Electrical Machines Multiple Choice Questions on “Losses and Efficiency – 2”.
1. A consideration of the power losses in electrical machines is essential for which of the following reasons?
A. Operating cost
B. Temperature rise
C. Voltage drops
D. All of the mentioned
Answer: D
Clarification: A machine with lower efficiency has more losses, and therefore increased operating cost. Also, losses cause heating of the machine and therefore, its temperature rise. Similarly, voltage drop IR is associated with ohmic loss.
2. To determine the efficiency of the machine, direct load test is not advantageous because of which of the following reasons?
A. Cost of providing large inputs
B. Difficulty of dissipating the large outputs
C. Both cost of providing large inputs and difficulty in dissipating the large outputs
D. None of the mentioned
Answer: C
Clarification: Also, a small error in the measurement of either output or input causes the same amount of error in the computed efficiency.
3. Which of the following statements are correct regarding brush contact losses?
A. In DC machine: proportional to armature current
B. In synchronous machine: neglected
C. In induction machine: neglected
D. Any of the mentioned
Answer: D
Clarification: There is brush contact loss at the contacts between the brushes and commutator (in DC machine) or between the brushes and slip rings (synchronous and induction machines). However, in practical, the brush contact loss is neglected for synchronous and induction machines.
4. In rotating electrical machines, when the armature rotates, there are continuous magnetic reversals and power required for their reversals is called _____________
A. Eddy current loss
B. Hysteresis loss
C. Resistance or ohmic losses
D. Mechanical loss
Answer: B
Clarification: Hysteresis loss is directly proportional to the number of magnetic reversals per second or the speed.
5. The usual lamination thickness selected to minimize the eddy current loss in rotor is _____________
A. 0.1 mm to 0.2 mm
B. 0.3 mm to 0.4 mm
C. 0.4 mm to 0.5 mm
D. 0.9 mm to 0.10 mm
Answer: C
Clarification: If lamination thickness is made less than 0.4 mm, the reduction in eddy current losses is achieved, but at the cost of additional labor charges in assembling the rotor.
6. In an induction motor, which of the following is correct?
A. Stator core loss < rotor core loss
B. Stator core loss = rotor core loss
C. Stator core loss > rotor core loss
D. Any of the mentioned
Answer: C
Clarification: The rotor core loss is almost negligible because of reduced frequency of the flux reversals (equal to slip frequency, sf) in the rotor, and Pe∝f2 and Ph∝f. (Pe = eddy current losses, Ph = hysteresis losses).
7. The pole shoes in DC and synchronous machines are laminated to reduce _____________
A. Resistance losses
B. Pulsation or pole-face losses
C. Mechanical losses
D. None of the mentioned
Answer: B
Clarification: The pulsations in flux density wave arising from slot openings cause losses in the field iron, particularly in the pole faces. This loss occurring due to relative motion, between field poles and slotted armature, is referred to as pulsation loss.
8. What percentage of the rated output for DC machine and synchronous machine is taken as stray load losses respectively?
A. 1% and 0.5%
B. 0.5% and 1%
C. 3% and 0.1%
D. 0.1% and 5%