Multiple choice question for engineering
Set 1
1. An electro-mechanical energy conversion device is one which converts _______
a) Electrical energy to mechanical energy only
b) Mechanical energy to electrical energy only
c) All of the mentioned
d) None of the mentioned
Answer
Answer: c [Reason:] The operating principles of electrical to mechanical and mechanical to electrical conversion devices are similar, hence, the common name electro-mechanical device.However, their structural details differ depending on their function.
2. What is the coupling field used between the electrical and mechanical systems in an energy conversion devices?
a) Magnetic field
b) Electric field
c) Magnetic field or Electric field
d) None of the mentioned
Answer
Answer: c [Reason:] Either electric field or magnetic field can be used, however most commonly we use magnetic field because of its greater energy storage capacity.
3. The energy storing capacity of magnetic field is about ________ times greater than that of electric field?
a) 50,000
b) 25,000
c) 10,000
d) 40,000
Answer
Answer: b [Reason:] As the energy storage capacity of the magnetic field is higher, it is most commonly used as coupling medium in electro-mechanical energy conversion devices.
4. The formula for energy stored in the mechanical system of linear motion type is ______
a) 1/2 Jwr2
b) 1/2 mv2
c) 1/2 mv
d) Jwr2
Answer
Answer: b [Reason:] Energy stored is kinetic energy, since the system is of linear motion.
5. In an electro-mechanical energy conversion device, the coupling field on the
(i) electrical side is associated with emf and current
(ii) electrical side is associated with torque and speed
(iii) mechanical side is associated with emf and current
(iv) mechanical side is associated with torque and speed
From the above, the correct statements are
a) (i) & (ii)
b) (ii) & (iii)
c) (iii) & (iv)
d) (i) and (iv)
Answer
Answer: d [Reason:] The coupling field will be associated with the electrical quantities on electrical side and vice versa.
6. A coupling magnetic field must react with
(i) electrical system in order to extract energy from mechanical system
(ii) mechanical system in order to extract energy from mechanical system
(iii) electrical system in order to extract energy from electrical system
(iv) mechanical system in order to extract energy from electrical system
(v) electrical or mechanical system for electro-mechanical energy conversion
From the above, the correct statements are
a) (i), (ii) & (iii)
b) (ii), (iii) & (v)
c) (ii), (iii) & (iv)
d) (ii), (iii) & (v)
Answer
Answer: b [Reason:] To convert electrical to mechanical, the coupling magnetic field must take energy from input, the electrical system and vice versa.
7. The developed electromagnetic force and/or torque in electromechanical energy conversion system, acts in such a direction that tends to ___________
a) increase the stored energy at constant mmf
b) decrease the stored energy at constant mmf
c) decrease the co-energy at constant mmf
d) increase the stored energy at constant flux
Answer
Answer: b [Reason:] fe=-(∂Wfld (φ,x))/∂x= -(∂Wfld (∅,x))/∂x and Te=-(∂Wfld (φ,θ))/∂θ=-(∂Wfld(∅,θ))/∂θ.
8. The developed electromagnetic force and/or torque in electromechanical energy conversion systems, acts in a direction that tends to ___________
(i) increase the co-energy at constant flux
(ii) increase the co-energy at constant mmf
(iii) decrease the stored energy at constant mmf
(iv) decrease the stored energy at constant flux
Which of the above statements are correct?
a) (ii), (iv)
b) (i), (iii)
c) (ii), (iii)
d) (i), (iv)
Answer
Answer: a [Reason:] fe=-(∂Wfld (φ,x))/∂x and fe= (∂Wfld1(F,x))/∂x.
9. A physical system of electromechanical energy conversion, consists of a stationary part creating a magnetic field with electric energy input, and a moving part giving mechanical energy output. If the movable part is kept fixed, the entire electrical energy input will be _______
a) stored in the magnetic field
b) stored in the electric field
c) divided equally between the magnetic and electric fields
d) zero
Answer
Answer: a [Reason:] As the movable part is fixed, Wmech=0, we know, Welec=Wmech+ Wfld,therefore, Welec=Wfld.
Set 2
1. The electromagnetic torque in non salient pole machines depends on
a) number of poles
b) machine dimensions at the air gap
c) peak value of stator and rotor MMF
d) Any of the mentioned
Answer
Answer: d [Reason:] Te also depends on the sine of the angle between the axis of the two fields(or MMFs) Te =-(P/2)(μoΠrl/g)(FsFr)sinλNm.
2. The torque equation for non salient pole machine is Te =-(P/2)(μoΠrl/g)(FsFr)sinλNm. What does the negative sign in the torque expression indicates?
a) stator and rotor MMF tend to reduce angle λ
b) stator and rotor MMF tend to increase angle λ
c) stator tend to reduce the air gap g
d) rotor tend to reduce the air gap g
Answer
Answer: a [Reason:] The torque acts in such a direction as to align the two MMFs Fs and Fr and thereby reduce the angle λ between them.
3. Which of the following statements are correct regarding the torque equation in non salient pole machines, Te =-(P/2)(μoΠrl/g)(FsFr)sinλNm?
a) stator experiences less torque than rotor and in opposite directions
b) rotor experiences less torque than stator and in same directions
c) stator and rotor experiences equal and opposite torques
d) stator and rotor experiences equal torque and in same directions
Answer
Answer: c [Reason:] Both stator and rotor experience equal and opposite torques.
4. In practice, electromagnetic torque acting on the stator
a) rotate in opposite direction to rotor
b) is transmitted to ground
c) makes it rotate in same direction as of rotor
d) None of the mentioned
Answer
Answer: b [Reason:] As stator is fixed, Teis transmitted to ground through its foundation.
5. Torque angle λ is the angle between ____________
a) stator MMF Fs and resultant MMF FR
b) stator MMF Fs and rotor MMF Fr
c) rotor MMF Fr and resultant MMF FR
d) Any of the mentioned
Answer
Answer: b [Reason:] The electrical space angle λ between stator and rotor MMFs is called the torque angle.
6. Load angle δ is the angle between ____________
a) stator MMF Fs and resultant MMF FR
b) stator MMF Fs and rotor MMF Fr
c) rotor MMF Fr and resultant MMF FR
d) stator MMF Fs and resultant MMF FR (or) rotor MMF Fr and resultant MMF FR
Answer
Answer: d [Reason:] The electrical space angle δs between Fs and FR ; and δr between Fr and FR are called load angles.
7. In the design of electromagnetic devices, the maximum value of MMF is limited from the considerations of ____________
a) temperature rise
b) limitation on flux density in teeth
c) torque production rise
d) power rating
Answer
Answer: a [Reason:] The maximum value of MMF is limited from a consideration of temperature rise.
8. Consider the principle that the torque in a rotating machine is proportional to peak of stator MMF(Fs), rotor MMF(Fr) and the angle between them(δ). Following statements relate to Fs,Fr and δ in different machine:
(i) δ is fixed in DC machine
(ii) Fs is fixed in DC shunt machine
(iii) δ is variable in induction machine
(iv) Fr is fixed in synchronous machne
(v) Fr is variable in DC machine
(vi) Fr is fixed in induction motor
(vii) resultant of Fs and Fr is fixed in synchronous motor
Which of these statements are correct?
a) (i),(ii),(iii),(iv) and (vi)
b) (i),(ii),(iii),(v) and (vii)
c) (ii),(iii),(v),(vi) and (vii)
d) (i),(ii),(iv),(v) and (vii)
Answer
Answer: b [Reason:] Fr is not fixed in synchronous and induction machine.
9. In all rotating electrical machine, electrical torque is developed when relative speed between stator field and rotor field is ____________
a) zero
b) equal to rotor speed
c) equal and opposite to rotor speed
d) dependent upon the type of electrical machine
Answer
Answer: a [Reason:] If the relative speed between stator field and rotor field is not zero, then the load angle δr varies with time.
From equation Te=-(Π/8)P2ΦFrsinδr Nm, we can say that average torque over a complete cycle is zero, implies Te=0.
Set 3
1. What is the angle between stator direct axis and quadrature axis ?
a) 90°
b) 0°
c) 45°
d) any of the mentioned
Answer
Answer: a [Reason:] In reluctance motor, direct axis is horizontal axis and quadrature axis is at 90° to the direct axis.
2. Space angle, θr is measured between stator d-axis and _____
a) quadrature axis
b) direct d-axis
c) long rotor axis
d) none of the mentioned
Answer
Answer: c [Reason:] Space angle is measured between stator direct axis and long rotor axis.
3. The reluctance offered to the stator flux by two very large air gaps in series with high permeability iron, in reluctance machine is maximum, when the space angle θr = ______
a) 0°
b) 45°
c) 90°
d) 180°
Answer
Answer: c [Reason:] Only when θr = 90°, the long rotor axis is perpendicular to the stator d-axis, and thus reluctance is maximum.
4. The reluctance offered to the stator flux by two small air gaps in series with high permeability iron, in reluctance machine is minimum, when the space angle θr = ______
a) 0°
b) 45°
c) 90°
d) 270°
Answer
Answer: a [Reason:] Only when θr= 0°, the long rotor axis is coincident with the stator d-axis, and thus reluctance is minimum.
5. The variation of reluctance Rl with space angle θr depends on the shape of __________
a) stator poles
b) rotor poles
c) stator or rotor poles
d) both stator and rotor poles
Answer
Answer: d [Reason:] Also, the variation is assumed to be a function of space angle θr.
6. Reluctance motor can produce torque at ________
a) any speed less than synchronous speed
b) synchronous speed only
c) any speed greater than synchronous speed
d) any of the mentioned
Answer
Answer: b [Reason:] Te(av)=-1/4 ∅max2(Rlq – Rld)(1/2 sin(-2δ)), w= synchronous speed
if wr≠w then Te(av)=0 ,and if wr=w then Te(av)≠0, as the last term in the equation 1/2 sin(-2δ) does not vary with time, and hence average torque is not equal to zero.
7. For a reluctance motor , the maximum average torque occurs when δ= __________
a) 45°
b) 90°
c) 0°
d) 180°
Answer
Answer: a [Reason:] The equation for average torque is Te(av)=1/8 ∅max2(Rlq – Rld)sin(2δ), and it is maximum when δ=45°.
8. For a given reluctance motor, Rld and Rlq are ________
a) constant
b) varying
c) zero
d) any of the mentioned
Answer
Answer: a [Reason:] Rld and Rlq are constant for a given motor, as they depend on the geometry of the magnetic circuit.
9. The single phase reluctance machine acts as a generator when angle δ is _______
a) positive
b) negative
c) zero
d) any of the mentioned
Answer
Answer: b [Reason:] If δ is positive, then the machine acts as a motor and if δ is negative, the machine acts as a generator.
10. Single phase reluctance motors are extensively used in ________
a) grinder applications
b) driving electric clocks and other timing devices
c) welding applications
d) lifts/ elevators
Answer
Answer: b [Reason:] The single phase reluctance motors operate at constant synchronous speed, in case the supply frequency remains constant, and hence timing devices mostly use these motors.
11. If the salient pole rotor in a single phase reluctance motor is replaced by a cylindrical rotor, then
(i) reluctance offered to stator flux remains constant for all rotor positions
(ii) no reluctance torque will be developed
(iii) reluctance torque will be developed
(iv) reluctance offered to stator flux changes for all rotor positions
Which of the above statements are true ?
a) (i), (ii)
b) (ii), (iii)
c) (iii), (iv)
d) (i), (iv)
Answer
Answer: a [Reason:] The reluctance torque is developed only when the reluctance is seen but the stator flux varies with rotor movement, and it depends on the air gap. For a cylindrical rotor, the air gap remains same, and hence no change in reluctance and no torque is developed.
12. Which of the following are applications of singly excited magnetic systems ____________
a) electromagnets, relays
b) moving-iron instruments
c) reluctance motors
d) any of the mentioned
Answer
Answer: d [Reason:] All of the applications mentioned above needs singly excited magnetic systems.
Set 4
1. For a p-pole machine, which of the following statements are correct regarding the rotating field speed?
a) The rotating field speed is 1/(p/2) revolutions in one cycle
b) f/(p/2) revolutions in f cycles
c) f/(p/2) revolutions in one second
d) Any of the mentioned
Answer
Answer: d [Reason:] f/(p/2) revolutions in one second is also correct because f cycles are completed in one second. ns(rotating field speed in revolutions per second) = 2f/p or Ns = 120f/p r.p.m.
2. The speed at which rotating magnetic field revolves is called
a) Induction speed
b) Synchronous speed
c) Relative speed
d) Rotating speed
Answer
Answer: b [Reason:] Rotating field speed (Ns) = 120f/(p r.p.m) = synchronous speed.
3. What is the amplitude of rotating MMF produced as a result of m-phase currents flowing in m-phase windings?
a) (2/m)Fm
b) mFm
c) (m/2)Fm
d) Fm
where Fm is maximum MMF in any one phase, when current is maximum in that phase.
Answer
Answer: c [Reason:] In general m phase current flowing in m phase winding results in rotating MMF wave of constant amplitude, equal to m/(2Fm).
4. The peak of rotating MMF wave (FR) is directed along which of the following axis?
a) The axis of that phase which carries the maximum current at that instant
b) The axis of that phase which carries half of the maximum current at that instant
c) The axis of that phase which carries minimum current at that instant
d) Any of the mentioned
Answer
Answer: a [Reason:] The resultant MMF FR = 3/(2Fm) is directed along the axis of that phase, which carries the maximum current at that instant.
5. If the phase sequence of supply currents are reversed, then the direction of rotation of the resultant MMF wave will
a) Be reversed
b) Remains unchanged
c) Cannot be determined
d) None of the mentioned
Answer
Answer: a [Reason:] A reversal of the phase sequence of supply currents will reverse the direction of rotation of the resultant MMF wave.
6. Which of the following statements are correct regarding individual phase MMF in rotating machines ?
a) It is a rotating MMF wave
b) It is not a rotating MMF wave and its amplitude doesn’t alternate along its own phase axis
c) It is not a rotating MMF wave but its amplitude merely pulsates
d) None of the mentioned
Answer
Answer: c [Reason:] It should be remembered that individual phase MMF is not a rotating MMF wave, its amplitude merely pulsates or alternates along its own phase axis.
7. The effect of poly phase currents in poly phase winding can be compared to _______________
a) Mechanical rotation of permanent magnets at synchronous speed
b) Mechanical rotation of DC excited field poles at synchronous speed
c) Mechanical rotation of either permanent magnets or DC excited field poles, at synchronous speed
d) None of the mentioned
Answer
Answer: c [Reason:] The speed at which rotating magnetic field revolves is called the synchronous speed and this effect is equivalent to mechanical rotation of permanent magnets or DC excited field poles.
8. The magnitude of rotating flux _____________ at all instants of time.
a) Changes
b) Remains constant
c) Pulsates
d) Any of the mentioned
Answer
Answer: b [Reason:] FR = 3/(2Fm) (for a 3 phase machine). A constant amplitude rotating MMF or rotating field is produced in the air gap of a three phase machine.
9. The amplitude of rotating MMF wave is proportional to
a) Nph and P
b) I and P
c) Nph and I
d) Nph, I and P
Answer
Answer: c [Reason:] FR = 3/(2Fm) = 3/(2 4√2/(ΠKw NphI/P)
where Nph = series turns per phase and I = RMS current.
10. If F1 is the constant amplitude of fundamental rotating MMF wave, the space harmonics of order 6K+1 (where K=1,2,3….), the harmonic MMF wave is of _____________
a) Constant amplitude and stationary in space
b) Constant amplitude and rotates along F1
c) Varying amplitude and rotates along F1
d) Constant amplitude and rotates against F1
Answer
Answer: b [Reason:] Space harmonics of the order of 6K+1(7,13,19,…) have their constant amplitude equal to 1/(6K+1)F1 and rotate in the direction of fundamental component F1 at a speed of 1/(6K+1) of the fundamental synchronous speed.
Set 5
1. If F1 is the constant amplitude of fundamental rotating MMF wave, the for the space harmonics of order 6K-1 (where K=1,2,3….), the harmonic MMF wave is of _____________
a) Constant amplitude and stationary in space
b) Constant amplitude and rotates along F1
c) Varying amplitude and rotates along F1
d) Constant amplitude and rotates against F1
Answer
Answer: b [Reason:] Space harmonics of the order of 6K-1(5,11,17,…) have their constant amplitude equal to 1/(6K-1)F1 and rotate in the direction of fundamental component F1 at a speed of 1/(6K+1) of the fundamental synchronous speed.
2. Which of the following harmonics are present in the resultant MMF produced by 3 phase currents flowing in the 3 phase windings?
a) Triplen harmonics (3K), where K is an integer
b) 6K+1 harmonics only, where K is an integer
c) 6K-1 harmonics only, where K is an integer
d) 6k+1 and 6K-1 harmonics, where K is an integer
Answer
Answer: d [Reason:] Space harmonics of the order 3,9,15,… i.e, triplen harmonics are absent.
3. When a single-phase winding is excited by an alternating current, then which of the following statements are correct?
a) It produces one stationary MMF wave
b) Produced MMF wave pulsates along its magnetic axis
c) It doesn’t produce two counter rotating MMF wave
d) All of the mentioned
Answer
Answer: d [Reason:] It is the decomposition of the MMF of a single phase winding into two counter rotating MMF waves, but are not actually produced and only one stationary MMF wave pulsating along its magnetic axis is produced.
4. What does the space angle α in the equation Fr(α,t)=3/2Fmcos(α-ωt) for MMF produced when 3-phase balanced windings excited by three phase balanced currents represent?
a) MMF wave at any moment is sinusoidally distributed in space
b) Entire MMF waveform is travelling at (constant angular speed) synchronous speed
c) MMF wave is of constant amplitude
d) None of the mentioned
Answer
Answer: a [Reason:] The space angle α means that MMF wave at any moment is sinusoidally distributed in space along the air-gap periphery.
5. Three phase voltages are applied to the three windings of an electrical machine. If any two supply terminals are interchanged, then the rotating MMF wave _____________
a) Direction reverses, amplitude alters
b) Direction reverses, amplitude unaltered
c) Direction remains same, amplitude alters
d) Direction remains same, amplitude unaltered
Answer
Answer: b [Reason:] The direction of rotating MMF wave is reversed, though its amplitude remains unaltered.
6. The rotating MMF wave of constant amplitude can be produced, only if the time angle displacement between currents is _____________
a) Less than the space angle displacement between winding axis
b) More than the space angle displacement between winding axis
c) Equal to the space angle displacement between winding axis
d) Any of the mentioned
Answer
Answer: c [Reason:] If the time angle displacement between currents and space angle displacement between winding axis are not equal, the magnitude of resultant MMF wave is always zero.
7. A 3 phase star connected winding is fed from symmetrical 3 phase supply with their neutrals connected together. If one of the 3 supply lines gets disconnected, then the revolving MMF wave will have a ___________
a) Constant amplitude but sub synchronous speed
b) Varying amplitude but synchronous speed
c) Constant amplitude but super synchronous speed
d) Varying speed and a speed fluctuating around the synchronous speed
Answer
Answer: d [Reason:] With one supply phase disconnected, a rotating MMF wave of varying amplitude is produced, but at a speed fluctuating around the synchronous speed.
8. In case of the 3 phase winding with delta or star connection without neutral, if one of the supply lines gets disconnected, then the MMF becomes __________
a) Stationary and pulsating
b) Rotating with synchronous speed
c) Rotating with sub synchronous speed
d) Rotating with super synchronous speed
Answer
Answer: a [Reason:] Disconnection of one of the supply leads, results in single-phase operation of the machine.
9. If φm is the maximum value of flux due to any one phase, then resultant flux in 2 phase and 3 phase AC machines would respectively be given by _________
a) φm and 1.5φm, both rotating
b) φm and 1.5φm, both stand still
c) φm stand still and 1.5φm rotating
d) 1.5φm and 2φm, both rotating
Answer
Answer: a [Reason:] In a 2 phase and 3 phase AC machine, rotating MMF is produced with magnitudes of φm and 3/2φm.
