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Multiple choice question for engineering

Set 1

1. The 3-phase induction motor operates at lagging power factor due to
I. Leading magnetizing current necessary to produce the working flux.
II. Voltage drops in the leakage impedance.
III. Lagging magnetizing current which lags by 90° to the voltage
a) II, III
b) I,II
c) II
d) I, II, III

View Answer

Answer: a [Reason:] Magnetizing current is lagging in nature and also the leakage impedance increases the power factor angle.

2. Choose the most appropriate option regarding the power factor control of the induction motor.
electrical-machines-questions-answers-power-control-three-phase-motors-q2
a) A: with capacitor
b) B: with capacitor
c) A: with capacitor, B: with capacitor
d) A: with no capacitor, B: with capacitor

View Answer

Answer: a [Reason:] Leading stator current shifts to the left.

3. Which tests are to be conducted to draw circle diagram for a given induction motor?
A. Running light test
B. Blocked rotor test
C. Voltmeter-Ammeter test
a) A, B, C
b) A, B
c) B, C
d) A,C

View Answer

Answer: a [Reason:] All the three tests results are needed to plot the circle diagram.

4. The tests that are to be conducted to draw circle diagram for a given induction motor are
A. Running light test
B. Slip test
C. Voltmeter-Ammeter test
a) A, C
b) A, B, C
c) B, C
d) A, B

View Answer

Answer: a [Reason:] Slip test is conducted on the synchronous machine.

5. If an unbalanced supply is fed to an induction motor, it will cause excessive heating of
a) rotor
b) stator
c) rotor shaft
d) All of the mentioned

View Answer

Answer: a [Reason:] Unbalanced supply will cause most heating of rotor winding than any other part.

6. If one of the three phases of the induction motor fails due to fault. The motor will
a) stop and do permanent damage if it was running overloaded
b) continue running but will draw more current if it was operating at rated load
c) will not be affect the operation if light loaded
d) any of the mentioned

View Answer

Answer: d [Reason:] All the possibilities can happen if one of the phase is removed.

7. Which of the following will not happen if one of the phases to the induction motor is not available?
a) Motor will start but operate at lower speed
b) It will hum but not start
c) It will continue to operate below 57.7% of rated load
d) External means are needed to make it run at rated speed

View Answer

Answer: a [Reason:] Motor will not start if one of the phases is missing.

8. Induction motor can be fed from
a) either rotor or stator
b) stator
c) rotor
d) neither stator nor rotor

View Answer

Answer: a [Reason:] It can be fed from either side at one time.

9. In double cage induction motor, inner cage has
a) small resistance and large reactance
b) high resistance and high reactance
c) small resistance and small reactance
d) high resistance and small reactance

View Answer

Answer: a [Reason:] In the bottom cage, the air gap is less, so the leakage reluctance is less and more the leakage reactance.

10. Static capacitors are used to control power factor of slip ring induction motor only.
a) True
b) False

View Answer

Answer: b [Reason:] Static capacitors are also used to improve power factor of the slip ring as well as wound rotor induction motors.

11. A 3-phase, 400 V, 50 Hz delta connected induction motor has power factor of 0.35 when delivering 30% of its load. If it is star connected, then its
a) p.f. increased and the stator current decreases
b) p.f. worsened and the stator current decreases
c) p.f. increased,and the stator current increases
d) p.f. does not change,and the stator current increase

View Answer

Answer: a [Reason:] In star connection, resistance decrease than delta connected.

12. A 3-phase induction machine is operating on 3-phase fixed frequency ac mains at a per unit slip of 1.5. Then which of the following statements follow the working condition?
I. It draws electrical power from the mains.
II. It draws mechanical power through the shaft.
III. It delivers electrical power to the mains.
IV. It delivers mechanical power through the shaft.
a) I, IV
b) I, II
c) II, III
d) III, IV

View Answer

Answer: a [Reason:] The slip is positive so it is operating as a motor and in the braking mode.

13. A 3-phase induction machine is operating on 3-phase fixed frequency ac mains at a per unit slip of 1.5. Then machine is operating in generating mode.
a) True
b) False

View Answer

Answer: b [Reason:] It is braking mode.

14. As per the condition below,
If a 3-phase squirrel-cage induction motor is operating at a slip of -0.05 then machine will
a) draw mechanical strength and deliver electrical power to mains
b) draw electrical power from mains and deliver mechanical strength to shaft
c) draw electrical power from shaft and deliver mechanical strength through mains
d) any of the mentioned

View Answer

Answer: a [Reason:] It is in generating mode as the slip is negative.

15. If the induction motor connected to a power grid draws real power P and reactive power Q. If it is made to run as a generator, then P and Q will be
a) negative, positive
b) positive, positive
c) positive, negative
d) negative, negative

View Answer

Answer: a [Reason:] Induction motor will always need reactive power as it is single excited machine.

Set 2

1. The magnetization current drawn from an AC supply a synchronous motor is used to _________
a) set up flux in magnetic circuit of device
b) compensate core losses
c) set up magnetizing armature reaction
d) all of the mentioned

View Answer

Answer: a [Reason:] Magnetizing current sets up the flux needed in the machine.

2. A 3 phase synchronous motor is working at normal excitation, then the flux deficient in circuit is ________
a) given by armature winding mmf
b) given by field winding mmf
c) supplied to armature winding mmf
d) supplied to field winding mmf

View Answer

Answer: a [Reason:] it is given by armature winding mmf.

3. The excess flux in synchronous motor is neutralized by
a) armature winding by drawing demagnetizing component of the current from ac supply
b) armature winding by drawing magnetizing component of the current from ac supply
c) field winding by drawing demagnetizing component of the current from dc supply
d) field winding by drawing magnetizing component of the current from dc supply

View Answer

Answer: a [Reason:] Excess flux is neutralized y the demagnetizing component.

4. The V-curves of synchronous motor is plotted between ________
a) Ia Vs If with constant shaft load
b) If Vs Ia with constant shaft load
c) power factor vs If
d) power factor vs Ia

View Answer

Answer: a [Reason:] The V-curves of synchronous motor is plotted between Ia Vs If with constant shaft load.

5. When a constant power output is desired to maintain
a) Ef*sinδ and Ia*cosθ are to be maintained constant
b) Ef*sinδ is be maintained constant
c) Ia*cosθ is to be maintained constant
d) Ef*sinδ and Ia*sinθ are to be maintained constant

View Answer

Answer: a [Reason:] For constant real power output the component of the excitation voltage lagging to Vt should remain constant as per the phasor diagram.

6. When the excitation voltage is increased from 1 to 1.3 pu of a 3-phase synchronous motor. Then load angle for the constant power operation
a) must be decreased
b) can also be increased
c) increased
d) decreased

View Answer

Answer: a [Reason:] Efsinδ is to be maintained constant. so if excitation voltage increases then the load angle must decrease.

7. The armature current is ________ at UPF when compared to leading power factor for a motor.
a) minimum
b) maximum
c) equal
d) none of the mentioned

View Answer

Answer: a [Reason:] At leading pf armature current will be high as the emf will be high.

8. The armature current is _______ at UPF than motor at lagging power factor.
a) minimum
b) maximum
c) zero
d) none of the mentioned

View Answer

Answer: a [Reason:] At leading pf armature current will be high as the emf will be high for a synchronous motor.

9. The inverted V plots is plotted between
a) power factor vs field current for constant shaft load
b) field current vs power factor for variable shaft load
c) armature current vs field current
d) terminal voltage vs power factor

View Answer

Answer: a [Reason:] inverted v-curves are plotted between power factor and field current for constant shaft load.

10. For a power system having induction motor loads, an overexcited synchronous motor is also attached. Then the over all power factor
a) improves
b) degrades
c) becomes upf
d) remains same

View Answer

Answer: a [Reason:] An over excited synchronous motor acts as a source of lagging reactive power and so the overall power factor improves.

11. For a power system having induction motor loads, an overexcited synchronous motor is also attached. The induction motor will now operate at
a) lagging
b) leading
c) reduced power factor
d) increased power factor

View Answer

Answer: a [Reason:] Adding synchronous motor will improve the pf of the system but then the induction motor will still work at lagging pf only.

12. Synchronous compensators are
a) overexcited synchronous motor with no mechanical load
b) overexcited synchronous motor with mechanical load
c) underexcited synchronous motor with no mechanical load
d) normal excited synchronous motor with no mechanical load

View Answer

Answer: a [Reason:] Synchronous compensators are overexcited synchronous motor with no mechanical load.

13. For a synchronous motor, mark the appropriate.
electrical-machines-questions-answers-power-factor-control-synchronous-machines-q13
a) A-Inductor, B-Capacitor
b) A-Capacitor, B-Resistor
c) A-Inductor, B-Inductor
d) A-Capacitor, B-Inductor

View Answer

Answer: a [Reason:] A will act as the inductor curve consuming the reactive power while the B curve is like capacitor.

14. (I) Speed of an isolated alternator can be altered.
(II) Speed of an alternator connected to IBB can be also altered.
a) I is true, II is false
b) Both the statements are true
c) I is false while II is true
d) Both are false

View Answer

Answer: a [Reason:] Speed of the alternator connected to the infinite bus bas can not be changes as it has to be of fixed frequency.

15. (I)For constant power of an isolated alternator, V-curves can be obtained.
(II)For constant power of an alternator connected to IBB, V-curves can be obtained
a) I is true, II is false
b) Both the statements are true
c) I is false while II is true
d) Both are false

View Answer

Answer: c [Reason:] For an isolated alternator working at constant power, we can not alter the armature current to have a v-curve while the is possible for the machine connected to infinite bus bar.

16. The operating frequency and voltage of an isolated alternator ________, if the prime mover input to motor is increased.
a) increases
b) decreases
c) remains constant
d) no relation exists between the power and frequency

View Answer

Answer: a [Reason:] By varying the prime mover torque, the load angle as well as the operating frequency also get affected.

17. The active power delivered by an alternator will _______ when the prime mover input is increased for the alternator connected to infinite bus bar.
a) increases
b) decreases
c) remains constant
d) no relation exists between the power and prime mover droop characteristic

View Answer

Answer: a [Reason:] The load angle will increase and so the real power will increase in case the prime mover input is increased.

Set 3

1. The magnetization current drawn from an AC supply a synchronous motor is used to _________
a) set up flux in magnetic circuit of device
b) compensate core losses
c) set up magnetizing armature reaction
d) all of the mentioned

View Answer

Answer: a [Reason:] Magnetizing current sets up the flux needed in the machine.

2. A 3 phase synchronous motor is working at normal excitation, then the flux deficient in circuit is ________
a) given by armature winding mmf
b) given by field winding mmf
c) supplied to armature winding mmf
d) supplied to field winding mmf

View Answer

Answer: a [Reason:] it is given by armature winding mmf.

3. The excess flux in synchronous motor is neutralized by
a) armature winding by drawing demagnetizing component of the current from ac supply
b) armature winding by drawing magnetizing component of the current from ac supply
c) field winding by drawing demagnetizing component of the current from dc supply
d) field winding by drawing magnetizing component of the current from dc supply

View Answer

Answer: a [Reason:] Excess flux is neutralized y the demagnetizing component.

4. The V-curves of synchronous motor is plotted between ________
a) Ia Vs If with constant shaft load
b) If Vs Ia with constant shaft load
c) power factor vs If
d) power factor vs Ia

View Answer

Answer: a [Reason:] The V-curves of synchronous motor is plotted between Ia Vs If with constant shaft load.

5. When a constant power output is desired to maintain
a) Ef*sinδ and Ia*cosθ are to be maintained constant
b) Ef*sinδ is be maintained constant
c) Ia*cosθ is to be maintained constant
d) Ef*sinδ and Ia*sinθ are to be maintained constant

View Answer

Answer: a [Reason:] For constant real power output the component of the excitation voltage lagging to Vt should remain constant as per the phasor diagram.

6. When the excitation voltage is increased from 1 to 1.3 pu of a 3-phase synchronous motor. Then load angle for the constant power operation
a) must be decreased
b) can also be increased
c) increased
d) decreased

View Answer

Answer: a [Reason:] ‘Ef*sinδ’ is to be maintained constant. so if excitation voltage increases then the load angle must decrease.

7. The armature current is ________ at UPF when compared to leading power factor.
a) minimum
b) maximum
c) equal
d) none of the mentioned

View Answer

Answer: a [Reason:] electrical-machines-questions-answers-power-factor-correction-synchronous-motors-q7

8. The armature current is _______ at UPF than motor at lagging power factor.
a) minimum
b) maximum
c) zero
d) none of the mentioned

View Answer

Answer: a [Reason:] electrical-machines-questions-answers-power-factor-correction-synchronous-motors-q7

9. The inverted V plots is plotted between
a) power factor vs field current for constant shaft load
b) field current vs power factor for variable shaft load
c) armature current vs field current
d) terminal voltage vs power factor

View Answer

Answer: a [Reason:] Inverted v-curves are plotted between power factor and field current for constant shaft load.

10. For a power system having induction motor loads, an overexcited synchronous motor is also attached. Then the over all power factor
a) improves
b) degrades
c) becomes upf
d) remains same

View Answer

Answer: a [Reason:] An over excited synchronous motor acts as a source of lagging reactive power and so the overall power factor improves.

11. For a power system having induction motor loads, an overexcited synchronous motor is also attached. The induction motor will now operate at
a) lagging
b) leading
c) reduced power factor
d) increased power factor

View Answer

Answer: a [Reason:] Adding synchronous motor will improve the pf of the system but then the induction motor will still work at lagging pf only.

12. Synchronous compensators are
a) over excited synchronous motor with no mechanical load
b) over excited synchronous motor with mechanical load
c) under excited synchronous motor with no mechanical load
d) normally excited synchronous motor with no mechanical load

View Answer

Answer: a [Reason:] Synchronous compensators are synchronous motor with no mechanical load which are overexcited.

13. For a synchronous motor, mark the appropriate.
electrical-machines-questions-answers-power-factor-correction-synchronous-motors-q13
a) A-Inductor, B-Capacitor
b) A-Capacitor, B-Resistor
c) A-Inductor, B-Inductor
d) A-Capacitor, b-Inductor

View Answer

Answer: a [Reason:] A will act as the inductor curve consuming the reactive power while the B curve is like capacitor.

14. (I) Speed of an isolated alternator can be altered.
(II) Speed of an alternator connected to IBB can be also altered.
a) I is true, II is false
b) Both the statements are true
c) I is false while II is true
d) Both are false

View Answer

Answer: a [Reason:] Speed of the alternator connected to the infinite bus bas can not be changes as it has to be of fixed frequency.

15. (I)For constant power of an isolated alternator, V-curves can be obtained.
(II)For constant power of an alternator connected to IBB, V-curves can be obtained
a) I is true, II is false
b) Both the statements are true
c) I is false while II is true
d) Both are false

View Answer

Answer: c [Reason:] For an isolated alternator working at constant power, we can not alter the armature current to have a v-curve while the is possible for the machine connected to infinite bus bar.

16. The operating frequency and voltage of an isolated alternator ________ if the prime mover input to motor is increased.
a) increases
b) decreases
c) remains constant
d) no relation exists between the power and frequency

View Answer

Answer: a [Reason:] By varying the prime mover torque, the load angle as well as the operating frequency also get affected.

17. The active power delivered by an alternator will _______ when the prime mover input is increased for the alternator connected to infinite bus bar.
a) increases
b) decreases
c) remains constant
d) no relation exists between the power and prime mover droop characteristic

View Answer

Answer: a [Reason:] The load angle will increase and so the real power will increase in case the prime mover input is increased.

Set 4

1. Which is the correct relation below?
a) Pim = -Pog
b) Pim = (Pog)*
c) Pim = Pog
d) Pim = -(Pog)*

View Answer

Answer: a [Reason:] Electrical input to the motor is equal output power of the generator but in opposite direction.

2. The condition for maximum power output for an alternator having its load impedance as (2+j) ohms is
a) 26.56°
b) 63.43°
c) 90°
d) 116.56°

View Answer

Answer: a [Reason:] Maximum power output is arctan(1/2)= 26.56°.

3. For an alternator, the specified load angle should be _________ for an impedance angle of ‘θ’.
a) δ=θ
b) δ=90-θ
c) δ=90+θ
d) δ=-θ

View Answer

Answer: a [Reason:] dP/dδ= EVcos(δ+α)=0 δ = 90-α=θ.

4. For a 3 phase synchronous motor, the load angle should be equal to impedance angle.
a) True
b) False

View Answer

Answer: a [Reason:] dP/dδ= EVcos(δ+α)/Z=0 δ = 90-α=θ.

5. With constant E,V,Z, the maximum input power can be obtained at load angle of value _______
a) 180-θ
b) 90+θ
c) θ
d) 180

View Answer

Answer: a [Reason:] dPi/dδ = EVcos(δ-α)=0 δ = 90+α= 2α+θ=180-θ.

6. for a 3 phase 315V alternator having impedance angle of 60 degree, the required load angle for the maximum power input to the generator is _______
a) 120°
b) 60°
c) 150°
d) 30°

View Answer

Answer: a [Reason:] δ = 90+α= 2α+θ=180-θ So δ = 180-60=120°.

7. For a 3 phase synchronous generator of terminal voltage as 1 pu and field excitation of 1.2 pu. If the resistive drop is 0.2 pv, then what can be concluded for power factor?
a) upf
b) zpf
c) lagging pf
d) leading pf

View Answer

Answer: a [Reason:] The Value for Ef-Ia*ra = Vt; Here it is Vt = 0. So it is upf.

8. For a 3 phase synchronous generator of terminal voltage as 1 pu and field excitation of 1 pu. If the resistive drop is 0.2 pv, then what can be concluded for power factor?
a) Leading
b) Lagging
c) Upf
d) Zpf

View Answer

Answer: a [Reason:] Here Vt > Ef-Ia*ra.

9. For a 3 phase synchronous generator of terminal voltage as 1 pu and field excitation of 1.5 pu. If the resistive drop is 0.2 pv, then what can be concluded for power factor?
a) Leading
b) Lagging
c) Upf
d) Zpf

View Answer

Answer: b [Reason:] Here Vt < Ef-Ia*ra.

10. For a three phase synchronous motor, if the following condition is satisfied, it can be said that it is operating at
Vt < Ef-Ia*ra
a) Leading
b) Lagging
c) Upf
d) Zpf

View Answer

Answer: a [Reason:] The reactive power will be positive for this condition.

11.For a three phase synchronous motor, if the following condition is satisfied, it can be said that it is operating at
Vt = Ef-Ia*ra
a) Leading
b) Lagging
c) Upf
d) Zpf

View Answer

Answer: c [Reason:] The net reactive power flow will be zero.

12. For a three phase synchronous motor, if the following condition is satisfied, it can be said that it is operating at
Vt > Ef-Ia*ra
a) Leading
b) Lagging
c) Upf
d) Zpf

View Answer

Answer: b [Reason:] The motor will absorb the reactive power which will be lagging in nature.

13. Reactive power is a matter of interest at
a) output terminals of generator
b) input terminal of motor
c) output terminals of generator and input terminal of motor
d) none of the mentioned

View Answer

Answer: a [Reason:] Reactive power is a matter of interest at output terminals of generator and input terminal of motor.

14. The reactive power flow is outward from an alternator, then it is
a) over excited
b) under excited
c) critically excited
d) none of the mentioned

View Answer

Answer: a [Reason:] Because Q > 0; For an alternator, Q = V(Ecosδ -V)/Z.

15. The reactive power flow is outward from an alternator, then the alternator works at
a) lagging power factor
b) leading power factor
c) zero power factor
d) normal power factor

View Answer

Answer: a [Reason:] As the reactive power is positive the power factor based on the standard convention should be lagging.

Set 5

1. The rotor of a three phase induction motor can never attain synchronous speed.
a) True
b) False

View Answer

Answer: a [Reason:] Because then there will be no torque developed and flux cutting.

2. The direction of rmf when a single phase supply is given to stator of three phase induction motor is
a) zero
b) Ns
c) 2Ns
d) -Ns

View Answer

Answer: a [Reason:] There will be no rmf generated as a single phase is only fed to the three phase of induction motor.

3. The direction of rmf when a three phase supply is given to stator of three phase induction motor is
a) zero
b) Ns
c) 2Ns
d) -Ns

View Answer

Answer: b [Reason:] There will be rotating magnetic field will be produced causing the emf to be induced and the current will flow in the rotor at a speed of synchronous speed.

4. Two three phase induction motors A and B are identical in all respects except that motor A has a larger air-gap than motor B. Which motor will have more no load current?
a) A
b) B
c) Both A and B
d) Neither A nor B

View Answer

Answer: a [Reason:] As the air gap of motor A is more, it will require more flux to excitation and so the no load current.

5. Two three phase induction motors A and B are identical in all respects except that motor A has a larger air-gap than motor B. Which motor will have poorer power factor?
a) A
b) B
c) Both A and B
d) Neither A nor B

View Answer

Answer: a [Reason:] Due to larger air gap of motor A, it will need more of exciting current. So the no load angle will increase and power factor will deteriorate.

6. Two three phase induction motors A and B are identical in all respects except that motor A has a larger air-gap than motor B. Which motor will have better full-load power factor?
a) A
b) B
c) Both A and B
d) Neither A nor B

View Answer

Answer: b [Reason:] At full load conditions, the excitation current is not very high and so the power factor improves.

7. A three phase induction motor is sometimes calles a generalized transformer in so far as voltage and frequency transformation has been concerned. How a three phase induction motor operates when rotor frequency is equal to stator frequency?
a) It will not operate
b) It will operate as induction motor only
c) It will operate as induction generator
d) It will operate in braking mode

View Answer

Answer: a [Reason:] As the rotor and stator frequencies are equal, there will be no slip and so the zero emf will be produced.

8. A three phase induction motor is sometimes called a generalized transformer in so far as voltage and frequency transformation has been concerned. How a three phase induction motor operates when rotor frequency is greater than stator frequency?
a) Rotor will be driven against rmf
b) Rotor will be driven in direction of rmf
c) No emf will be induced
d) The losses will be maximum

View Answer

Answer: a [Reason:] As the rotor rotates at larger speed than the stator, the rmf speed is less than the rotor speed and it tries to rotate in the opposite direction to rmf.

9. A three phase induction motor is sometimes called a generalized transformer in so far as voltage and frequency transformation has been concerned. How a three phase induction motor operates when rotor frequency is less than stator frequency?
a) It operates as induction motor
b) Emf is induced in rotor
c) Rmf will rotate at synchronous speed
d) All of the mentioned

View Answer

Answer: d [Reason:] It will operate as induction motor.

10. A three phase induction motor is sometimes called a generalized transformer in so far as voltage and frequency transformation has been concerned. Then the rotor emf E2 and the rotor current I2 are zero at
a) Ns
b) 2Ns
c) zero
d) slip of 50%

View Answer

Answer: a [Reason:] The rotor emf and current are zero at the speed of synchronous speed of the machine.

11. During no load test the wattmeter reading will be
I. stator copper loss
II. stator core loss
III. rotor core loss
IV. friction and windage losses
V. Rotor copper loss
a) I, II, IV
b) II,III,IV
c) I, III, II, IV
d) II,IV

View Answer

Answer: a [Reason:] Blocked rotor test will give stator losses and the friction and windage losses.

12. If use auto transformer method starting to start an induction motor to replace star-delta method, then the required tapping on the transformer will be
a) 57.73%
b) 86.7%
c) 57%
d) 66.66%

View Answer

Answer: a [Reason:] k = 1/1.73 = 57.73%.

13. During the blocked rotor test the wattmeter reading will be
I. stator copper loss
II. stator core loss
III. rotor core loss
IV. friction and windage losses
V. Rotor copper loss
a) I, V
b) II,III,IV
c) I, III, II, IV
d) II,IV

View Answer

Answer: a [Reason:] Blocked rotor test will account for copper losses.

14. The starting method for a 3-phase squirrel-cage induction motor which is inferior in view of poor starting torque per ampere of line current drawn,
a) series-inductor method of starting
b) direct-on-line starting
c) auto-transformer method
d) star-delta method

View Answer

Answer: a [Reason:] Due to inductor method, reactance increases and the starting torque decreases.

15. Reactor method of starting is preferred over resistance method because
I. power factor improves
II. increase starting torque
III. lower losses
IV. more effective in reducing voltage
a) III,IV
b) I,II,III
c) II,III,IV
d) I,II

View Answer

Answer: a [Reason:] By reactor method of starting, losses will be lesser and voltage reduction is effective than resistance method of starting.