This set of Electrical Machines Question Bank focuses on “Operating Characteristics of Alternators and their Ratings”.
Explanation: At no load Ia = 0. So Ef = Vt.
Explanation: Ia should lag Ef.
Explanation: Ia will lead the excitation.
Explanation: Ia in phase with excitation voltage.
Explanation: Ia has to be lagging by 90°.
6. A 315V, 3-phase 400 MVA alternator is running at zero power factor leading at constant excitations. The most appropriate representaion in space domian is
d) none of the mentioned
Explanation: Ia has to be leading by 90°.
7. With the alternator running at rated terminal voltage Vt, at rated current Ia = 1 per unit. What happens if load current is reduced to zero, if it was operating at lagging power factor?
Explanation: Check for the Ia lagging Ef and Ef > Vt.
8. With the alternator running at rated terminal voltage Vt, at rated current Ia = 1 per unit. What happens if load current is reduced to zero, if it was operating at leading power factor?
Explanation: Check for the Ia leading Ef and Ef < Vt.
9. 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
Explanation: The new excitation will be more than 1 pu to compensate for the flux required.
10. 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
Explanation: A is zpf lead for the alternator and D will be zpf lag.
Explanation: After the maximum excitation the alternator will have a fall out if the excitation is increased beyond synchronous speed.
12. Alternator compounding characteristic is obtained by
a) keeping Vt constant
b) keeping Ef constant
c) keeping Ia constant
d) varying Vt
Explanation: Alternator compounding characteristic is obtained by keeping Vt constant.
13. At zpf lag, the excitation should be increased in order to maintain the armature terminal voltage constant.
Explanation: Due to demagnetizing armature reaction at zpf lag in an alternator.
14. The rating of the alternator is decided by
c) armature current
Explanation: The losses determine the rating to be used for alternator.
15. The core losses in synchronous machine is _______ dependent.
Explanation: Core losses depend on the voltage applied to the machine.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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