# Design of Electrical Machines Questions and Answers – Relation Between Rating, Dimensions, Variation of Output of Rotating Machines

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This set of Design of Electrical Machines online quiz focuses on “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

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

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

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.
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4. How many terms can be used to express the output of a machine?
a) 3
b) 4
c) 5
d) 6

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

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.
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6. How many factors are used to determine the specific magnetic loading?
a) 2
b) 3
c) 4
d) 5

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.

7. How many factors are used to determine the specific electric loading?
a) 2
b) 3
c) 4
d) 5

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

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

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

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 I2R loss?
a) I2R loss = number of conductors / copper loss in each conductor
b) I2R loss = number of conductors + copper loss in each conductor
c) I2R loss = number of conductors – copper loss in each conductor
d) I2R loss = number of conductors * copper loss in each conductor

Explanation: The number of conductors and copper loss in each conductor is first calculated. On substitution the I2R 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 – output + losses
d) efficiency = output + output + losses

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

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.

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