DC Machines Multiple Choice Questions on “Construction”.
1. Where is field winding mounted in a DC machine?
a) Stator
b) Rotor
c) Absent
d) Anywhere on stator or rotor
Answer: a
Clarification: The field winding (concentrated type) is mounted on salient-poles on the stator and the armature winding (distributed type) is wound in slots on a cylindrical rotor. In AC machines field winding is mounted on rotor.
2. What are the materials used for brushes in dc machines?
a) Iron
b) Carbon
c) Aluminum
d) Steel
Answer: b
Clarification: On some extent carbon brush can act as a self-lubricating brush. On moment, polishes the commutator segments. Damage to the commutators is less when copper brushes are used on occurrence of sparkover.
3. Function of yoke is to provide the return path for magnetic flux.
a) True
b) false
Answer: a
Clarification: The function of yoke is that it protects the entire machine from dust and dirt. It also provides mechanical support for the magnetic poles. It acts as the return path for the magnetic flux.
4. The angle (electrical) made by brushes with axes of adjoining filed poles is ______________
a) 450
b) 1800
c) 900
d) 300
Answer: c
Clarification: Brushes in a DC machine are normally placed electrically in the interpolar regions and therefore make an angle of 900 electrical with the axes of adjoining filed poles.
5. In a DC machine, rectification process is carried out in order to get unidirectional output (DC). This rectification process is carried out by ______________
a) Half wave rectifier
b) Full wave rectifier
c) Mechanical rectification
d) Centre tapped rectifier
Answer: c
Clarification: In a DC machine electronic rectification is not used. Instead they use mechanical rectification with the help of commutator-brush assembly.
6.Which of the following part is used in construction of DC machine but not in AC machine?
a) Armature Winding
b) Field winding
c) Commutator
d) Shaft
Answer: c
Clarification: Commutator is used in mechanical rectification process, to convert induced AC to output DC. In AC machine, we don’t need rectification process.
7. In a DC machine fractional pitch winding is used to _______________
a) To improve cooling
b) To reduce sparking
c) To reduce copper losses
d) To increase generated EMF
Answer: b
Clarification: Due to poor performance of brush, poor undercutting of commutator, incorrect spring pressure sparking at brush faces happen. To overcome this sparking fractional pitch winding is used.
8. In normal dc machines operating at full-load conditions, the most powerful electromagnet is _________
a) Field winding
b) Interpole Winding
c) Interpole and compensating winding together
d) Armature winding
Answer: a
Clarification: Electromagnet is more powerful when its MMF is high. At full-load condition, field winding contains maximum ampere turns, hence it is most powerful electromagnet in a DC machine.
9. If a DC motor is connected to AC supply what will happen then?
a) Not run
b) Burn
c) Run at normal speed
d) Run at extremely low speed
Answer: b
Clarification: If a DC motor is connected to AC supply, an alternating current pass through the brushes and commutator to the armature winding, while it passes through the commutator it is converted into DC so that the group of conductors under successive field poles carry current in same direction. So, the flux per pole will remain constant and not vary. There will be production of heat due to flow of eddy current in field winding and the motor will be burned.
10. The armature of DC motor is laminated to ____________
a) To reduce mass
b) To reduce hysteresis loss
c) To reduce eddy current loss
d) To reduce inductance
Answer: c
Clarification: The armature is built up in a cylindrical or drum shape high grade silicon steel in form of lamination. By using laminations, the circular path of eddy currents is terminated. Hence heating and ultimately damage to the armature can be reduced by lamination.