This set of DC Machines Interview Questions and Answers focuses on “Brushes and Commutator Assembly – 2”.
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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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?
b) Armature conductors
c) Winding elements
7. Spacing between the brushes for a 4-pole machine in terms of commutator segments for 12 conductor segments is _____
Explanation: 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?
Explanation: 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
Explanation: 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.
Explanation: 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?
Explanation: 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
Explanation: 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.
Sanfoundry Global Education & Learning Series – DC Machines.
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