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Multiple choice question for engineering

Set 1

1. Stator equivalent circuit for polyphase induction motor is
a) electrical-machines-questions-answers-equivalent-circuit-q1a
b) electrical-machines-questions-answers-equivalent-circuit-q1b
c) electrical-machines-questions-answers-equivalent-circuit-q1c
d) electrical-machines-questions-answers-equivalent-circuit-q1d

View Answer

Answer: a [Reason:] The Rc and Xm represent the copper losses and the core losses in the stator.

2. The actual rotor equivalent of an induction motor is best depicted by
a) electrical-machines-questions-answers-equivalent-circuit-q1a
b) electrical-machines-questions-answers-equivalent-circuit-q1b
c) electrical-machines-questions-answers-equivalent-circuit-q1c
d) electrical-machines-questions-answers-equivalent-circuit-q1d

View Answer

Answer: b [Reason:] Taking the actual reactance which is slip dependent in the circuit.

3. Rc in the below circuit represents
electrical-machines-questions-answers-equivalent-circuit-q1c
a) core loss of induction motor
b) no load losses of induction motor
c) iron losses of induction motor
d) copper losses of induction motor

View Answer

Answer: a [Reason:] The shunt resistance will represent only the core loss of the induction motor.

4. The electrical anlogous of the mechanical load in the induction machine is given by
a) (r2)*(1-s)/s
b) (r2)
c) (r2)/s
d) (r2)/(1-s)

View Answer

Answer: a [Reason:] Mechanical load is represented by a factor (1-s)/s of the rotor resistance.

5. The actual rotor resistance referred to the stator if the induction motor is
a) r2
b) (r2)/s
c) (r2)/(1-s)
d) (r2)*(1-s)/s

View Answer

Answer: a [Reason:] The actual resistance will be be r2 only.

6. The leakage reactances are less than their values at the full load operating conditions. Why?
a) Due to large inrush current during starting
b) Due to very low saturation
c) Due to speed is not same as synchronous speed
d) All of the mentioned

View Answer

Answer: a [Reason:] During the start, there will be large inrush currents flowing in the machine, which cause saturation and so the leakage reactance decreases at start.

7. The leakage reactance and the rotor resistance are constant throughout the operation in an induction motor.
a) True
b) False

View Answer

Answer: b [Reason:] No, the reactance changes with the variation in the slip of the machine.

8. At standstill the equivalent of the 3-phase induction motor is similar to
a) short-circuited two winding transformer
b) open circuit two winding transformer
c) stand still synchronous motor
d) none of the mentioned

View Answer

Answer: a [Reason:] For s=1 at standstill, so it same as the short circuit of the transformer for the induction motor.

9. The graph for the induction motor operating with load torque proportional to square of the shaft speed.
a) electrical-machines-questions-answers-equivalent-circuit-q9a
b) electrical-machines-questions-answers-equivalent-circuit-q9b
c) electrical-machines-questions-answers-equivalent-circuit-q9c
d) electrical-machines-questions-answers-equivalent-circuit-q9d

View Answer

Answer: a [Reason:] Torque α (speed)^2; Power = torque*speed

10. An induction motor is operating for the load with torque developed proportional to the shaft speed. The characteristics below is
a) electrical-machines-questions-answers-equivalent-circuit-q9a
b) electrical-machines-questions-answers-equivalent-circuit-q9b
c) electrical-machines-questions-answers-equivalent-circuit-q9c
d) electrical-machines-questions-answers-equivalent-circuit-q9d

View Answer

Answer: b [Reason:] Torque α (speed); Power = torque*speed

11. The characteristics required of the induction motor to match that of the load for load torque independent of the shaft speed is
a) electrical-machines-questions-answers-equivalent-circuit-q9a
b) electrical-machines-questions-answers-equivalent-circuit-q9b
c) electrical-machines-questions-answers-equivalent-circuit-q9c
d) electrical-machines-questions-answers-equivalent-circuit-q9d

View Answer

Answer: b [Reason:] Torque = constant ; Power = torque*speed;

12. The characteristics required of the three phase induction motor to match that of the load with the criteria of load torque inversely proportional to the shaft speed is
a) electrical-machines-questions-answers-equivalent-circuit-q9a
b) electrical-machines-questions-answers-equivalent-circuit-q9b
c) electrical-machines-questions-answers-equivalent-circuit-q9c
d) electrical-machines-questions-answers-equivalent-circuit-q9d

View Answer

Answer: d [Reason:] Torque α 1/(speed); Power = torque*speed;

13. In a 3-phase induction generator which is self excited, load has been increased.To keep the frequency of the generated voltage constant, speed of the induction machine should be
a) increased
b) decreased
c) maintained less than rated synchronous speed
d) maintained more than rated synchronous speed

View Answer

Answer: a [Reason:] Frequency should be increased to maintain the speed constant.

14. A 3-phase induction machine is running at super synchronous speed. For the machine to be self excited, it
a) draws reactive power from mains
b) draws real power from mains
c) feeds reactive power to mains
d) generates emf by gradual voltage build up

View Answer

Answer: a [Reason:] To provide the reactive power for the machine, it will draw reactive power from a source.

15. Induction generators will deliver power at
a) leading p.f.
b) lagging p.f.
c) unity p.f.
d) zero p.f.

View Answer

Answer: a [Reason:] Induction generator will deliver power at leading p.f.

16. A star connected capacitor bank is connected to the induction motor having capacitance C and VA rating as VAs.
If the capacitor bank is replaced by a delta bank then
a) C = C/3, VA = VAs
b) C = 3C, VA = VAs
c) C = C, VA = VAs/3
d) C = C/3, VA = VAs/3

View Answer

Answer: a [Reason:] The VA rating will not change due the configuration, but the capacitance will change due to the delta configuration to C/3.

17. A delta connected capacitance bank is preferred over a star connected bank. This is done to ensure
a) lesser capacitance
b) easy calculation
c) easier connection
d) all of the mentioned

View Answer

Answer: a [Reason:] Delta connected bank will have lesser capacitance than star connected and so the better power factor improvement.

18. If it is desired to approximate the efficiency of an induction motor operating at slip ‘s’, then its efficiency is given by
a) 1-s/1+s
b) 1/1+s
c) s/s+1
d) s/1-s

View Answer

Answer: a [Reason:] Approximately efficiency is given as 1-s/1+s for an IM.

Set 2

1. Consider a DC generator running at the rated speed of 2000 rpm, suddenly there is an insulator falls on the field circuit and breaks it. Then
a) the motor stops in a few rounds
b) it continues to run, but as dc motor
c) it continues to run as a motor
d) none of the mentioned

View Answer

Answer: a [Reason:] As the field circuit gets broken there is interruption in the flux to the dc machine, and as a result the motor stops in few seconds.

2. If a self excited DC generator is failed to run, this refers to
a) zero residual voltage
b) field MMF, that it is not cumulative
c) resistance is greater than critical resistance
d) any of the mentioned

View Answer

Answer: a [Reason:] Any of the reasons are valid for no build up of an emf.

3. The self-excited dc generator with gradual build up of residual voltage and EMF corresponds to
a) positive feedback
b) negative feedback
c) saturation
d) any of the mentioned

View Answer

Answer: a [Reason:] It responds to a positive feedback because the gain is always more than one in the linear region.

4. The critical resistance refers to
a) the resistance above which machine does not excite
b) the resistance below which machine does not excite
c) the resistance at which machine does not excite
d) any of the mentioned

View Answer

Answer: a [Reason:] After taking a glance at the magnetization curve, it can be concluded that the emf will not build up if the resistance is greater than critical resistance.

5. If a DC shunt generator fails to start, the most primitive action would be
a) reversing field connection to armature
b) reversing direction of rotation
c) any of the mentioned applicable ways
d) none of the mentioned

View Answer

Answer: a [Reason:] The first measure is to check if the field winding as properly connected with additive polarity so that the flux is aiding in nature.

6. Flashing is used to
a) temporarily excited field from a battery source for large DC generator
b) temporarily excited field from a battery source for small DC generator
c) temporarily excited field from a battery source for large DC motor
d) temporarily excited field from a battery source for large DC motor

View Answer

Answer: a [Reason:] It is a method to give a kick start to dc machine when its residual magnetism is vanished.

7. A series generator, having external characteristics which is a straight line through 0 to 50 V at 200 A is connected as a booster, between a station bus-bar and feeders of 0.5 ohm resistors. The voltage at the far end of feeder at current of 50 A is
a) 25 V
b) 17 V
c) 8 V
d) 50 V

View Answer

Answer: a [Reason:] V = voltage drop-voltage added up by boosters = (0.3*50 – 50*50/200) = 15-12.50 = 2.5 V

8. The number of parallel paths in the armature is increased by
a) increasing number of magnetic poles
b) decreasing number of magnetic poles
c) lap number of magnetic poles
d) using more brushes

View Answer

Answer: a [Reason:] It is increased by increasing magnetic poles which give parallel path to the magnetic circuit.

9. A dc series generator as armature and field connections are reversed, the generator
a) stops
b) opposite direction
c) same direction
d) none of the mentioned

View Answer

Answer: a [Reason:] It happens due to non build up voltage.

10. ____ is responsible for mechanical power output of a DC motor
a) Electrical input power
b) Any of the mentioned
c) Air-gap flux
d) Armature emf

View Answer

Answer: a [Reason:] Primarily it is the electrical input power which starts the motoring action.

Set 3

1. Below is the torque slip characteristic of the induction machine. What can be inferred from it?
electrical-machines-questions-answers-experienced-q1
a) R22 > R21
b) R22 < R21
c) R22 = R21
d) R22 = -R21

View Answer

Answer: a [Reason:] As rotor resistance increases, the starting torque will increase but not the maximum torque. But the slip at which the maximum torque occurs will change.

2. Choose the most appropriate from the following characteristics of 3 phase induction motor torque-slip characteristics.
a) electrical-machines-questions-answers-experienced-q2a
b) electrical-machines-questions-answers-experienced-q2b
c) electrical-machines-questions-answers-experienced-q2c
d) electrical-machines-questions-answers-experienced-q2d

View Answer

Answer: a [Reason:] As rotor resistance increases, the starting torque will increase but not the maximum torque. But the slip at which the maximum torque occurs will change.

3. If the reactance of the 3-phase slip ring induction motor is increased then the torque-slip characteristic varies as
a) electrical-machines-questions-answers-experienced-q3a
b) electrical-machines-questions-answers-experienced-q3b
c) electrical-machines-questions-answers-experienced-q2c
d) electrical-machines-questions-answers-experienced-q3d

View Answer

Answer: a [Reason:] Torque is inversely proportional to reactance

4. The reactance vs slip is given by
a) electrical-machines-questions-answers-experienced-q4a
b) electrical-machines-questions-answers-experienced-q4b
c) electrical-machines-questions-answers-experienced-q4c
d) electrical-machines-questions-answers-experienced-q4d

View Answer

Answer: a [Reason:] Slip is inversely proportional to the slip

5. Induction generator is equivalent to
a) underexcited alternator
b) dc generator
c) over excited alternator
d) leading over excited alternator

View Answer

Answer: a [Reason:] It is similar to underexcited alternator

6. When an induction generator is connected to an infinite bus, it
a) requires no capacitor bank
b) requires star connected capacitor bank
c) requires delta connected capacitor bank
d) requires any capacitor bank

View Answer

Answer: a [Reason:] There is no requirement of the capacitor bank when induction generator is connected to infinite bus.

7. Torque produced by induction generator is negative.
a) True
b) False

View Answer

Answer: a [Reason:] Because the developed torque is produced opposite to the rotating field.

8. Slip greater than unity is obtained practically by
a) plugging
b) changing phase sequence
c) changing stator leads
d) any of the mentioned

View Answer

Answer: d [Reason:] The direction of the rotating field can be reversed by any one of the methods.

9. Changing the rotor resistance will also affect
a) starting torque, speed, current
b) maximum torque, speed, current
c) maximum torque, speed, current, starting torque
d) speed and current

View Answer

Answer: a [Reason:] Rotor resistance does not alter the maximum torque of the induction machine.

10. At starting, developed mechanical power is _____ and developed torque is ______
a) zero, not zero
b) zero,zero
c) not zero, zero
d) not zero, maximum

View Answer

Answer: a [Reason:] At start, torque is not zero. P α (1-s)/s; so it is zero at s = 1.

11. Mechanical power developed in the braking region of the induction motor is
a) negative
b) positive
c) zero
d) any of the mentioned

View Answer

Answer: a [Reason:] slip is more than one, so Pm will be negative.

12. Developed torque during plugging is
a) negative
b) positive
c) zero
d) any of the mentioned

View Answer

Answer: b [Reason:] Torque will not be negative in braking mode in the induction motor.

13. In the 3-phase induction motor developed torque during plugging is ______ mechanical power developed in the braking region of the induction motor is ______
a) positive, negative
b) negative, positive
c) positive, positive
d) negative, negative

View Answer

Answer: a [Reason:] Tem is positive while Pm will be negative in braking or plugging operation.

14. The power slip characteristic is given as
a) electrical-machines-questions-answers-experienced-q14a
b) electrical-machines-questions-answers-experienced-q14b
c) electrical-machines-questions-answers-experienced-q14c
d) electrical-machines-questions-answers-experienced-q14d

View Answer

Answer: a [Reason:] Mechanical power developed is negative in braking and generating mode and it is zero at starting.

Set 4

1. A 3 phase synchronous machine is _________ excited machine.
a) doubly
b) single
c) two stage
d) feedback based

View Answer

Answer: a [Reason:] The 3 phase synchronous machine has two excitations. Ac excitation at armature while dc excitation at field winding.

2. Field winding of a dc machine
a) always absorbs power
b) absorbs real power at lagging load
c) delivers power while behaving as generator
d) always delivers power

View Answer

Answer: a [Reason:] Field is always given supply from dc system so it always absorbs power as it is necessary to produce the flux.

3. In large synchronous machine, field winding is placed on ______ and ac supply on ______
a) rotor, stator
b) stator, rotor
c) armature, slots
d) pole shoes, stator

View Answer

Answer: a [Reason:] The field winding is placed on the stator as it will carry less current while the ac supply is fed at stator as it will have to carry large amount of current.

4. The pilot exciter in dc exciters is ________
a) dc shunt generator feeding field winding of mains
b) universal motor
c) stepper motor feeding the field winding of mains
d) any of the mentioned

View Answer

Answer: a [Reason:] Pilot exciter is dc shunt generator which supplies the field of main winding.

5. The main problems faced by conventional DC exciters are/is _______
a) cooling and maintainance
b) wear and tear
c) additional parasitic losses
d) all of the mentioned

View Answer

Answer: d [Reason:] The conventional dc exciters have all the above mentioned losses which make them out dated and least efficient.

6. The static excitations mainly comprises of _______
a) brushless excitation system
b) thyrister based excitation system
c) synchronous motor excitation system
d) brushless and thyristor based excitation system

View Answer

Answer: b [Reason:] SCRs are used in the rectifier configuration as a static system for excitation for dc field winding.

7. The excitation system and speed of alternator has _______
a) linear response
b) exponential response
c) rectangular hyperbola response
d) circular response

View Answer

Answer: a [Reason:] As the excitation energy is directly taken from alternator terminals, excitation voltage is directly proportional to speed.

8. The brushless excitation and conventional DC excitation systems are comparable but one significant change is _______
a) pilot exciter is brushless permanent magnet motor
b) three phase rectifier feeds directly to main exciter
c) a three phase rectifier is fed by main exciter
d) no direct rectification is provided to the pilot exciter

View Answer

Answer: a [Reason:] The brushless excitation involves a brushless permanent motor which acts as an auxiliary exciter.

9. The main exciter used in DC excitation is ________
a) field on stator
b) armature on stator
c) field on rotor
d) field on poles

View Answer

Answer: a [Reason:] The main exciter used in DC excitation is field on stator.

10. If the DC excitation is suddenly dropped to 0 ,the three phase alternator _________
a) runs as motor
b) stops to zero speed in few seconds
c) continues to run as motor but at lower speed
d) no change in the operating conditions

View Answer

Answer: a [Reason:] When the dc excitation is disconnected from the supply, it will start acting as synchronous motor as the prime mover will still be mechanically coupled.

Set 5

1. A dc motor is connected in the short-shunt configuration, but the series and the shunt windings get interchanged by mistake and the motor is connected to the rated voltage,
a) neither of the windings get short-circuit
b) series winding gets overloaded
c) shunt winding gets overloaded
d) both of the winding get overloaded

View Answer

Answer: a [Reason:] In the short shunt configuration, the winding interchanging does not change the configuration of the set up so none of the winding get affected.

2. The main concern before doing hopkinson’s test for finding efficiency is
a) needs one motor and one generator is that it
b) ignores iron and stray losses
c) needs one motor
d) requires identical shunt machines

View Answer

Answer: d [Reason:] It is must that both the machines are identical as required by the calculations.

3. Retardation test on DC shunt motor is used for finding _______
a) stray losses
b) copper losses
c) friction losses
d) iron losses

View Answer

Answer: a [Reason:] Retardation test is used for finding the stray losses.

4. The quantities needed to complete retardation test are/is
a) dw/dt and moment of inertia
b) dw/dt
c) current
d) any of the mentioned

View Answer

Answer: a [Reason:] We need the angular acceleration and the moment of inertia of the machine.

5. ______ test is used for determining the efficiency of a traction motor
a) Field
b) Retardation
c) Hopkinson
d) Swineburne’s

View Answer

Answer: a [Reason:] Field test is used for finding efficiency of a traction motor.

6. If the field current and armature current are reversed, then
a) direction of rotation remains same
b) direction of rotation reverses
c) stops
d) none of the mentioned

View Answer

Answer: a [Reason:] When both field current and armature current are reversed then the direction will not change.

7. Mark the possible causes of overheating of commutator in a DC machine.
a) Overload
b) Restricted ventilation
c) Shorted winding
d) Any of the mentioned

View Answer

Answer: a [Reason:] All the factors can cause the overheating of commutator.

8. In the hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has field current of 1.3A. Which machine acts as motor?
a) B
b) A
c) A,B
d) None of the mentioned

View Answer

Answer: a [Reason:] Ea/Eb = 1.4/1.3 Ea= 1.077 Eb > Eb So, the machine will act as motor.

9. In the Hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has field current of 1.3A. Which machine acts as generator?
a) B
b) A
c) A,B
d) None of the mentioned

View Answer

Answer: b [Reason:] Ea/Eb = 1.4/1.3 Ea= 1.077 Eb > Eb So, the machine will act as generator.

10. Hopkinson’s test is also called regenerative test due to
a) energy of one machine is used to drive other
b) feedback
c) losses are least
d) extra motor is used

View Answer

Answer: a [Reason:] Regenerative phenomena utilizes the energy of the running machine to tap the dynamically produced energy and use it to charge the battery.

11. The most appropriate relation to find efficiency of the generator
a) output/(output + losses)
b) (output – losses)/input
c) (output – losses)/(output + losses)
d) any of the mentioned

View Answer

Answer: a [Reason:] For a generator the output can be measured and so this expression is used.

12. The efficiency in the swineburne’s test can be found.
a) True
b) False

View Answer

Answer: a [Reason:] Because the constant losses are known.

13. The possible assumption is/are made while doing swineburne’s test
(i) mechanical loss constant
(ii) Armature reaction neglected
(iii) Increases in flux
a) (i) and (ii)
b) (iii) and (i)
c) (i), (ii), and (iii)
d) (i)

View Answer

Answer: a [Reason:] There is decrease in flux due to positive temperature coefficient of resistance in shunt machine.

14. Swineburne’s test is applicable to those machines in which flux is practically _______
a) constant
b) linear
c) non-linear
d) none of the mentioned

View Answer

Answer: a [Reason:] Because the constant losses are known in advance of the test is conducted.

15. Similarity between Hopkinonson’s and field’s test is that
a) both need similar mechanically coupled motor
b) both need similar electrically coupled motor
c) regenerative power
d) negligible power

View Answer

Answer: a [Reason:] The machines have to be mechanically coupled together.