This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Frames of D.C. & A.C. Machines”.

1. What is the work of the frame of dc machines?

a) to reduce the voltage

b) to reduce the flux

c) to carry the flux

d) to carry the current

View Answer

Explanation: The main function of the frame of dc machines is to carry the flux. Thus the frame must be large enough to carry flux.

2. Why is the length of the yoke made larger?

a) to protect the armature windings

b) to cover the armature windings

c) to cover the field windings

d) to cover and protect the field windings

View Answer

Explanation: The length of the yoke is usually made larger than the pole cores. It is because to cover and protect the field windings.

3. What is the formula for the depth of the yoke?

a) depth of yoke = thickness/2

b) depth of yoke = thickness

c) depth of yoke = 2*thickness

d) depth of yoke = 3*thickness

View Answer

Explanation: The depth of yoke is equal to the thickness of the yoke. It is calculated to give the required cross-section for the magnetic circuit.

4. In large machines, the thickness is relatively larger to the diameter.

a) true

b) false

View Answer

Explanation: The thickness is used in the calculation of the depth of the yoke. In large machines, the thickness is relatively smaller to the diameter.

5. What is the formula in order to check the rigidity?

a) moment of inertia ≥ (weight of magnetic frame * radius^{2} * 10^{-6}) / 225

b) moment of inertia ≤ (weight of magnetic frame * radius^{2} * 10^{-6}) / 225

c) moment of inertia = (weight of magnetic frame * radius^{2} * 10^{-6}) / 225

d) moment of inertia < (weight of magnetic frame * radius^{2} * 10^{-6}) / 225

View Answer

Explanation: The moment of inertia, the weight of magnetic frame and the radius is calculated first. The machine is highly rigid if the moment of inertia is greater than or equal to the product of weight of magnetic frame and square of radius divided by 225.

6. What is the formula for the thickness of the ac machines?

a) thickness = 40 * inner diameter of frame/12

b) thickness = 40 + inner diameter of frame/12

c) thickness = 40 – inner diameter of frame/12

d) thickness = 40 * inner diameter of frame*12

View Answer

Explanation: The thickness of the ac machines depend upon the inner diameter of the frame. On obtaining the inner diameter of frame and on substitution gives the thickness of ac machines.

7. What is the formula for the breadth of the ac machine?

a) breadth = 6 + 0.01 * inner diameter of frame

b) breadth = 6 – 0.01 * inner diameter of frame

c) breadth = 6 * 0.01 * inner diameter of frame

d) breadth = 6 / 0.01 * inner diameter of frame

View Answer

Explanation: The breadth of the ac machines also depends upon the inner diameter of the frame. On substituting the values the breadth is calculated.

8. What is the formula for the checking of rigidity of induction machines?

a) moment of inertia ≥ radius / length of stator core * 90

b) moment of inertia ≥ radius * length of stator core * 90

c) moment of inertia ≥ radius * length of stator core / 90

d) moment of inertia ≤ radius / length of stator core * 90

View Answer

Explanation: The radius and length of the stator core along with the moment of inertia is calculated. If the moment of inertia is greater than or equal to the product of length and radius divided by 90, the machine is more rigid.

9. What is the formula for the radius at the centre of gravity?

a) radius at the centre of gravity = inner diameter^{1.5}/6.3

b) radius at the centre of gravity = inner diameter^{2}/6.3

c) radius at the centre of gravity = outer diameter^{1.5}/6.3

d) radius at the centre of gravity = outer diameter^{2}/6.3

View Answer

Explanation: The outer diameter of stator core is first calculated. On substituting the values the radius at the centre of gravity is obtained.

10. What is the formula of the centrifugal force?

a) centrifugal force = weight of revolving body * 39.43 * speed^{2} * radius of circular path

b) centrifugal force = weight of revolving body / 39.43 * speed^{2} * radius of circular path

c) centrifugal force = weight of revolving body * 39.43 / speed^{2} * radius of circular path

d) centrifugal force = weight of revolving body * 39.43 * speed^{2} / radius of circular path

View Answer

Explanation: The weight of revolving body, speed, radius of circular path is calculated. On substitution the centrifugal force is obtained.

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