DC Machines Multiple Choice Questions on “Brushes and Commutator Assembly”.
1.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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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.
11. 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.
12. 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.