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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