# 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

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

_{dn}=(sinqnγ/2)/(qsinnγ/2), k

_{dn}

_{1}, implies it has the effect of reducing n

^{th}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

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

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

_{d}= 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

7. A 3-phase machine has integral slot winding with fundamental distribution factor k_{d1}, the distribution factor for n^{th} harmonic k_{dn} is ____________

a) less than k_{d1}

b) more than k_{d1}

c) equal to k_{d1}

d) depends upon the number of slots and poles

### View Answer

_{dn}=(sinqnγ/2)/(qsinnγ/2) and examples will show that k

_{dn}is less than k

_{d1}. This has the effect of reducing the n

^{th}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

_{k}/k.P

_{k}=S

_{k}/P

_{k}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 = S

_{k}/P

_{k}The distribution factor for fractional slot winding’s is obtained by replacing q by S

_{k}and k

_{dn}= (sinnγ/2)/(S

_{k}sinnγ/2S

_{k}), if n=3, k

_{d3}=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

_{pn}=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

_{p}= 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

_{p}=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° ⇒ K

_{p}=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° ⇒ K

_{p}=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° ⇒ K

_{p}=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

_{p}=cosε/2=0.9659. We know, K

_{d}=(sin(qγ/2))/qsin(γ/2), here q=48/4∗3 = 4 ⇒ K

_{d}=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

_{w}=K

_{p}&lowest;K

_{d}= cos7.5(sin(4∗15/2))/(4∗sin(15/2)) = (cot7.5)/8.

4. The chording angle for eliminating 5^{th} harmonic should be ____________

a) 30°

b) 34°

c) 36°

d) 35°

### View Answer

^{th}harmonic k

_{p5}must be zero, k

_{pn}= cosnε/2 ⇒ k

_{p5}= cos5ε/2=0=cos90° ⇒ 5ε/2=90° ⇒ ε =36°.

5. Consider the following statements:

(i) breadth factor for third harmonic k_{d3} is more than that for fundamental k_{d1}

(ii) k_{d3} < k_{d1}

(iii) k_{d3} may be less or more than k_{d1} depending upon the number of slots and poles

(iv) coil-span factor for third harmonic k_{p3}>k_{p1} (coil span factor for fundamental)

(v) k_{p3} < k_{p1}

(vi) k_{p3} may be less or more than k_{p1} 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

_{dn}< k

_{d1}and has the effect of reducing the n

^{th}harmonic EMF in comparison with the fundamental EMF, similarly k

_{pn}< k

_{p1}.

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

_{p}=cosε/2=cos15° and K

_{d}=sin(γq/2)/q(sinγ/2)=sin45/(3sin15) K

_{w}=K

_{p}∗K

_{d}=cot15°/3√2

7. For eliminating n^{th} 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

_{pn}=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

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

_{dA}=(sinσ/2)/σ/2, σ=60° ⇒ k

_{dA}=0.9556 k

_{dB}=(sinσ/2)/σ/2, σ=120° ⇒ k

_{dB}=0.827 thus, k

_{dA}/ k

_{dB}=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

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2. The EMF equation e = Nω_{r}Φsinω_{r}t 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

_{r}Φsinω

_{r}t-NdΦ/dtcosω

_{r}t, for time invariant field flux dΦ/dt = 0 ⇒ e = Nω

_{r}Φsinω

_{r}t, 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 = E_{max}/√2 = √2πf_{r}NΦ, f_{r} 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

_{r}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

_{max}cos(ω

_{r}t-π/2) Ψ; = NΦcosω

_{r}t 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 k_{p}, is given by ____________

a) k_{p} = cosε

b) k_{p} = cosε/2

c) k_{p} = √2πf_{r}NΦcosε

d) k_{p} = cosε/2cosω_{r}t

### View Answer

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 = √2πk_{p}f_{r}NΦ

b) E = 2πk_{p}f_{r}NΦ

c) E = NΦω_{r}k_{p}sinω_{r}t

d) None of the mentioned

### View Answer

_{r}k

_{p}sinω

_{r}t, E

_{max}= NΦω

_{r}k

_{p}when sinω

_{r}t = 1 RMS value, E = E

_{max}/√2 = √2πf

_{r}k

_{p}NΦ

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

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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

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 k_{d}. This factor is called ____________

a) distribution/speed factor

b) coil pitch factor

c) winding factor

d) any of the mentioned

### View Answer

_{p}k

_{d}f

_{r}NΦ, k

_{d}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

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11. The equation for slip speed is ____________

a) (ω-ω_{r})/ω

b) ω-ω_{r}

c) (ω_{r}-ω)/ω

d) ω_{r}

### View Answer

_{r}) is referred to as the slip speed in rad/sec slip speed = (ω-ω

_{r}) 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

_{r}) = ω+ω(1-s) = ω(2-s) and E = 2πf(2-s)N

_{phr}k

_{w}Φ

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

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

_{ph}= √2πfk

_{w}N

_{ph}Φ, 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

_{r}NΦ, Φ = flux per pole = 4/pB

_{p}l

_{r}dimensions are doubled ⇒ l

^{1}= 2l and r

^{1}= 2r ⇒ Φ

^{1}= 4Φ No. of turns doubled ⇒ N

^{1}= 2N, f

_{r}changes only when the poles are changed ⇒ f

_{r}

^{1}= f

_{r}⇒ E

^{1}=√2πf

_{r}(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

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

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

_{p}, 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

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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

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

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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

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

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

_{e}∝f

^{2}and P

_{h}∝f. ( P

_{e}= eddy current losses, P

_{h}= 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

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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%

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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

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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

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

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

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

## 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

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

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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

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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

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

_{α1}=(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 F

_{1p}=(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

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

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

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

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

### View Answer

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

### View Answer

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

### View Answer

13. In a DC machine, z,p,I_{a} 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)(I_{a}/a)].

b) 8/Π^{2}[(z/p)(I_{a}/a)].

c) 4/Π[(z/2p)(I_{a}/a)].

d) 8/Π^{2}[(z/p)(I_{a}/a)].

### View Answer

_{a}for a DC machine is F

_{a}=8/&Pi

^{2};[(z/p)(I

_{a}/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)