Electrical Machines - Synchronous Machines

Synchronous Machines - IES Previous year wise questions......

1. If the dimensions of all the parts of a synchronous generator, and the number of field and armature turns are doubled, then the generated voltage will change by a factor of  [IES-1997]
a. 1         b. 2        c. 4       d. 8

2. The phasor diagram of a synchronous machine connected to an infinite bus is shown i above figure The machine is acting as a (on graph x-axis terminal voltage 'v' and at some angle induced voltage 'E' & angle is approxmatly at 45) [IES-1998]
a. Generator and operating at leading power factor
b. Generator and operating at lagging power factor
c. Motor and operating at leading power factor
d. Motor and operating at lagging power factor

3. 1) Match List I (Power factor) with list II (Armature reaction of an alternator) and select the correct answer :
List I
A. UPF
B. ZPF lagging
C. ZPF leading
D. Intermediate Lagging
List II
1. Fully demagnetising
2. Fully magnetising
3. Cross - magnetising
4. Partly cross-magnetising and
5. partly demagnetising [IES-2000]

        A.       B.        C.       D
a.     3.        1.        4.       2
b.     3.        1.        2.       4
c.     1.        3.        2.       4
d.     1.        3.        4.       2

4. Match List - I (equivalent circuit parameter) with List -II(Values) for a 50 MVA three phase alternator and select the correct answer
List - I
A. Armature resistance
B. Synchronous reactance
C. Leakage reactance
List - II
1. 1pu
2. 0.1 pu
3. 0.01 pu [IES-2001]
a. A-1,B-2,C-3
b. A-3,B-1,C-2
c. A-3,B-2,C-1
d. A-1,B-3,C-2

5. When the excitation of normally operating unloaded salient-pole synchronous motor suddenly gets disconnected. it continues to run as a [IES-2002]
a. S charge motor
b. Spherical motor
c switched reluctance motor
d variable reluctance motor

6. If the field of a synchronous motor is under-excited, the power factor will be [IES-2003]
a. Leading
b. Lagging
c. Unity
d. More than unity

7.  Which one of the following is the primary reason for placing field on rotor in an alternator? [IES-2004]
a. Low current in field circuits
b. Insulation of high voltage is made easy on stator than on rotor
c. Large power in stator
d. Large current in stator

8. A synchronous motor is operated from a bus zero power factor lading current. Its synchronous reactance is 0.5u. What is the excitation emf of the motor? [IES-2005]
a. 2.0    b. 1.5.    c. 1.0    d. 0.5

9. A loss-less cylindrical rotor synchronous generator is floating on the system bus bar after being properly synchronized and is running at no-load. Now, if the prime mover is acting as a mechanical load on the machine, then [IES-2006]
a. the torque developed by the machine will act in the same direction as the prime-mover torque
b. the torque developed by the machine will act in the opposite direction to that of the prime mover torque
c. the machine rotor would fall-back and the induced emf would lag behind the terminal voltage
d.  the machine rotor would fall-back and the induced emf would lead the terminal voltage

10. A 10 pole, 25 Hz alternator is directly coupled to and is driven by 60Hz synchronous motor. What is the number of poles for the synchronous motor [IES-2007]
a. 48     b. 12       c. 24      d. 16

11.  Assertion (A) : AC armature winding are short chorded by selecting value of coil span more than the pole pitch.
Reason (R) : Short chording is done to eliminate harmonics in the induced e.m.f  [IES-2008]
a. Both are true and reason is correct explanation for assertion
b. Both are true and reason is not correct explanation for assertion
c. Assertion is correct and reason is false
d. Assertion is false and  reason is true

12. The resultant flux density in the air gap of a synchronous generator is the lowest during
a. Open circuit
b. Solid Short circuit
c. At half-load
d. At full-load

13. A three phase 50 Hz, 11KV and 37.5 MW at 0.9 p.f. capacity synchronous generator has its stator bore diameter of 765 cm and an axial core length of 80 cm. For which power plants this generator is suitable? [IES-2009]
a. Thermal coal fired plant
b. Hydroelectric power plant
c. Nuclear power plant
d. pumped Storage power plant

14. Consider the following :
1.EMF
2. Reversal
3. MMF
4.Direct
which method among these are for the determination of voltage regulation of an alternator?  [IES-2010]
a. 1,2,3 and 4
b. 1 and 3
c.  1 and 2
d.  2 and 3

15. Assertion (A) : While running on light load a salient pole synchronous machine can continue to operate on infinite bus - bars even if its excitation fails.
Reason (R) : A Salient pole machine develops reluctance power due to the variable reluctance offered by the salient poles  [IES-2011]
a. Both are true and reason is correct explanation for assertion
b. Both are true and reason is not correct explanation for assertion
c. Assertion is correct and reason is false
d. Assertion is false and  reason is true

16.  A synchronous generator has its effective internal impedance Zs = 10 ohm and resistance Ra = 1.0 ohm . Its generated voltage Ef and terminal voltage Vt are both 500 V. The maximum power out put is [IES-2012]
a. 5000w
b. 4550W
c. 3000 W
d. 2250 W

17. The maximum power delivered by 1500 kW, 3-phase, star-connected, 4 kV, 48 pole 50 Hz synchronous motor, with synchronous reactance of 4n/phase and unity power factor [IES-2013]
a. 4271.2 kw
b. 35.5 kw
c. 120.61 kw
d. 2078 kw