Design of Electrical Machines Questions and Answers – Design of Commutator and Brushes

This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design of Commutator and Brushes”.

1. What is a commutator in DC machines?
a) electrical device which reverses the current direction between the rotor and external circuit
b) mechanical device which reverses the current direction between the rotor and external circuit
c) electrical device which allows the current flow between the rotor and external circuit
d) mechanical device which allows the current flow between the rotor and external circuit

Explanation: Commutator is an electrical device found only in DC machines. It is used to reverse the current direction between rotor and external circuit. It also changes dc voltage to alternating voltage.

2. What is the use of brushes in DC machines?
a) to connect the parts of the machine to the external circuit
b) to conduct current between moving parts
c) to conduct current between stationary wires and moving parts
d) used for smooth conduction of current

Explanation: Brushes are another important part in the construction of DC machine. They are connected in the lower end of the machine to allow the current flow between the moving parts and stationary wires.

3. Which material is commonly used in brushes?
a) copper
b) carbon
c) silicon
d) steel

Explanation: Carbon is the most commonly used material in the manufacture of brushes. It is because carbon has high melting point, and is also less prone to high temperatures.

4. What is the total number of design steps available for the commutators in DC machines?
a) 4
b) 3
c) 2
d) 5

Explanation: There are basically 4 design steps available for the commutators. They include the finding of the number of commutator segments, voltage across the commutator, width of the commutator and length of the commutator.

5. What is the total number of design steps for the brushes in DC machines?
a) 4
b) 3
c) 5
d) 6

Explanation: There are basically 5 steps in the brush design. They include the calculation of the brush current, number of brushes, thickness of the brush, width of the brush, total commutator losses.
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6. What factor does the diameter of the commutator depend on?
a) length of the commutator
b) speed of the commutator
c) peripheral speed of the commutator
d) opening of the commutator

Explanation: Peripheral speed is a term which is used in the design of the commutators of DC machines. If the peripheral speed of the machine gets crossed, then the diameter should be reduced.

7. For a DC machine, given Diameter of the commutator= 0.48 m, Speed = 600 rpm. What is the voltage across the commutator?
a) 15 V
b) 15.5 V
c) 15.2 V
d) 15.1 V

Explanation: Voltage across the commutator = 3.14*(Diameter of the commutator)*Speed
Voltage = 3.14*0.48*600 = 15.1 V.

8. What is the other name for commutator segment pitch of DC machines?
a) width of the commutator
b) length of the commutator
d) height of the commutator

Explanation: Commutator segment pitch is otherwise known as the width of the commutator. It is one of the commutator design steps.

9. What is the formula for finding out the width of the commutator of the DC machines?
a) width of the commutator = 3.14*(Diameter of the commutator)
b) width of the commutator = 3.14*(Diameter of the commutator)*(Number of armature coils)
c) width of the commutator = 3.14*(Diameter of the commutator) / (Number of armature coils)
d) width of the commutator = Number of armature coils / 3.14*(Diameter of the commutator)

Explanation: First by the design of the armature, the number of armature coils is found out. Then the diameter of the commutator is measured.

10. The diameter of the commutator should be 0.2-0.4 times the main diameter for a good design.
a) true
b) false

Explanation: For a good design of the commutator, the diameter of the commutator should be in the range of 0.6-0.8 times of the main diameter. If the value goes above or below this range, we should not choose that value.

11. What should be the range for the width of the commutator in a good design?
a) < 4 mm
b) > 4 mm
c) 3-4 mm
d) < 3 mm

Explanation: For a good design of the commutator, the width of the commutator should be greater than 4 mm. If the value goes below 4 mm, we should not choose that value.

12. The design of commutator and the brushes of DC machines are interconnected.
a) true
b) false

Explanation: The design of brushes and commutators are interconnected, because in the calculation of the length of the commutator, the width of the brush, number of brushes are used. In the same way, brush values are also used in calculation of commutator loss.

13. What is the formula for calculation of brush current of DC machine for wave winding?
a) brush Current = 2*(Armature Current) / P
b) brush Current = (Armature Current) / P
c) brush Current = Armature Current
d) brush Current = P/2*(Armature Current)

Explanation: Brush Current = 2*(Armature Current) / P is the formula for the calculation of brush current of DC machine for lap winding.

14. What is the formula for brush contact loss of DC machines?
a) brush contact loss = armature current + brush contact voltage
b) brush contact loss = armature current – brush contact voltage
c) brush contact loss = armature current * brush contact voltage
d) brush contact loss = (armature current + brush contact voltage) / 2

Explanation: For calculation of the brush contact loss, first the armature current is obtained. Then the voltage through the brush contacts is calculated and the product gives the brush contact loss.

15. What is the formula for total commutator loss for DC machine?
a) brush contact loss + brush friction loss
b) brush contact loss – brush friction loss
c) brush contact loss * brush friction loss
d) brush contact loss / brush friction loss

Explanation: Brush contact loss = brush contact loss = armature current * brush contact voltage
Brush friction loss = (Coefficient f friction) * Brush Pressure * Area of the brush*Commutator voltage.

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