Alternator Short Question Answer

1. The zero power factor characteristic for the Potier diagram can be obtained by loading the alternator using

(a) lamp load.
(b) synchronous motor.
(c) water load.
(d) dc motor. [U.P.S.C. I.E.S. 1999]

2. Modern alternators are designed to have poor regulation as it

(a) increases the value of short-circuit current.
(b) limits the value of short-circuit current.
(c) increases the efficiency of operation.
(d) both (a) and (c).

3. The phasor diagram by ASA method gives

(a) reliable results for both regulation and power angle of a salient pole synchronous generator.
(b) reliable result for power angle but erroneous result for regulation. (c) reliable result for regulation but erroneous result for power angle. (d) erroneous results for both regulation and power angle of a salient-pole synchronous generator.

4. Armature reaction mmf and leakage reactance of a synchronous machine are determined by

(a) open-circuit and short-circuit tests.
(b) open-circuit and zero pf tests.
(c) open-circuit test only.
(d) zero pf test only.

5. Consider the following:

1. EMF
2. Reversal
3. MMF
4. Direct

Which methods among these are for the determination of voltage regulation of an alternator?
(a) 1, 2, 3 and 4.
(b) 1 and 2 only.
(c) 2 and 3 only.
(d) 1 and 3 only. [U.P.S.C. I.E.S. E.E.-II, 2010]

6. Consider the following:

1. LPF method.
2. EMF method.
3. ZPF method.
4. MMF method.

Which of the above methods are correct for determination of voltage regulation of an alternator?
(a) 1, 2 and 3.
(b) 2, 3 and 4.
(c) 2 and 3 only.
(d) 3 and 4 only. [U.P.S.C. I.E.S. E.E.-II, 2009]

7. Which one of the following methods would give higher than actual value of regulation of an alternator?

(a) ZPF method.
(b) MMF method.
(c) EMF method.
(d) ASA method. [U.P.S.C. I.E.S. E.E.-II, 1998]

8. Which one of the following methods gives more accurate result for determination of voltage regulation of an alternator?

(a) MMF method.
(b) Synchronous impedance method.
(c) Potier triangle method.
(d) American Institution Standard method. [U.P.S.C. I.E.S. E.E.-11, 2004]

9. The four methods of calculation of voltage regulation of a 3-phase alternator are

1. EMF method.
2. Saturated synchronous reactance method.
3. New ASA method.
4. MMF method.

The correct sequence of the ascending order of the values of regulation obtained by these methods is
(a) 3, 4, 2, 1
(b) 4, 3, 1, 2
(c) 3, 4, 1, 2
(d) 4, 3, 2, 1 [U.P.S.C. I.E.S. 1993]

10. By slip test on a 3-phase synchronous generator the maximum and minimum impressed voltages per phase are found to be V_max and V_min respectively whereas the maximum and minimum phase currents are found to be I_max and l_min respectively. The values of the direct axis synchronous reactance X_d and quadrature axis synchronous reactance X_q are given as

(a) X_d = V_max/I_min ; X_q = V_min/I_max
(b) X_d = V_max/I_min ; X_q = V_min/I_min
(c) X_d = V_max/I_min ; X_q = V_max/I_max
(d) X_d = V_min/I_max ; X_q = V_max/I_min

11. Which one of the following statements is not correct in respect of synchronous machines?

(a) In salient pole machines, the direct-axis synchronous reactance, is greater than the quadrature axis synchronous reactance.
(b) The damper bars help the motor to self start.
(c) Short-circuit ratio is the ratio of field current required to produce the rated voltage on open circuit to the rated armature current.
(d) The V-curve of a synchronous motor represents the variation in armature current with field excitation at a given output power. [GATE E.E. 2005]

12. Slip test is performed to determine

(a) slip.
(b) direct-axis reactance and quadrature axis reactance.
(c) positive-sequence reactance and negative-sequence reactance.
(d) sub-transient reactance. [U.P.S.C. I.E.S. E.E.-11, 1999]

13. For maximum current during slip test on a synchronous machine, the armature mmf aligns along

(a) d-axis.
(b) q-axis.
(c) 45° to d-axis.
(d) 45° to q-axis. [U.P.S.C. I.E.S. E.E.-II, 2002]

14. While conducting a “slip” test for determination of direct- and quadrature-axis synchronous reactance X_d and X_q of salient pole synchronous machine, the rotor of the machine is run with a slip ‘s’ and stator supply frequency ‘f’ . The frequency of

1. voltage induced across open-field terminals
2. envelope of armature terminal voltage
3. envelope of armature current and
4. armature current will be respectively

(a) sf, sf, sf and f.
(b) sf, f sf and f
(c) f, sf, f and sf.
(d) f, (1 — s) f, (2 — s) f and sf. [U.P.S.C. I.E.S. E.E.-II, 1997]

15. In which one of the following is reluctance power developed?

(a) Salient pole alternator.
(b) Non-salient pole alternator.
(c) Squirrel cage induction motor.
(d) Transformer. [U.P.S.C. I.E.S. E.E.-II, 2006]

16. Which is the value of load angle when the power output of a salient pole synchronous generator is maximum?

(a) 0°
(b) 45°
(c) 90°
(d) none of the above. [U.P.S.C. I.E.S. 2006]

17. Higher synchronous reactance is preferred in the present day alternators, because one can have

(a) reduced subtransient current.
(b) reduced harmonic currents.
(c) reduced transient currents.
(d) higher voltage regulation with load. [GATE E.E. 1999]

18. When a 3-phase alternator is suddenly short-circuited at its terminals, the initial value of short-circuit current is limited by which one of the following:

(a) Subtransient reactance X”_d.
(b) Transient reactance X’_d.
(c) Synchronous reactance X_s.
(d) Sum of X”_d , X’_d and X_s. [U.P.S.C. I.E.S. E.E.-11, 2004]

19. X_d, X’_d and X”_d are steady-state d-axis synchronous reactance, transient d-axis reactance and subtransient d-axis reactance of a synchronous machine respectively. Which of the following statements is true?

(a) X_d > X’_d > X”_d
(b) X”_d > X’_d > X_d
(c) X’_d > X”_d > X_d
(d) X_d > X”_d > X’_d [GATE E.E. 2001]

20. An alternator with higher value of SCR has

(a) poor voltage regulation and lower stability limit.
(b) better voltage regulation and higher stability limit.
(c) poor voltage regulation and higher stability limit.
(d) better voltage regulation and low stability limit. [A.M.I.E. Sec B. Winter 1994]

21. A 3-φ alternator has negligible stator resistance. A short-circuit test is conducted on this alternator. At a particular speed a field current of I_f1 is required to drive the rated armature current. If the speed of the alternator is reduced to half, the field current required to maintain rated armature current

(a) would be equal to I_fi.
(b) would be equal to 2I_f1.
(c) would be equal to I_f1/2 .
(d) cannot be predicted due to insufficient data. [GATE 1993]

22. Which one of the following is the correct statement?

(a) The armature current upon symmetrical 3-phase, short-circuit of a synchronous machine (armature resistance is negligible) constitutes q-axis current only.
(b) The armature current upon symmetrical 3-phase short-circuit of a synchronous machine (armature resistance is negligible) constitutes d-axis current only.
(c) The armature current upon symmetrical 3-phase short-circuit of a synchronous machine (armature resistance is negligible) has both d-axis and q-axis component.
(d) Short circuit current cannot be divided into d- and q-axes components. [U.P.S.C. I.E.S. E.E.-II, 2007]

23. If the excitation of a 3-phase alternator operating on infinite bus bars is changed, which one of the following shall alter?

(a) Active power of machine.
(b) Reactive power of machine.
(c) Terminal voltage of machine.
(d) Frequency of machine. [U.P.S.C. I.E.S. 2008]

24. A 3-phase synchronous generator with constant steam input supplies power to an infinite bus at a lagging power factor. If the excitation is increased

(a) both power angle and power factor decrease.
(b) both power angle and power factor increase.
(c) the power angle decreases while power factor increases.
(d) the power angle increases while power factor decreases. [U.P.S.C. IES. 2005]

25. A turbo-alternator set feeds power to a 3-phase constant voltage, constant frequency bus. If the steam supply to the set is cut off, then the set will

(a) continue to run at rated speed in the same direction.
(b) continue to run at a reduced speed in the same direction.
(c) run at rated speed in the reverse direction.
(d) come to stop. [U.P.S.C. I.E.S. E.E.-II 1998]

26. A stand alone engine driven synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nulify the inductive current. For this operating condition

(a) the field current and fuel input have to be reduced.
(b) the field current and fuel input have to be increased.
(c) the field current has to be increased and fuel input left unaltered. (d) the field current has to be reduced and fuel input left unaltered. [GATE. E.E. 2003]

 

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