## electrical

### Alternator MCQ Questions and Answers Page - 3

1.
 The seventh space harmonics in the mmf produced by balanced fundamental frequency armature currents rotate at ____times the synchronous speed with respect to the field A 5/7 B 8/7 C 6/7 D 7/6 Answer : C Explanation :
2.
 The phase sequence of a three-phase alternator will reverse if A The filed current is reversed keeping the direction of rotation same B The field current remains the same but the direction of rotation is reversed C The field current is reversed and the number of poles is doubled D The number of poles is doubled without reversing the field current. Answer : B Explanation :
3.
 Power factor of an alternator driven by constant prime mover input can be changed by changing its A Speed B Load C Field excitation D Phase sequence Answer : C Explanation :
4.
 How can the reactive power delivered by a synchronous generator be controlled? A By changing the prime mover input B By changing the excitation C By changing the direction of rotation D By changing the prime mover speed Answer : B Explanation :
5.
 The rating of the prime mover driving an alternator is determined entirely by its ____rating A Voltage B Current C kW output D speed Answer : C Explanation :
6.
 When an alternator designed for operation at 60 Hz is operated at 50 Hz A kVA rating will increase in the ratio of 1.2 B operating voltage will reduce in the ratio of 5/6 C operating voltage will increase in the ratio of 1.2 D operating voltage will reduce in the ratio of (5/6)2 Answer : B Explanation :
7.
 The rating of a synchronous machine is usually governed by its A Speed B Temperature rise C Weight D None of these Answer : B Explanation :
8.
 The flux set up by the armature current, which does not cross the air gap and takes a different path is called the ____flux A Leakage B Main C Cross magnetizing D Demagnetizing Answer : A Explanation :
9.
 The magnitude of leakage flux depends on A The magnitude of armature current B Phase angle between armature current and terminal voltage C Air gap thickness D Both (a) and (b) Answer : D Explanation :
10.
 Leakage reactance of the armature of a salient pole synchronous machine which varies with the position of rotor is due to leakage flux A Of end connections B Which comes out of slot teeth, crosses the air gap and enters the pole faces C Which remains within the slot D None of the above Answer : B Explanation :
11.
 In a synchronous generator A The armature mmf leads the air gap flux and air gap flux leads the field mmf B The armature mmf leads the air gap flux and the air gap flux lags behind the field mmf C The armature mmf large behind the air gap flux and airgap flux lags behind the field mmf D The armature mmf lags behind the air gap and air gap flux leads the field mmf. Answer : C Explanation :
12.
 Armature reaction effect is / are ____on the main field flux A Distorting (or cross magnetizing) as well as demagnetizing B Only assisting C Only demagnetizing D Only distorting Answer : A Explanation :
13.
 The armature reaction effect in a synchronous machine depends on A Load current B Power factor of the load C Speed of the machine D Both (a) and (b) Answer : D Explanation :
14.
 In a synchronous generator, the effect of cross magnetization is to make the generated voltage A Truly sinusoidal B Non sinusoidal C Free from harmonics D None of these Answer : B Explanation :
15.
 In an alternator, the armature mmf will be in phase with the main field flux only when the load is purely A Resistive B Inductive C Capacitive D None of these Answer : C Explanation :
16.
 In an alternator, the armature reaction is considered to be equivalent to a fictitious A Reactance B Resistance C Impedance D Admittance Answer : A Explanation :
17.
 A synchronous generator is feeding a zero-power factor (lagging) load at rated current. The armature reaction is A Magnetizing B Demagnetizing C Cross magnetizing D Ineffective Answer : B Explanation :
18.
 Armature reaction AT of a synchronous generator supplying power at rated voltage with zero power factor lagging is A Magnetizing B Demagnetizing C Cross magnetizing D Both magnetizing and cross magnetizing Answer : B Explanation :
19.
 The synchronous reactance is the A Reaction 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 Answer : C Explanation :
20.
 Drop in terminal voltage of an alternator due to armature reaction is countered by A Damper winding B Effect of saliency C Increased prime mover output D Automatic voltage regulator Answer : D Explanation :
21.
 The saturated synchronous reactance of an alternator is _____its unsaturated synchronous reactance A More than B Less than C Equal to D None of these Answer : B Explanation :
22.
 In an alternator, short circuit current is limited by A Saturated synchronous impedance B Unsaturated synchronous impedance C Either of the above D None of these Answer : B Explanation :
23.
 When the load (inductive) is thrown off, the terminal voltage will A Increase B Decrease C Remain unchanged D None of these Answer : A Explanation :
24.
 As the leading power factor of the load of an alternator decreases, the magnitude of generated voltage required to give rated terminal voltage A Increases B Decreases C Remains unchanged D None of these Answer : B Explanation :
25.
 In a synchronous generator delivering lagging power factor load A The excitation emf leads terminal voltage by the power angle B The excitation emf lags the terminal voltage by the power angle C Excitation emf leads the terminal voltage by the power factor angle D None of the above Answer : A Explanation :
26.
 The power factor on which an alternator operates depends on the A Speed of the prime mover B Nature of the load being supplied C Armature losses D Copper losses Answer : B Explanation :
27.
 Unbalanced 3-phase stator currents cause A Vibrations B Heating of rotor C Double frequency currents in the rotor D All of the above Answer : D Explanation :
28.
 Regulation of an alternator supplying resistive or inductive load is A Always +ve B Always -ve C Either of the above or zero D None of the above Answer : A Explanation :
29.
 Alternators of a central power station will have A Revolving field winding B Revolving armature winding C Either of the above D None of the above Answer : A Explanation :
30.
 Non-salient pole type of rotor construction is usually provided in the alternators used in A Hydropower stations B Thermal power stations C Either of the above D None of the above Answer : B Explanation :
31.
 In an alternator terminal voltage rise will be more A When leading load is throw off B When lagging load is thrown off C When unity power factor load is thrown off D None of the above Answer : B Explanation :
32.
 Turbo-alternator usually have A 12 poles B 8 poles C 4 poles D 2 poles Answer : D Explanation :
33.
 In an alternator short pitch coils are used A To reduce the stray losses B To reduce the size of the machine C To provide accurate phase difference of 120° between each phase D To reduce the harmonic in generated e.m.f Answer : D Explanation :
34.
 If the input prime mover of an alternator is kept constant but the excitation is increased then A KVA will be lagging B The power factor of the load remains constant C KVA will be leading D KVA will be changed Answer : A Explanation :
35.
 Generated e.m.f for same field current and double speed will be A Same B Double C Less than double D More than double Answer : B Explanation :
36.
 If the space flux distribution is non-sinusoidal e.m.f induced in the distributed winding A Will be less sinusoidal than flux distribution B Will be more sinusoidal than flux distribution C Will be equally non-sinusoidal D None of the above Answer : B Explanation :
37.
 Voltage regulation obtained by synchronous impedance method is A Equal to that given by actual test B Less than that given by actual tests C More than that given by actual tests D None of the above Answer : C Explanation :
38.
 In an alternator short-circuit current is limited by A The saturated synchronous impedance B The unsaturated synchronous impedance C Either of the above D None of the above Answer : B Explanation :
39.
 In an alternator, armature reaction is considered equivalent to A Fictitious impedance B Fictitious conductance C Fictitious reactance D Fictitious resistance Answer : C Explanation :
40.
 Synchronous impedance method gives more regulation as compared to ampere turn method because A Armature reaction is considered negligible B Saturation effect is ignored in the synchronous impedance C Saturation effect is taken into account in the synchronous impedance method D None of these Answer : A Explanation :
41.
 Leakage reactance of the armature of a salient pole synchronous machine which varies with the position of the rotor is due to A Leakage flux which remains within the slot B Leakage flux of end connection C Leakage flux which comes out of slot teeth, crosses the air gap and enters the pole faces D None of the above Answer : C Explanation :
42.
 Turbo-rotor is made up of A Solid steel forging having milled slots for field winding B Large spider to which laminated pole shoe are secured C Laminations D None of the above Answer : A Explanation :
43.
 A commercials alternator has A Stationary armature and rotary field B Rotating armature and stationary field C Both armature and field rotary D Both armature and field fixed Answer : A Explanation :
44.
 For the alternators operating in parallel, if the load shared by one of them is to be increased, its field excitation is A To be weekend keeping input torque same B To be strengthened keeping input torque should be increased C To be kept constant but input torque should be increased D To be kept constant but input torque should be decreased Answer : C Explanation :
45.
 Which of the following methods is best for finding the voltage regulation? A Synchronous impedance method B M.M.F method C Potier triangle method D All are equally good Answer : C Explanation :
46.
 For a 3-phase winding with 5 slots per pole per phase and with coil span of 12 slot pitch, the value of pitch factor is A 0.851 B 0.951 C 0.98 D 1.05 Answer : B Explanation :
47.
 Salient pole rotors are used where A High frequency current is required B Floor space is available plenty C Low and medium speed prime-mover are available D High speed prime movers are available Answer : C Explanation :
48.
 The power factor of an alternator is determined by its A Prime mover B Excitation C Speed D Load Answer : D Explanation :
49.
 If the input to the prime-mover of an alternator is kept constant but excitations changed then A The power factor of the load remains constant B The reactive component of the output is changed C The active component of the output is changed D None of the above Answer : B Explanation :
50.
 For parallel operation, A.C polyphase alternators must have the same A KVA rating B Excitation C Speed D Voltage rating Answer : D Explanation :
51.
 Unlike D.C generator kW rating, alternator are rated in A MW B kVAR C kWh D kVA Answer : D Explanation :
52.
 Dirt accumulation in generator can cause A flashover B overheating C poor voltage regulation D all of the above Answer : D Explanation :
53.
 An alternator running in parallel with other alternators all having automatic voltage regulators is to be taken off the bus. The usual procedure before opening the switch is to A Reduce the power fed to the prime mover B Reduce alternator excitation C Increase alternator excitation D None of the above Answer : A Explanation :
54.
 Due to which of the following reasons concentrated windings are not used in alternators? A Concentrated windings increase voltage harmonic B Concentrated windings decrease induced e.m.f/phase C Concentrated windings increase copper to iron ratio thereby decreasing the capacity D Concentrated windings require deep slots for accommodation which leads to increased armature leakage and reactance Answer : D Explanation :
55.
 A stationary alternator should not be connected to alive bus bar because it A Will get short-circuit B Will get short-circuit C Is likely to run as a synchronous motor D Will decrease bus bar voltage though momentary Answer : A Explanation :
56.
 The power drawn by the prime-mover of an alternator, under no-load condition, goes to A Meet copper losses both in armature and rotor winding B Produce power in armature C Meet no-load losses D Produce e.m.f in armature winding Answer : C Explanation :
57.
 In a large generator, damper A Reduce frequency fluctuations B Increase stability C Reduce voltage fluctuations D None of the above Answer : B Explanation :
58.
 The choice of field construction, in an alternator is basically determined by A Generated voltage needed by the customer B Comparative cost of the field systems C Supply frequency required by the consumers D The kind of energy source available in the geographic locations Answer : D Explanation :