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# Multiple choice question for engineering

## Set 1

1. Voltage induce in the induction motor is highest at
a) starting
b) standstill
c) rated speed
d) any of the mentioned

Answer: a [Reason:] At starting when slip is 1, the voltage induced is maximum.

2. Starters are required in the induction motor because
a) of high starting current
b) they are not self starting
c) torque produced is very low at starting to overcome inertia
d) all of the mentioned

Answer: a [Reason:] Starters are needed in induction motors to limit the starting current.

3. Full voltage starting of induction motor may cause
a) dip in the voltages
b) high inrush current
c) high losses
d) all of the mentioned

Answer: a [Reason:] Starting induction motor on full load will cause high inrush current which furthe causes dip in the bus voltage.

4. For a 4-pole, 50 Hz induction motor has starting current as 5 times to that of the full load current working at a speed of 1450rpm.
The ratio of starting to full load torque will be
a) 0.83
b) 0.17
c) 0.67
d) 1

Answer: a [Reason:] Tst/Tfl = (Ist/Ifl)^2*slip Tst/Tfl= 25*0.033 =0.83

5. Starting methods applicable to both squirrel-cage and slip ring induction motors is/are
A. DOL starting
B. Auto transformer starting
C. Rotor resistance starting
a) A, B
b) A, B, C
c) B, C
d) A, C

Answer: a [Reason:] Rotor resistance method can only be used with slip ring induction motor.

6. The method which employs reduction of the voltage while induction motor starting is/are
A. DOL starting
B. Auto transformer starting
a) Only A
b) A, B
c) Only B
d) Neither A nor B

Answer: a [Reason:] Direct online starting uses reduction in voltage.

7. The method which employs raising of the voltage while induction motor starting is/are
A. DOL starting
B. Auto transformer starting
a) Only A
b) A, B
c) Only B
d) Neither A nor B

Answer: d [Reason:] None of the starting methods use increase in the starting voltage.

8. External methods of induction motor starting is employed to
a) Reduce the voltage
b) Reduce the current
c) improve efficiency
d) all of the mentioned

Answer: a [Reason:] External methods like auto transformer is used to mainly reduce voltage.

9. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the
starting current reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current

Answer: a [Reason:] The current will be reduced by a fraction of x.

10. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the
starting torque reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current

Answer: b [Reason:] Tst/Tfl = (x^2)(Ist/Ifl)^2*slip.

11. Series reactor method of starting has main disadvantage of
a) lowering the starting power
b) increasing the starting current
c) increasing starting torque
d) all of the mentioned

Answer: a [Reason:] Main disadvantage of using reactors is that ot reduces the starting power of the machine and so the efficiency.

12. Series reactor method of starting has main disadvantage of lowering the starting power. This can be overcome by
a) using series resistance
b) using parallel reactor
c) using parallel resistance
d) using series reactor

Answer: a [Reason:] Adding series resistance instead od reactors will help improving the starting power.

13. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the
starting current reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current

Answer: b [Reason:] The starting current reduces by a fraction of x^2.

14. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the
starting torque will be
a) x *Tfl
b) x^2 *Tfl
c) 1/x *Tfl
d) no change in the starting current

Answer: a [Reason:] Tst/Tfl = (x^2)(Ist/Ifl)^2*slip.

15. For the application requiring high starting torque and minimum starting current, the most suitable starting method will be
a) Auto transformer starting method
b) Reactance starting method
c) DOL method
d) Star-Delta method

Answer: a [Reason:] Auto transformer provides the maximum starting torque.

16. Auto transformer method of starting has most acceptable method for the starting of the induction motor due to
a) maximum torque
b) maximum reduction of inrush current
c) variable tapping
d) all of he mentioned

Answer: a [Reason:] Maximum torque, reduction of inrush current and variable turns ratio makes it popular method of starting of IM.

## Set 2

1. Voltage induce in the induction motor is highest at
a) starting
b) standstill
c) rated speed
d) any of the mentioned

Answer: a [Reason:] At starting when slip is 1, the voltage induced is maximum.

2. Starters are required in the induction motor because
a) of high starting current
b) they are not self starting
c) torque produced is very low at starting to overcome inertia
d) all of the mentioned

Answer: a [Reason:] Starters are needed in induction motors to limit the starting current.

3. Full voltage starting of induction motor may cause
a) dip in the voltages
b) high inrush current
c) high losses
d) all of the mentioned

Answer: a [Reason:] Starting induction motor on full load will cause high inrush current which furthe causes dip in the bus voltage.

4. For a 4-pole, 50 Hz induction motor has starting current as 5 times to that of the full load current working at a speed of 1450rpm.
The ratio of starting to full load torque will be
a) 0.83
b) 0.17
c) 0.67
d) 1

Answer: a [Reason:] Tst/Tfl = (Ist/Ifl)^2*slip Tst/Tfl= 25*0.033 =0.83

5. Starting methods applicable to both squirrel-cage and slip ring induction motors is/are
A. DOL starting
B. Auto transformer starting
C. Rotor resistance starting
a) A, B
b) A, B, C
c) B, C
d) A, C

Answer: a [Reason:] Rotor resistance method can only be used with slip ring induction motor.

6. The method which employs reduction of the voltage while induction motor starting is/are
A. DOL starting
B. Auto transformer starting
a) Only A
b) A, B
c) Only B
d) Neither A nor B

Answer: a [Reason:] Direct online starting uses reduction in voltage.

7. The method which employs raising of the voltage while induction motor starting is/are
A. DOL starting
B. Auto transformer starting
a) Only A
b) A, B
c) Only B
d) Neither A nor B

Answer: d [Reason:] None of the starting methods use increase in the starting voltage.

8. External methods of induction motor starting is employed to
a) Reduce the voltage
b) Reduce the current
c) improve efficiency
d) all of the mentioned

Answer: a [Reason:] External methods like auto transformer is used to mainly reduce voltage.

9. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the
starting current reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current

Answer: a [Reason:] The current will be reduced by a fraction of x.

10. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by reactance method of starting with a fraction of ‘x’, then the
starting torque reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current

Answer: b [Reason:] Tst/Tfl = (x^2)(Ist/Ifl)^2*slip.

11. Series reactor method of starting has main disadvantage of
a) lowering the starting power
b) increasing the starting current
c) increasing starting torque
d) all of the mentioned

Answer: a [Reason:] Main disadvantage of using reactors is that ot reduces the starting power of the machine and so the efficiency.

12. Series reactor method of starting has main disadvantage of lowering the starting power. This can be overcome by
a) using series resistance
b) using parallel reactor
c) using parallel resistance
d) using series reactor

Answer: a [Reason:] Adding series resistance instead od reactors will help improving the starting power.

13. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the
starting current reduces by
a) x
b) x^2
c) 1/x
d) no change in the starting current

Answer: b [Reason:] The starting current reduces by a fraction of x^2.

14. A 3-phase, 4-pole, 50 Hz induction motor is uses DOL method of starting. It is later replaced by auto transformer method of starting with a tapping ratio of ‘x’, then the
starting torque will be
a) x *Tfl
b) x^2 *Tfl
c) 1/x *Tfl
d) no change in the starting current

Answer: a [Reason:] Tst/Tfl = (x^2)(Ist/Ifl)^2*slip.

15. For the application requiring high starting torque and minimum starting current, the most suitable starting method will be
a) Auto transformer starting method
b) Reactance starting method
c) DOL method
d) Star-Delta method

Answer: a [Reason:] Auto transformer provides the maximum starting torque.

16. Auto transformer method of starting has most acceptable method for the starting of the induction motor due to
a) maximum torque
b) maximum reduction of inrush current
c) variable tapping
d) all of he mentioned

Answer: a [Reason:] Maximum torque, reduction of inrush current and variable turns ratio makes it popular method of starting of IM.

## Set 3

1. Synchronous motors are
a) not self starting
b) self starting
c) single excited
d) none of the mentioned

Answer: a [Reason:] The average torque developed in the synchronous motor is zero.

2. I. Auxiliary motor
II. Dc motor
III. induction motor
The method is/are used to perform magnetic locking
a) I,II,III
b) II,III
c) II,I
d) II

Answer: a [Reason:] A sub motor is needed to make the synchronous machine run till the speed of synchronous speed and to achieve that we need additional motor.

3. Auxiliary method of starting is not possib1e due to the fact that
a) it can not be started under load
b) it can be only started under load
c) it is less efficient
d) all of the mentioned

Answer: a [Reason:] Auxiliary motors usually reduces the performance indices of the synchronous machine when started under load.

4. Rating of the dc motor to be used in the starting of SM, should be higher than the synchronous motor.
a) True
b) False

Answer: b [Reason:] No, the dc motor used as pony motor is of lower rating than the synchronous motor it starts.

5. A synchronous machine with its field winding on stator and polyphase armature winding on rotor.
At steady state, which of the following is not true for its air gap field
a) stationary w.r.t. stator
b) rotating at double the speed Ns w.r.t. rotor
c) rotating in direction opposite to rotor
d) rotating at Ns w.r.t. rotor

Answer: d [Reason:] The air gap will not be at synchronous speed w.r.t. rotor.

6. A synchronous machine with its field winding on rotor and polyphase armature winding on stator. At steady state running condition, its air gap field is
I. stationary w.r.t. stator
II. rotating at double the Ns w.r.t. rotor
III. rotating at -Ns w.r.t. rotor
IV. stationary w.r.t. rotor.
a) IV
b) I,II,IV
c) II,IV
d) I

Answer: a [Reason:] The excitation is given on rotor so the air gap field will be stationary w.r.t. rotor.

7. Consider a 3-phase cylindrical-rotor alternator
A. E.m.f. generated by armature reaction lags armature current by 90°.
B. Air gap voltage leads the field flux by 90°
C. Air gap voltage lags the field flux by 90°
D. Armature reaction due to intermediate lagging p.f. is partly cross magnetizing and partly magnetizing
a) A,B
b) A,C,D
c) B,D
d) A,B,D

Answer: a [Reason:] Having cross magnetizing armature reaction in the machine means partial magnetization and demagnetization, adding another clause is useless.

8. These days alternators are designed to have larger air gaps for
a) stable parallel operation
b) higher stability limit
c) sinusoidal mmf distribution
d) all of the mentioned

Answer: a [Reason:] Large air gap ensures more SCR value and better preformances.

9. Consider the following statements and mark appropriately
I. In a cylindrical-rotor synchronous machine, armature flux is in phase with armature current.
II. In salient pole machine, armature flux lags the armature current.
a) I is true
b) II is true
c) I and II are true
d) None of them are true

Answer: c [Reason:] In CRSM, there is no saliency and the armature flux follows armature current in phase.

10. In a 3-phase cylindrical-rotor alternator, synchronous reactance is sum of mutual and leakage reactance.
a) True
b) False

Answer: b [Reason:] Xs = magnetizing reactance + leakage reactance.

11. A 3-phase, 400V, synchronous motor is providing load at 0.8 p.f. lagging. If the field current of the motor is continuously increased, then
a) power factor increases and then decreases.
b) power factor is not affected
c) power factor decreases upto a certain value of field current and then increases.
d) power facor increases simply

Answer: a [Reason:] The magnetization current is increased as we increase the excitation, so the power factor will increase upto the saturation and then again it will decrease.

12. A synchronous machine is operating at constant load and at unity power factor.
If its excitation is increased and then it operates as
a) motor at leading p.f
b) motor at lagging p.f
c) absorber of reactive power
d) generator at leading p.f

Answer: a [Reason:] It will operate at leading p.f. if it was in motoring mode.

13. A synchronous machine is operating at constant load and at unity power factor. If its excitation is increased, then it operates as
a) generator at lagging p.f
b) motor at lagging p.f
c) absorber of reactive power
d) generator at leading p.f

Answer: a [Reason:] It will operate at lagging p.f. if it was working as generator.

14. The reactive power output of a synchronous generator is limited by
a) armature current and field current
b) field current and load angle
c) load angle and prime mover input
d) armature current and prime mover input

Answer: a [Reason:] Q=f(Ia,If).

15. Power factor of a synchronous motor varies when
a) applied voltage is varied
b) load is changed
c) supply frequency and field excitation is changed
d) all of the mentioned

Answer: a [Reason:] p.f. = f(V,E,f,Field current).

## Set 4

1. One of the most prominent component of base loads is
a) synchronous generator
b) induction generator
c) dc series generator
d) all of the mentioned

Answer: a [Reason:] Synchronous machines are the main generating section of the power plants.

2. ______ alternator is used in thermal stations and ______ alternator is used in hydel plants.
a) Cylindrical-rotor, salient pole
b) Salinet pole, cylindrical-rotor
c) Salient pole, Reluctance
d) Cylindrical-rotor, cylindrical-rotor

Answer: a [Reason:] Due to salient poles, the machine will not run at very large speed and it becomes less rugged.

3. Synchronous compensators are mainly employed for
a) compensating reactive power
b) compensating real power
c) improving power factor
d) compensating field flux

Answer: a [Reason:] Synchronous compensator is a source for the reactive power.

4. We prefer induction motor over synchronous motor due to
a) no requirement of dc supply
b) starting problems
c) sensitive to environment
d) all of the mentioned

Answer: a [Reason:] All the listed factors are disadvantages for using a synchronous motor.

5. The condition which allows the torques to be taken care for choosing a synchronous motor is/are
a) starting torque
b) pull-in torque
c) pull-out torque
d) all of the mentioned

Answer: d [Reason:] All the performance torques are needed to know before using any synchronous motor.

6. A synchronous machine with its field winding on stator and polyphase armature winding on rotor. At steady state, its air gap field is
a) stationary w.r.t. stator
b) rotating at double the speed Ns w.r.t. rotor
c) rotating in direction opposite to rotor
d) all of the mentioned

Answer: d [Reason:] All are correct.

7. A synchronous machine with its field winding on rotor and polyphase armature winding on stator. At steady state running condition, its air gap field is
a) stationary w.r.t. stator
b) rotating at double the Ns w.r.t. rotor
c) rotating at -Ns w.r.t. rotor
d) stationary w.r.t. rotor.

Answer: d [Reason:] As the field is on rotor itself, the sir gap will be stationary w.r.t. rotor.

8. Choose the appropriate
The synchronous generators used in the thermal power stations have
(i) cylindrical rotor
(ii)slip rings and brushes
(iii)laminated rotor
(iv)stator slots in the multiple of 6
(v)salient rotor
a) (i),(ii),(iv)
b) (ii),(iii),(v)
c) (i),(iii)
d) (ii),(iii),(iv),(v)

Answer: a [Reason:] Thermal plants use cylindrical rotor synchronous machines.

9. The practical speed ranges for the thermal, nuclear and hydel power plants is (in rpm)
a) 3000,3000,300
b) 3000,300,300
c) 1500,1500,3000
d) 1000,500,3000

Answer: a [Reason:] Thermal and nuclear power plants have high speed requirements and hydel power plants have salient pole generators so the speed is also less.

10. Synchronous motor has been preferred for driving the loads requiring high power at low speeds.
a) True
b) False

Answer: a [Reason:] Synchronous motor acts a capacitor when operated at leading p.f. with no load.

11. The zero power characteristics used in the Potier plot can be obtained by loading the alternator using
a) synchronous motor
c) dc motor
d) induction motor

Answer: a [Reason:] To obtain mmf and emf drops the loading is done as a synchronous motor.

12. Below are the statements about cylindrical rotor alternator.
A. E.m.f. generated by armature reaction lags armature current by 90°.
B. Air gap voltage leads the field flux by 90°
a) Only A is true
b) Only B is true
c) A is true, B is false
d) A and B both are true

Answer: d [Reason:] Both the statements are right.

13. Out of the given statements about the cylindrical rotor alternator, mark the correct
A. Air gap voltage lags the field flux by 90°
B. Armature reaction mmf lags the field flux by (90°+ internal p.f. angle)
a) A is false, B is true
b) Only A is true
c) Only B is true
d) Both A and B are true

Answer: a [Reason:] Air gap voltage will lead the field flux by 90° in a cylindrical-rotor alternator.

14. The power factor of an alternator under short circuit condition will be almost near
a) zero lagging
c) unity
d) depends on the type of the alternator

Answer: a [Reason:] During short circuit condition, the magnetizing current flows to provide the core losses compensation. So the pf is slightly zpf lagging.

15. In a 3-phase cylindrical rotor alternator
a) field mmf leads air gap flux, air gap flux leads armature flux
b) field mmf leads air gap flux, air gap flux lags armature flux
c) armature mmf leads air gap flux and air gap flux leads field flux
d) armature mmf leads air gap flux and air gap flux lags field flux

Answer: a [Reason:] Check the phasor diagram.

## Set 5

1. Steady state stability limit is defined as maximum power flow possible through a particular point without loss of stability when
a) power is increased gradually
b) power is increased suddenly
c) power is reduced gradually
d) power is reduces suddenly

Answer: a [Reason:] Steady state analysis involves study of stability at gradual changes in the machine.

2. If the load angle is increased till 135°, the consequence will be
a) loss of synchronism
b) no alternation of synchronism
c) increase in stability limit
d) reduction in stability limit

Answer: a [Reason:] Because load angle is more than 90°.

3. The steady state limit can be improved by
i. Increasing excitation
ii. reducing reactance
iii. using series capacitors
a) i, ii, iii
b) i, ii
c) iii only
d) ii, iii

Answer: a [Reason:] Steady state limit = EV/X.

4. Series capacitors will improve the
a) SSSL
b) Transient stability
c) Hunting
d) Stiffness

Answer: a [Reason:] Steady state limit = EV/X.

5. Transient stability limit is less important than SSSL
a) True
b) False

Answer: b [Reason:] Transient stability is more of concern due to the reason that the machine may loose synchronism due to sudden disturbances.

6. (i) Transient stability is lower than SSSL
(ii) Transient disturbances are more common
a) i and ii both are true
b) i false, ii true
c) i true, ii false
d) Both are false

Answer: a [Reason:] Both are correct statements.

7. The supply voltage to an induction motor is decreased by 10%. By what percentage, approximately, will the maximum torque decrease?
a) 5%
b) 10%
c) 20%
d) 40%

Answer: c [Reason:] Torque is proportional to V^2. Torque will change by twice percentage of the change in voltage i.e. 20%.

8. In a salient pole synchronous machine the air gap is least under the middle of the pole shoe and increases outwards.
a) True
b) False

Answer: a [Reason:] Air gap for a salient pole structure is minimum at mid of a given pole.

9. If it is planned to bring an incoming alternator to the operating alternator in parallel, then which of the criteria can be ignored?
a) Prime mover speed
b) Voltage magnitude
c) Frequency
d) Phase sequence of the alternator