Electrical Machines Questions and Answers – Operating Characteristics of Alternators and their Ratings

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This set of Electrical Machines Question Bank focuses on “Operating Characteristics of Alternators and their Ratings”.

1. A 315V, 3-phase 400 MVA alternator is running at no load at constant excitations. The most appropriate representation in space domain is?
a) A 315V, 3-phase 400 MVA alternator is running at no load at constant excitation - option a
b) A 315V, 3-phase 400 MVA alternator is running at no load at constant excitation - option b
c) A 315V, 3-phase 400 MVA alternator is running at no load at constant excitation - option c
d) A 315V, 3-phase 400 MVA alternator is running at no load at constant excitation - option d
View Answer

Answer: a
Explanation: At no load Ia = 0. So Ef = Vt.
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2. A 315V, 3-phase 400 MVA alternator is running at lagging power factor at constant excitations. The most appropriate representation in space domain is?
a) A 315V, 3-phase 400 MVA alternator is at lagging power factor - option a
b) A 315V, 3-phase 400 MVA alternator is at lagging power factor - option b
c) A 315V, 3-phase 400 MVA alternator is at lagging power factor - option c
d) A 315V, 3-phase 400 MVA alternator is at lagging power factor - option d
View Answer

Answer: a
Explanation: Ia should lag Ef.

3. A 315V, 3-phase 400 MVA alternator is running at leading power factor load at constant excitations. The most appropriate representation in space domain is?
a) A 315V, 3-phase 400 MVA alternator is running at leading power factor load - option a
b) A 315V, 3-phase 400 MVA alternator is running at leading power factor load - option b
c) A 315V, 3-phase 400 MVA alternator is running at leading power factor load - option c
d) A 315V, 3-phase 400 MVA alternator is running at leading power factor load - option d
View Answer

Answer: a
Explanation: Ia will lead the excitation.
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4. A 315V, 3-phase 400 MVA alternator is running at unity power factor at constant excitations. The most appropriate representation in space domain is?
a) A 315V, 3-phase 400 MVA alternator at unity power factor at constant excitation - option a
b) A 315V, 3-phase 400 MVA alternator at unity power factor at constant excitation - option b
c) A 315V, 3-phase 400 MVA alternator at unity power factor at constant excitation - option c
d) A 315V, 3-phase 400 MVA alternator at unity power factor at constant excitation - option d
View Answer

Answer: a
Explanation: Ia in phase with excitation voltage.

5. A 315V, 3-phase 400 MVA alternator is running at zero power factor lagging at constant excitations. The most appropriate representation in space domain is?
a) A 315V, 3-phase 400 MVA alternator is at zero power factor lagging - option a
b) A 315V, 3-phase 400 MVA alternator is at zero power factor lagging - option b
c) A 315V, 3-phase 400 MVA alternator is at zero power factor lagging - option c
d) A 315V, 3-phase 400 MVA alternator is at zero power factor lagging - option d
View Answer

Answer: a
Explanation: Ia has to be lagging by 90°.
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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?
a) A 315V, 3-phase 400 MVA alternator is running at zero power factor leading - option a
b) A 315V, 3-phase 400 MVA alternator is running at zero power factor leading - option b
c) A 315V, 3-phase 400 MVA alternator is running at zero power factor leading - option c
d) none of the mentioned
View Answer

Answer: a
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?
a) The alternator at rated terminal voltage Vt at rated current Ia = 1 per unit - option a
b) The alternator at rated terminal voltage Vt at rated current Ia = 1 per unit - option b
c) The alternator at rated terminal voltage Vt at rated current Ia = 1 per unit - option c
d) The alternator at rated terminal voltage Vt at rated current Ia = 1 per unit - option d
View Answer

Answer: a
Explanation: Check for the Ia lagging Ef and Ef > Vt.
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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?
a) Load current is reduced to zero if operating at leading power factor - option a
b) Load current is reduced to zero if operating at leading power factor - option b
c) Load current is reduced to zero if operating at leading power factor - option c
d) Load current is reduced to zero if operating at leading power factor - option d
View Answer

Answer: a
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
d) zero
View Answer

Answer: a
Explanation: The new excitation will be more than 1 pu to compensate for the flux required.
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10. The variation of Vt vs Ia is for the alternator is given below.
A is zpf lead for the alternator & D will be zpf la in variation of Vt vs Ia
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
View Answer

Answer: a
Explanation: A is zpf lead for the alternator and D will be zpf lag.

11. Mark the plot for terminal voltage vs armature current for the alternator.
a) The plot for terminal voltage vs armature current for the alternator - option a
b) The plot for terminal voltage vs armature current for the alternator - option b
c) The plot for terminal voltage vs armature current for the alternator - option c
d) The plot for terminal voltage vs armature current for the alternator - option d
View Answer

Answer: a
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
View Answer

Answer: a
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.
a) True
b) False
View Answer

Answer: a
Explanation: Due to demagnetizing armature reaction at zpf lag in an alternator.

14. The rating of the alternator is decided by _________
a) losses
b) voltage
c) armature current
d) temperature
View Answer

Answer: a
Explanation: The losses determine the rating to be used for alternator.

15. The core losses in synchronous machine is _______ dependent.
a) voltage
b) current
c) temperature
d) insulation
View Answer

Answer: a
Explanation: Core losses depend on the voltage applied to the machine.

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice Electrical Machines Question Bank, here is complete set of 1000+ Multiple Choice Questions and Answers.

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