# Design of Electrical Machines Questions and Answers – Main Dimensions

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This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Main Dimensions”.

1. What are the parameters which come under the term “Main Dimensions”?
a) diameter
b) length
c) diameter and length
d) area

Explanation: The main dimensions entirely deal with the calculation of the diameter and length. They are obtained from the output equation of the machine.

2. What is the range of the ratio of the core length to pole pitch for minimum cost?
a) 1.5-2
b) 1-2
c) 1.0-1.25
d) 2-2.5

Explanation: 1.0-1.25 is the range of the ratio of the core length to pole pitch for good power factor. For minimum cost, 1.5 is the minimum range for the ratio and 2 is the maximum range for the ratio.

3. What is the value of the ratio of the core length to pole pitch for good efficiency?
a) 1
b) 1.5
c) 2
d) 3

Explanation: 1 is the value of the ratio of the core length to pole pitch for good overall design. 1.5 is the value of the ratio of the core length to pole pitch for good efficiency.

4. What is the relation between motors and ratio of core length to pole pitch?
a) for small motors high ratio is preferred
b) for big motors high ratio is preferred
c) for small motors small ratio is preferred
d) for big motors small ratio is preferred

Explanation: For small motors, high values of the ratio of core length to pole pitch which may not be able to accommodate even a small amount of slots. Thus small motors prefer only small ratio to accommodate slots.

5. What are the factors the value of core length to pole pitch depend upon?
a) area of the slots
b) size of machine
c) size of conductors
d) size of machine, minimum cost, good efficiency

Explanation: The value of core length to pole pitch varies from 0.6 to 2. It depends on the cost, efficiency, power factor, overall design.

6. What is the Relation between pole pitch and the core length in terms of the best power factor?
a) pole pitch = (0.18 * core length)3
b) pole pitch = (0.18 * core length)2
c) pole pitch = (0.18 * core length)1/2
d) pole pitch = (0.18 * core length)1/3

Explanation: For the best power factor, the pole pitch is equal to the square root of the product of 0.18 and core length. This value however is not dimensionally correct and is valid for the values in meters.

7. What is the range of the permissible peripheral speeds in the 3 phase induction machine?
a) 60-75 m per s
b) 60-70 m per s
c) 40-70 m per s
d) 50-70 m per s

Explanation: For the standard rotor construction can generally be used for peripheral speeds upto 60 m per s. For special rotor construction, the peripheral speeds upto 75 m per s are permissible.

8. What is the maximum permissible level for the peripheral speed for a normal design?
a) < 30 m per s
b) > 30 m per s
c) <=30 m per s
d) >=30 m per s

Explanation: For the normal design, the diameter value is chosen such that the peripheral speed value doesn’t exceed 30 m per s. The diameter value is directly proportional to the peripheral speed.

9. What is the range of the core length for which the stator is provided with the ventilating ducts?
a) 105-120 mm
b) 100-120 mm
c) 100-150 mm
d) 100-125 mm

Explanation: The stator is provided with the ventilating ducts if the core length exceeds 100-125 mm. If the value is less than 100 mm, then no ventilating ducts are provided.

10. The width of each duct is about 8 to 10 mm.
a) true
b) false 