This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Voltage Regulation of an Alternator”.
1. When the load on an alternator is varied, its terminal voltage also changes due to _____________
a) armature resistance
b) leakage reactance
c) armature reaction
d) all of the mentioned
Explanation: Armature reactance, leakage reactance and armature reaction cause the terminal voltage to differ from the excitation voltage.
2. Voltage regulation for the alternator operating at leading power factor is negative.
Explanation: Voltage regulation for an alternator operating at leading power factor is negative due to magnetizing nature of armature reaction.
3. Voltage regulation for an alternator operating at leading power factor is negative due to ___________
a) magnetizing nature of armature reaction
b) demagnetizing nature of armature reaction
c) cross-magnetizing nature of armature reaction
d) all of the mentioned
Explanation: Field flux gets aided by the armature reaction leading to higher Vt.
4. Alternator on infinite bus bar has constant ___________
a) terminal voltage and frequency
c) power factor
d) power factor and terminal voltage
Explanation: Infinite bus bar has constant voltage and frequency.
5. The emf method of the voltage regulation is applicable only to cylindrical rotor alternator due to ___________
a) resultant air gap flux is not affected by angular position of rotor
b) uniform angular position of rotor
c) non uniform angular position of rotor
d) saliency of the poles is a trouble while estimating the emf
Explanation: It is so because resultant air gap is unaffected by rotor position.
6. Emf method is also known as ___________
a) pessimistic method
b) optimistic method
c) zero power factor method
d) none of the mentioned
Explanation: Unsaturated impedance is greater than saturated impedance, so the drop accounted will be less than the actual making the voltage regulation more than actual.
7. In mmf method ___________
a) all the emf is scaled to mmf
b) only the mmf values are considered neglecting impedance drop
c) all the emf are taken zero
d) emf is converted to saturated impedance drops
Explanation: In mmf method all the emf is scaled to mmf
8. Mmf method of voltage regulation is called _______ while the emf method is __________
a) optimistic, pessimistic
b) pessimistic, pessimistic
c) optimistic, optimistic
d) pessimistic, optimistic
Explanation: In the mmf method the calculated value is lesser than the actual value so it is less misleading than emf method which predicts a greater value.
9. The preferred order of calculating the voltage regulation is ___________
a) ZPF > ASA > MMF > EMF
b) ZPF > MMF > ASA > EMF
c) ASA > MMF > ASA > EMF
d) EMF > ASA > ZPF > MMF
Explanation: Emf method is least preferred over the other methods and zpf method gives the accurate results as it does not alter the values.
10. In a calculation, the actual voltage regulation is 33.1% while the calculated value of the voltage regulation is 56.8%. This infers that the chosen method has been ___________
Explanation: As the given value is more than actual, so it must be emf method.
11. In a calculation, the actual voltage regulation is 33.1% while the calculated value of the voltage regulation is 25%. This infers that the chosen method has been ___________
Explanation: As the given value is less than actual, so it must be mmf method.
12. In a calculation, the actual voltage regulation is 33.1% while the calculated value of the voltage regulation is 32.9%. This infers that the chosen method has been ___________
a) zpf or asa
d) mmf or zpf
Explanation: As the given value is very close to actual, so it must be zpf or asa method of regulation.
13. If the emf from the air gap line is the 3-phase alternator is 440V per phase and armature current is 110 A. The synchronous reactance is?
a) 4 ohms
b) 2 ohms
c) 6.92 ohms
d) 2.32 ohms
Explanation: Z = V/I = 440/110 = 4 ohms.
14. If the machine size increases, the three phase alternator resistance _______ and synchronous reactance _______
a) increases, decreases
b) decreases, increases
c) remains constant, decreases
d) decreases, decreases
Explanation: If the machine size increases, the copper amount will increase but the air gap will also increase having more reluctance so the reactance will reduce.
15. Ideally the voltage regulation of an alternator should be ___________
Explanation: Ideally the terminal voltage and the excitation voltage should be same.
So V.R. = E-V/V = 0%.
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