# Design of Electrical Machines Questions and Answers – Separation of D and L, Standard Frames

This set of Design of Electrical Machines Questions and Answers for Entrance exams focuses on “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 – pole pitch

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

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

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) 1.3–1.8
c) 1.0-1.25
d) 1.1-1.6

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

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.
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6. The core length should be high to obtain high short circuit ratio.
a) true
b) false

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

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

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

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

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

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

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.

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