# DC Machines Questions and Answers – Characteristics of Separately Excited Generators

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This set of DC Machines test focuses on “Characteristics of Separately Excited Generators”.

1. Characteristics of separately excited DC generator are drawn by keeping _____
a) Field current and speed both constant
b) Field current and speed both variable
c) Field current constant and speed variable
d) Field current variable and speed constant

Explanation: The operation considered here assumes that the armature is driven at constant speed (by means of prime mover) and the field excitation (If) is adjusted to give rated voltage at no-load and is then held constant at this value throughout the operation considered.

2. What is the reason behind dropping down of Ea with load?
a) Field resistance
c) Internal factors
d) Armature reaction

Explanation: In spite of fixed excitation, Ea drops off with load owing to the demagnetizing effect of the armature reaction. As the voltage drop is caused by magnetic saturation effect, it increases with load non-linearity.

3. External characteristic differ from an internal characteristic in separately excited DC generator by _________
a) Ia *Ra
b) If *Ra
c) IL *RL
d) If *Rf

Explanation: Internal characteristic is drawn across armature generated voltage, it doesn’t account the presence of armature resistance. External characteristic takes into account the presence of armature resistance as it is drawn across terminal voltage.

4. The variation in terminal voltage of DC shunt generator with respect to variation in separately excited DC generator is ___________
a) Much rapid
b) Much slower
c) Remains constant
d) Can’t say

Explanation: The terminal voltage drops off much more rapidly with load in a shunt generator than in a separately-excited generator because of fall in field current with terminal voltage. The external characteristic is a double-valued curve with a certain IL (max).

5. Which of the following characteristic lies above of all others?
a) Differential compound
b) Under compound
c) Level compound
d) Over compound

Explanation: All the graphs when drawn across voltage and load current, start from the same point but with increase in values of load current all machines show different elevation according to the values of series field resisitor.

6. Why differential compound generator is not used in practice?
a) High cost
b) High maintenance
c) High drop down in voltage
d) Difficult construction

Explanation: At a given value of load current, differential compound machine gives lowest voltage output. As the load current increases drop down in terminal voltage of DC differential compound generator is maximum.

7.How compounding level in a compound machine is adjusted?
a) By adding variable resistance in series with series field resistance
b) By adding variable resistance in parallel with series field resistance
c) By adding fixed resistance in parallel with series field resistance
d) By adding fixed resistance in series with series field resistance

Explanation: When fixed resistance is added in parallel with series field resistance we get only one other compounding level. So, by adding variable resistance in parallel with series field we can get various other compounding levels.

8. Which of the following have different external characteristic than other?
a) Self excited DC shunt generator
b) Separately excited generator
c) Compound DC generator
d) Series DC generator

Explanation: Unlike Self excited DC shunt generator, separately excited generator, compound DC generator the voltage at zero load current for DC series motor doesn’t start from some positive non-zero value, instead it starts from origin.

9. External characteristic differ from internal characteristic in DC series motor by ______
a) Ia *(Ra + RSE)
b) Ia *Ra
c) Ia *RSE
d) If *RSE

Explanation: In series DC generator series resistor RSE is added with armature circuit, thus total drop in terminal voltage from generated armature voltage is equal to Ia *(Ra + RSE). While drawing internal characteristics presence of armature resistance and series resistance is not taken into account.

10. For series DC generator, internal/external characteristic start from ____________
a) Positive non-zero voltage
b) Zero voltage
c) Negative non-zero voltage
d) Can start from anywhere

Explanation: For all other DC generators other than series DC generators, the external and internal characteristic as well, start from non-zero positive value of voltage. While in series DC generator both internal and external characteristic start from origin.

11. For a DC series generator what is the condition for self-excitation?
a) (Ra+Rse) > RC
b) (RSE+Rse+RL) < RC
c) (RSE+Rse+RL) > RC
d) (Rse+RL) > RC

Explanation: Summation of armature resistance, series field resistance, load resistance must be less than the critical resistance. As seen in self-excited DC generator characteristics if this value is greater than critical resistance, voltage build-up will not be possible.

12. For a given DC series generator with critical resistance equal to 100 Ω, armature resistance is equal to 50 Ω, and series field resistance is equal to 20 Ω, is connected across load of 50 Ω. What will be the load voltage?
a) 20 kV
b) 0 V
c) 2 kV
d) Data insufficient

Explanation: Here, DC series motor fails to excite as addition of armature resistance, load resistance and field resistance is greater than the critical resistance of the machine by 20 Ω. Thus, machine fails to self-excite, as a result we’ll get zero terminal voltage.

Sanfoundry Global Education & Learning Series – DC Machines.

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