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Multiple choice question for engineering

Set 1

1. Which of the following two windings produces/induces the working flux and working EMF respectively?
a) field winding and armature winding
b) armature winding and field winding
c) both are produced in field winding
d) both are produced in armature winding

View Answer

Answer: a [Reason:] Field winding produces the working flux and armature winding has the working EMF, which is induced by the working flux.

2. Armature winding carries which of the following current?
a) magnetizing current only
b) load current only
c) both magnetizing current and load current
d) none of the mentioned

View Answer

Answer: b [Reason:] The winding on the machine that carries only load current is called armature winding.

3. Field winding carries which of the following current?
a) exciting current only
b) load current only
c) both exciting current and load current
d) none of the mentioned

View Answer

Answer: a [Reason:] The winding that handles only exciting current is called field winding.

4. Which of the following statements are correct regarding exciting current?
(i) exciting current varies with the load
(ii) exciting current doesn’t vary with load
(iii) exciting current produces only a working magnetic flux
(iv) exciting current is not responsible for the production of working magnetic flux
a) (i),(iv)
b) (ii),(iii)
c) (i),(iii)
d) (ii),(iv)

View Answer

Answer: c [Reason:] The current that produces only a working magnetic flux, and does not vary with the load on the machine is called exciting current, magnetizing current or field current.

5. Current in the field winding is ___________
a) AC always
b) DC always
c) Both AC and DC
d) Either AC or DC

View Answer

Answer: b [Reason:] Field winding is always given DC supply and hence current is also DC.

6. The rating of the armature winding is ___________
a) 1/2 to 2% of the rated power of the machine
b) less than the power rating of the machine
c) greater than the power rating of the machine
d) equal to the power rating of the machine

View Answer

Answer: d [Reason:] The armature winding handles all the power that is being converted or transformed.

7. The power rating of the field winding is ___________
a) equal to the power rating of the machine
b) 1/2 to 2% of the rated power of the machine
c) greater than the power rating of the machine
d) less than the power rating of the machine

View Answer

Answer: b [Reason:] The power input to the DC field winding is dissipated as i2R loss in the field winding (once the required field current is established).

8. What is the reason behind armature structure for both DC and AC machines being laminated?
a) reduce i2R losses
b) reduce the leakage flux
c) reduce the eddy current losses
d) for better operating power factor

View Answer

Answer: c [Reason:] The armature windings of both the DC and AC machines have to deal with alternating current only; this is the reason why the armature structure of all rotating machines are laminated in order to reduce the eddy current losses.

9. Armature winding is one in which working ___________
a) flux is produced by field current
b) flux is produced by the working emf
c) emf is produced by the working flux
d) emf is produced by the leakage flux

View Answer

Answer: c [Reason:] In an electrical machine, armature winding is one in which the working emf is induced by the working flux.

10. Why is the air gap between stator and rotor should be kept as small as mechanically possible in induction motor?
a) to reduce the leakage flux between stator and rotor
b) as it leads to better operating power factor of the induction motor
c) reduce the eddy current losses
d) both, to reduce leakage flux between stator and rotor, and also for better operating power factor

View Answer

Answer: d [Reason:] The leakage flux and power factor depends on the air gap and it should be as small as possible.

Set 2

1. Which type of slots are used in the construction of large size and small size induction motors respectively?
a) open slots and semiclosed slots
b) semiclosed slots and open slots
c) open slots and open slots
d) semiclosed slots and semiclosed slots

View Answer

Answer: a [Reason:] Large size motors use open slots, so that already prepared and properly insulated coils can be inserted in open slots. Small size induction motors use semiclosed slots, so as to reduce the effective gap length between stator and rotor.

2. In which of the following applications, wound rotor type of induction motor is used?
a) where the driven load requires speed control
b) where high starting torque is required
c) when external resistance is to be inserted
d) any of the mentioned

View Answer

Answer: d [Reason:] Only in wound rotor type of induction motor, external resistance can be inserted in series with the rotor winding, for speed and starting torque control.

3. For an induction motor,
(i) squirrel cage type is simpler and more economical in construction
(ii) wound rotor type requires less maintenance
(iii) squirrel cage type is more rugged and requires less maintenance
(iv) no external resistance can be inserted in the rotor circuit of squirrel cage induction motor
(v) no external resistance can be inserted in the rotor circuit of a wound rotor induction motor
Which of the above statements are correct?
a) (ii),(v),(iii)
b) (ii),(iii),(v)
c) (i),(iii),(iv)
d) (i),(ii),(iv)

View Answer

Answer: c [Reason:] The squirrel cage type is simpler and more economical in construction, requires less maintenance than wound rotor type, since the former doesn’t require slip rings and carbon brushes.

4. What are the advantages of providing the field winding on rotor and armature winding on the stator?
a) more economical
b) more efficient
c) efficient cooling
d) all of the mentioned

View Answer

Answer: d [Reason:] If the armature winding is on the stator, only two slip rings are required which implies less losses, more efficiency and more economy. Also, stationary armature winding can be cooled more efficiently.

5. The stator frame and end covers in synchronous and induction machines are designed to ___________
a) carry the magnetic flux
b) to serve as a mechanical support
c) to provide cooling or to carry induced EMF
d) any of the mentioned

View Answer

Answer: b [Reason:] The stator frame and end covers serve merely as a mechanical support to the stator core, and are not designed to carry the magnetic flux.

6. What is the advantage of connecting two coils in parallel?
a) reduce the amount of copper required
b) increase the current per parallel path to double the value
c) increase the voltage capacity
d) all of the mentioned

View Answer

Answer: b [Reason:] The parallel connection of two coils will increase the current per parallel path to double the value.

7. In a synchronous machine, salient pole construction has been found to suit best for ___________
a) low speed prime movers
b) high speed prime movers
c) medium speed prime movers
d) any of the mentioned

View Answer

Answer: a [Reason:] For accommodating large number of poles, the rotor diameter must be comparatively increased, and from mechanical considerations, salient pole construction has been found to suit best for low speed prime movers.

8. What is the equation for frequency of generated EMF?
a) f = PN/120 Hz
b) f = 120/PN Hz
c) f = P/120 Hz
d) f = N/120 Hz

View Answer

Answer: a [Reason:] f= PN/120 Hz in India and many other countries, alternators feeding the power system generates at a frequency of 50 Hz.

9. How many poles should the alternators driven by the oil engines and hydraulic turbine prime movers must have, for better operation?
a) fewer number of poles
b) larger number of poles
c) medium number of poles
d) any of the mentioned

View Answer

Answer: b [Reason:] Oil engines and hydraulic turbines operate best at relatively low speeds, therefore, the alternators driven by these prime movers must have relatively large number of poles, as prop 1/N.

10. Large synchronous machines are constructed with armature winding on the stator because stationary armature winding
(i) can be insulated satisfactorily for higher voltages
(ii) can be cooled more efficiently
(iii) would lead to reduced slip ring losses
(iv) would have no slot harmonics
(v) would have reduced armature reactance
Which of the above statements are correct ?
a) (i),(ii),(iii),(iv)
b) (i),(ii),(iii),(v)
c) (i),(ii),(iii),(iv),(v)
d) (i),(ii),(iv),(v)

View Answer

Answer: b [Reason:] If the armature winding is on the stator, only two slip rings are required, which implies less losses, more efficiency and more economy. Also, stationary armature windings can be cooled more efficiently.

Set 3

1. How many poles should the alternators, driven by the steam and gas turbines prime movers must have, for better operation?
a) fewer number of poles
b) large number of poles
c) medium number of poles
d) any of the mentioned

View Answer

Answer: a [Reason:] Steam and gas turbines have best operating characteristics at relatively high speeds, therefore, alternators driven by these must have a fewer number of poles, say 2 or 4, as f prop 1/N.

2. In a synchronous machine, cylindrical pole construction has been found to suit best for ___________
a) low speed prime movers
b) high speed prime movers
c) medium speed prime movers
d) any of the mentioned

View Answer

Answer: b [Reason:] For high speed prime movers, if salient pole construction is used, the rotor structure may not be able to withstand the enormous centrifugal forces, developed by the salient poles. Therefore, cylindrical rotor construction is best suited from mechanical considerations.

3. Commercially, which of the following generators is called turbo-alternators or turbo-generators?
a) salient pole rotor synchronous generators
b) cylindrical pole rotor induction generators
c) salient pole rotor induction generators
d) cylindrical rotor synchronous generators

View Answer

Answer: d [Reason:] Cylindrical rotor synchronous generators are used commercially at high speed, driven by steam and gas turbines.

4. For relatively large sizes of synchronous machines, which of the following are used for providing closed circuit cooling?
a) air
b) water
c) hydrogen/helium
d) any of the mentioned

View Answer

Answer: d [Reason:] For synchronous machines of large sizes, adequate cooling arrangements must be incorporated to dissipate the heat produced by various losses.

5. In a synchronous generator, the frequency of the generated EMF depends on
a) rotational speed
b) poles
c) both rotational speed and poles
d) none of the mentioned

View Answer

Answer: c [Reason:] f = PN/120 Hz

6. A synchronous machine can be connected to an energy system only when ___________
a) frequency/rotational speed matches
b) voltage matches
c) current matches
d) power matches

View Answer

Answer: a [Reason:] For successful operation, the rotational speed/frequency of the synchronous machine must synchronize with the frequency of the energy system, and this is the reason for calling it a synchronous machine.

7. In larger DC machines, the reason behind using fabricated steel instead of cast iron in the manufacture of yoke is ?
a) economical considerations
b) permeability considerations
c) mechanical considerations
d) insulation considerations

View Answer

Answer: b [Reason:] Fabricated steel has higher permeability than cast iron.

8. In a DC machine, the pole core is usually of smaller cross section than the pole shoe because ___________
(i) smaller cross section requires less copper
(ii) larger pole shoe increases the air gap reluctance
(iii) larger pole shoe increases the flux per pole entering the armature
(iv) larger pole shoe reduces the mechanical strength
(v) larger pole shoe supports the field winding
Which of the above statements are correct?
a) (i),(ii),(v)
b) (ii),(iii),(v)
c) (i),(iii),(v)
d) (ii),(iv),(v)

View Answer

Answer: c [Reason:] Larger pole shoe area increases the flux per pole entering the armature, due to the reduction in air gap reluctance, and it also provides mechanical strength.

9. In a DC generator, the commutator serves as a _________
a) mechanical rectifier for alternating EMF to direct EMF
b) mechanical inverter to invert the direct applied voltage to alternating voltage
c) any of the mentioned
d) none of the mentioned

View Answer

Answer: a [Reason:] A DC generator is equivalent to an AC machine plus a mechanical rectifier, i.e, commutator.

10. In a DC motor, the commutator serves as a ___________
a) mechanical rectifier for alternating EMF to direct EMF
b) mechanical inverter to invert the direct applied voltage to alternating voltage
c) any of the mentioned
d) none of the mentioned

View Answer

Answer: b [Reason:] A DC motor is equivalent to an AC machine plus a mechanical inverter, i.e, commutator.

Set 4

1. Which of the following are examples of doubly-excited magnetic systems?
a) Synchronous Machines
b) Loudspeakers and Tachometers
c) D.C Shunt Machines
d) Any of the mentioned

View Answer

Answer: d [Reason:] All of the above applications require two independent sources of excitation.

2. Most of the electromagnetic energy conversion devices belong to __________
a) singly excited magnetic systems
b) doubly excited magnetic systems
c) multiply excited magnetic systems
d) both doubly excited magnetic systems and multiply excited magnetic systems

View Answer

Answer: d [Reason:] Widely used motor/generators such as synchronous and induction machines belong to doubly excited systems, and many other practical applications require multiply excited magnetic systems.

3. In a doubly excited magnetic system with salient pole type stator and rotor, if the rotor is not allowed to move, then the equation for magnetic field stored energy in establishing the currents from zero to is and ir is __________
a) Wfld=1/2 is2Ls+1/2 ir2Lr
b) Wfld=1/2 is2Ls+Mrs is ir
c) Wfld=1/2 is2Ls+1/2 ir2Lr+Mrs is ir
d) Wfld=1/2 ir2Lr+Mrs is ir

View Answer

Answer: c [Reason:] As the rotor is not allowed to move, dWmech=0, thus dWelec=0+dWfld=isdΨ;;s+irdΨ;r, if in this equation, we introduce the self and mutual inductance terms, (Ψ;s=Lsis) and integrate the resulting equation from 0 to is, 0 to ir and 0 to iris, the respective terms, finally we get Wfld=1/2 is2Ls+1/2 ir2Lr+Mrs is ir

4. In a doubly excited magnetic system of salient pole stator and rotor, the magnetic torque (Te) depends on
(i) the instantaneous values of currents is and ir
(ii) the angular rate of change of inductances
(iii) the differential changes of current dis and dir
(iv) only the instantaneous values of self inductance

Which of the above statements are true?
a) (i),(iii)
b) (i),(ii)
c) (iii),(iv)
d) (i),(iv)

View Answer

Answer: b [Reason:] Te=1/2is2dLs/dθr+1/2ir2dLr/dθr+isirdMsr/dθr.

5.In a doubly excited magnetic systems, the magnetic torques and forces act in such a direction as to tend to
a) decrease the field energy at constant currents
b) decrease the field co-energy at constant currents
c) increase the field energy at constant currents
d) none of the mentioned

View Answer

Answer: c [Reason:] Te= ∂Wfld(is,irr)/∂θr = ∂Wfld1(is,irr)/∂θr fe= ∂Wfld(is,ir,x)/∂x = ∂Wfld1(is,ir,x)/∂x The positive sign in the formula indicates that force/torque acts in a direction as to tend to increase both field energy and co-energy.

6. In a doubly excited magnetic system of salient pole type stator and rotor, the reluctance torque is present only when _____________
a) both stator and rotor currents are acting
b) stator current is acting alone
c) rotor current is acting alone
d) any of the stator or rotor currents acting alone

View Answer

Answer: d [Reason:] Equation for magnetic torque is Te=1/2is2dLs/dθr+1/2ir2dLr/dθr+isirdMsr/dθr , if ir=0, Te=1/2is2dLs/dθr and if is=0, then Te= 1/2ir2dLr/dθr, and these equations for torque are called reluctance torques.

7. Which component of torque in the following equation is called the electromagnetic torque of electromagnetic energy conversion device?
Te=1/2is2dLs/dθr+1/2ir2dLr/dθr+isirdMsr/dθr
a) 1/2is2dLs/dθr
b) 1/2ir2dLr/dθr
c) isirdMsr/dθr
d) any of the mentioned

View Answer

Answer: c [Reason:] The torque developed by the interaction of stator and rotor magnetic fields is the electromagnetic torque or interaction torque.

8. Which components of torque in the following equation are called the reluctance torque terms?
Te=1/2is2dLs/dθr+1/2ir2dLr/dθr+isirdMsr/dθr
a) 1/2is2dLs/dθr and isirdMsr/dθr
b) 1/2is2dLs/dθr and 1/2ir2dLr/dθr
c) 1/2ir2dLr/dθr and isirdMsr/dθr
d) 1/2is2dLs/dθr, 1/2ir2dLr/dθr and isirdMsr/dθr

View Answer

Answer: b [Reason:] Because the change of reluctance (Rls or Rlr) are responsible for the production of these torques.

9. Which of the following statements are true about electromagnetic torques and reluctance torques:
(i) electromagnetic torque can exist only if both windings carry current
(ii) reluctance torque depend on the direction of current in stator or rotor windings
(iii) reluctance torque doesn’t depend on the direction of current in stator or rotor windings
(iv) electromagnetic torque depend on the direction of currents is and ir
(v) electromagnetic torque doesn’t depend on the direction of currents is and ir

a)(i),(ii),(iii)
b)(ii),(iii),(v)
c)(i),(iii),(iv)
d)(ii),(iii),(iv)

View Answer

Answer: c [Reason:] Reluctance torque: [Reason:] 1/2is2dLs/dθr and 1/2ir2dLr/dθr Positive or negative value of current doesn’t affect the torque direction. The interaction/electromagnetic torque = isirdMsr/dθr, here the direction of is and ir changes the torque.

10. Singly and doubly excited magnetic systems applications are respectively:
a) loud speakers and tachometers
b) synchronous motors and moving iron instruments
c) DC shunt machines and solenoids
d) reluctance motors and synchronous motors

View Answer

Answer: d [Reason:] Reluctance motors can work on single excitation, and synchronous motors require double excitation.

11. Electromagnetic torque in rotating electrical machinery is present when ________
a) stator winding alone carries current
b) rotor winding alone carries current
c) air gap is uniform
d) both stator and rotor windings carry current

View Answer

Answer: d [Reason:] Electromagnetic torque = isirdMsr/dθr and if either is or ir =0, then Te=0.

12. All practical electromechanical energy conversion devices make use of the magnetic field rather than the electric field as the coupling medium.This is because
a) electric field systems present insulation difficulties
b) electric field systems have more dielectric loss than the magnetic loss, for the same power rating of the machine
c) in electric field systems, for normal electric field strength, the stored energy density is high
d) in magnetic field systems, for normal magnetic flux density, the stored energy density is high

View Answer

Answer: d [Reason:] As the energy storing capacity of the magnetic field is higher, magnetic field as coupling medium is most common in electromechanical energy conversion devices.

Set 5

1. Consider a single phase synchronous machine of cylindrical rotor type. A single phase supply is given to the stator winding and a voltmeter is connected across rotor winding. For what value of θr, the voltmeter reads maximum?
a) 90°
b) 0°
c) 45°
d) 30°

View Answer

Answer: b [Reason:] When θr=0, the flux linkages with the rotor are maximum, and this is indicated by a maximum voltmeter reading.

2. Consider a single phase synchronous machine of cylindrical rotor type. A single phase supply is given to the stator winding, and a voltmeter is connected across rotor winding. For what value of θr the voltmeter reads a zero?
a) 0°
b) 30°
c) 45°
d) 90°

View Answer

Answer: d [Reason:] When θr=90°, the voltmeter reading is zero, because the stator winding axis is perpendicular to the rotor winding axis, and the mutual inductance Msr is zero.

3. Which of the following equation best represents the mutual inductance between stator and rotor in a single phase cylindrical rotor synchronous machine?
a) Msr=0
b) Msr=Mmax
c) Msr=Mmaxcosθr
d) Msr=Mmaxsinθr
where θr= space angle between stator and rotor field axis

View Answer

Answer: c [Reason:] In a single phase cylindrical rotor synchronous machine, the Msr is maximum, when θr=0, and is zero when θr=90° and cosine function represents it.

4. In a single phase cylindrical rotor synchronous machine, the torque Te tends to ____________ the space angle θr.
a) reduce
b) increase
c) maintain constant
d) all of the mentioned

View Answer

Answer: a [Reason:] Wfld(is,irr)=1/2 is2Ls+1/2 ir2Lr+ isirMmaxcosθr Torque, Te=∂Wfld(is,irr)/∂θr= -isirMmaxsinθr Negative sign indicates that the torque Te tends to reduce angle θr.

5. The following torque expression: Te=-isirMmaxsinθr is applicable in a single phase cylindrical rotor synchronous machine when the rotor is __________
a) revolving
b) stationary
c) any of the mentioned
d) none of the mentioned

View Answer

Answer: c [Reason:] The torque expression is applicable whether the rotor is revolving or stationary.

6. In a two phase cylindrical rotor synchronous machine, the self inductances Lα, Lβ for phases α,β respectively are __________
a) constant and equal
b) constant and unequal
c) varying and equal
d) varying and unequal

View Answer

Answer: a [Reason:] Constant and equal, as the reluctance offered to the fluxes produced by phase α and β winding doesn’t vary with rotor movement.

7. In a two phase cylindrical rotor synchronous machine, the mutual inductance Mαβ between α phase and β phase winding is __________
a) Mmax
b) 0
c) Mmin
d) any of the mentioned

View Answer

Answer: b [Reason:] Since the axes of phase α and phase β windings are normal to each other and air gap is uniform, the mutual inductance Mαβ between α,β winding is zero.

8. In a two phase cylindrical rotor synchronous machine, torque Te is _____________
a) constant at some instant of time
b) changing at every instant of time
c) constant at every instant of time
d) changing at some instant of time

View Answer

Answer: c [Reason:] Te= IfImMmaxsinδ We can observe that it remains constant at every instant of time.

9. Which of the following equation represents the reluctance torque of a salient pole synchronous machine?
a) Te(av)= 1/4 Im2(Ld-Lq)sin2δ
b) Te(av)= 2 Im2(Ld-Lq)sinδcosδ
c) Te(av)= 1/8 Im2(Ld-Lq)
d) Te(av)= 1/8 Im2(Ld-Lq)sin2δ

View Answer

Answer: d [Reason:] Te(av)= 1/8 Im2(Ld-Lq)sin2δ+1/2 IfImMmaxsinδ If field current If is reduced to zero, the reluctance torque Te(av)= 1/8 Im2(Ld-Lq)sin2δ.

10. Which component of torque equation, Te(av)= 1/8 Im2(Ld-Lq)sin2δ+1/2 IfImMmaxsinδ represents the electromagnetic/interaction torque in single phase salient pole synchronous machine?
a) 1/2 IfImMmaxsinδ
b) 1/8 Im2(Ld-Lq)sin2δ
c) 1/8 Im2(Ld-Lq)sin2δ+ 1/2 IfImMmaxsinδ
d) none of the mentioned

View Answer

Answer: a [Reason:] The torque developed by the interaction of both stator and rotor magnetic fields is the electromagnetic/interaction torque.

11. An electromechanical energy conversion device has cylindrical stator but salient pole rotor. If δ is the angle between stator field and rotor field, the average torque developed is proportional to (A and B are constants) __________
a) Asinδ
b) Asin2δ
c) Asinδ+Bsin2δ
d) δ

View Answer

Answer: c [Reason:] Te(av)= 1/8 Im2(Ld-Lq)sin2δ+1/2 IfImMmaxsinδ = Asinδ+Bsin2δ.

12. An electromechanical energy conversion device has cylindrical stator but salient pole rotor. Rotor is not excited. If δ is the angle between stator field and rotor long axis, then average torque developed is proportional to (A and B are constants)
a) Asinδ
b) Asin2δ
c) Asinδ + Bsin2δ
d) zero

View Answer

Answer: b [Reason:] We know, Te= 1/8 Im2(Ld-Lq)sin2δ+1/2 IfImMmaxsinδ, rotor is not excited implies If=0, and thus Te= 1/8 Im2(Ld-Lq)sin2δ = Asin2δ

13. The self and mutual inductances of a doubly excited magnetic system are Ls=0.6+0.20cos2θr H and Lr=0.75+0.30cos2θr H and Msr=0.8cosθr H. For a stationary rotor at an angular position of θr=60°,what is the magnitude of torque when the currents is=20A DC and ir=10A DC?
a) -233.88N-m
b) +233.88N-m
c) -467.76N-m
d) +467.76N-m

View Answer

Answer: a [Reason:] For θr=60°; Ls, Lr, Msr and their derivatives with respect to θr have the following values: Ls=0.50H, Lr=0.60H, Msr=0.40H, dLs/dθr=-0.3464, dLr/dθr=-0.52, dMsr/dθr=-0.693 Substituting the numerical values in Te= 1/2is2dLs/dθr+1/2ir2dLr/dθr+isrdMsr/dθr= -233.88N-m.