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Multiple choice question for engineering

Set 1

1. Blocked rotor test comprises of
a) rotor and stator copper losses.
b) rotor copper and stator core losses.
c) rotor core losses
d) stator copper losses

View Answer

Answer: a [Reason:] Blocked rotor test consists of copper losses of stator and rotor winding copper losses.

2. A 10kW,400V, 4-pole delta connected induction motor gave the following test results:
No-load test: 400V, 8A, 250 W
Blocked rotor test: 90V,35A,1350 W
The dc resistance per phase is 0.6 ohms. The no load rotational losses is
a) 204W
b) 304W
c) 196W
d) 186W

View Answer

Answer: a [Reason:] Pr = 250-3*64/3*(0.6*1.2) = 204W.

3. A 10kW,400V, 4-pole delta connected induction motor gave the following test results:
No-load test: 400V, 8A, 250 W
Blocked rotor test: 90V,35A,1350 W
The dc resistance per phase is 0.6 ohms. The effective stator resistance is
a) 0.72 ohms
b) 0.6 ohms
c) 0.3 ohms
d) 1.2 ohms

View Answer

Answer: a [Reason:] Reff = 1.2*r; Due to delta connected rotor.

4. While conducting the tests on the 3-phase induction motor, the variation of rotational losses with respect to voltage can be obtained as
a) electrical-machines-questions-answers-aptitude-test-q4a
b) electrical-machines-questions-answers-aptitude-test-q4b
c) electrical-machines-questions-answers-aptitude-test-q4c
d) electrical-machines-questions-answers-aptitude-test-q4d

View Answer

Answer: a [Reason:] Core loss is zero for , zero applied voltage but the friction and windage losses will be present in the machine which need to be overcome.

5. The external rotor resistance is
a) gradually cut out as motor picks up the speed
b) fixed for a stable point
c) removed instantalLy as the motor is started
d) added or reduced as per the efficiency requirement

View Answer

Answer: a [Reason:] External resistance is gradually cut out after picking up the speed.

6. The speed of the squirrel-cage induction motor can be changed by varying slip.
a) True
b) False

View Answer

Answer: b [Reason:] For squirrel-cage induction motor, the rotor get shorted and it can not be altered.

7. The speed characteristic of the induction motor is analogous to
a) dc shunt motor
b) synchronous motor
c) cumulative compound motor
d) stepper motor

View Answer

Answer: a [Reason:] As the speed practically remains constant and lesser than than the rated speed, it is similar to dc shunt motor.

8. If the squirrel-cage induction motor is replaced by a solid cylinder in an induction motor its slip will
a) increase
b) decrease
c) be same
d) zero

View Answer

Answer: a [Reason:] Reactance of the rotor will decrease as the leakage wil reduce, so the slip will increase.

9. Number of the slip rings required for the satisfied operation of an induction motor is
a) number of phases
b) number of poles
c) halve of number of poles
d) double the phases

View Answer

Answer: a [Reason:] Slip rings number is same as the phase of the supply.

10. I. An induction can never operate at leading power factor.
II. Speed of the induction motor can be made more than synchronous speed and still it will work as motor.
a) I is true, II is false
b) I is true, II is true
c) I is false, II is true
d) I is false, II is also false

View Answer

Answer: a [Reason:] Speed more than Ns will be an induction generator not motor.

Set 2

1. While comparing potential transformer to an auto transformer, a potential transformer transfers power ________
a) conductively
b) inductively
c) both conductively as well as inductively
d) electromagnetic induction

View Answer

Answer: a [Reason:] Potential divider is resistance division and it does not take part in induction processes.

2. The statements which support the points that auto transformers are advantageous
I. Weight of conductor reduces
II. Ohmic losses reduces
III. Leakage reactance reduces
IV. Lower short-circuit current
a) I,II,III
b) II,III,IV
c) I,II,III,IV
d) I,IV

View Answer

Answer: a [Reason:] Short circuit current of the auto transformer is higher than the corresponding 2-winding transformer.

3. The statements which support the points that auto transformers are advantageous
I. Weight of conductor reduces
II. Direct electrical contacts
III. Leakage reactance reduces
IV. Lower short-circuit current
a) I,III
b) II,III
c) I,II,III,IV
d) I,IV

View Answer

Answer: a [Reason:] Direct electrical contacts is a disadvantage to the auto transformer. Short circuit current of the auto transformer is higher than the corresponding 2-winding transformer.

4. The statements which support the points that auto transformers are disadvantageous as compared to 2-winding transformer
I. Weight of conductor reduces
II. Direct electrical contacts
III. Leakage reactance reduces
IV. Lower short-circuit current
a) I,III
b) II,III
c) II,IV
d) I,II,IV

View Answer

Answer: c [Reason:] Direct electrical contacts is a disadvantage to the auto transformer. Short circuit current of the auto transformer is higher than the corresponding 2-winding transformer.

5. I. KVA rating : 1/(1-k)
II. Losses : (1-k)
III. Impedance drop = 1/(1-k)
Which of the above are correct for an auto transformer when compared to the identical rating two winding transformer?
a) I,II
b) II,III
c) 1,III
d) I,II,III,IV

View Answer

Answer: a [Reason:] KVA(auto)/KVA(2-W) = V2*I2/V2*(I2-I1) = 1/(1-I1/I2) = 1/(1-k) Losses(auto)= (1-k) *Losses(2-W) Impedance drop(auto)= (1-k)*Impedance drop(2-W).

6. The voltage regulation of a transformer at full-load 0.8 p.f leading is -2%. Its voltage regulation at full load 0.8 p.f lagging
a) will be positive
b) will be negative
c) may be positive
d) may be negative

View Answer

Answer: a [Reason:] The leading p.f. has negative v.r. and lagging p.f. has major portion of positive voltage regulation.

7. The voltage regulation of a transformer depends on its
(A)equivalent reactance (B) equivalent resistance
(C)load power factor (D)transformer size
(E) load current
a) A,B,C,E
b) A,B,C,D,E
c) A,B,D,E
d) A,B,C,D

View Answer

Answer: a [Reason:] Voltage regulation is independent of the size of the transformer.

8. Three transformers having identical dimensions but with core of iron, aluminium and wood are wound with same number of turns and have same supply.
Then choose the order for hysteresis losses.
a) wood > aluminium > iron
b) aluminium > iron > wood
c) iron > wood > aluminium
d) iron > aluminium > wood

View Answer

Answer: d [Reason:] Hysteresis losses occur maximum in the ferromagnetic material.

9. Three transformers having identical dimensions but with core of iron, aluminium and wood are wound with same number of turns and have same supply.
Then choose the order for eddy current losses.
a) wood > aluminium > iron
b) aluminium > iron > wood
c) iron > wood > aluminium
d) iron > aluminium > wood

View Answer

Answer: a [Reason:] The eddy current losses are dependent on resistance offered to the currents. Wood is an insulator so it will get heated up most.

10. Maximum efficiency of a transformer for a constant load current , occurs at
a) at any p.f
b) zero p.f leading
c) zero p.f lagging
d) unity p.f

View Answer

Answer: d [Reason:] Efficiency = KVA*p.f/(KVA*p.f + Losses); So the efficiency is maximum at unity power factor.

11. A 1-phase tranformer has a leakage impedance of 1+ j4 Ω for primary and 3+ j11 Ω for secondary windings. This transformer has
a) H.V primary
b) Medium voltage primary
c) L.V primary
d) L.V secondary

View Answer

Answer: c [Reason:] The side which has lower impedance will have lower number of turns and so the low voltage side.

12. If a transformer is at no load , then it will act like
a) a resistor at p.f =0
b) an inductive reactor at 0.2 lagging
c) a capacitive reactor at0.2 leading
d) an inductive reactor at 0.8 lagging

View Answer

Answer: b [Reason:] Transformer is nothing but the arranged windings which are magnetically coupled. The windings will inductive predominantly with very low resistance.

13. The tappings on the transformers is provided on
a) h.v side at one end of the winding
b) l.v. side at one end of the winding
c) h.v side at middle
d) l.v side at middle

View Answer

Answer: c [Reason:] High voltage side winding has more voltage so the variation of the voltage will be more and better applicable. And it is better to place at the middle because it gives uniform voltage distribution by eliminating axial forces.

14. In a transformer, we place the tapping windings at the middle because
a) of radial forces on the windings
b) axial forces on the windings
c) insulation level of the winding
d) to provide a mechanical balance to the windings

View Answer

Answer: b [Reason:] Due to two different voltages, different currents will flow in the windings and it will cause the axial forces due to currents interaction appear causing damage.

15. When a transformer winding suffers a short circuit, then inter turns of the same winding experience ______ forces.
a) an attractive
b) a repulsive
c) no force
d) may be attractive or repulsive type

View Answer

Answer: a [Reason:] As the current will flow in one direction, attractive force will appear across the inter turns as per Biot-savart’s law.

16. Two tranformer having same voltage but different KVA are operating in parallel. For a good load sharing,
a) impedance must be equal
b) pu impedances must be equal
c) pu impedance and X/R ratios must be equal
d) impedance and X/R ratios must be equal

View Answer

Answer: c [Reason:] Both the conditions are to be met in order to have an efficient operation while in parallel condition.

Set 3

1. A three phase induction motor is not self starting.
a) True
b) False

View Answer

Answer: b [Reason:] A three phase induction motor is self starting. Because air gap flux and rotor mmf produce electromagnetic torque in the direction of rotating magnetic field.

2. Mark the correct waveform for the emf produced in an induction motor and the resultant flux density wave for a resistive load.
a)electrical-machines-questions-answers-campus-interviews-q2a
b)electrical-machines-questions-answers-campus-interviews-q2b
c)electrical-machines-questions-answers-campus-interviews-q2c
d)electrical-machines-questions-answers-campus-interviews-q2d

View Answer

Answer: a [Reason:] The flux will lead the induced current and emf by 90°.

3. Mark the most appropriate phasor for the induction machine operation as motor.
a)electrical-machines-questions-answers-campus-interviews-q3a
b)electrical-machines-questions-answers-campus-interviews-q3b
c)electrical-machines-questions-answers-campus-interviews-q3c
d) none of the mentioned

View Answer

Answer: a [Reason:] The flux will lead the induced current and emf by 90°.

4. In a three phase induction motor, rotor current in each phase lags behind the generated emf in that phase to rotor by a power factor angle of
a) atan(r2/x2)
b) atan(x2/r2)
c) atan(x1/r1)
d) atan(r1/x1)

View Answer

Answer: b [Reason:] The power factor angle is atan(x2/r2).

5. In a three phase induction motor of rotor impedance as (0.2+j0.2), rotor current in each phase lags behind the generated emf in that phase to rotor by a power factor angle of
a) 45°
b) 135°
c) 90°
d) 0°

View Answer

Answer: a [Reason:] The power factor angle would be atan(0.2/0.2) = 45°.

6. For the three phase induction motor having leakage impedance, the maximum generated emf would be attained at an angle of θ having a purely resistive load
a) 90 + θ
b) θ
c) 90 – θ
d) -θ

View Answer

Answer: a [Reason:] Due to leakage reactance there will be a lagging of angle θ in the excitation voltage with the main flux. So the maximum is attained at 90°+θ.

7. For a slip ring induction motor having leakage impedance of (1+j), the maximum generated emf would be attained at
a) 135°
b) 45°
c) -45°
d) 0°

View Answer

Answer: a [Reason:] Due to leakage reactance there will be a lagging of angle θ in the excitation voltage with the main flux. So the maximum is attained at 90°+θ.

8. In an induction motor, the stator is also known as _________ and the rotor as __________
a) field winding, armature winding
b) armature winding, field winding
c) armature winding, compensating winding
d) armature winding, interpole winding

View Answer

Answer: a [Reason:] In an induction motor, the stator is also known as field winding, and the rotor as armature winding.

9. For a 6-pole three phase squirrel cage induction motor has flux density wave of four poless, then the rotor induced poles will be
a) 4
b) 6
c) 2
d) 10

View Answer

Answer: a [Reason:] For a squirrel-cage induction motor the rotor will have the induced poles will be same as flux density poles which are induced inside the machine while working.

10. The external resistance can be inserted in rotor circuit of
a) wound rotor induction motor
b) slip ring induction motor
c) wound rotor as slip ring induction motor
d) neither of motors

View Answer

Answer: a [Reason:] External resistance can be inserted only in the wound rotor as does not get shorted as like squirrel cage motor.

11. Starting torque induced in a three phase induction motor in a direction of rotating magnetic field depends on
a) number of poles squared
b) inverse of number of poles
c) number of poles
d) number of number poles cubic

View Answer

Answer: a [Reason:] T = (pi/8).(P2) φ (F2)cos(θ)

12. Starting torque induced in a three phase induction motor in a direction of rotating magnetic field depends on
(i) rotor mmf per pole
(ii) rotor power factor angle
(iii)type of rotor winding
a) (i), (ii)
b) (ii), (iii)
c) (i), (iii)
d) (i), (ii), (iii)

View Answer

Answer: a [Reason:] Starting torque induced in a three phase induction motor depends on rotor mmf per pole and rotor power factor angle.

Set 4

1. Circle diagram of an induction motor is graphical representation of
a) its equivalent circuit
b) its rotor equivalent
c) its stator equivalent
d) its stationary equivalent circuit

View Answer

Answer: a [Reason:] Circle diagram gives the information about the equivalent circuit of the induction motor.

2. Circle diagram depicts the relation between
a) stator current and the slip variation
b) rotor current and stator current
c) slip variation and the power factor
d) slip variation and the losses of the machine

View Answer

Answer: a [Reason:] Circle diagram shows extermeties of stator current and the slip variation.

3. A 10kW, 50 Hz, 3-phase induction motor develops the rated torque at 1440rpm. If the load torque is reduced to half, then the motor speed is
a) 1470rpm
b) 1410rpm
c) 1400rpm
d) 1444rpm

View Answer

Answer: a [Reason:] Slip = 0.04 T2/T1 = s2/s1 s2 = (1/2)*0.04 = 0.02 Nr = (1-0.02)*1500 = 1470 rpm

4. A 10kW, 50 Hz, 3-phase induction motor develops the rated torque at 1440rpm. If the load torque is reduced to half, then the power output that can now be obtained is
a) 5 kW
b) 5.3 KW
c) 4.6 kW
d) 8 kW

View Answer

Answer: a [Reason:] s2 = (1/2)*0.04 = 0.02 P = (10000*60*2*3.14*1470)/(2*2*3.14*1460*60) P= 5.03 kW/

5. In a 3-phase induction motor, the flux per pole will vary with the slip as
a) electrical-machines-questions-answers-circle-diagrams-induction-machines-q5a
b) electrical-machines-questions-answers-circle-diagrams-induction-machines-q5b
c) electrical-machines-questions-answers-circle-diagrams-induction-machines-q5c
d) electrical-machines-questions-answers-circle-diagrams-induction-machines-q5d

View Answer

Answer: a [Reason:] Flux remains constant throughout the operation of the induction motor.

6. The rotor mmf per pole in the 3-phase, 4-pole induction motor varied with slip as
a) electrical-machines-questions-answers-circle-diagrams-induction-machines-q6a
b) electrical-machines-questions-answers-circle-diagrams-induction-machines-q6b
c) electrical-machines-questions-answers-circle-diagrams-induction-machines-q6c
d) electrical-machines-questions-answers-circle-diagrams-induction-machines-q6d

View Answer

Answer: a [Reason:] As slip decreases, the rotor current decreases so the rotor mmf.

7. Blocked rotor test is conducted to find the
a) leakage impedance
b) leakage reactance
c) stator impedance
d) rotor impedance

View Answer

Answer: a [Reason:] Blocked rotor test determines the leakage impedance and so the core losses.

8. In the below circle diagram, OG represents
electrical-machines-questions-answers-circle-diagrams-induction-machines-q8
a) Rated torque
b) Maximum torque
c) Rated power output
d) Losses

View Answer

Answer: a [Reason:] OG is the rated torque.

9. In the following circle diagram of the induction motor, QN is the
electrical-machines-questions-answers-circle-diagrams-induction-machines-q8
a) rotor current referred to stator
b) stator current
c) stator current referred to rotor
d) no load input to rotor

View Answer

Answer: a [Reason:] It is rotor current referred to stator.

10. Stator resistance can be measured using Kelvin’s double bridge.
a) True
b) False

View Answer

Answer: a [Reason:] Stator has very low resistance, so it is most accurately observed by using a kelvin’s double bridge.

11. If the no load test is conducted at its rated frequency but less than the rated voltage, then the wattmeter reading will be
a) reduced
b) more than previous reading
c) remain same
d) none of the mentioned

View Answer

Answer: a [Reason:] No load losses will reduce, so the power consumption.

12. If the no load test is conducted at its rated frequency but less than the rated voltage, then the mechanical losses will _____ and the constant losses will _________
a) constant, reduce
b) constant, rise
c) reduce, rise
d) constant, constant

View Answer

Answer: a [Reason:] Mechanical losses remain always constant, but the stator losses will reduce as the voltage has been decreased.

13. No load power factor of the induction motor will __________ if the no load test is conducted at its rated frequency but less than the rated voltage.
a) improve
b) degrade
c) remain same
d) none of the mentioned

View Answer

Answer: a [Reason:] It will improve.

14. If the no load test is conducted at its rated voltage but less than the rated frequency.
A. The wattmeter reading will be more than previous.
B. The power factor will also improve.
a) Only A is true
b) Only B is true
c) Both A as well as B are true
d) None of the mentioned

View Answer

Answer: a [Reason:] Flux increases as flux = V/f. So the magnetizing current also increases. Power factor decreases.

15. Circle diagram for the fixed rotor resistance and variable reactance is most appropriately shown by
a) electrical-machines-questions-answers-circle-diagrams-induction-machines-q15a
b) electrical-machines-questions-answers-circle-diagrams-induction-machines-q15b
c) electrical-machines-questions-answers-circle-diagrams-induction-machines-q15c
d) electrical-machines-questions-answers-circle-diagrams-induction-machines-q15d

View Answer

Answer: a [Reason:] |I| = |V/Z| θ = atan(x/r), Here reactance is variable, which maps a semicircle in its locus diagram.

16. Circle diagram for the variable rotor resistance and fixed reactance is most appropriately shown by
a) electrical-machines-questions-answers-circle-diagrams-induction-machines-q16a
b) electrical-machines-questions-answers-circle-diagrams-induction-machines-q15d
c) electrical-machines-questions-answers-circle-diagrams-induction-machines-q15c
d) electrical-machines-questions-answers-circle-diagrams-induction-machines-q15a

View Answer

Answer: a [Reason:] |I| = |V/Z| θ = atan(x/r), Resistance is varying, so the locus will be a semi circle.

17. Use the below circle diagram to choose the correct option
electrical-machines-questions-answers-circle-diagrams-induction-machines-q8
a) CF: Rotor copper loss, FG: Stator copper loss, GE: Fixed losses.
b) FG:Rotor copper loss, CF: Stator copper loss, GE: Fixed losses.
c) GE:Rotor copper loss, FG: Stator copper loss, CF: Fixed losses.
d) FG:Rotor copper loss, CF: Stator copper loss, CF: Core losses.

View Answer

Answer: a [Reason:] GE is fixed losses. From the F to top the rated rotor copper losses occur.

18. In the following circle diagram, CC’ is
electrical-machines-questions-answers-circle-diagrams-induction-machines-q8
a) full load condition
b) no load condition
c) fixed losses
d) rated full load losses

View Answer

Answer: a [Reason:] It is full load condition.

Set 5

1. Inspite of heavy initial investments, dc motors are used due to
a) flexibility and ease of control
b) lower losses
c) improved power factor of the system
d) all of the mentioned

View Answer

Answer: a [Reason:] The control of dc machines is very much simplified when compared to other machines. So this makes it very useful when compared.

2. Separately excited dc generators are used in
a) Ward leonard system of speed control
b) Hopkinson’s testing
c) Voltage control
d) None of the mentioned

View Answer

Answer: a [Reason:] Separately excited dc generators are used in Ward leonard system of speed control.

3. Maximum torque in dc series motor is limited by
a) commutation
b) heating
c) field control
d) all of the mentioned

View Answer

Answer: a [Reason:] Commutation is the process which reduces the induced emf and so the torque.

4. Hoists, cranes and battery powered vehicles use _________ motors in the locomotive.
a) dc series
b) dc shunt
c) induction
d) reluctance

View Answer

Answer: a [Reason:] Hoists, cranes require large starting torque which can be provided by dc series motor.

5. There is an application which required pulsating loads, punch presses. The most preferred machines would be
a) compound dc machine
b) series dc machine
c) shunt dc machine
d) all of the mentioned

View Answer

Answer: a [Reason:] It is compound machine whose flux can varied higher as well as lower in order to have variable torques.

6. The manufacturer has mentioned a medium starting torque and 15% speed regulation. The most appropriate motor for his requirement is
a) dc shunt motor
b) induction motor
c) differential motor
d) none of the mentioned

View Answer

Answer: a [Reason:] The dc shunt motor provides a medium values of the torque.

7. Centrifugal pumps, fans-blowers use
a) shunt as well as induction motor
b) only shunt motors
c) only induction motor
d) none of the mentioned

View Answer

Answer: a [Reason:] Centrifugal pumps, fans-blowers use shunt as well as induction motor.

8. Most commercial compound dc generator are normally supplied by manufacturers as over compound machines because
a) degree of compounding can be adjusted by diverters acrosss series field
b) they have ideally best for HVDC
c) cost effective than shunt
d) zero percent regulation

View Answer

Answer: a [Reason:] It is usually over compounded so that degree of compounding can be adjusted by diverters across series field.

9. Which of the following are applied with differential compound motor?
a) Hoist
b) Cranes
c) Drilling machines
d) None of the mentioned

View Answer

Answer: a [Reason:] Practically it is never used as the speed can rise very high.

10. Most commercial compound dc generator are normally supplied by manufacturers as ______ compounded machines.
a) over
b) under
c) level
d) none of the mentioned

View Answer

Answer: a [Reason:] Most commercial compound dc generator are normally supplied by manufacturers as over compounded.

11. Long back ago, magnetic casette were in use, having permanent magnet dc motors which had
a) magnets on stator and armature on rotor
b) magnet on rotor and armature on stator
c) electronic commutation
d) all of the mentioned

View Answer

Answer: a [Reason:] PMDC motors have field fixed on the stator and rotor has coils.

12. If a dc shunt motor is stopped by forcing starter handle back to OFF position, then
a) dangerous sparking occurs at last stud
b) dangerous sparking occurs at all stud
c) it stops normally
d) heavy sparking

View Answer

Answer: a [Reason:] The sparking will occur at the last stud not all the studs of the starter.

13. Interpoles are used to start the dc motor above the base speed.
a) True
b) False

View Answer

Answer: b [Reason:] Interpoles do not reduce the flux across the windings, hence the speed does not rise above base speed.

14. A 200 V dc motor has external resistances of Ra and Rf in armature and field circuits respectively. The starting current is reduced when
a) Ra is maximum and Rf minimum
b) Rf is maximum and Ra minimum
c) Ra is minimum and Rf minimum
d) Ra is maximum and Rf maximum

View Answer

Answer: a [Reason:] When Ra is maximum and Rf minimum then the current to the armature is reduced. And hence the sparkings are reduced while starting.

15. A d.c. generator has been provided tappings on the armature winding at intervals of 120 degrees from the side of commutator. The connection will be
a) delta connected alternator
b) star connected alternator
c) star connected induction motor
d) any of the mentioned

View Answer

Answer: a [Reason:] Dc generator if has displaced by 120 degrees physically then it will be act as ac machine.