DC Machines Problems on “External and Armature Characteristics of DC Generator”.
1. Armature characteristic is _____________
a) No-load characteristic
b) Load characteristic
c) OCC Current
d) Cannot be determined
Answer: b
Clarification: It is the presentation of Ia vs If with Vt held constant (at rated value) and generator run at constant n and load varied. It reveals the armature reaction effect on the flux/pole. It is also called regulation characteristic.
2. External characteristics is drawn with constant term/s ____________
a) Field current
b) Speed
c) Both Field current and Speed
d) Load
Answer: c
Clarification: External characteristic is a load characteristic. In this, variation of terminal voltage across armature current is drawn keeping speed and field current constant. Load is generally kept variable.
3.Armature characteristic is drawn across ________
a) Ia vs If
b) Ia vs Vt
c) If vs Vt
d) Ea vs Ia
Answer: a
Clarification: Armature characteristic is drawn across Ia vs If. This characteristic is on load characteristic, drawn when terminal voltage and speed of the generator is kept constant. Armature characteristic is regulation characteristic.
4. what is the first step in drawing the armature characteristic?
a) If is made 0
b) Field current is adjusted to give Voc equal to rated value
c) Switch connecting armature circuit with external circuit is made open
d) Cannot be determined
Answer: c
Clarification: While drawing armature characteristic, switch of armature circuit is made open so that armature current is set to 0. Then, field current is adjusted to give Voc equal to rated value. Switch its then closed.
5. In armature characteristic, nature of If at lower values of armature current is ______
a) Very large
b) Almost constant
c) Decreasing rapidly
d) Increasing then decreasing
Answer: b
Clarification: It is seen from the characteristic that at low values of Ia, the increase in If is very small to provide for increasing Ia *Ra drop. At large values of Ia there is a sharp increase in If to compensate for voltage drop caused by armature reaction.
6. At some point If On OCC, the value of voltage obtained which is more than rated voltage is _______
a) Ia *Ra + If *Rf
b) Ia *Ra
c) Vd + Ia *Ra
d) Can’t tell
Answer: c
Clarification: The difference in the observed value and rated value of voltage can be obtained from the y-axis that is, from Voc. This difference is equal to Vd + Ia *Ra. Thus, by subtracting voltage drop in armature Vd can be found.
7. Which of the following is the correct formula for cumulative compound generator?
a) Nse= Nf (ΔIa/If)
b) Nse= Nf (ΔIf/Ia)
c) Nse= 2Nf (ΔIa/If)
d) Nse= Nf (ΔIa/If)/2
Answer: b
Clarification: According to proportionality constant obtained by analysis of OCC, various value of armature current can be obtained. From ΔIf values we can compute number of turns in series of armature in cumulative compound machine.
8. External Characteristic is a plot of _____________
a) Generated emf and load current
b) Terminal voltage and load current
c) Generated voltage and field current
d) Armature current and field current
Answer: b
Clarification: The load characteristic of a dc generator at a particular speed is the relationship between its terminal voltage and load current (line current) and is also termed as the external characteristic. The internal characteristic is the plot between the generated emf and load current.
9. External characteristics for all separately DC generators (except series generator) can be summarized as ____________
a) Parabola opening in +x direction
b) Parabola opening in -x direction
c) Parabola opening in +y direction
d) Parabola opening in -y direction
Answer: b
Clarification: All parabolas for separately excited DC generator, shunt generator, compound generator are parabolas which are opening in -x directions, only centres of all parabolas lie at different places depending on the type of generator.