DC Machines Questions and Answers for Entrance exams on “Efficiency of DC Machine – 2”.
1. The condition for maximum efficiency for a DC generator is __________
a) Eddy current losses = stray losses
b) Hysteresis losses = eddy current losses
c) Copper losses = 0
d) Variable losses = constant losses
Clarification: All losses in a given DC machine can be categorized into variable losses and constant losses. Variable losses are proportional to the square of armature current while constant losses are almost constant for a given DC machine throughout its application.
2. DC generators are normally designed for maximum efficiency at or near ____________
b) Minimum load
c) Rated voltage
d) At all loads
Clarification: The efficiency of a machine is different at different values of power output. All electrical machines are generally designed to give maximum efficiency at or near the rated output of the machine. Thus, maximum efficiency occurs at rated voltage.
3. In a DC generator, the iron losses mainly take place in ____________
c) Armature conductors
d) Armature rotor
Clarification: Iron losses take place in the form of hysteresis loss and eddy current loss. These losses are maximum, where field is maximum. Thus, when armature is rotated in presence of flux we get maximum iron loss.
4. If DC generators are located near load centres, which losses can be minimised?
a) Iron losses
b) Eddy current losses
c) Line losses
d) Corona losses
Clarification: Line losses occur in long transmission lines while sending output power to the loading stations. Thus, by locating the generators near loading stations losses occurring due transmission line can be eliminated.
5. Nature of efficiency curve of DC machine is _________________
a) First decreases then increases
b) First constant then decreases
c) First constant then increases
d) First increases then decreases
Clarification: The efficiency of a DC machine is different at different values of power output. As the output increases, the efficiency increases till it reaches to a maximum value. As the output is further increased, the efficiency starts decreasing slowly.
6. Why retardation test is carried on DC machine?
a) To find stray losses
b) To find eddy current losses
c) To find field copper losses
d) To find windage losses
Clarification: This test is generally employed to shunt generators and shunt motors. From this method we can get stray losses of a machine. Thus, if armature and shunt copper losses at any given load current are known then efficiency of a DC machine can be easily estimated.
7. In the DC motor the iron losses occur in __________
Clarification: Armature winding in a DC machine is located on rotor. Iron losses occur in an armature, hence in rotor. Iron losses are hysteresis loss and eddy current loss, which are seen prominently in armature rotor.
8. Which of the following losses is not under constant losses?
a) Friction and windage losses
b) No load core losses
c) Shunt field losses
d) Hysteresis losses
Clarification: All the losses that is friction and windage losses, no load core losses, shunt field core losses in shunt field and compound motors come under the category of constant losses, while iron losses come under category of variable losses.
9. Variable losses are proportional to ________
a) Armature current
b) Square of armature current
c) Inverse of armature current
d) Inverse of square of armature current
Clarification: Variable losses include losses in armature resistance and losses in series resistance, which are directly proportional to the square of armature current. It also includes stray load loss (iron plus copper) which is proportional to square of armature current.