DC Machines Multiple Choice Questions on “Methods of Excitation”.
1. Which winding has large number of turns?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement
Answer: a
Clarification: The shunt field winding is provided with a large number (hundreds or even thousands) of turns of thin wire and is excited from a voltage source. The series field winding has a few turns of thick wire and is excited from armature current by placing it in series with armature.
2. Which winding contains wire with higher thickness?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement
Answer: b
Clarification: Series field winding is used when wire is thick. Thus, by making minimum number of turns it can be used to excite a DC machine. For a given field current, control of this field is achieved by means of a diverter, a low resistance connected in parallel to series winding.
3. Which winding have higher resistance?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement
Answer: a
Clarification: The shunt field winding is provided with a large number (hundreds or even thousands) of turns of thin wire and is excited from a voltage source. The field winding, therefore, has a high resistance and carries a small amount of current. It is usually excited in parallel with armature circuit.
4. Which is more practical method used in control of series field?
a) Diverter
b) Tappings
c) Switch relay
d) Cannot be determined
Answer: b
Clarification: For a given field current, control of series field is achieved by means of a diverter, a low resistance connected in parallel to series winding. A more practical way of a series field control is changing the number of turns of the winding by suitable tappings which are brought out for control purpose.
5. In compound excitation, which winding/s is/are excited?
a) Shunt
b) Series
c) Both
d) Cannot be determined
Answer: c
Clarification: In compound excitation, both series and shunt windings are connected to the armature. For compound excitation both of these windings are excited. Two types are cumulative excitation and differential excitation.
6. In which type of excitation air gap flux increases with armature current?
a) Differential compound
b) Cumulative compound
c) Differential and Cumulative
d) Cannot be determined
Answer: b
Clarification: In compound excitation both shunt and series field are excited. If the two field aid each other (their ampere-turn is additive), the excited is called cumulative compound. The shunt field is much stronger than the series field. The air gap flux increases with armature current.
7. In differential compound excited machine, what is the variation in air gap flux per pole with respect to armature current?
a) Increases
b) Decreases
c) Remains constant
d) Always varies
Answer: b
Clarification: If the two fields oppose each other, the excitation is called differential compound. The air gap flux/pole decreases with armature current. The series field is so designed that the increase or decrease in flux/pole is to a limited extent.
8. Which winding in compound excitation is responsible for change in air gap flux per pole?
a) Series
b) Parallel
c) Interconnected
d) No coil is responsible