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Multiple choice question for engineering

Set 1

1. The angular slot pitch, γ is given by
a) γ= πP/Total number of slots Elect radians
b) γ= 180P/Total number of slots Elect degrees
c) γ= 180/Slots per pole Elect degrees
d) Any of the mentioned

View Answer

Answer: d [Reason:] Angle γ is the angle between adjacent slots.

2. Consider the following statements regarding the design of distributed armature winding in a 3-phase alternator:
(i) it reduces the phase-belt harmonics
(ii) it increases the utilization of the armature iron and copper
(iii) it increases rigidity and mechanical strength of the winding
(iv) it reduces copper in the overhang of the winding
Which of these statements are correct?
a) (i),(iii),(iv)
b) (ii),(iii),(iv)
c) (i),(ii),(iii)
d) (i),(ii),(iii),(iv)

View Answer

Answer: c [Reason:] Distributed armature winding does not reduce copper in the overhang of the winding, also kdn=(sinqnγ/2)/(qsinnγ/2), kdnd1, implies it has the effect of reducing nth harmonic EMF, where q=slots per pole per phase.

3. What is the coil span for a 2 pole 18 slot machine?
a) 9
b) 4.5
c) 18
d) 6

View Answer

Answer: a [Reason:] Coil span= number of slots/number of poles= 18/2=9.

4. In a machine with coil span 9, if a coil has its one coil-side in slot 1, then its other coil-side must be in ____________
a) slot 9
b) slot 8
c) slot 10
d) slot 2

View Answer

Answer: c [Reason:] The other coil-side must be in slot 10=1+coil span=1+9 .

5. The belt factor is defined as the ratio of
a) arithmetic sum of coil EMF to the phasor sum of coil EMFs
b) phasor sum of EMF per coil to the arithmetic sum of EMF per coil
c) phasor sum of coil EMFs to the arithmetic sum of coil EMFs
d) phasor sum of coil EMFs to per phase voltage

View Answer

Answer: c [Reason:] Belt factor or distribution factor or breadth factor = kd= phasor sum of coil EMFs/arithmetic sum of coil EMFs.

6. The armature winding of a 2-pole, 3-phase alternator for each phase is distributed in a number of slots per phase. The RMS value of the voltage per phase is less than the RMS value of the voltage per coil multiplied by the number of coils in series because the
a) RMS value of the voltage in different coils of the phase is different
b) equal RMS voltages in different coils of the phase has mutual phase difference
c) maximum value of the induced voltage in different coils of the phase are different
d) different coils of the phase pass through different saturated regions of the magnetic circuit

View Answer

Answer: b [Reason:] The phase displacement will be given by slot angular pitch γ.

7. A 3-phase machine has integral slot winding with fundamental distribution factor kd1, the distribution factor for nth harmonic kdn is ____________
a) less than kd1
b) more than kd1
c) equal to kd1
d) depends upon the number of slots and poles

View Answer

Answer: a [Reason:] kdn=(sinqnγ/2)/(qsinnγ/2) and examples will show that kdn is less than kd1. This has the effect of reducing the nth harmonic EMF in comparision with fundamental EMF. This is certainly an advantage of distributing the winding in slots.

8. If a stator has 48 slots, 6 poles and 3 phase with narrow spread winding, then the third harmonic belt factor is ____________
a) 0.9556
b) 0.6407
c) 0.1944
d) None of the mentioned

View Answer

Answer: b [Reason:] q= (s/m)/p=k.Sk/k.Pk=Sk/Pk for fractional slot winding where k is the highest common factor between s/m and P, here q= 48/3*6 =16/6= 8/3 = Sk/Pk The distribution factor for fractional slot winding’s is obtained by replacing q by Sk and kdn = (sinnγ/2)/(Sksinnγ/2Sk), if n=3, kd3=0.6407.

9. A fraction pitch winding is used to reduce
(i) amount of copper in the winding
(ii) size of the machine
(iii) harmonics in the generated EMF
(iv) cost of the machine
From these, the correct answer is
a) (i),(ii),(iii),(iv)
b) (i),(ii),(iii)
c) (ii),(iii),(iv)
d) (i),(iii),(iv)

View Answer

Answer: d [Reason:] The fractional pitch winding results in reduction of copper in overhang and it results in less cost of the machine. kpn=cos(nε)/2 implies reduces harmonics and thereby rendering the output EMF wave almost a sine wave.

10. The pitch factor, in rotating electrical machinery, is defined as the ratio of resultant EMF of a
a) full-pitched coil to that of a chorded coil
b) full-pitched coil to the phase EMF
c) chorded coil to the phase EMF
d) chorded coil to that of a full-pitched coil

View Answer

Answer: d [Reason:] Kp= Resultant EMF of a chorded coil/Resultant EMF of a full pitched coil = cosε/2

Set 2

1. The winding’s for a 3-phase alternator are:
(i) 36 slots, 4 poles, span 1 to 8
(ii) 72 slots, 6 poles, span 1 to 10
(iii) 96 slots, 4 poles, span 1 to 21
The winding’s having pitch factor of more than 0.97 are
a) (i) and (ii) only
b) (ii) and (iii) only
c) (i) and (iii) only
d) (i),(ii) and (iii)

View Answer

Answer: c [Reason:] Kp=cosε/2 (i) Slots per pole= 36/4=9, for a coil span of 8 slots, the coil is short pitched by 1 slot and the chording angle is ε=γ= 20° ⇒ Kp=cos10°=0.985 (ii) Slots per pole= 72/6=12, for a coil span of 10 slots, the coil is short pitched by 2 slots and the chording angle is ε=2γ and γ=180/12 ⇒ ε= 30° ⇒ Kp=cos15°=0.9659 (iii) Slots per pole= 96/4=24, for a coil span of 21 slots, the coil is short pitched by 3 slots and the chording angle is ε=3γ and γ=180/24 ⇒ ε= 24.5° ⇒ Kp=cos12.25°=0.97723.

2. In 48 slot, 4-pole,3 phase alternator, the coil-span is 10. Its distribution and pitch factors are respectively ____________
a) 0.9717,0.966
b) 0.9822, 0.9814
c) 0.9577, 0.9814
d) 0.9577, 0.966

View Answer

Answer: d [Reason:] Slots per pole= 48/4=12, for a coil span of 10 slots, the coil is short pitched by 2 slots and the chording angle is ε=2γ and γ=180/12 ⇒ ε= 30° ⇒ Kp=cosε/2=0.9659. We know, Kd=(sin(qγ/2))/qsin(γ/2), here q=48/4∗3 = 4 ⇒ Kd=0.957662.

3. A 3-phase, 4-pole alternator has 48 stator slots carrying a 3-phase distributed winding. Each coil of the winding is short chorded by one slot pitch. The winding factor is given by ______________
a) (cos7.5)/16
b) (cot7.5)/8
c) 1/(8sin7.5)
d) (cot7.5)/16

View Answer

Answer: b [Reason:] Slots per pole= 48/4 =12 ⇒ γ=180/12=15° and q=48/4&lowest;3=4 coil of the winding is short chorded by one slot pitch ⇒ ε=γ=15°, Kw=Kp&lowest;Kd= cos7.5(sin(4∗15/2))/(4∗sin(15/2)) = (cot7.5)/8.

4. The chording angle for eliminating 5th harmonic should be ____________
a) 30°
b) 34°
c) 36°
d) 35°

View Answer

Answer: c [Reason:] To eliminate 5th harmonic kp5 must be zero, kpn= cosnε/2 ⇒ kp5= cos5ε/2=0=cos90° ⇒ 5ε/2=90° ⇒ ε =36°.

5. Consider the following statements:
(i) breadth factor for third harmonic kd3 is more than that for fundamental kd1
(ii) kd3 < kd1
(iii) kd3 may be less or more than kd1 depending upon the number of slots and poles
(iv) coil-span factor for third harmonic kp3>kp1 (coil span factor for fundamental)
(v) kp3 < kp1
(vi) kp3 may be less or more than kp1 depending upon the number of slots and poles

From these, the correct answer is
a) (ii),(v)
b) (i),(iv)
c) (iii),(vi)
d) (i),(iii),(iv),(vi)

View Answer

Answer: a [Reason:] Examples will show that kdn < kd1 and has the effect of reducing the nth harmonic EMF in comparison with the fundamental EMF, similarly kpn < kp1.

6. A 6-pole alternator with 36 slots carries a 2-phase distributed winding. Each coil is short pitched by one slot. The winding factor is given by
a) cot15°/3√2
b) cot15°/4
c) cot15°/2√2
d) cot15°/4

View Answer

Answer: a [Reason:] Slots per pole =36/6=6, q=6/2=3 ⇒ γ=180°/6=30°, coil is short pitched by one slot ⇒ ε=γ=30° and Kp=cosε/2=cos15° and Kd=sin(γq/2)/q(sinγ/2)=sin45/(3sin15) Kw=Kp∗Kd=cot15°/3√2

7. For eliminating nth harmonic from the EMF generated in the phase of a 3-phase alternator, the chording angle should be
a) n∗full pitch
b) (1/n)∗full pitch
c) (2/n)∗full pitch
d) (3/n)∗full pitch

View Answer

Answer: a [Reason:] kpn=cosnε/2=0 ⇒ nε/2=90° ⇒ ε=180°/n= full pitch/n.

8. Which among the given harmonics are called belt harmonics?
a) 5,7,11,13
b) 3,6,9,12
c) 5,6,11,12
d) 7,11,13,15

View Answer

Answer: a [Reason:] The odd harmonics of the order 5,7,11,13 etc are called belt harmonics.

9. Machine A has 60° phase spread and machine B has 120° phase spread. Both the machines have uniformly distributed winding. The ratio of distribution factors of machine A to machine B is ____________
a) 0.866
b) 1.1
c) 1.55
d) 1.155

View Answer

Answer: c [Reason:] kdA=(sinσ/2)/σ/2, σ=60° ⇒ kdA=0.9556 kdB=(sinσ/2)/σ/2, σ=120° ⇒ kdB=0.827 thus, kdA/ kdB=1.155.

Set 3

1. Which of the following statements is/are correct regarding the generation of EMF in rotating electrical machines in the armature winding? EMF is generated ____________
a) by rotating armature windings through a magnetic field
b) by rotating magnetic field with respect to the armature windings
c) by designing the magnetic circuit to have variable reluctance with rotor rotation
d) any of the mentioned

View Answer

Answer: d [Reason:] Any of the above mentioned methods will produce EMF in the armature windings.

2. The EMF equation e = NωrΦsinωrt is applicable to ____________
a) AC systems with time variant field flux
b) DC systems with time variant field flux
c) Both AC and DC systems with time invariant field flux
d) Both AC and DC systems with time variant field flux

View Answer

Answer: c [Reason:] e = NωrΦsinωrt-NdΦ/dtcosωrt, for time invariant field flux dΦ/dt = 0 ⇒ e = NωrΦsinωrt, is the general equation, and is applicable to both AC and DC systems.

3. In the equation for RMS value of the generated EMF in a full pitched coil of an AC machine,
E = Emax/√2 = √2πfrNΦ, fr depends on
a) rotating speed of the armature coil
b) rotating speed of the flux density wave
c) relative velocity between the flux density wave and armature coil
d) any of the mentioned

View Answer

Answer: c [Reason:] fr is called the rotational or speed frequency, since its value depends upon the relative velocity between the flux density wave and the armature coil.

4. In AC rotating machines, the generated or speed EMF
a) leads Φ by 90°
b) lags Φ by 90°
c) is in phase with working flux
d) lags Φ by 180°

View Answer

Answer: b [Reason:] e = Emaxcos(ωrt-π/2) Ψ; = NΦcosωrt It reveals that the speed or generated EMF lags by 90° the flux that generates it, and is true when flux is time invariant and is sine distributed in space.

5. In a short pitched coil, the coil pitch factor kp, is given by ____________
a) kp = cosε
b) kp = cosε/2
c) kp = √2πfrNΦcosε
d) kp = cosε/2cosωrt

View Answer

Answer: b [Reason:] The effect of short pitched coil is to reduce the generated EMF, and this reduction factor is cosε/2, and is referred to as coil pitch factor.

6. Which of the following equations represents the RMS value of the generated EMF in a short-pitched N-turn armature coil of an AC machine ____________
a) E = √2πkpfr
b) E = 2πkpfr
c) E = NΦωrkpsinωrt
d) None of the mentioned

View Answer

Answer: a [Reason:] EMF induced = NΦωrkpsinωrt, Emax = NΦωrkp when sinωrt = 1 RMS value, E = Emax/√2 = √2πfrkp

7. The effect of short pitched coil on the generated EMF is _____________
a) increasing
b) decreasing
c) either increasing or decreasing
d) none of the mentioned

View Answer

Answer: b [Reason:] The effect of short pitched coil is to reduce the generated EMF and this reduction factor is cosε/2 and is referred to as coil pitch factor.

8. A winding is distributed in the slots along the air gap periphery
(i) to add mechanical strength to the winding
(ii) to reduce the amount of conductor material required
(iii) to reduce the harmonics in generated EMF
(iv) to reduce the size of the machine
(v) for full utilization of iron and conductor materials
From these, the correct answer is
a) (i),(iii),(iv),(v)
b) (i),(ii),(iv),(v)
c) (i),(ii),(iii),(v)
d) (i),(iii),(v)

View Answer

Answer: d [Reason:] In rotating electrical machines, the armature turns are usually distributed in slots rather than concentrated in single slot. This is essential from the view point of utilizing the armature periphery completely, and add mechanical strength.

9. In an AC machine, the effect of distributing the turns in different slots, results in a further reduction of generated EMF by the factor kd. This factor is called ____________
a) distribution/speed factor
b) coil pitch factor
c) winding factor
d) any of the mentioned

View Answer

Answer: a [Reason:] E = √2πkpkdfrNΦ, kd is the distribution factor.

10. A polyphase induction motor of the slip ring or wound rotor type can be used ____________
a) for high start-up torque applications
b) as a frequency converter
c) any of the mentioned
d) none of the mentioned

View Answer

Answer: c [Reason:] A polyphase induction motor can be used as a frequency converter (or changer) for changing the supply frequency f to other frequencies sf and (2-s)f at the slip ring terminals.

11. The equation for slip speed is ____________
a) (ω-ωr)/ω
b) ω-ωr
c) (ωr-ω)/ω
d) ωr

View Answer

Answer: b [Reason:] The relative speed between rotor and rotating flux wave, i.e, (ω-ωr) is referred to as the slip speed in rad/sec slip speed = (ω-ωr) rad/sec

12. If the rotor of an induction motor is made to revolve in a direction opposite to the rotating flux wave, then RMS value of EMF induced in one phase of rotor E is proportional to ____________
a) 2-s
b) s
c) 1-s
d) 2+s

View Answer

Answer: a [Reason:] Relative velocity between rotor winding and rotating flux wave becomes (ω+ωr) = ω+ω(1-s) = ω(2-s) and E = 2πf(2-s)NphrkwΦ

13. In an alternator, frequency per revolution is equal to
a) number of poles
b) twice the number of poles
c) speed in rps
d) number of pole-pairs

View Answer

Answer: d [Reason:] For a P-pole machine, in n rev/sec, P/2n cycles/second are generated and cycles per second is referred to as frequency f of the EMF wave. Here in this question n=1 ⇒ frequency per revolution = P/2 ⇒ number of pole pairs.

14. The EMF generated in an alternator depends upon
(i) speed
(ii) maximum flux per pole
(iii) series turns per phase
(iv) phase spread
(v) coil-span
(vi) type of alternator
Which of the above statements are correct?
a)(i),(ii),(iii),(iv),(v),(vi)
b) (i),(iii),(iv),(v)
c) (i),(ii),(iii),(v),(vi)
d) (i),(iii),(iv),(v),(vi)

View Answer

Answer: b [Reason:] Eph = √2πfkwNphΦ, here Φ = total flux per pole.

15. If the dimensions of all the parts of a synchronous generator and the number of field and armature turns are doubled, then the generated voltage will change by a factor of
a) 1
b) 2
c) 4
d) 8

View Answer

Answer: d [Reason:] E = √2πfrNΦ, Φ = flux per pole = 4/pBplr dimensions are doubled ⇒ l1 = 2l and r1 = 2r ⇒ Φ1 = 4Φ No. of turns doubled ⇒ N1 = 2N, fr changes only when the poles are changed ⇒ fr1 = fr ⇒ E1=√2πfr(2N)(4Φ) = 8E The generated voltage will change by a factor of 8.

16. A 6 pole machine is rotating at a speed of 1200rpm. This speed in mechanical rad/sec and electrical radians per second is respectively
a) 40π, 40π/3
b) 120π, 40π
c) 20π, 60π
d) 40π, 120π

View Answer

Answer: d [Reason:] N = 1200rpm,P = 6 and f = PN/120 = 60 cycles per sec. 1cycle = 360° electrical degrees = 2π electrical radians f in electrical radians per sec = 60∗2π electrical radians/sec = 120π elect radians/sec speed in mechanical radians/sec = 2/P(speed in electrical rad/sec) = 2/P(120π)=40π mechanical rad/sec.

17.The short pitch winding for an alternator gives
(i) improved wave form of the generated EMF per phase
(ii) reduced value of self inductance of the winding
(iii) increased rating of machine
(iv) reduced tooth ripples
(v) increased total generated EMF
(vi) saving in winding copper
Which of the above statements are correct?
a) (i),(ii),(iv),(v),(vi)
b) (i),(ii),(vi)
c) (i),(ii),(iv),(vi)
d) (i),(iii),(vi),(iv)

View Answer

Answer: b [Reason:] The effect of short pitched coil is to reduce the generated EMF, and it improves waveform of EMF generated per phase, and as the windings are placed closed to each other, copper will be saved.

18. The DC machines are designed with flat topped flux density waves because
a) average value of brush voltage is more for a flat topped B-wave
b) average value of brush voltage is less for a flat topped B-wave
c) losses for flat topped B-wave are less
d) none of the mentioned

View Answer

Answer: a [Reason:] For the same value of peak flux density Bp, it is easy to see that average value of brush voltage could be more for a flat topped B-wave than for a sinusoidal flux density wave.

Set 4

1. A consideration of the power losses in electrical machines is essential for which of the following reasons?
a) Operating cost
b) Temperature rise
c) Voltage drops
d) Any of the mentioned

View Answer

Answer: d [Reason:] A machine with lower efficiency has more losses, and therefore increased operating cost. Also, losses cause heating of the machine and therefore, its temperature rise. Similarly, voltage drop IR is associated with ohmic loss.

2. To determine the efficiency of the machine, direct load test is not advantageous because of which of the following reasons?
a) Cost of providing large inputs
b) Difficulty of dissipating the large outputs
c) Both cost of providing large inputs and difficulty in dissipating the large outputs
d) None of the mentioned

View Answer

Answer: c [Reason:] Also, a small error in the measurement of either output or input causes the same amount of error in the computed efficiency.

3. Which of the following statements are correct regarding brush contact losses?
a) In DC machine: proportional to armature current
b) In synchronous machine: neglected
c) In induction machine: neglected
d) Any of the mentioned

View Answer

Answer: d [Reason:] There is brush contact loss at the contacts between the brushes and commutator (in DC machine) or between the brushes and slip rings (synchronous and induction machines). However, in practical, the brush contact loss is neglected for synchronous and induction machines.

4. In rotating electrical machines, when the armature rotates, there are continuous magnetic reversals and power required for their reversals is called _____________
a) Eddy current loss
b) Hysteresis loss
c) Resistance or ohmic losses
d) Mechanical loss

View Answer

Answer: b [Reason:] Hysteresis loss is directly proportional to the number of magnetic reversals per second or the speed.

5. The usual lamination thickness selected to minimize the eddy current loss in rotor is _____________
a) 0.1 mm to 0.2 mm
b) 0.3 mm to 0.4 mm
c) 0.4 mm to 0.5 mm
d) 0.9 mm to 0.10 mm

View Answer

Answer: c [Reason:] If lamination thickness is made less than 0.4 mm, the reduction in eddy current losses is achieved, but at the cost of additional labor charges in assembling the rotor.

6. In an induction motor, which of the following is correct:
a) Stator core loss < rotor core loss
b) Stator core loss = rotor core loss
c) Stator core loss > rotor core loss
d) Any of the mentioned

View Answer

Answer: c [Reason:] The rotor core loss is almost negligible because of reduced frequency of the flux reversals (equal to slip frequency, sf) in the rotor, and Pe∝f2 and Ph∝f. ( Pe = eddy current losses, Ph = hysteresis losses)

7. The pole shoes in DC and synchronous machines are laminated to reduce _____________
a) Resistance losses
b) Pulsation or pole-face losses
c) Mechanical losses
d) None of the mentioned

View Answer

Answer: b [Reason:] The pulsations in flux density wave arising from slot openings cause losses in the field iron, particularly in the pole faces. This loss occurring due to relative motion, between field poles and slotted armature, is referred to as pulsation loss.

8. What percentage of the rated output for DC machine and synchronous machine is taken as stray load losses respectively?
a) 1% and 0.5%
b) 0.5% and 1%
c) 3% and 0.1%
d) 0.1% and 5%

View Answer

Answer: a [Reason:] Stray load losses cannot be determined accurately. In DC machine, by convention, it is taken as 1% of the rated output for rating above 150 KW. For synchronous and induction machine, it is taken as 0.5% of their rated output.

9. Which of the given losses are directly proportional to square of speed?
a) Windage loss
b) Eddy current loss
c) Both Windage and eddy current loss
d) Hysteresis loss and brush loss

View Answer

Answer: c [Reason:] The windage loss includes the power required to circulate air through the machine and ventilating ducts, and is approximately proportional to the square of speed. Also, eddy current losses are proportional to square of speed.

10. Consider the following statements regarding efficiency of electrical machines:
(i) efficiency should be calculated by measuring output and input
(ii) efficiency is maximum when constant losses = variable losses
(iii) electrical machines are designed to have maximum efficiency at full load
(iv) electrical machines are designed to have maximum efficiency at near about full load
(v) efficiency should be calculated by measuring their losses
(vi) efficiency is maximum when constant losses = x times of (variable losses)
Which of the above statements are correct?
a) (i),(iii),(vi)
b) (ii),(iv),(v)
c) (i),(ii),(iii)
d) (iv),(v),(vi)

View Answer

Answer: b [Reason:] The machine efficiency rises with load. but at a particular load, efficiency is maximum and beyond this load, efficiency diminishes. Also, for both motors and generators, machine efficiency is maximum when variable loss = constant loss.

11. The electromechanical energy conversion devices used in power systems are never operated to deliver maximum power output, because at maximum power output, _____________
a) Efficiency is less than 50%
b) Temperature of the power devices is much more than the specified allowable temperature rise
c) Half of the power input appears as losses
d) Any of the mentioned

View Answer

Answer: b [Reason:] In practice, devices are operated at a load somewhat less than rated load, at which the efficiency is maximum.

12. For the same rating machines, which of the following statement is correct regarding the efficiency (η)?
a) η of low speed machine > η of high speed machine
b) η of low speed machine < η of high speed machine
c) η of low speed machine = η of high speed machine
d) Any of the mentioned

View Answer

Answer: b [Reason:] The amount of conductor and iron materials required for a machine of given rating is inversely proportional to its speed. Also, more iron and conductor would entail more losses.

13. No load rotational losses in electrical machine consists of _____________
a) Friction and windage losses
b) Stator core, friction and windage losses
c) Rotor core, friction and windage losses
d) No load core, friction and windage losses

View Answer

Answer: d [Reason:] The sum of friction and windage loss under no load is called no load rotational losses.

Set 5

1. The winding MMF in rotating machines depends on
a) winding arrangement
b) winding current
c) air gap length,slot openings etc
d) both winding arrangement and winding current

View Answer

Answer: d [Reason:] The winding MMF depends only on the winding arrangement and the winding current.

2. A knowledge of the air gap flux distribution in a machine helps in determining the
a) generated EMF waveform and its magnitude
b) electrical torque
c) winding MMF
d) both generated EMF waveform and electrical torque

View Answer

Answer: d [Reason:] The winding mmf depends only on the winding arrangement and the winding current.

3. If the current in the coil is DC, then MMF doesn’t vary with ____________
a) space
b) time
c) both space and time
d) None of the mentioned

View Answer

Answer: c [Reason:] If the coil current is DC, the magnitude of MMF wave does not vary with time and space.

4. If the current in a coil is AC, the amplitude of MMF
a) varies with time but not with space
b) varies with space but not with time
c) varies with both space and time
d) doesn’t vary with both space and time

View Answer

Answer: a [Reason:] For AC in the coil, the air-gap MMF wave is time variant but space invariant.

5. Peak value of fundamental component of MMF produced by one N-turn coil carrying a current ‘i’ is ____________
a) 4/Π(Ni)
b) 2/Π(Ni)
c) 1/Π(Ni)
d) 1/2Π(Ni)

View Answer

Answer: b [Reason:] The fundamental component of rectangular wave is found to be Fα1=(4/Π)(Ni/2)cosα, for 2-pole machine where α=electrical space angle The peak value of the sine MMF wave for a 2 pole machine is given by F1p=(4/π)(Ni/2)AT’s per pole= 2/π(Ni) AT.

6. The armature MMF wave in a DC machine is ____________
a) sinusoidal and depends on the speed
b) square and independent of speed
c) triangular and depends on speed
d) triangular and independent of speed

View Answer

Answer: d [Reason:] The armature MMF for a DC machine remains constant in amplitude and does not depend on the armature speed, also the MMF wave produced is seen to be a triangluar MMF wave.

7. Which of the following statements are correct regarding the current sheet?
a) it is a thin strip of conducting material carrying the current in the dot
b) it is wrapped around stator or rotor
c) it develops MMF identical to that produced by the winding it replaces
d) Any of the mentioned

View Answer

Answer: d [Reason:] The MMF produced by uniformly distributed current sheet is identical with the MMF produced by uniformly distributed winding. If the distributed winding in the slots is on the rotor, then the uniformly distributed current sheet would be wrapped around the rotor accordingly.

8. A current sheet with sinusoidal current produces?
a) sinusoidal MMF wave lagging it by 90°
b) sinusoidal MMF wave leading it by 90°
c) trapezoidal MMF wave leading it by 90°
d) trapezoidal MMF wave lagging it by 90°

View Answer

Answer: b [Reason:] A uniform current sheet produces trapezoidal MMF wave and sinusoidal current sheet produces sinusoidal MMF wave leading by 90°.

9. A uniformly distributed winding on the stator has three full pitched coils, each coil having N turns and each turn carrying a current i. The MMF produced by the winding is
a) sinusoidal in waveform with an amplitude 3Ni
b) sinusoidal in waveform with an amplitude 3Ni/2
c) trapezoidal in waveform with an amplitude of 3Ni
d) trapezoidal in waveform with an amplitude of 3Ni/2

View Answer

Answer: d [Reason:] When the machine has more than 3 slots per pole per phase, the steps in the MMF wave are neglected and MMF variation is taken as smooth over these slots and as a consequence, stepped MMf wave changes to trapezoidal MMF wave. The trapezoidal MMF wave amplitude =3Ni/2.

10. MMF produced by one N-turn coil carrying a current i is ____________
a) rectangular of amplitude Ni/2
b) trapezoidal of amplitude Ni/2
c) rectangular of amplitude Ni
d) trapezoidal of amplitude Ni

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Answer: a [Reason:] The MMF variation due to one N-turn coil is seen to be a rectangular wave of amplitude +1/2(Ni) or -1/2(Ni).

11. A winding of 20 full pitched series turns, distributed over a band of 60° under each pole, carries a current of 3A. The winding produces a uniform current sheet of density (in AT’s per electrical radiangs) of
a) 180/Π
b) 120/Π
c) 90/Π
d) 60/Π

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Answer: a [Reason:] Uniform current density J=(20∗3)/(Π/3)=180/Π.

12. A sinusoidal current sheet in a rotating electrical machine has peak value along q-axis. The peak value of sinusoidal MMF would be
a) in phase with current peak
b) 90° lagging the current peak
c) 90° leading the current peak
d) may lag or lead depending upon the type of machine

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Answer: c [Reason:] Sinusoidal current sheet produces sinusoidal MMF wave leading by 90°.

13. In a DC machine, z,p,Ia and a are respectively the number of conductors, number of poles, armature current and number of parallel paths. The peak value of fundamental component of armature MMF wave is ____________
a) 8/Π[(z/2p)(Ia/a)].
b) 8/Π2[(z/p)(Ia/a)].
c) 4/Π[(z/2p)(Ia/a)].
d) 8/Π2[(z/p)(Ia/a)].

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Answer: d [Reason:] The peak value of the fundamental sine component Fa for a DC machine is Fa=8/&Pi2;[(z/p)(Ia/a)] AT/pole.

14. The armature MMF wave in a DC machine is triangluar and
(i) rotates with respect to stator
(ii) is stationary with respect to stator
(iii) rotates with respect to armature
(iv) is stationary with respect to armature
Which of the above statements are correct?
a) (ii) only
b) (iii) only
c) (i) and (iv)
d) (ii) and (iii)

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Answer: d [Reason:] In a DC machine, armature current doesn’t vary with time. Therefore, the armature MMF wave for a DC machine remains constant in amplitude and doesn’t depends on the armature speed. Hence the armature MMF wave is stationary with respect to stator and armature.