class brush mcq
DC Machines Interview Questions and Answers on “Brushes and Commutator Assembly”.
1. In a DC generator the ripples in the direct emf generated can be reduced by________
a) Using conductor of annealed copper
b) Using commutator with large number of segments
c) Using carbon brushes of superior quality
d) Using equalizer rings
Answer: c
Clarification: Brushes carry current to/from rotating parts from/to stationary part. Ripples can be avoided if brushes are maintained. Else, brushes will have some voltage drop in it and we’ll not get simple repeating part in emf.
2. The drop in the voltage for which of the following types of brush can be expected to be least?
a) Graphite brushes
b) Carbon brushes
c) Metal graphite brushes
d) Cannot be determined
Answer: c
Clarification: Metal graphite brushes are ideal for a variety of applications because of their low resistivity. Thus, drop will be less in metal graphite brushes. Metal graphite brushes are used on commutators of plating generators where low voltage and high brush current densities are encountered.
3. What is the requirement of the good commutation?
a) Brushes should be of proper grade, size and material
b) Brushes should smoothly run in the holders
c) Smooth, concentric commutator properly undercut
d) Brushes should be of proper grade, size, material, run smoothly in the holders and concentric commutator properly undercut
Answer: d
Clarification: Brushes are in contact with commutator. So, for good commutation brushes must be of superior quality so that brushes will give/receive appropriate current to and from commutator. Also, the contact between brushes and commutator must be smooth for proper commutation process.
4. How to avoid grooves in the commutation of DC machine with the help of brush?
a) Brushes of opposite polarity should track each other
b) Brushes of same polarity should track each other
c) Brush position has no effect on the commutator grooving
d) Brushes should not track each other
Answer: a
Clarification: Brushes are located such that they are displaced 900 electrically from the axes of main poles. The two positive and two negative brushes are respectively connected in parallel for feeding the external circuit.
5. Reason behind the rapid wear of brushes is __________
a) Abrasion from dust
b) Excessive spring pressure
c) Rough commutator bars
d) Abrasion from dust, excessive spring pressure and rough commutator bars
Answer: d
Clarification: Brushes undergo various forces due to their location in a DC machine, they are in contact with rotating and stationary part of the machine. Hence, rough contact between commutator and brushes, inappropriate pressure on brush to rotating part may affect quality of commutation process.
6. For both lap and wave windings, what is the number of commutator bars equal to?
a) Slots
b) Armature conductors
c) Winding elements
d) Poles
Answer: c
7. Spacing between the brushes for a 4-pole machine in terms of commutator segments for 12 conductor segments is _____
a) 48
b) 3
c) 2
d) 6
Answer: b
Clarification: The spacing between adjacent brushes in terms of the commutator segment is ratio of number of commutator segments with poles for a given DC machine.
C/P= 12/4= 3.
It may also be noted that C/P need not necessarily be an integer.
8. Spacing between the brushes for a 4-pole machine in terms of commutator segments is equal to 6. What will be the number of armature slots?
a) 48
b) 3
c) 24
d) 6
Answer: c
Clarification: The spacing between adjacent brushes in terms of the commutator segment which is also equal to armature slots is ratio of number of commutator segments with poles for a given DC machine.
C= P*Spacing= 4*6= 24.
9. What is the range of the brush friction coefficients for medium category?
a) 0.40 and above
b) 0.22 to 0.40
c) 0.11 to 0.22
d) 0.08 to 0.11
Answer: b
Clarification: Brush friction is influenced by many variables including brush temperature, spring force, current, atmospheric conditions, mechanical conditions, ring or commutator materials, surface films, speed and other factors. Brush friction is of medium category when, coefficient of friction lies in between 0.22 to 0.44.
10. Specific resistance for a brush is given by _________
a) R = (E * W * T) / (I * L)
b) R = (E * W * I) / (T * L)
c) R = (E * W * I) * (T * L)
d) Doesn’t depend on E, W, T, I, L.
Answer: a
Clarification: Specific resistance is measured in the length direction of the brush, since resistance in the direction of width or thickness may be considerably different. For, E = voltage drop over length L, I = amps of current passed through the sample, W = width of sample, T = thickness on sample, L = that portion of the length, over which the voltage drop E is measured, R is calculated by R = (E * W * T) / (I * L).
11. How many poles to be used in DC machine if brushes are placed 4 commutator segments apart for 16 commutator segments?
a) 8
b) 12
c) 2
d) 4
Answer: d
Clarification: Number of poles can be found by dividing the total commutator segments to spacing between brushes. Hence, number of poles = 16 commutator segments/ 4 commutator spacing= 4 poles.
12. DC generators are disconnected/connected from/to the busbars only under the floating condition because ____________
a) To avoid sudden loading of the prime mover
b) To avoid burning of all switch contacts
c) To avoid mechanical jerk to the shaft
d) To avoid sudden loading of the prime mover, burning of all switch contacts and avoid mechanical jerking to the shaft
Answer: d
Clarification: Brushes are the medium between rotating and non-rotating part of the DC machine. If sudden change in connections are done, whole machine undergoes change in all electrical quantities, which may damage machine. Thus, machines are connected and disconnected only at floating condition.
13.The armature in DC machines is always placed on rotor because _______
a) Otherwise commutation will not be possible
b) Otherwise there will not be any induced emf
c) Otherwise current will not flow
d) Can be placed anywhere
Answer: a
Clarification: A DC machine is a heteropolar structure with stationary poles and the rotating armature. The armature winding of a DC machine is placed on the rotor to improve commutation i.e. to convert the alternating voltage produced in the winding into direct voltage at the brushes.
14. In a DC machine, rectification provided with commutator is ___________
a) Half wave rectification
b) Full wave rectification
c) Semi controlled rectification
d) Uncontrolled rectification
Answer: b
Clarification: In any electromagnetic machine the voltage generated is always alternating one as per Faraday’s law. For a DC machine the output must be unidirectional. This is carried out by a commutator. Hence, commutator provides full wave rectification.
15. Commutator performs rectification so that output of the machine is bi-directional.
a) True
b) False
Answer: b
Clarification: Commutator and brush assembly of the DC machine performs the mechanical rectification process so induced AC is converted into DC (Unidirectional). Commutation process provides full wave rectification.
16. Which of the following method is used to connect commutator segments to armature conductors?
a) Brazing
b) Normal wires
c) Insulation pads
d) Copper lugs
Answer: d
Clarification: Commutator in connected to the armature using lugs. Generally, they are made with copper. They are tightly bolted to the armature in order to prevent the centrifugal forces from causing the segments to fly away.
17. In D.C. generators, rapid brush ware causes due to _________
a) Severe sparking
b) Rough commutator surface
c) Imperfect contact
d) Severe sparking, rough commutator surface or imperfect contact
Answer: d
Clarification: Brushes are the parts in a DC machine which are always in contact with rotating and stationary parts. Thus, imperfect contact, rough surfaces, sparking all these may reduce the life of brushes.
18. What are the number of the brushes in the lap winding?
a) Double the number of poles
b) Same as the number of poles
c) Half the number of poles
d) Two
Answer: b
Clarification: In a lap winding, the number of parallel paths, A, is always equal to the number of poles, P, and also to the number of brushes. In wave windings, the number of parallel paths, a, is always two (2), and there may be two or more brush positions.
19. When Copper brushes are used in DC machine?
a) Where low voltage and high currents are involved
b) Where high voltage and small currents are involved
c) Cannot be determined
d) Where low voltage and low currents are involved
Answer: a
Clarification: Due to various limitations, copper brushes are used in low voltage applications. For various other voltage ratings, different carbon-graphite proportions are used in manufacturing of brush materials.
20. In DC generators, current is fed up to the external circuit from armature through _______
a) Commutator
b) Direct wire
c) Slip rings
d) Cannot be determined
Answer: a
Clarification: In any rotary machine current is induced in Sine wave format, according to Faraday’s law. For DC machine commutator provides mechanical rectification so that output is in the unidirectional format.
21. What are the number of the brushes in the wave winding?
a) Same or more than the number of poles
b) Same or less than the number of poles
c) Exactly half the number of poles
d) Depends on other parameters
Answer: a
Clarification: In wave windings, the number of parallel paths, A, is always two (2), and there may be two or more brush positions. When two adjacent commutator bars make contact with a particular brush, p/2 coils are shorted by the brush in the wave winding.
22. Which conductors are in point of contact with brushes?
a) Lie under south pole
b) Lie under north pole
c) Lie in inter polar region
d) Are farthest from the poles
Answer: c
Clarification: Because of the diamond shape of coils, the brushes which are physically opposite the pole centres are electrically connected to coil-sides lying close to the interpolar region. Thus, electrically the brushes are displaced 90° elect. From the axes of the main poles.
23. In case of DC machine winding, number of commutator segments is equal to ______
a) Number of armature coils
b) Number of armature coil sides
c) Number of armature conductors
d) Number of armature turns
Answer: a
Clarification: Armature current is induced in a DC machine, which is fed up to the external circuit. Thus, it needs to be unidirectional. So, for converting bidirectional current to unidirectional commutators which are equal in number of armature coils need to be used.
24. How total number of brushes in a commutator are determined in a given DC machine?
a) Speed of armature
b) Type of winding
c) Voltage
d) Amount of current to be collected
Answer: d
Clarification: Brushes are in contact with rotating part and stationary part. Thus, if more amount of current is to be carried, it requires more number of brushes. Hence brush number depends directly on the amount of current that needs to be collected and fed up in or out.