Synchronous machine_objectives

1.A 3-phase alternator with high rotational inertia and negligible armature resistance was delivering power to an isolated load when the armature terminals got short circuited. After a while, the mechanical power input to the shaft of the machine is terminated at time t0. The short-circuit current Is circulating in the armature will very till the machine comes to rest at time ts. Which one of the curves shown in the following figures best reflects this variation?


2.When a synchronous motor is running at synchronous speed, the damper winding
produces
a. Damping torque
b. Eddy current torque
c. Torques aiding the developed to torque
d. No torque

3.For a given developed power, a synchronous motor operating from a constant voltage and constant frequency supply, will draw the minimum and maximum armature currents, Imin and Imax respectively, corresponding to
a. Imm at unity pf, but Imax at zero pf
b. Imax at unity pf, but Imin at zero pf
c. Both Imin and Imax at unity pf
d. Both Imin and Imax at zero pf

4.Match list-I (Type of winding) with list-Il (Feature) and select the correct answer
using the codes given below the Lists:
List I
A. Fractional slot windings
B. Chorded and distributed windings
C. Damper windings
D. Single-phase windings
List II
1. Only two- third of the pole pitch is occupied by the winding
2. Effects of slot harmonics are reduced or even eliminated
3. The effects of phase belt harmonics are reduced
4. Additional asynchro nous torque capability in projected pole motors
---A B C D
a. 2 3 4 1
b. 2 3 1 4
c. 3 2 1 4
d. 3 2 4 1


5.While conducting a “slip’ test for the determination of direct and quadratureaxis synchronous reactance’s Xd and Xq of a salient pole synhronous 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 the armature terminal voltage,
3. Envelope of the 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

6.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
a. 1
b. 2
c. 4
d. 8


7.Consider the following statements
The effects of slot harmonics are
1. skewing the slots
2. fractional slot winding.
3. short - chorded winding
4. distributed winding.
Of these statements
a. 1 and 2 are correct
b. 2, 3 and 4 correct
c. 1, 3 and 4 are correct
d. 1 and 3 are correct

8.Which one of the following methods would give a higher than actual value of regulation of an alternator?
a. ZPE method
b. MMF method
c. EMF method
d. ASA method


9.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 sped 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 a stop


10.An ideal synchronous motor has no starting torque because the
a. rotor is made up of salient poles
b. relative velocity between the stator and the rotor mmf’s is zero
c. relative velocity between stator and rotor mmf’s is not zero
d. rotor winding is highly reactive


11.A synchronous motor is operating on noload at unity power factor. If the field current is increased, the power factor will become
a. leading and the current will decrease
b. lagging and the current will increase
c. lagging and the current will decrease
d. leading and the current will increase

12.A synchronous motor with negligible armature resistance runs at a load angle of 20° at the rated frequency. If supply frequency is increased by 10%, keeping other parameters constant, the new load angle will be
a. 16°
b. 18°
c. 20°
d. 22°


13.A synchronous motor is running from an infinite bus of voltage Vt in steady-stat, at about 50% of its rated load with a powerangle , between Vt and Ef (induced voltage). The load is suddenly decreased to 25%. Ef attains its new steady-state powerangle with Vt by initially,
a. falling behind and making a complete rotation
b. advancing and making a complete rotation
c. falling behind, followed by oscillation about
d. advancing, followed by oscillation about


14.The two-reaction phasor diagram of a salient-pole synchronous generator is considered, where Ia = armature current per phase, X1 = armature leakage reactance per phase, Xd = direct-axis synchronous reactance, Xq = quadrature-axis synchronous reactance and V = generator terminal voltage, taken as reference. The voltage in phase with the direct-axis will be the phasor-sum of
a. V + Iara + IaX1
b. V + Iara + IaXd
c. V + Iara + IaXq
d. V + Iara + Ia(Xd + Xq)

15.Consider the following statements : The synchronous generators used in thermal power stations have
1. cylindrical rotors
2. slip rings and brushes
3. laminated rotor
4. stator slots in multiples of 6
5. 3-phase star connected stator windings
Which of these statements are correct?
a. 1, 2, 3 and 4
b. 1, 2, 3 and 5
c. 3, 4 and 5
d. 1, 2, 4 and 5

16.Slip test is performed to determine
a. Slip
b. direct axis reactance and quadrature axis
c. positive sequence reactance and negative sequence reactance
d. sub-transient reactance


17.Match List I (methods of full-load regulation of 25 MVA alternator at 0.8 Power factor lagging) with list II (% regulation):
List I
A. emf
B. mmf
C. ZPF
List II
1. 13%
2. 18%
3. 32%

---A B C
a. 3 1 2
b. 1 3 2
c. 2 3 1
d. 1 2 3

18.The phasor diagram of a synchronous machine connected to an infinite bus is shown in figure The machine is acting as a


a. generator and operating at a lagging power factor
b. generator and operating at a leading power factor
c. motor and operating at a leading power factor
d. motor and operating at a lagging power factor

19.An induction motor and synchronous motor are connected to a common feeder line. To operate the feeder line at unity pf, the synchronous motor should be
a. under-excited
b. over-excited
c. normally excited
d. disconnected from the common terminals


20.Consider the following statements regarding the design of distributed armature winding in a 3-phase alternator :
1. It reduces the phase belt harmonics
2. It increases utilisation of armature iron and copper
3. It increases rigidity and mechanical strength of the winding
4. It reduces copper in the overhang of the winding
Which of these statements are correct?
a. 1, 3 and 4
b. 2, 3 and 4
c. 1, 2 and 3
d. 1, 2 and 4


21.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

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

22.In a synchronous machine, damper windings are used to
a. help in starting as a motor
b. run it as an induction motor
c. help in starting as a motor and to reduce hunting
d. increase efficiency

23.Match List I with list II and select the correct answer:

List I
Flux due to
A. currents in damper windings
B. currents in field windings
C. armature currents with unity internal pf
D. armature currents with zero pf

List II
1. Direct axis synchronous reactance
2. Quadrature axis synchronous reactance
3. Transient reactance
4. Sub-transient reactance

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

24.In the ‘V’ curve shown in the figure for a synchronous motor, the parameter of x and y coordinates are, respectively

a. Armature current and field current
b. Power factor and field current
c. Armature current and torque
d. Torque and field current

25.Match List I (Equivalent circuit parameter) with List II (Values) for a 50 MVA three phase alternator arid select the correct answer:

List I
A. Armature resistance
B. Synchronous reactance
C. Leakage reactance

List II
1. 1 Pu
2. 0.1 Pu
3. 0.01 pu

---A B C
a. 1 2 3
b. 3 1 2
c. 3 2 1
d. 1 3 2


26. Synchronous condenser means
a. A synchronous motor with capacitor connected across stator terminals to improve pf
b. A synchronous motor operating at full load with leading pf
c. An over-excited synchronous motor partially supplying mechanical load, and also improving. Pf of the system to which it is connected
d. An over-excited synchronous motor operating at no-load with leading pf used in large power stations for improvement of pf


27. Match List I with List II aid select the correct answer:

List I (Tests)
A. No-load and blocked rotor test
B. Sumpner’s test
C. Swinburne’s test

List II (Machines)
1. Transformer
2. Induction motor
3. Synchronous motor
4. dc motor

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


28. Match List 1 with List II and select the correct answer:

List I (Regulation method)
A. Synchronous impedance method
B. Mmf method
C. Zero power factor (ZPF) method
D. American standard Association method

List II (Relevant phasor)
1. emf phasor
2. Predominantly mmf phasor
3. Both emf and mmf phasors
4. emf and mmf phasors including saturation

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

29. In a cylindrical rotor synchronous machine, the phasor addition of stator and
rotor mmfs is possib1 because
a. Two mmfs are rotating in opposite direction
b. Two mmfs are rotating in same direction at different speeds
c. Two mmfs are stationary with respect to each other
d. One mmf is stationary and the other mmf is rotating

30.The ‘synchronous - impedance method’ of finding the voltage regulation by a cylindrical rotor alternator is generally considered
a. a pessimistic method because saturation is not considered V
b. an optimistic method because saturation is not considered
c. a fairly accurate method even if power factor is not taken into account while
determining synchronous impedance
d. a fairly accurate method when power factor is taken into account while determining synchronous impedance

31.Generally the no-load losses of an electrical machine is represented in its equivalent circuit by a
a. parallel resistance with a low value
b. series resistance with a low value
c. parallel resistance with a high value
d. series resistance with a high value


32.The power factor of a synchronous motor
a. improves with increase in excitation and may even become leading at higher excitations
b. decreases with increase in excitation
c. is independent of its excitation
d. increases with loading for a given excitation


33.When the excitation of normally operating unloaded salient-pole synchronous motor suddenly gets disconnected, it continues to run as a
a. Schrage motor
b. Spherical motor
c. Switched- reluctance motor
d. Variable - reluctance motor


34.A 6-pole, 3-phase alternator running at 1000 rpm supplies to an 8-pole, 3-phase induction motor which has a rotor current of frequency 2 Hz. The speed at which the motor operates is
a. 1000 rpm
b. 960 rpm
c. 750 rpm
d. 720 rpm


35.For maximum current during ‘Slip Test’ on a synchronous machine, the armature and aligns along
a. d-axis
b. q-axis
c. 45° to d-axis
d. 45° to q-axis


36.Match List I with List II and select the correct answer:

List I (Name of test)
A. Open circuit and short circuit tests
B. Open circuit and zero power factor tests
C. Slip test
D. Maximum lagging current test

List II (Result)
1. Leakage reactance
2. Direct axis synchronous reactance
3. Quadrature axis synchronous reactance
4. Ratio, of direct axis synchronous reactance to quadrature axis synchronous reactance

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


37.A 3-phase 50 MVA 10 kV generator has a reactance of 0.2 ohm per phase. Hence the per-unit value of the reactance on. a base of 100 MVA 25 kV will be
a. 1.25
b. 0.625
c. 0.032
d. 0.32


38. The results of a ‘Slip Test’ for determining direct-axis (Xd) and quadrature-axis (Xq) reactance’s of a star-connected salient - pole alternator are given below
phase values: Vmax =108V; Vmin = 96V,Imax =12A, Imin = 10A. Hence the two reactance will be

a. Xd = 10.8 ohms and Xq = 8 ohms
b. Xd = 9 ohms and Xq = 9.6 ohms
c. Xd = 9.6 ohms and Xq = 9 ohms
d. Xd = 8 ohms and Xq = 10.8 ohms

39.If the field of a synchronous motor is under excited, the power factor will be
a. Lagging
b. Leading
c. Unity
d. More than unity


40.Which of the following graphs represents the speed-torque characteristic of a synchronous motor?



41.Match List I (Machine Components) with List II (Functions) and select the correct
answer :

List I
A. Damper Winding
B. Interpole Winding
C. End Ring
D. Conservator

List II
1. Used in synchronous motor to provide starting torque
2. Used in d.c. generator to neutralize the reactance voltage and cross magnetizing effect of armature reaction
3. Used to short circuit rotor bars of cagetype induction motor
4. Used to protect transformers from damage when oil expands due to rise in temperature

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

42.The synchronous reactance is the
a. Reactance due to armature reaction of the machine
b. Reactance due to leakage flux.
c. Combined reactance due to leakage flux and armature reaction
d. Reactance either due to armature reaction or leakage flux


43. Match List I (Type of AC Winding) with List II (Feature of AC Windings) and
select the correct answer :

List I
A. Single Layer
B. Double Layer
C. Full Pitch
D. Chorded

List II
1. Coil span and pole pitch are. equal
2. Coil span is shorter than pole pitch
3. Number of slots is equal to number of coils
4. Number of slots is twice the number of coils

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

44. Match List I (Machines) with List II (Tests) and select the correct answer :

List I
A. Transformer V
B. DC Motor
C. Alternator
D. Induction Motor

List II
1. Slip Test
2. Blocked Rotor Test
3. Sumpner’s Test
4. Swinburne’s Test

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


45.The leakage reactance of a three-phase alternator determined by performing
a. Open circuit and zero power factor tests
b. Zero power factor and slip tests
c. Open- and short-circuit tests
d. Short-circuit and slip tests

46.Four important parameters of alternators have comparatively larger or smaller values. In comparison to a steam turbine driven alternator, a hydraulically driven machine will have which one of the following combinations?
---Number of | Axial | Number | Operating
---armature | length of | of poles | speed
---conductors | conductors | |
a. Smaller | larger | smaller | higher
b. Larger | smaller | larger | lower
c. Larger | larger | smaller | lower
d. Smaller | smaller | larger | higher


46. Which one of the following is the primary reason for placing field on rotor in an alternator?
a. Small power .in field circuit
b. Insulation of high voltage is made easy on stator than on rotor
c. Large power in stator
d. Large current in the stator

47.Consider the following:
1. Supply voltage
2. Excitation current
3. Maximum value of load angle

The maximum power developed by a synchronous motor is a function of which of the bove?
a. 1 and 2
b. 1 and 3
c. 2 and 3
d. 1, 2 and 3


48.When a 3–phase alternator is suddenly short–circuited at its terminals, the initial value of the short–circuit current is limited by which one of the following ?
a. Subtransient reactance x”d
b. Transient reactance x’d
c. Synchronous reactance xs
d. Sum of x”d, x’d and xs


49.Which one of the following methods gives more accurate result for determination of voltage regulation of an alternator?
a. m.m.f. method
b. Synchronous impedance method
c. Potier triangle method
d. American Institution Standard method


50.Which one of the following statements is correct?

In a salient pole synchronous machine the air gap is
a. uniform under the whole pole shoe
b. least under the middle of the pole shoe and increases outwards
c. largest under the middle of the pole shoe and decreases outwards
d. least at one end of the pole shoe and increases to the maximum Value at the other end


51.Which one of the following is not a necessary condition to be satisfied for synchronizing an incoming alternator to an already operating alternator?
a. Same voltage magnitude
b. Same frequency
c. Same prime mover speed
d. Same phase sequence

52.A synchronous motor is operated from a bus voltage of 1.0 pu and is drawing 1.0 pu zero power factor leading current. Its synchronous reactance is 0.5 pu. What is the excitation emf of the motor?
a. 2.0
b. 1.5
c. 1.0
d. 0.5

53.In the measurement of Xd, Xq (in ohms),following data are obtained by the slip test on a salient pole machine:
Id max = 10 A Id min = 6.5 A
Vd max = 30 V Vd min = 25 V
Which one of the following is correct?
a. Xd = 3, Xq = 3.86
b. Xd = 4.615, Xq = 2.5
c. Xd = 3, Xq = 2.5
d. Xd = 4.61, Xq = 3.86


54. What are the conditions to be satisfied for alternator to be synchronized with an
incoming supply?
1. Equal voltage
2. Equal frequency
3. Same power rating
4. Same phase sequence

Select the correct answer using the code
given below:
a. 2, 3 and 4
b. 3 and 4
c. 1, 2 and 3
d. 1, 2 and 4

55.What is the value of the 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

56.Which one of the following statements is not correct in respect of synchronous
machines ?
a. In salient pole machines, the directaxis 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 the 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 the armature current with field excitation at a given output power.


57.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


58.Which of the following conditions are to be satisfied for proper synchronization of alternators?
1. Equal terminal voltage
2. Same frequency
3. Same phase sequence
4. Same kVA rating
5. Same phase displacement

Select the correct answer using the code given below:
a. 1, 3, and 4 only
b. 1, 2, 4 and 5 only
c. 2, 3, 4 and 5 only
d. 1, 2, 3 and 5 only

59. 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