# Design of Electrical Machines Questions and Answers – Design of Field Winding – 2

This set of Tricky Design of Electrical Machines Questions and Answers focuses on “Design of Field Winding – 2”.

1. What is the formula for the winding height in the design of the field windings?
a) winding height = height of the pole – height of shoe + space taken by the spool, flanges, etc
b) winding height = height of the pole + height of shoe + space taken by the spool, flanges, etc
c) winding height = height of the pole + height of shoe – space taken by the spool, flanges, etc
d) winding height = height of the pole – height of shoe – space taken by the spool, flanges, etc

Explanation: The height of the pole and the height f the shoe is calculated. Next the space taken by the spool, flanges is calculated and the winding height is obtained.

2. What is the approximate value of the space taken by spools, flanges, etc?
a) 15 mm
b) 10 mm
c) 12 mm
d) 20 mm

Explanation: The value of the space taken by spools, flanges are required for the calculation of the winding height. The approximate value of the space taken by spools, flanges, etc. is 20 mm.

3. What is the winding depth for the pole pitch of 0.1 mm?
a) 25 mm
b) 35 mm
c) 45 mm
d) 50 mm

Explanation: The winding depth is 25 mm for the pole pitch of 0.1 mm. The winding depth is 35 mm for the pole pitch of 0.2 mm. The winding depth is 45 mm for the pole pitch is 0.3 mm.

4. What is the formula for the voltage across each field coil?
a) voltage across each field coil = field current * resistance of each field at 75°C
b) voltage across each field coil = field current / resistance of each field at 75°C
c) voltage across each field coil = field current + resistance of each field at 75°C
d) voltage across each field coil = field current – resistance of each field at 75°C

Explanation: The field current is first calculated from the machine. Next the resistance value of each field at 75°C is calculated and this gives the voltage across each field coil.

5. What is the range of the current density in the field conductors?
a) 3 to 5 A per mm2
b) 3 to 4 A per mm2
c) 4 to 5 A per mm2
d) 3 to 6 A per mm2

Explanation: The minimum range of the current density in the field conductors is 3 A per mm2. The maximum value of the current density in the field conductors is 4 A per mm2.
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6. What is the formula for the field current of the synchronous machines?
a) field current = current density * area of conductors
b) field current = current density / area of conductors
c) field current = current density – area of conductors
d) field current = current density + area of conductors

Explanation: For the calculation of the field current, first the current density is calculated. Next, the area of conductors is calculated and the field current is calculated.

7. What is the formula for the number of field turns of the field windings?
a) number of field turns = field mmf per pole at full load * field current
b) number of field turns = field mmf per pole at full load / field current
c) number of field turns = field mmf per pole at full load + field current
d) number of field turns = field mmf per pole at full load – field current

Explanation: The field mmf per pole at full load is calculated from the voltage across each field coil. Next, the field current is calculated and from these values the number of field turns is calculated.

8. What is the relation between winding space and the depth?
a) winding space is directly proportional to the depth
b) winding space is indirectly proportional to the depth
c) winding space is directly proportional to the square of the depth
d) winding space is indirectly proportional to the square of the depth

Explanation: The winding space is indirectly proportional to the depth. If the winding space is less, then the depth is increased.

9. What is the formula of the resistance of the winding is calculated at 75°C?
a) resistance of the winding = (Number of field turns * pole proportion * length of mean turns of the coil) / area of the field conductors
b) resistance of the winding = (Number of field turns * pole proportion * length of mean turns of the coil) * area of the field conductors
c) resistance of the winding = (Number of field turns / pole proportion * length of mean turns of the coil) / area of the field conductors
d) resistance of the winding = (Number of field turns * pole proportion / length of mean turns of the coil) / area of the field conductors

Explanation: The number of field turns is calculated along with the pole proportion. The length of mean turns of the coil and the area of the field conductors is calculated and on substituting the values the resistance of the winding is obtained.

10. What is the formula of the dissipating surface of the coil?
a) dissipating surface of the coil = 2*length of mean turns of the coil*(winding height * diameter of winding)
b) dissipating surface of the coil = 2*length of mean turns of the coil*(winding height / diameter of winding)
c) dissipating surface of the coil = 2*length of mean turns of the coil*(winding height + diameter of winding)
d) dissipating surface of the coil = 2*length of mean turns of the coil/(winding height * diameter of winding)

Explanation: First the length of the mean turns of the coil is calculated. Then the winding height and the diameter of the winding is calculated to obtain the dissipating surface of the coil.

11. What is the formula for the cooling co-efficient to the rotating field coils?
a) cooling coefficient of rotating field coils = 0.05 to 0.08 / 1 + armature voltage
b) cooling coefficient of rotating field coils = 0.05 to 0.08 / 1 – armature voltage
c) cooling coefficient of rotating field coils = 0.08 to 0.12 / 1 + armature voltage
d) cooling coefficient of rotating field coils = 0.08 to 0.12 / 1 – armature voltage

Explanation: The armature voltage is first calculated for the calculation of the cooling coefficient of rotating field coils. The cooling coefficient is used to calculate the temperature rise.

12. What is the formula for the temperature rise in the design of field windings?
a) temperature rise = 1 / copper loss in each field coil at 75°C * cooling coefficient of rotating field coils * dissipating surface of the coil
b) temperature rise = copper loss in each field coil at 75°C * cooling coefficient of rotating field coils * dissipating surface of the coil
c) temperature rise = copper loss in each field coil at 75°C / cooling coefficient of rotating field coils * dissipating surface of the coil
d) temperature rise = copper loss in each field coil at 75°C * cooling coefficient of rotating field coils / dissipating surface of the coil

Explanation: The copper loss in each field coil is first calculated using its formula. Next, the cooling coefficient of rotating field coils is calculated. Finally dissipating surface of the coil is calculated and this gives the temperature rise.

13. If the temperature increases beyond the acceptable limits the depth of the winding should be decreased.
a) true
b) false

Explanation: The temperature rise = copper loss in each field coil at 75°C * cooling coefficient of rotating field coils / dissipating surface of the coil is calculated. If the temperature rise crosses the specified limits, the depth of the winding is increased.

14. The increase in the depth of the winding increases the heat dissipating surface.
a) true
b) false

Explanation: The increase in the depth of the windings increase the heat dissipating surface. The increase in the heat dissipation decreases the temperature rise.

15. What is the minimum clearance between adjacent field coils and pole drawing?
a) 14 mm
b) 15 mm
c) 13 mm
d) 12 mm

Explanation: The final step in the design of field windings is the checking of the clearance between the adjacent field coils and pole drawing. The minimum value of clearance should be 15 mm.

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