always absorb the reactive power to meet its flux requirements to generate the real power.
6. Under the condition for 3 phase alternator, maximum active power output, the delivered reactive power output is ________ while under maximum reactive power output, the delivered reactive power is ________
a) negative, negative
b) positive, negative
c) negative, positive
d) positive, positive
Answer
Answer: a [Reason:] Under both the situation, there is need of reactive power which will set up the flux in the machine to make it run.
7. For an overexcited 3 phase synchronous motor, connected to infinite bus is
a) motor is delivering reactive power to IBB
b) motor is absorbing reactive power to IBB
c) motor is delivering zero reactive power to IBB
d) none of the mentioned
Answer
Answer: a [Reason:] An over excited synchronous motor will act as a source of the reactive power.
8. If the synchronous motor connected to the infinite bus bar delivers positive reactive power to it, then it must be operating at
a) leading pf
b) lagging pf
c) zero pf
d) unity pf
Answer
Answer: a [Reason:] An over excited synchronous motor will act as a source of the reactive power and it will operate at leading pf.
9. For an underexcited 3 phase synchronous motor, connected to IBB is
a) absorbing reactive power from IBB
b) delivering reactive power to IBB
c) absorbing zero reactive power from IBB
d) delivering zero reactive power to IBB
Answer
Answer: a [Reason:] An under excited synchronous motor will act as a source of the reactive power.
10. If the synchronous motor connected to the infinite bus bar absorbs reactive power to it, then it must be operating at
a) leading pf
b) lagging pf
c) zero pf
d) unity pf
Answer
Answer: b [Reason:] An under excited synchronous motor will act as a sink of the reactive power and it will operate at leading pf.
11. Synchronous motor
a) absorbs reactive power for maximum input real power
b) absorbs reactive power for maximum input reactive power
c) delivers reactive power for maximum input real power
d) delivers reactive power for maximum input reactive power
Answer
Answer: a [Reason:] Synchronous motor always absorbs reactive power for maximum real as well as reactive power.
12. Under the conditions of maximum values of active or reactive power, a synchronous machine always absorbs reactive power.
a) True
b) False
Answer
Answer: a [Reason:] Synchronous motor always absorbs reactive power for maximum real as well as reactive power.
13. An ________ synchronous machine delivers reactive power whereas an ________ one absorbs reactive power.
a) over excited, under excited
b) under excited, over excited
c) over excited, over excited
d) under excited, normal excited
Answer
Answer: a [Reason:] An over excited synchronous machine delivers reactive power whereas an under excited one absorbs reactive power.
14. An alternator, with synchronous reactance of 0.8 pv is connected to an IBB at rated voltage. With its excitation emf adjusted to 1.3 pv, the alternator delivers an output of 0.5 pv. The load angle is __________
a) 17.92
b) 72.08
c) 34.92
d) 90
Answer
Answer: a [Reason:] δ = atan(0.5*0.8/1.3) = 17.92°.
15. An 3 phase alternator star connected 400 V synchronous motor takes a power input of 5472 watts at rated voltage. Its synchronous reactance is 10** per phase and negligible resistance.
If its excitation voltage is adjusted equal to the rated voltage of 400 V, its power factor is _______
a) cos(10°)
b) cos(20°)
c) cos(40°)
d) cos(30°)
Answer
Answer: a [Reason:] Vt = Ef=400/1.73 = 231 V
V*Esinδ/X = P
δ=20°
θ=δ/2.
Set 2
1. The relative speed between rotor and stator field is zero. This means
a) mechanical speed of rotor is equal to stator field speed
b) mechanical speed is zero
c) stator field is stationary
d) none of the mentioned
Answer
Answer: a [Reason:] The relative speed is between the rotor and the stator field.
2. The satisfactory operation of the synchronous machine means
a) mechanical speed of rotor is equal to stator field speed
b) mechanical speed is zero
c) stator field is stationary
d) none of the mentioned
Answer
Answer: a [Reason:] The relative speed should be zero between the rotor and the stator field.
3. Variation of load angle delta, after sudden loading of an unloaded synchronous motor with no damping is
a) 
b)
c)
d)
Answer
Answer: a [Reason:] With no damping there will be oscillations in the rotor angle.
4. Variation of load angle delta, with loading of an unloaded synchronous motor with damping is
a)
b)
d)
Answer
Answer: a [Reason:] With damping the load angle damps out to nominal value.
5. Effects of hunting are/is
a) mechanical stress
b) surge in current
c) rise in temperature
d) all of the mentioned
Answer
Answer: d [Reason:] Hunting causes all the mentioned effects on the synchronous machine.
6. The probable causes of hunting is
a) sudden change in load
b) fault in supply
c) sudden change in field current
d) all of the mentioned
Answer
Answer: d [Reason:] Changing the load suddenly, excitation or the faults will cause transients in the machine.
7. Using a _____ can be used to limit hunting
a) Flywheel
b) DC Exciter
c) Large load
d) Any of the mentioned
Answer
Answer: d [Reason:] Any method mentioned will diminish the transients in the machine.
8. Employment of damper winding will reduce hunting.
a) True
b) False
Answer
Answer: a [Reason:] Damper winding are mechanical windings which also reduce the hunting.
9. Damper windings are made of
a) copper
b) iron
c) silicon
d) cast iron
Answer
Answer: a [Reason:] They are made of copper as the current also flows through them.
10. The damper winding are placed in
a) pole shoes
b) series with armature
c) series with field
d) rotor slots
Answer
Answer: a [Reason:] The damper winding are placed in pole shoes.
11. Turbo generators do not use damper winding due to
a) solid steel rotor core
b) laminated armature
c) soft iron core
d) small current flow in pole shoes
Answer
Answer: a [Reason:] Because eddy current in the q-axis produce the same effect as the damper in salient pole machine.
12. Damper winding should have
a) high resistance
b) low starting torque
c) low resistance
d) low reactance
Answer
Answer: c [Reason:] For desired hunting, the damper winding should have low resistance.
13. At synchronous speed, relative speed between rotating air-gap flux and damper is
a) zero
b) Ns
c) lesser than Ns
d) more than zero
Answer
Answer: a [Reason:] It is zero.
14. When the rotor speed becomes greater than synchronous speed, which type of torque is produced in damper winding?
a) Induction generator
b) Synchronous generator
c) Reluctance motor
d) Induction motor
Answer
Answer: a [Reason:] The slip will become negative and it will operate in generating mode.
15. When an alternator is loaded suddenly the torque produced in damper is?
a) Induction motor
b) Induction generator
c) Synchronous motor
d) Synchronous generator
Answer
Answer: a [Reason:] Sudden loading will reduce the speed of the machine and so there will be small slip induced in the machine, making it operate as induction motor.
Set 3
1. The coupling angle or load angle of a synchronous motor is defined as the space angle between
a) rotor and stator poles of opposite polarity
b) rotor and stator poles of same polarity
c) rotor and stator teeth
d) rotor and resultant magnetic field
Answer
Answer: a [Reason:] It is measured between stator and rotor poles of opposite polarity.
2. Synchronizing power is
(i) Stiffness coefficient of electromagnetic coupling between stator and rotor
(ii) Stability measure of synchronous machine
(iii) Power consumption factor
a) i,ii
b) ii, iii
c) iii, i
d) i, ii, iii
Answer
Answer: a [Reason:] Synchronizing power is stiffness coefficient representation as well the a measure of stability.
3. Large stiffness coefficient represents
a) motor speed remains practically constant
b) large fluctuations due to mechanical load
c) motor speed is variable to changes
d) no such significance with speed
Answer
Answer: a [Reason:] Large stiffness represents the tight coupling in the machine and more stability.
4. An over excited synchronous machine is more stable than under excited
a) True
b) False
Answer
Answer: a [Reason:] Synchronizing power is directly proportional to the excitation.
5. Consider the below remarks for an alternator
(i) An over excited machine is more stiff
(ii) Machine with longer air-gap is less stable than one with smaller air-gap
a) (i) is True, (ii) is False
b) (i) is False, (ii) is True
c) (i) is True, (ii) is True
d) (i) is False, (ii) is False
Answer
Answer: a [Reason:] Larger the air gap smaller the reactance.
6. As load angle is increased, degree of stability __________
a) reduces
b) increases
c) remains same
d) no change in stability
Answer
Answer: a [Reason:] Synchronizing power = EV*cosδ/X.
7. Synchronizing power is
a) transient in nature
b) steady state in nature
c) pulsating at small frequencies
d) constant for a machine
Answer
Answer: a [Reason:] It is transient in nature and determines the stability while disturbances.
8. Which of the following makes synchronizing power to come to play in the machine?
a) Disturbance in power mover
b) Disturbance in field current
c) Disturbance in load
d) All of the mentioned
Answer
Answer: a [Reason:] Any disturbance will lead to transients to arise and so the synchronizing power.
9. Which of the following is/are used in synchronous machines to maintain mechanical stability?
a) Damper winding
b) Interpole winding
c) Compensating winding
d) Equalizer rings
Answer
Answer: a [Reason:] Damper windings are the dummy windings which help to stablize the machine during transient instability.
10. For a 4-pole 3 phase, 400V alternator has synchronizing power of 300 units. Then the synchronizing power per mechanical degree is
a) 300
b) 1200
c) 900
d) 600
Answer
Answer: a [Reason:] P(sy, mech) = Poles* P(sy,ele)/2
P(sy,ele)=300 units.
11. Electromagnetic torque is present in rotating machines when
a) both stator and rotor windings carry current
b) rotor windings carry current
c) stator windings carry current
d) any of the mentioned
Answer
Answer: a [Reason:] Synchronous machine is a doubly excited machine.
12. The operation of a synchronous motor operating on constant excitation across infinite bus will not be stable if power angle δ
a) exceeds internal angle θ
b) is less than θ
c) is more than θ/2
d) is less than θ/2
Answer
Answer: a [Reason:] Power angle should not exceed θ, to operate under normal conditions.
13. In a synchronous motor the back emf peak set up in the stator depends on
a) rotor excitation
b) supply voltage
c) rotor speed
d) load on the motor
Answer
Answer: a [Reason:] It will depend on the excitation.
14. Variation in the dc excitation of a synchronous motor causes variation in
a) armature current and power factor
b) power factor
c) armature current
d) speed
Answer
Answer: a [Reason:] Armature current and power factor both will vary by varying the dc excitation.
15. Which motor can conveniently operate at lagging as well as leading power factor?
a) Synchronous motor
b) Slip ring induction motor
c) Squirrel cage induction motor
d) Stepper motor
Answer
Answer: a [Reason:] Its the synchronous motor which can favourably operate both at leading and lagging power factor.
Set 4
1. Commutator is a
a) rotating segment to which brushes are attached
b) stationary segment to which rotating brushes are attached
c) rotating segment to which rotating brush segment is attached
d) any of the mentioned
Answer
Answer: a [Reason:] Commutator is a rotating segment to which brushes are attached.
2. Commutator action is analogous to _________ in analog electronics.
a) full wave rectifier
b) half wave rectifier
c) comparator
d) amplifier
Answer
Answer: a [Reason:] Commutator converts the alternating voltage induced at the armature to the unidirectional voltage at the slip rings.
3. Inductive kick is
a) reactance voltage in commutator segments
b) reactance voltage in shorted commutator segments
c) impulsive voltage in commutator segments
d) none of the mentioned
Answer
Answer: a [Reason:] Inductive kick is the name given to the reactance voltage which occurs at the commutation taking place in the commutator.It occurs due to the high circulating current during the commutation.
4. Inductive kick causes
a) sparking at the brushes of the machine
b) similar problems when neutral phase shift takes place in the armature
c) large current to flow in the equalizer rings
d) none of the mentioned
Answer
Answer: a [Reason:] Inductive kick is the name given to the reactance voltage which occurs at the commutation taking place in the commutator.It occurs due to the high circulating current during the commutation.
5. Operation of a dc machine will
a) not be affected by commutating poles
b) be affected by commutating poles
c) be affected significantly upto 30% spread of commutating poles
d) none of the mentioned
Answer
Answer: a [Reason:] Commutating poles are placed under the armature poles and it compensates only poles under these regions.
6. A 200 V dc machine working at rated speed. Voltage across the commutating terminals should ideally be
a) zero
b) 2 V
c) 1 V
d) 200 V
Answer
Answer: a [Reason:] The commutating poles undergoing commutation should have zero voltage across it so that there is no reactive voltage induced.
So it will not cause large circulating current in the winding.
7. For a four pole, 16 slot armature with two coil-side/slot having lap connected progressive winding has single turn coils. Then number of commutator segment is
a) 16
b) 32
c) 4
d) 8
Answer
Answer: a [Reason:] Commmutator segment is equal to number of coil slots.
8. For a four pole, 16 slot armature with two coil-side/slot having wave connected progressive winding has single turn coils. The commutator pitch for the machine should be
a) 1
b) -1
c) +1 and 1
d) none of the mentioned
Answer
Answer: a [Reason:] Commutator pitch for the wave connected progressive winding should be 1.
9. The brushes in dc machine are normally placed electrically in interpolar region, to
a) make an angle of 90 deg. electrical
b) make an angle of 90 deg. mechanical
c) make an angle of 0 deg. electrical
d) none of the mentioned
Answer
Answer: a [Reason:] To obtain proper commutation without any sparks, the magnetic neutral axis and geometrical neutral axis should be aligned to each other,such that coils undergoing commutation experience zero emf across them.
10. The armature rotation in the dc machine and the magnetic field are independent of each other and obey laws of conservation of energy.
a) True
b) False
Answer
Answer: a [Reason:] The armature rotation does not affect the stored magnetic energy at the steady state of operation.
11. Under commutation results
a) sparking at the trailing edge of brush
b) sparking at the leading edge of brush
c) sparking at the middle of brush
d) any of the mentioned
Answer
Answer: a [Reason:] Under commutation takes place when the commutation speed is slow and it takes more time than critical defined time for commutation. Because of this the trailing edge of the brush still remains under the commutation.
12. While the operation of dc motor, there are sparkings at the commutation observed. These can be happening due to
a) under commutation
b) over commutation
c) too rapid reversal of the current
d) any of the mentioned
Answer
Answer: d [Reason:] The over commutation, over commutation or rapid reversal of the current, any of these reasons can lead to sparking at the brushes.
Set 5
1. Wound rotor induction motor is most appropriate for the applications requiring
a) high starting torque
b) variable starting torque
c) fixed starting torque
d) all of the mentioned
Answer
Answer: b [Reason:] A wound rotor induction motor can have variable starting torque.
2. Relatively a squirrel-cage induction motor is advantageous over wound rotor type induction motor of identical rating due to
a) less conductor material used.
b) lower leakage flux
c) ruggedness
d) all of the mentioned
Answer
Answer: d [Reason:] A squirrel-cage induction motor is more efficient than a wound rotor induction motor.
3. Relatively a squirrel-cage induction motor is advantageous over wound rotor type induction motor of identical rating due to
a) less conductor material used
b) larger leakage flux
c) large starting torque
d) all of the mentioned
Answer
Answer: a [Reason:] Squirrel cage induction motor has lesser starting torque.
4. We can not operate a cage induction motor on a variable frequency load due to
a) no availability of varying the speed
b) availability of variable speed
c) low starting torque
d) fixed slip
Answer
Answer: a [Reason:] We can not increase or decrease the rotor resistance of a cage induction motor, hence it is not suitable for a variable frequency.
5. Speed control is possible for _______ and not possible for __________
a) induction motor, synchronous motor
b) induction motor, differential motor
c) synchronous motor, synchronous-induction motor
d) dc motor, induction motor
Answer
Answer: a [Reason:] Speed can be adjusted for a induction motor while it can not be altered for a synchronous motor operating at normal speed.
6. Which of the following can be done using a synchronous motor but not by induction motor?
a) Power factor improvement
b) Supplying mechanical load
c) Power factor improvement and supply mechanical load
d) None of the mentioned
Answer
Answer: a [Reason:] Induction motor is a singly excited machine and it always needs exciting current to set up the flux in the machine.
7. The torque in an induction motor varies as ______ and in synchronous motor as _________
a) square of voltage, proportion to voltage
b) proportion to voltage, proportion to voltage
c) proportion to voltage, square of voltage
d) square of voltage, square of voltage
Answer
Answer: a [Reason:] Torque in induction motor varies as square of the voltage and in synchronous motor as linear to voltage.
8. If a 3-phase 350V, 50Hz squirrel-cage induction motor is run at 55 Hz supply,then
A. Starting torque will increase
B. Maximum torque will increase
C. Maximum torque will remain same
D. Starting torque will decrease
E. Operating speed will rise
a) C, D, E
b) A, C, E
c) A, B
d) E
Answer
Answer: a [Reason:] Frequency is increased, so the starting torque will decrease. Speed will increase as slip will reduce.
9. When a 3-phase 350V, 50Hz squirrel-cage induction motor is run at 35 Hz supply,then
A. Starting torque will increase
B. Maximum torque will increase
C. Maximum torque will remain same
D. Starting torque will decrease
E. Operating speed will rise
a) A, C
b) A, B, E
c) A, B
d) C, D, E
Answer
Answer: a [Reason:] Frequency is decreased so the slip increases. Starting torque will increase and the maximum torque will remain same.
10. Reducing the poles of a 3-phase induction motor, it
a) will decrease maximum power factor
b) will increase maximum power factor
c) will make no change in power factor
d) can not be depicted
Answer
Answer: a [Reason:] If number of poles reduces, speed increases, slip decreases, reactance increases, power factor angle increases and so the power factor decreases.
11. When the poles of a 3-phase wound rotor induction motor increased, it
a) will decrease maximum power factor
b) will increase maximum power factor
c) will make no change in power factor
d) can not be depicted
Answer
Answer: b [Reason:] If number of poles increases, speed decreases, slip increases, reactance decreases, power factor angle decreases and so the power factor increases.
12. A 3-phase induction motor taking a line current of 200 A, is started by direct switching. If an auto transformer of with 50% tapping is
made to be used, the motor line current and supply line current will be respectively
a) 100, 50
b) 50, 100
c) 50, 200
d) 50*1.73, 200
Answer
Answer: a [Reason:] The motor line current will reduce by a fold of four times and the line current by half.
13. A 3-phase induction motor is running for a constant load torque at rated voltage and frequency. If both the frequency and voltage are halved
following will be the related statements ignoring the stator losses.
A. The difference between actual speed and synchronous speed remains same.
B. Per unit slip remains same
C. Stator current remains same
D. Air gap flux remains same
a) A, C, D
b) A, B, C, D
c) B, C, D
d) C, D
Answer
Answer: a [Reason:] Per unit slip varies as the reactance will change as the frequency has been halved.
14. A 3-phase induction motor is running for a constant load torque at rated voltage and frequency. If both the frequency and voltage are halved
following will be the related statements ignoring the stator losses.
A. The difference between actual speed and synchronous speed remains same.
B. Per unit slip remains same
C. Stator current increases
D. Air gap flux remains same
a) A, D
b) A, C, D
c) B, C, D
d) C, D
Answer
Answer: a [Reason:] Stator current will remain same and pu slip varies.
15. A 3-phase induction motor is running for a constant load torque at rated voltage and frequency. If both the frequency and voltage are halved
following will be the related statements ignoring the stator losses.
A. The difference between actual speed and synchronous speed remains same.
B. Per unit slip remains varies
C. Stator current remains same
D. Air gap flux remains same
a) A, C, D
b) A, B, C, D
c) B, C, D
d) C, D
Answer
Answer: b [Reason:] Slip varies, stator current remains same as the net impedance is same.
16. No load test has been conducted on an induction motor for different supply voltages and input power vs voltage plot has been drawn.
The curve is extrapolated to intersect the y-axis. This point is
a) friction and windage losses
b) core losses
c) stray losses
d) all of the mentioned
Answer
Answer: a [Reason:] The extrapolated point is friction and windage losses of the induction machine.
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