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Multiple choice question for engineering

Set 1

1. How is the selection of number of poles made in AC and DC machines?
a) any number of poles can be used for both AC and DC machines
b) fixed number of poles in both AC and DC machines
c) fixed number of poles in DC machines and any number of poles in AC machines
d) fixed number of poles in AC machines and any number of poles in DC machines

View Answer

Answer: d [Reason:] When it comes to DC machines, any number of poles can be made use of, but it should be within a range. Whereas in AC machines the number of poles is fixed by supply frequency and speed.

2. How many considerations are present in the selection of number of poles?
a) 4
b) 5
c) 6
d) 7

View Answer

Answer: d [Reason:] There are 7 consideration in selection of number of poles. They are frequency, weight of iron parts, weight of copper, length of commutator, labor charges, flash over, distortion of field form.

3. What is the formula for frequency of flux reversals?
a) f = p*n
b) f = p/n
c) f = n/p
d) f = (p*n)/2

View Answer

Answer: d [Reason:] Frequency is directly proportional to the number of poles (p). It is also proportional to the speed of the machine as well.

4. What is the range of frequency during the selection of number of poles?
a) 20-50 Hz
b) 25-40 Hz
c) 25-50 Hz
d) >50 Hz

View Answer

Answer: c [Reason:] While selecting the number of poles, the lowest value of frequency should be minimum 25 Hz. The highest value of frequency should be limited to 50 Hz.

5. What is the relation of hysteresis loss and weight of iron parts with respect to increase of number of poles?
a) decrease in hysteresis loss, increase in weight
b) decrease in hysteresis loss, decrease in weight
c) increase in hysteresis loss, increase in weight
d) increase in hysteresis loss, decrease in weight

View Answer

Answer: b [Reason:] With larger number of poles, the area of cross section can be reduced, henceforth decreasing the hysteresis loss. Also by increasing pole number, weight of iron parts is reduced.

6. What happens to the weight of copper in both armature and field windings when the number of poles increase?
a) weight of copper in armature winding decreases and weight of copper in field winding increases
b) weight of copper in armature winding increases and weight of copper in field winding decreases
c) weight of copper in armature winding and field winding decreases
d) weight of copper in armature winding and field winding increases

View Answer

Answer: c [Reason:] The weight of copper is indirectly proportional to the number of poles. As the number of poles increases, the weight of the copper decreases.

7. What happens to the length of the commutators with the increase in number of poles?
a) The length of commutators are increased
b) The length of commutators are decreased
c) The length of commutators are stable
d) The length of commutators are higher

View Answer

Answer: b [Reason:] The area of the brushes decreases if the number of poles are being increased. As the area of the brushes are decreased, the length of the commutators also decrease.

8. What happens to the labor charges when there is an increase in number of poles?
a) labor charges are reduced
b) labor charges are increased
c) labor charges are fixed always
d) labor charges vary

View Answer

Answer: b [Reason:] With increase in the number of poles, the armature windings increase, and more work increases to insulate. The commutator segments also increase, and the work increases.

9. What is the effect of the distortion of field form with respect to the small number of poles?
a) small number of poles cause no distortions
b) small number of poles clears all distortions
c) small number of poles reduces distortions
d) small number of poles increases distortions

View Answer

Answer: d [Reason:] When there is small number of poles, that time the armature mmf per pole increases. As the armature mmf increases, it results in increase of distortion.

10. Large number of poles lead to large flashover between brushes?
a) true
b) false

View Answer

Answer: a [Reason:] The number of brushes is equal to number of poles. For the same diameter of the commutator, the distance between the adjacent brush arms decreases and this increases the possibility of flashover.

11. What is the dependency of the cost of the armature and field windings with respect to large number of poles?
a) high cost for armature windings, low cost for field windings
b) high cost for armature windings, high cost for field windings
c) low cost for armature windings, high cost for field windings
d) low cost for armature windings, low cost for field windings

View Answer

Answer: d [Reason:] With large number of poles the armature and the field windings reduce in number. Thus the cost of the field and armature windings also decrease.

12. Lower values of frequency are used for small machines?
a) true
b) false

View Answer

Answer: b [Reason:] Lower values of frequency are actually used for the large machines. Also higher values of frequency are actually used for small machines.

13. What is the range of the current per parallel path for the choice of number of poles?
a) limited to 100 A
b) limited to 150 A
c) limited to 200 A
d) limited to 250 A

View Answer

Answer: c [Reason:] The current per parallel path should be limited to maximum of 200 A. If the limit gets exceeded then there occurs damage to the machine.

14. What should be the range of the current per brush arm?
a) limited to 400 A
b) limited to 200 A
c) limited to 100 A
d) limited to 300 A

View Answer

Answer: a [Reason:] The current per brush arm should be limited to maximum of 400 A. If the limit gets exceeded then there occurs damage to the machine.

15. What should be the armature mmf per pole for output over 1500 kW?
a) 5000 A
b) 5000-7500 A
c) 7500-10000 A
d) upto 12500

View Answer

Answer: d [Reason:] 5000 A is for output of about 100 kW. 5000 to 7500 A for output voltage of 100 to 500 kW. 7500 to 10000 A is for the output of 500 to 1500 kW.

Set 2

1. How many steps are involved in the construction of single phase induction motor?
a) 3
b) 4
c) 5
d) 6

View Answer

Answer: c [Reason:] There are 5 steps in the construction of the single phase induction motor. They are stator, stator windings, rotor, starting switches, electrolytic capacitor.

2. What is the lamination used for the stator?
a) cast iron
b) die cast aluminium alloy frame
c) cast iron or die cast aluminium alloy frame
d) cast iron and die cast aluminium alloy frame

View Answer

Answer: c [Reason:] The stator is made up of a block of laminations. The block of laminations are made up of cast iron or die cast aluminium alloy frame.

3. What type of coils are used for winding the single phase induction motor generally?
a) rectangular coils
b) square coils
c) cruciform coils
d) circular coils

View Answer

Answer: d [Reason:] The slots house the starting and running windings. The single phase induction motors are generally wound with concentric coils.

4.How many kinds of single phase windings are present?
a) 2
b) 3
c) 4
d) 5

View Answer

Answer: b [Reason:] There are basically 3 kinds of single phase windings. They are concentric, progressive and skein.

5. How are the poles and pitches in the concentric windings?
a) single pole, different pitches
b) different pole, different pitches
c) different pole, single pitch
d) single pole, single pitch

View Answer

Answer: a [Reason:] The concentric windings have a single pole for a common centre. They have different pitches for each individual coil.

6. What is the form of the progressive windings?
a) double layer diamond coil windings
b) single layer diamond coil windings
c) multi layer diamond coil windings
d) three layer diamond coil windings

View Answer

Answer: b [Reason:] The progressive windings is one kind of the stator windings. They are in the form of the single layer diamond coil windings.

7. When is the skein winding made use of?
a) when small amount of relatively small size wire is used
b) when large amount of relatively small size wire is used
c) when large amount of relatively large size wire is used
d) when small amount of relatively large size wire is used

View Answer

Answer: a [Reason:] Skein winding is one of the 3 kinds of single phase windings used. It is used when small amount of relatively small size wire is used.

8. What kind of motor employs the skein winding made use of?
a) maximum horse power single phase induction motor
b) fractional horse power single phase induction motor
c) minimum horse power single phase induction motor
d) zero horse power single phase induction motor

View Answer

Answer: b [Reason:] The skein winding is one of the 3 kinds of single phase induction motor. The skein winding is used when fractional horse power single phase induction motor is used.

9. Which winding is mostly used winding in the single phase induction motor?
a) circular winding
b) concentric winding
c) progressive winding
d) skein winding

View Answer

Answer: b [Reason:] The concentric winding is the most widely used winding. It is also the most flexible winding of the windings used in the single phase induction motor.

10. What is/are the advantages of the skein winding?
a) low cost to wind
b) low cost to insert
c) permits some freedom of choice of distribution
d) low cost to wind, low cost to insert, permits some freedom of choice of distribution

View Answer

Answer: d [Reason:] The skein winding is the low cost to wind and to insert. It also permits some freedom of choice of distribution.

11. What material is used in the tunnel of the rotor of the single phase induction motor?
a) aluminium
b) copper
c) steel
d) wood

View Answer

Answer: a [Reason:] The rotor consists of a block of slotted laminations. The slots form a series of tunnels which are filled with aluminium in its molten state.

12. What type of operations are used in the starting switches?
a) mechanical operation
b) electrical operation
c) centrifugal operation and mechanical operation
d) centrifugal operation

View Answer

Answer: c [Reason:] The starting switch is used to cut the auxillary winding when the motor attains 75% of the full load speed. The switches operate in both the centrifugal as well as mechanical operation.

13. The ac electrolytic capacitor is formed by winding two sheets of etched aluminium foil?
a) true
b) false

View Answer

Answer: a [Reason:] Modern capacitor start motors employ ac electrolytic capacitors. The ac electrolytic capacitor is formed by winding two sheets of etched aluminium foil, separated by two layers of insulating paper, into a cylindrical shape.

14. The electrolytic capacitor and insulator unit is impregnated using ethylene glycol or a derivative?
a) true
b) false

View Answer

Answer: a [Reason:] The electrolytic capacitor and insulator unit is impregnated using the ethylene glycol. It is also impregnated using the derivative of ethylene glycol.

15. What is the range of the power factor of electrolytic capacitors?
a) 2-4
b) 4-6
c) 6-8
d) 7-9

View Answer

Answer: c [Reason:] The minimum power factor of the electrolytic capacitor is 6. The maximum power factor of the electrolytic capacitor is 8.

Set 3

1. What is the range of the flux density in the teeth at no load?
a) 1.7-1.8 Wb per mm2
b) 1.3-1.5 Wb per mm2
c) 1.3-1.6 Wb per mm2
d) 1.4-1.6 Wb per mm2

View Answer

Answer: a [Reason:] The minimum value of the flux density in the teeth is given to be 1.7 Wb per mm2. The maximum value of the flux density in the teeth is given to be 1.8 A per mm2.

2. What is the formula for the minimum width of tooth?
a) minimum width of tooth = flux * pole proportion * (number of stator slots / number of poles) * length * 1.8
b) minimum width of tooth = flux / pole proportion * (number of stator slots / number of poles) * length * 1.8
c) minimum width of tooth = flux * pole proportion / (number of stator slots / number of poles) * length * 1.8
d) minimum width of tooth = flux * pole proportion * (number of stator slots / number of poles) * length / 1.8

View Answer

Answer: b [Reason:] The flux value, pole proportion and the length values are first obtained. Then the ratio of the number of stator slots to number of poles is obtained and on substitution gives the minimum width of tooth.

3. Name the slots that are commonly used.
a) parallel sided
b) square sided
c) rectangular
d) circular

View Answer

Answer: a [Reason:] The most commonly used type of slots are parallel sided. The other type of slots may be used for the purposes required.

4. How is the teeth and the minimum width designed in the machines?
a) teeth is tapered and minimum width is across the medium
b) teeth is sharpened and minimum width occurs across the air gap
c) teeth is widened and minimum width occurs across the air gap
d) teeth is reduced and minimum width occurs across the medium

View Answer

Answer: a [Reason:] Parallel sided slots are made use of. Hence, the teeth is tapered and their minimum width occurs at the air gap surface.

5. What is the formula for the maximum permissible width of slot?
a) maximum permissible width = slot pitch * minimum width of the teeth
b) maximum permissible width = slot pitch + minimum width of the teeth
c) maximum permissible width = slot pitch / minimum width of the teeth
d) maximum permissible width = slot pitch – minimum width of the teeth

View Answer

Answer: d [Reason:] The slot pitch is first calculated with its respective formula. Next the minimum width of the teeth is calculated and the difference between both gives the maximum permissible width.

6. By how much should the depth of slot not exceed the width?
a) two times
b) three times
c) four times
d) six times

View Answer

Answer: b [Reason:] The depth of the slot depends upon the width of the slot. The depth should not exceed three times the width of the slot.

7. Why are slot made deeper in the machine?
a) to increase the short circuit current
b) to reduce the short circuit current
c) to increase the open circuit current
d) to reduce the open circuit current

View Answer

Answer: b [Reason:] The slots used in the machine are basically deeper slots. The slots are made deeper even more to increase the leakage reactance and to limit the short circuit current.

8. What is the formula for the height of length of mean turn of armature?
a) length of mean turn = 2*length + 2.5*pole pitch + 0.06 kV + 0.2
b) length of mean turn = 2*length + 2*pole pitch + 0.06 kV + 0.2
c) length of mean turn = 2*length + 2.5*pole pitch – 0.06 kV – 0.2
d) length of mean turn = 2*length – 2.5*pole pitch – 0.06 kV – 0.2

View Answer

Answer: a [Reason:] The length of the slots is obtained along with the pole pitch. The output kV is calculated and on substituting we get the length of mean turn.

9. The flux density in the armature core of salient pole machines lies between 1-1.2 Wb per m2?
a) true
b) false

View Answer

Answer: a [Reason:] The value of depth of core can be calculated by assuming a suitable value of flux density. The value of the flux density varies from 1-1.2 Wb per m2.

10. What is the formula for the depth of armature core?
a) depth of armature core = flux / length of the iron core * flux density
b) depth of armature core = flux * length of the iron core * flux density
c) depth of armature core = flux / 2 * length of the iron core * flux density
d) depth of armature core = flux * 2 * length of the iron core * flux density

View Answer

Answer: c [Reason:] The flux value is calculated along with the length of the iron core. Next the suitable flux density is chosen and the depth of armature core is calculated.

11. What is the formula for the outer diameter of the stator?
a) outer diameter = inner diameter + depth of the slots + depth of armature core
b) outer diameter = inner diameter + 2*depth of the slots + depth of armature core
c) outer diameter = inner diameter + 2*(depth of the slots + depth of armature core)
d) outer diameter = inner diameter + depth of the slots + 2*depth of armature core

View Answer

Answer: c [Reason:] The depth of the armature core is calculated and the depth of the slots is also calculated. The inner diameter is calculated and on substituting the outer diameter is obtained.

Set 4

1. What type of winding is generally used for the stators?
a) double layer wave winding
b) double layer lap winding
c) single layer wave winding
d) single layer lap winding

View Answer

Answer: b [Reason:] The double layer wave winding is generally used for stators. The wave winding is with diamond coils is used for stators.

2. What type of winding is made use of small motors?
a) single layer mush winding
b) single layer lap winding
c) single layer wave winding
d) double layer wave winding

View Answer

Answer: a [Reason:] Small motors consisting of small number of slots have a large number of turns per phase. These small motors use single layer mush windings.

3. What class does the slot and phase insulation belong to?
a) B
b) Y
c) H
d) E

View Answer

Answer: d [Reason:] The modern insulating materials for diamond coils belong to class E,B and F. The slot and phase insulation is Polyester foil coated with compressed fibre for Class E.

4. What class does the plastic foil baked with polyamide fibres belong to?
a) Y
b) B
c) F
d) H

View Answer

Answer: c [Reason:] The modern insulating materials for diamond coils belong to classes E,B, and F. The plastic foil baked with polyamide fibres belong to class F.

5. What is the formula for flux per pole?
a) flux per pole = average magnetic flux * pole pitch * length
b) flux per pole = average magnetic flux / pole pitch * length
c) flux per pole = average magnetic flux * pole pitch / length
d) flux per pole = average magnetic flux * pole pitch + length

View Answer

Answer: a [Reason:] First the average magnetic flux is calculated. Then the pole pitch is calculated and then the length of the pole is calculated.

6. What is the initial assumption for the value of winding factor?
a) 0.9
b) 0.95
c) 0.93
d) 0.92

View Answer

Answer: b [Reason:] The winding factor may be initially assumed as 0.955. It is the value of winding factor for infinitely distributed winding with full pitch coils.

7. What is the formula for stator turns per phase?
a) stator turns per phase = Stator voltage per phase / 4.44 * f * maximum flux / stator winding factor
b) stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux * stator winding factor
c) stator turns per phase = Stator voltage per phase / 4.44 * f * maximum flux * stator winding factor
d) stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux / stator winding factor

View Answer

Answer: c [Reason:] For the finding out of stator turns per phase, first the stator voltage per phase is obtained. Next the maximum flux is calculated, then the stator winding factor is calculated.

8. What should be the range of current density in the stator windings?
a) 2-5 A per mm2
b) 4-5 A per mm2
c) 3-5 A per mm2
d) 2-3 A per mm2

View Answer

Answer: c [Reason:] The minimum value for the current density in stator winding is 3 A per mm2. The maximum value of the current density in the stator windings should not exceed 5 A per mm2.

9. For the lower values of current, round conductors would be convenient to use?
a) true
b) false

View Answer

Answer: a [Reason:] For lower values of current, round conductors would be the most convenient to use while for higher current bars. It should be less than 2 or 3 mm in diameter or else it is difficult to wind.

Set 5

1. What is the formula for output equation in synchronous machines?
a) kVA output = output coefficient * diameter2 * length * synchronous speed
b) kVA output = output coefficient / diameter2 * length * synchronous speed
c) kVA output = output coefficient * diameter2 / length * synchronous speed
d) kVA output = output coefficient * diameter2 * length / synchronous speed

View Answer

Answer: a [Reason:] The output equation is found out by first calculating the output coefficient. Next the diameter and length are obtained, and the synchronous speed is calculated using the tacho-generator to obtain the kVA output.

2. What is the formula of the output coefficient?
a) output coefficient = 11 * specific magnetic loading / specific electrical loading * winding space factor * 10-3
b) output coefficient = 11 / specific magnetic loading * specific electrical loading * winding space factor * 10-3
c) output coefficient = 11 * specific magnetic loading * specific electrical loading * winding space factor * 10-3
d) output coefficient = 11 * specific magnetic loading * specific electrical loading / winding space factor * 10-3

View Answer

Answer: c [Reason:] The output coefficient is one of the terms required in the calculation of the output of the machine. The specific magnetic and electrical loading terms are first calculated along with the winding space factor.

3. What is the formula for the output equation with respect to the peripheral speed?
a) output = 1.11* specific magnetic loading * specific electrical loading * winding space factor * 10-3 * peripheral speed2 *Length * synchronous speed
b) output = 1.11* specific magnetic loading * specific electrical loading * winding space factor * 10-3 * peripheral speed2 *Length / synchronous speed
c) output = 1.11* specific magnetic loading * specific electrical loading * winding space factor / 10-3 * peripheral speed2 *Length * synchronous speed
d) output = 1.110 / specific magnetic loading * specific electrical loading * winding space factor * 10-3 * peripheral speed2 *Length * synchronous speed

View Answer

Answer: b [Reason:] The output equation with respect to the peripheral speed depends on the square of the peripheral speed of the machine. It doesn’t consist of the diameter term in the output equation.

4. How many factors does the choice of specific magnetic loading depend upon?
a) 4
b) 2
c) 5
d) 8

View Answer

Answer: c [Reason:] The choice of specific magnetic loading depends upon 5 factors basically. They are a) Iron Loss, b) Voltage, c) Transient short circuit current, d) Stability, e) Parallel Operation.

5. How is the iron loss related with the choice of specific magnetic loading?
a) choice of magnetic loading is directly proportional to the iron loss
b) choice of magnetic loading is indirectly proportional to the iron loss
c) choice of magnetic loading is directly proportional to the square of the iron loss
d) choice of magnetic loading is indirectly proportional to the square of the iron loss

View Answer

Answer: a [Reason:] The choice of specific magnetic loading is directly proportional to the iron loss. The iron loss increases with the increase in the air gap density.

6. How is the voltage related with the air gap density?
a) air gap density is directly proportional to the voltage
b) air gap density is indirectly proportional to the voltage
c) air gap density is directly proportional to the square of the voltage
d) air gap density is indirectly proportional to the square of the voltage

View Answer

Answer: b [Reason:] The air gap density is indirectly proportional to the voltage. High voltage machine should have low air gap density, to avoid excessive values of flux density in the teeth and core.

7. How is the transient short circuit current related with the air gap density?
a) air gap density is directly proportional to the short circuit current
b) air gap density is indirectly proportional to the short circuit current
c) air gap density is directly proportional to the square of the short circuit current
d) air gap density is directly proportional to the square of the short circuit current

View Answer

Answer: a [Reason:] The air gap density is directly proportional to the short circuit current. The air gap density should be kept low in order to reduce the initial electromagnetic forces under short circuit condition.

8. How is the steady state stability related with the air gap density?
a) air gap density is directly proportional to the steady state stability
b) air gap density is indirectly proportional to the steady state stability
c) air gap density is directly proportional to the square of the steady state stability
d) air gap density is directly proportional to the square of the steady state stability

View Answer

Answer: a [Reason:] The air gap density is directly proportional to the steady state stability. The steady state stability is improved if the air gap density is high.

9. The machines having high air gap density operates poorly when connected in synchronism?
a) true
b) false

View Answer

Answer: b [Reason:] The machines having high air gap density allows high amount of synchronizing power. Thus the machines having high air gap density provides high synchronism.

10. What is the range of the air gap density for salient pole machines?
a) 0.52-0.65 Wb per m2
b) 0.5-0.6 Wb per m2
c) 0.54-0.65 Wb per m2
d) 0.44-0.65 Wb per m2

View Answer

Answer: a [Reason:] The range of air gap density for salient pole machines is 0.52-0.65 Wb per m2. The range of air gap density for turbo-alternators is 0.54-0.65 Wb per m2.

11. How many factors influence the choice of specific electric loading?
a) 2
b) 3
c) 4
d) 5

View Answer

Answer: c [Reason:] There are 4 factors influence the choice of specific electric loading. They are a) Copper loss and temperature rise, b) voltage, c) synchronous reactance, d) stray load loss.

12. How is the specific electric loading related to copper losses and temperature rise?
a) high specific electric loading gives high copper losses and high temperature rise
b) high specific electric loading gives low copper losses and high temperature rise
c) high specific electric loading gives high copper losses and low temperature rise
d) high specific electric loading gives low copper losses and low temperature rise

View Answer

Answer: a [Reason:] The specific electric loading is directly proportional to the copper losses and the temperature rise. The high specific electric loading gives high copper losses and high temperature rise.

13. High value of the specific electric loading can be used for low voltage machines?
a) true
b) false

View Answer

Answer: a [Reason:] High value of specific electric loading can be used for low voltage machines. This is because the space required for insulation is small.

14. How is the specific electric loading related to the synchronous reactance of the machines?
a) specific electric loading is high, leakage reactance is high, giving low synchronous reactance
b) specific electric loading is high, leakage reactance is low, giving low synchronous reactance
c) specific electric loading is high, leakage reactance is high, giving high synchronous reactance
d) specific electric loading is low, leakage reactance is high, giving high synchronous reactance

View Answer

Answer: c [Reason:] The specific electric loading is directly proportional to the synchronous reactance. If the specific electric loading is high, the synchronous reactance becomes high.

15. What is the value of specific electric loading for the salient pole alternators?
a) 20,000-40,000 A per m
b) 50,000-75,000 A per m
c) 25,000-40,000 A per m
d) 20,000-45,000 A per m

View Answer

Answer: a [Reason:] The value of specific electric loading for the salient pole alternators is 20,000-40,000 A per m. The value of specific electric loading for the turbo alternators is 50,000-75,000 A per m.