# Design of Electrical Machines Questions and Answers – Bracing of Rotor Windings

This set of Design of Electrical Machines Questions and Answers for Aptitude test focuses on “Bracing of Rotor Windings”.

1. What is the use of the wire bands of rotor?
a) used for bracing the rotor windings
b) used for circulating the current in the rotor windings
c) used for the encircling of the rotor windings
d) used for the protecting the rotor windings

Explanation: Bands used on the rotors of electrical machines are intended for bracing the rotor windings. This is done against their shift in the radial direction under action of centrifugal forces.

2. Where are the wire bands placed?
a) active portions of rotor conductors
b) inactive portions of rotor conductors
c) active or inactive portions of rotor conductors
d) active and inactive portions of the rotor conductors

Explanation: The wire bands are placed on the active portions of the rotor conductors. They are also placed in the inactive portions of the rotor conductors.

3. What are the factors on which the sizes of bands placed on depend?
a) length of air gap
b) method of cooling of armatures
c) length of air gap and method of cooling of armatures
d) method of cooling of armatures or length of air gap

Explanation: The sizes of bands placed on the active portions of the conductors depend upon the length of air gap. They also depend upon the method of cooling of armatures.

4. In what machines are the wire bands along the active length of windings placed?
a) dc or ac machines
b) dc and ac machines
c) dc machines
d) ac machines

Explanation: Wire bands are generally placed on both the active and inactive portions of rotor conductors. The wire bands along the active length of windings are placed along the dc machines.

5. What is the range of the width of each band that should not be exceeded?
a) 10-15 mm
b) 15-20 mm
c) 20-25 mm
d) 18-23 mm

Explanation: Bands placed along the active length of windings are housed in the ring slots. The width of each band should not exceed 15 to 20 mm.

6. What is the maximum value above which the total width of the bands should not exceed?
a) 25-35% of the axial length of armature core
b) 30-35% of the axial length of armature core
c) 25-30% of the axial length of armature core
d) 35-40% of the axial length of armature core

Explanation: The total width should not exceed 25% of the axial length of the armature core. The total width should not exceed maximum of 35% of the axial length of the armature core.

7. What is the formula for the breadth of the ring slot?
a) breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire – 2*constant
b) breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire + 2*constant
c) breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire * 2*constant
d) breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire / 2*constant

Explanation: The number of turns in a band is first calculated along with the diameter of band wire. The value of constant is just fixed and on substitution gives the breadth of the ring slot.

8. What is the value of the constant used in the calculation of the breadth of the ring slot for the diameter of band wire < 1.5 mm?
a) 1 mm
b) 1.5 mm
c) 2 mm
d) 3 mm

Explanation: The value of the constant used in the calculation of the breadth of the ring slot is 1 mm for the diameter of band wire < 1.5 mm. The value of the constant used in the calculation of the breadth of the ring slot is 1.5 mm for the diameter of band wire > 1.5 mm.

9. What is the maximum width of the bands placed on the end windings of induction machines and high speed dc machines?
a) 30 mm
b) 35 mm
c) 40 mm
d) 45 mm

Explanation: The maximum width are obtained for the bands placed on the end windings of induction machines and high speed dc machines. The maximum width is 40 mm.

10. What is the diameter of the wire bands made of tin, steel or bronze wire?
a) 2 mm
b) 1 mm
c) 4 mm
d) 3 mm

Explanation: The wire bands are generally made up of tin, steel or bronze wires. The diameter of those wire bands are 3 mm.

11. What is the function of the bands when it is placed on overhang?
a) used to reduce the centrifugal forces
b) used to increase the centrifugal forces
c) used to balance the centrifugal forces
d) used to withstand the centrifugal forces

Explanation: The bands when placed on overhead only are used to withstand the centrifugal forces. The centrifugal forces are due to the weight of the overhang.

12. What is the function of the bands when they are distributed along the axial length of armature?
a) used to reduce the centrifugal forces
b) used to increase the centrifugal forces
c) used to decrease the centrifugal forces
d) used to withstand the centrifugal forces

Explanation: The bands are distributed along the axial length of the armature and they withstand the centrifugal forces. The centrifugal forces are due to the weight of both the active and inactive parts of armature.

13. What is the formula of the mean diameter at the position of centre of gravity?
a) mean diameter at the position of centre of gravity = Inner diameter + diameter of stator wires
b) mean diameter at the position of centre of gravity = Inner diameter * diameter of stator wires
c) mean diameter at the position of centre of gravity = Inner diameter / diameter of stator wires
d) mean diameter at the position of centre of gravity = Inner diameter – diameter of stator wires

Explanation: The inner diameter and the diameter of the stator wires is first calculated. Then on substitution gives the mean diameter at the position of centre of gravity.

14. What is the value of permissible stress for bronze wire for the diameter of branding wire of 1 mm?
a) 350 NM per m2
b) 250 NM per m2
c) 300 NM per m2
d) 450 NM per m2

Explanation: The permissible stress for bronze wire for the diameter of branding wire of 1 mm is 350 NM per m2. The permissible stress for bronze wire for the diameter of branding wire of 1.5 mm is 300 NM per m2.

15. What is the value of permissible stress for steel wire for the diameter of branding wire of 0.5-1.2 mm?
a) 570 NM per m2
b) 600 NM per m2
c) 650 NM per m2
d) 700 NM per m2

Explanation: The value of permissible stress for steel wire for the diameter of branding wire of 0.5-1.2 mm is 600 NM per m2. The value of permissible stress for steel wire for the diameter of branding wire of 1.5-2 mm is 570 NM per m2.

Sanfoundry Global Education & Learning Series – Design of Electrical Machines.

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