Electrical Machines Question Bank on “Operating Characteristics of Alternators and their Ratings”.
1. If the machine was operating at upf, then the new excitation voltage will be _________
A. greater than 1 pu
B. less than 1 pu
C. 1 pu
D. zero
Answer: A
Clarification: The new excitation will be more than 1 pu to compensate for the flux required.
2. The variation of Vt vs Ia is for the alternator is given below.
Choose the appropriate
A. A-zpf lead; B-0.8 lead; C-0.8 lag; D-zpf lag
B. A-zpf lag; B-0.8 lead; C-0.8 lag; D-zpf lead
C. A-zpf lead; B-0.8 lag; C-0.8 lead; D-zpf lag
D. A-zpf lead; B-0.8 lag; C-0.8 lead; D-zpf lag
Answer: A
Clarification: A is zpf lead for the alternator and D will be zpf lag.
3. Alternator compounding characteristic is obtained by _________
A. keeping Vt constant
B. keeping Ef constant
C. keeping Ia constant
D. varying Vt
Answer: A
Clarification: Alternator compounding characteristic is obtained by keeping Vt constant.
4. At zpf lag, the excitation should be increased in order to maintain the armature terminal voltage constant.
A. True
B. False
Answer: A
Clarification: Due to demagnetizing armature reaction at zpf lag in an alternator.
5. The rating of the alternator is decided by _________
A. losses
B. voltage
C. armature current
D. temperature
Answer: A
Clarification: The losses determine the rating to be used for alternator.
6. The core losses in synchronous machine is _______ dependent.
A. voltage
B. current
C. temperature
D. insulation
Answer: A
Clarification: Core losses depend on the voltage applied to the machine.