# DC Machines Questions and Answers – Methods of Excitation

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This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses 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

Explanation: 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

Explanation: 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

Explanation: 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.
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4. Which is more practical method used in control of series field?
a) Diverter
b) Tappings
c) Switch relay
d) Cannot be determined

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: In a compound excited machine, both series and parallel windings are connected with armature circuit. The series winding is specially designed for increasing or decreasing flux per pole in DC machine up to certain extent.

9. How short shunt and long shunt compound winding is selected?
a) Mechanical considerations
b) Switch reversal
c) Performance
d) Mechanical considerations and switch reversal

Explanation: In long shunt compound, the shunt field is connected across terminals. In short shunt compound, the shunt field is connected directly across the armature. There is no significant difference in machine performance for the two types of connections. The choice between them depends upon mechanical consideration or the reversing switches.

10. If a DC compound machine connected as a motor is about to use as a generator, we reverse the series field connections.
a) True
b) False

Explanation: If a dc compound machine connected as a generator is run as a motor, the series field connections must be reversed as the armature current reverses. The motoring action as cumulative/ differential would then be preserved (same as in the generator). This equally applies vice versa – motor to generator.

11. For a long-shunt compound motoring, which of the following equation is correct?
a) Vt= Ea+ Ia(Ra+ Rse)
b) Vt= Ea– Ia(Ra+ Rse)
c) Vt= Ea+ Ia(Ra– Rse)
d) Vt= -Ea+ Ia(Ra+ Rse)

Explanation: For a long shunt compound motor Rse is connected in series with armature, while in short shunt compound motors Rse is connected in series with terminal voltage. IL is supplied through the terminals which split into If and Ia.

12. Vt= Ea– Ia(Ra+ Rse) is the equation for _______________
a) Short shunt compound motoring mode
b) Short shunt compound generating mode
c) long shunt compound motoring mode
d) Cannot be determined

Explanation: In a DC machine, for a long shunt compound motor Rse is connected in series with armature, while in short shunt compound motors Rse is connected in series with terminal voltage. IL is supplied to the terminals which is equal to Ia – If.

13. How shunt field is controlled?
a) Diverter resistor in parallel
b) Tapped field winding
c) Series regulating resistance
d) Other methods

Explanation:
Control of Excitation: 1) Shunt field: by a series regulating resistance.2) Series field: For small armature by a diverter resistance connected in parallel with series field. For large armature by tapped field winding so the winding turns can be changed.

14. The generator is called flat compounded if _____________
a) The series field ampere turns are such as to produce the same voltage at rated load as at no load
b) The series field turns are such as that the no load voltage is smaller than the rated load voltage
c) The rated voltage is less than the no load voltage
d) Cannot be determined

Explanation: According to the operating characteristics of a DC compound generator, if series field mmf produces same voltage at rated load as that of no load then it is called as flat compounded generator.

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