**Design Of Electrical Machines MCQs – ****General Concepts and Constraints in Design of Rotating Machines MCQs ( Design Of Electrical Machines ) MCQs**

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**Design Of Electrical Machines MCQs – ****General Concepts and Constraints in Design of Rotating Machines ****MCQs ( Design Of Electrical Machines ) MCQs**

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# Relation Between Rating, Dimensions, Variation of Output of Rotating Machines

**1. How many factors are present in the relation between rating and dimension of rotating machines?**

a) 6

b) 7**c) 8**

d) 9**Answer: c****Explanation: There are 8 factors present in the relation between rating and dimension of rotating machines. They are symbols, main dimensions, total loadings, specific loadings, output equation, factors affecting size of rotating machine, choice of specific magnetic loading, choice of specific electric loading.****2. What is the formula of the total magnetic loading?****a) total magnetic loading = total flux around the armature periphery at the air gap**

b) total magnetic loading = total number of ampere conductors around the armature for periphery

c) total magnetic loading = number of poles/armature total flux

d) total magnetic loading = total flux/number of poles**Answer: a****Explanation: There are 2 types of loadings in the total loadings which are total magnetic loading and total electric loading. The total magnetic loading is the total flux around the armature periphery at the air gap.****3. What is the formula of the specific electric loading?**

a) specific electric loading = total armature ampere conductors * armature periphery at air gap

b) specific electric loading = total flux around the air gap / area of flux path at the air gap**c) specific electric loading = total armature ampere conductors / armature periphery at air gap**

d) specific electric loading = total flux around the air gap * area of flux path at the air gap**Answer: c****Explanation: The total flux around the air gap and the area of flux path at the air gap is calculated. On substitution, the specific electric loading is calculated.****4. How many terms can be used to express the output of a machine?**

a) 3**b) 4**

c) 5

d) 6**Answer: b****Explanation: There are 4 terms that can be used to express the output of a machine. They are main dimension, specific magnetic loading, specific electric loading, speed.****5. How many factors affecting the size of rotating machines?**

a) 3

b) 4

c) 5**d) 2****Answer: d****Explanation: There are 2 factors that affect the size of rotating machines. They are speed of the machines and the output coefficient of the machines.****6. How many factors are used to determine the specific magnetic loading?**

a) 2**b) 3**

c) 4

d) 5**Answer: b****Explanation: There are 3 factors are used to determine the specific magnetic loading. They are maximum flux density in iron parts of machine, magnetizing current and core losses.**

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Fundamental Aspects Of Electrical Machine Design MCQs

**7. How many factors are used to determine the specific electric loading?**

a) 2

b) 3**c) 4**

d) 5**Answer: c****Explanation: There are 4 factors used to determine the specific electric loading. They are temperature rise, cooling coefficient, size of machine, current density.****8. What is the relation between the output and the flux density?****a) flux density is directly proportional to the output**

b) flux density is indirectly proportional to the output

c) flux density is directly proportional to the square of the output

d) flux density is indirectly proportional to the square of the output**Answer: a****Explanation: Flux density is one of the factors used in the determination of the specific magnetic loading. The flux density is directly proportional to the output.****9. What is the relation between output and the specific electric loading?****a) specific electric loading is directly proportional to the output**

b) specific electric loading is indirectly proportional to the output

c) specific electric loading is directly proportional to the square of the output

d) specific electric loading is indirectly proportional to the square of the output**Answer: a****Explanation: The choice of specific electric loading is one of the factors under the relation between rating and dimensions of rotating machines. The specific electric loading is directly proportional to the output.****10. What is the relation of the specific electric loading and the diameter?**

a) specific electric loading is directly proportional to the diameter**b) specific electric loading is indirectly proportional to the diameter**

c) specific electric loading is directly proportional to the square of the diameter

d) specific electric loading is indirectly proportional to the square of the diameter**Answer: b****Explanation: The specific electric loading is one of the factors under the relation between rating and dimension of rotating machines. The specific electric loading is indirectly proportional to the diameter.****11. What is the formula of the I ^{2}R loss?**

a) I

^{2}R loss = number of conductors / copper loss in each conductor

b) I

^{2}R loss = number of conductors + copper loss in each conductor

c) I

^{2}R loss = number of conductors â€“ copper loss in each conductor

**d) I**

^{2}R loss = number of conductors * copper loss in each conductor**Answer: d**

**Explanation: The number of conductors and copper loss in each conductor is first calculated. On substitution the I**

^{2}R loss is obtained.**12. What is the formula of the efficiency of the machine?**

**a) efficiency = output / output + losses**

b) efficiency = output * output + losses

c) efficiency = output â€“ output + losses

d) efficiency = output + output + losses

**Answer: a**

**Explanation: The output is first calculated from the running of the machine and the different losses are calculated. On substitution the efficiency of the machine is calculated.**

**13. The fractional horsepower motors have efficiency of order of 98%.**

a) true

**b) false**

**Answer: b**

**Explanation: The fractional horse power motors have efficiency of order of 60% or less. The large turbo-alternators have efficiency of order of 98% because efficiency increases with an increase in dimensions.**

# Separation of D and L, Standard Frames

**1. What is the formula of the core length?**

a) core length = pole length * pole pitch**b) core length = pole length / pole pitch**

c) core length = pole length + pole pitch

d) core length = pole length â€“ pole pitch**Answer: b****Explanation: Core length is one of the factors used in the separation of D and L of the DC machines. It is the ratio of the pole length to the pole pitch.****2. What is the range of the ratio of the pole length to pole pitch?**

a) 0.60-0.70**b) 0.64-0.72**

c) 0.65-0.75

d) 0.70-0.80**Answer: b****Explanation: The minimum value of the ratio of the pole length to pole pitch is 0.64. The maximum value of the ratio of the pole length to pole pitch is 0.72.****3. What is the maximum value of the peripheral speed that should not exceed?**

a) 25 m per sec

b) 20 m per sec**c) 30 m per sec**

d) 35 m per sec**Answer: c****Explanation: The peripheral speed of the machine should not exceed 30 m per sec. If it exceeds it leads to the damage of the windings. The banding wires on the overhang have to be made so strong if the peripheral speed exceeds 30 m per sec.****4. What is the range of the pole length to pole pitch ratio for obtaining good power factor in induction motors?**

a) 1.5-2

b) 1.3â€“1.8**c) 1.0-1.25**

d) 1.1-1.6**Answer: c****Explanation: The range of the pole length to pole pitch ratio should be between 1.0-1.25 for the good power factor. The range of the pole length to pole pitch ratio should be between 1.5-2 for minimum cost.****5. What is the relationship between the diameter and number of poles?****a) diameter is directly proportional to the number of poles**

b) diameter is indirectly proportional to the number of poles

c) diameter is directly proportional to the square of the number of poles

d) diameter is indirectly proportional to the square of the number of poles**Answer: a****Explanation: The number of poles is one of the factors involved in the separation of D and L in the synchronous machines. The number of poles is directly proportional to the diameter.****6. The core length should be high to obtain high short circuit ratio.****a) true**

b) false**Answer: a****Explanation: The core length is directly proportional to the short circuit ratio. If high short circuit ratio is required, high core length should be used.****7. What is the factor which is by modern motors which are into a series of standard frames?**

a) current rating

b) voltage rating**c) power rating**

d) output rating**Answer: c****Explanation: The modern motors for industrial applications is concentrated into a series of standard frames. The standard frames are used to cover a wide range of power ratings.****8. What is the frame used in the standard frames?**

a) set of standard values

b) mechanical structure required to house a rotor of given outside diameter**c) mechanical structure required to house a stator of given outside diameter**

d) mechanical structure required to house a stator of given outside length**Answer: c****Explanation: The frame is a mechanical structure required to house a stator of given outside diameter. The frame also includes housing the bearings, end covers, terminal box and maximum length.****9. How can the variation in ratings be obtained?**

a) alternative windings**b) alternative core lengths**

c) alternative pole pitch

d) alternative pole length**Answer: b****Explanation: The frame is a mechanical structure required to house a stator of given outside diameter, housing the bearings, end covers, terminal box and maximum length. The variation in the ratings can be obtained using alternative core lengths.****10. What is the value of the alternative core lengths below which variation in rating can be obtained?**

a) 0.7 * length

b) 0.4 * length

c) 0.3 * length**d) 0.7 * length or 0.5 * length****Answer: d****Explanation: The variation in the ratings can be obtained using alternative core lengths. The value of the alternative core lengths below 0.5 * length or 0.7 * length provides the variation in the ratings.****11. What is the range of the central heights used in the standard frames?**

a) 50-1000 mm

b) 90-1000 mm

c) 100-1000 mm**d) 56-1000 mm****Answer: d****Explanation: The modern motors of small and medium sizes are built with standard frame sizes as specified in IEC 72. The IEC 72 lists a coherent range of main dimensions with the central heights ranging between 56-1000 mm.****12. The frame size is designated by a number which is its centre height expressed in mm.****a) true**

b) false**Answer: a****Explanation: The IEC 72 lists a coherent range of main dimensions with the central heights ranging between 56-1000 mm. The frame size is designated by a number which is its centre height expressed in mm. The frame designated 180 has a centre height of 180 mm.**