# Design of Electrical Machines Questions and Answers – Design of Rotor – 1

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design of Rotor – 1”.

1. How many factors does the design of rotor of synchronous machines depend upon?
a) 2
b) 3
c) 4
d) 5

Explanation: There are 4 factors which are associated with the design of rotor in the synchronous machines. They are height of pole, design of damper windings, height of pole shoe, pole profile drawing.

2. What is the formula for the flux in pole body?
a) flux in pole body = leakage coefficient * useful flux per pole
b) flux in pole body = leakage coefficient / useful flux per pole
c) flux in pole body = leakage coefficient – useful flux per pole
d) flux in pole body = leakage coefficient + useful flux per pole

Explanation: The leakage coefficient is obtained first from its formula. Next, the value of useful flux per pole is calculated and this gives the flux in pole body value.

3. What is the range of the permissible values of the flux densities in pole body?
a) 1.4-1.7 Wb per m2
b) 1.5-1.7 Wb per m2
c) 1.4-1.6 Wb per m2
d) 1.5-1.6 Wb per m2

Explanation: The minimum value of the flux density in the pole body is given to be 1.5 Wb per m2.The maximum permissible value of the flux density in the pole body is given to be 1.7 Wb per m2.

4. What is the range of the leakage coefficient in the pole body?
a) 1.1 to 1.2
b) 1.00 to 1.5
c) 1.15 to 1.2
d) 0.75 to 2.3

Explanation: The minimum value of the leakage coefficient in the pole body is 1.15. The maximum value of the leakage coefficient in the pole body is 1.2.

5. What is the formula for the area of cross-section of pole body for rectangular poles?
a) area of cross section of pole body = 0.98 * axial length of the pole * breadth of the pole
b) area of cross section of pole body = 0.98 / axial length of the pole * breadth of the pole
c) area of cross section of pole body = 0.98 * axial length of the pole / breadth of the pole
d) area of cross section of pole body = 1/0.98 * axial length of the pole * breadth of the pole

Explanation: The axial length of the pole and the breadth of the pole are calculated. Next by multiplying the two values with the stacking factor, we get the area of cross section of pole body.
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6. What is the formula for the copper area of the field windings?
a) copper area = full load field mmf * current density in the field winding
b) copper area = full load field mmf / current density in the field winding
c) copper area = full load field mmf + current density in the field winding
d) copper area = full load field mmf – current density in the field winding

Explanation: For the calculation of the copper area, first the current density in the field winding is calculated. Next the full load field mmf is calculated and the ratio gives the copper area of field windings.

7. What is the formula for the total space required for the winding?
a) total space = copper area + space factor
b) total space = copper area – space factor
c) total space = copper area / space factor
d) total space = copper area * space factor

Explanation: The copper area is calculated from its respective formula. Then the space factor is calculated and the ratio gives the value of total space.

8. What is the value of space factor for the strip on edge winding?
a) 0.8-0.9
b) 0.4
c) 0.65
d) 0.75

Explanation: The space factor for the strip on edge winding is 0.8-0.9. The space factor for small round wires is 0.4 and for large round wires it is 0.65. The space factor for large rectangular conductors is 0.75.

9. What is the formula for the height of winding?
a) height of winding = total winding area / depth of winding
b) height of winding = total winding area * depth of winding
c) height of winding = total winding area + depth of winding
d) height of winding = total winding area – depth of winding

Explanation: The total winding area is first calculated. Next the depth of the winding is calculated. The ratio of both gives the height of winding.

10. What is the formula for the radial length of the pole shoe?
a) radial length of the pole shoe = height of winding – height of pole shoe – 0.02
b) radial length of the pole shoe = height of winding + height of pole shoe – 0.02
c) radial length of the pole shoe = height of winding – height of pole shoe + 0.02
d) radial length of the pole shoe = height of winding + height of pole shoe + 0.02

Explanation: First the height of the winding is calculated from its formula. Next the height of pole shoe is calculated. Both the values are added with 0.02 to give the radial length of the pole shoe.

11. What is the formula for the height of pole body?
a) height of pole body = height of the winding + 0.02
b) height of pole body = height of the winding * 0.02
c) height of pole body = height of the winding – 0.02
d) height of pole body = height of the winding / 0.02

Explanation: The height of the pole body is one of the design factors in the design of rotor. It is obtained by adding the value of the height of winding with 0.02, which is the approximate space occupied by flanges.

12. What is the range of the ratio of radial length of pole to pole pitch?
a) 0.3-1
b) 0.3-1.5
c) 0.7-1
d) 0.7-1.5

Explanation: The minimum value of the ratio of radial length of pole to pole pitch is given to be 0.3. The maximum value of the ratio of radial length of pole to pole pitch is given to be 1.5.

13. The damper windings are made use of in synchronous generators to reduce the oscillations and to prevent hunting.
a) true
b) false

Explanation: The purpose of the damper windings is to reduce the oscillations and to prevent the hunting in synchronous generators. Next, the damper windings are used to suppress the negative sequence field in the synchronous generator.

14. The mmf of the damper windings depends on the pole pitch value.
a) true
b) false

Explanation: The mmf of the damper windings depends on the pole pitch value. The value for the mmf of the damper windings = 0.143 * specific electric loading * pole pitch.

15. What is the formula for the area per pole of damper pass provided?
a) area per pole of damper pass = 0.2 * specific electric loading * pole pitch * current density in damper bars
b) area per pole of damper pass = 0.2 * specific electric loading * pole pitch / current density in damper bars
c) area per pole of damper pass = 0.2 * specific electric loading – pole pitch / current density in damper bars
d) area per pole of damper pass = 0.2 + specific electric loading * pole pitch / current density in damper bars

Explanation: The specific electric loading and the pole pitch is calculated first. Next the current density in damper bars is next calculated. Substituting in the above formula gives the area per pole of damper pass provided.

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