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MCQ in AC Generators Part 3 | REE Board Exam

MCQ in AC Generators Part 3 | REE Board Exam

This is the Multiple Choice Questions Part 3 of the Series in AC Generators as one of the Electrical Engineering topic. In Preparation for the REE Board Exam make sure to expose yourself and familiarize in each and every questions compiled here taken from various sources including but not limited to past Board Exam Questions in Electrical Engineering field, Electrical Engineering Books, Journals and other Electrical Engineering References.

Continue Practice Exam Test Questions Part 3 of the Series

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MCQ in AC Generators Part 2 | REE Board Exam

101.  When a number of alternators are operating in parallel, the power factor at which each operates is determined by

A.  power factor of the load

B.  driving torque of the prime mover

C.  its field excitation

D.  none of these

View Answer:

Answer: Option C

Solution:

102.  If the excitation of an alternator operating in parallel with other alternators is increased above the normal value of excitation, its

A.  pf becomes more lagging

B.  pf becomes more leading

C.  output current decreases

D.  output kW decreases

View Answer:

Answer: Option A

Solution:

103.  The voltage of the bus-bar to which several alternators are paralleled may be raised by simultaneously ____ of all alternators.

A.  increasing field excitation

B.  decreasing field excitation

C.  increasing input to prime movers

D.  decreasing input to prime movers

View Answer:

Answer: Option A

Solution:

104.  The frequency of the system with which several alternators are paralleled can be increased by simultaneously ______ of all generators.

A.  increasing field excitation

B.  decreasing field excitation

C.  increasing the speed of prime movers

D.  decreasing the speed of prime movers

View Answer:

Answer: Option C

Solution:

105.  The alternators driven by ______ do not have a tendency to hunt.

A.  diesel engines

B.  water turbines

C.  steam turbines

D.  none of these

View Answer:

Answer: Option C

Solution:

106.  Damper windings are used in alternators to

A.  prevent hunting

B.  reduce windage losses

C.  achieve synchronism

D.  none of these

View Answer:

Answer: Option A

Solution:

107.  The rating of an alternator is expressed in

A.  kW

B.  HP

C.  kVA

D.  kVAR

View Answer:

Answer: Option C

Solution:

Unsolved problems

108.  REE Board Exam September 2004

A 3,600 V, 500 kVA, 60 Hz, 3-phase Y-connected generator on test yields the following results: Mechanical and iron losses is 10 kW; field current at full load 100% p.f. is 50 A; field current at full-load 80% p.f. is 70 A; resistance per phase of armature winding is 0.4 ohm. The exciter voltage is constant at 120 volts and voltage current is control is done by means of rheostat. Determine the full-load armature current at 80% lagging p.f.

A.  96.5 A

B.  99.8 A

C.  64.2 A

D.  80.2 A

View Answer:

Answer:

Solution:

109.  A 4-pole, 1200 rpm alternator will generate emf at

A.  50 Hz

B.  40 Hz

C.  60 Hz

D.  25 Hz

View Answer:

Answer: Option B

Solution:

110.  The span for a full-pitch coil wound for six poles is

A.  180 degrees mechanical

B.  90 degrees mechanical

C.  60 degrees mechanical

D.  45 degrees mechanical

View Answer:

Answer: Option C

Solution:

111.  The pitch factor for a two-thirds short-pitch coil is

A.  0.5

B.  0.66

C.  0.866

D.  0.707

View Answer:

Answer: Option C

Solution:

112.  The armature flux opposes the main field flux when the load power factor is

A.  unity

B.  zero lagging

C.  zero leading

D.  0.8 leading

View Answer:

Answer: Option B

Solution:

113.  The armature flux helps the main field flux when the load power factor is

A.  unity

B.  zero lagging

C.  0.8 lagging

D.  zero leading

View Answer:

Answer: Option D

Solution:

114.  A commercial alternator has

A.  rotating armature and stationary field

B.  stationary armature and rotating field

C.  both armature and field rotating

D.  both armature and field stationary

View Answer:

Answer: Option B

Solution:

115.  The stator core of a synchronous machine is made up of laminated sheets to

A.  increase the magnitude of flux produced

B.  make the machine lighter in weight

C.  minimize the eddy current loss

D.  minimize the hysteresis loss

View Answer:

Answer: Option C

Solution:

116.  In alternators damper windings are used to A.  reduce eddy current loss

B.  prevent hunting

C.  make the rotor dynamically balanced

D.  reduce armature reaction

View Answer:

Answer: Option B

Solution:

117.  A 4 –pole 50-Hz synchronous machine runs at

A.  750 rpm

B.  1500 rpm

C.  3000 rpm

D.  1440 rpm

View Answer:

Answer: Option B

Solution:

118.  The magnitude of the resultant magnetic field produced by a three-phase current flowing through a three-phase winding is equal to

A.  the maximum value of flux due to any one phase

B.  1.5 times the maximum value of flux due to any one phase

C.  half the value of maximum flux due to any one phase

D.  twice the maximum value of flux due to any one phase

View Answer:

Answer: Option B

Solution:

119.  Voltage regulation of an alternator may be negative when

A.  the load power factor is lagging

B.  the load power factor is leading

C.  it is loaded beyond its full-load capacity

D.  the machine is run at very low speeds

View Answer:

Answer: Option B

Solution:

120.  Pitch factor for 5/6 short pitch coil is

A.  0.966

B.  0.833

C.  1.0

D.  3.454

View Answer:

Answer: Option A

Solution:

121.  Distribution factor for a winding having 3 slots/pole/phase and a slot angle of 20 degrees is

A.  0.96

B.  1.0

C.  0.5

D.  0.707

View Answer:

Answer: Option A

Solution:

122.  Armature reaction in an electrical machine is the effect of

A.  armature flux on the main flux

B.  heat produced on the armature windings

C.  armature current on the output

D.  armature flux on the output

View Answer:

Answer: Option A

Solution:

123.  An infinite bus-bar should maintain

A.  infinite frequency and infinite voltage

B.  constant frequency and constant voltage

C.   constant frequency but variable voltage

D.  variable frequency and variable voltage

View Answer:

Answer: Option B

Solution:

124.  ______ refers to an AC generator.

A.  Commutator

B.  Motor (AC)

C.  Rotor

D.  Alternator

View Answer:

Answer: Option D

Solution:

125.  In huge alternators, the moving part is

A.  brushes

B.  armature

C.  poles

D.  none of these

View Answer:

Answer: Option C

Solution:

126.  In case of alternator, the stator frame serves

A.  to protect the whole machine

B.  as a return path for the flux

C.  to hold the armature stampings

D.  to ventilate the armature

View Answer:

Answer: Option B

Solution:

127.  Alternators mostly wotk on rotating field type principle which is different from d.c. machines where the field is stationary. Therefore alternators

A.  don’t obey Faraday’s law

B.  work on Faraday’s law

C.  do not obey Lenz’s law

D.  obey Lenz’s law

View Answer:

Answer: Option B

Solution:

128.  High speed alternators have a rotor construction

A.  similar to d.c. machines

B.  of non-salient type

C.  of salient-type

D.  none of these

View Answer:

Answer: Option B

Solution:

129.  The voltage applied to the field of a rotating cylindrical rotor type alternators is

A.  high d.c. voltage

B.  high a.c. voltage

C.  low d.c. voltage

D.  low a.c. voltage

View Answer:

Answer: Option C

Solution:

130.  Alternators are generally designed to generate

A.  variable frequency

B.  definite currents

C.  definite frequencies

D.  definite power factor

View Answer:

Answer: Option C

Solution:

131.  The standard practice, nowadays in alternators is to have

A.  rotating field

B.  rotating armature

C.  either rotating field or rotating armature

D.  none of these

View Answer:

Answer: Option A

Solution:

132.  In power generating stations (thermal stations) the usual number of poles employed in alternators are

A.  twenty four

B.  forty eight

C.  thirty two

D.  none of these

View Answer:

Answer: Option D

Solution:

133.  The diameter of high speed turbo alternators as compared to hydroelectric ones are

A.  smaller because speed is high

B.  smaller because speed is less

C.  larger because speed is high

D.  none of these

View Answer:

Answer: Option A

Solution:

134.  The slip rings employed in a three-phase alternators in hydrostation are insulated for

A.  extra high tension voltage

B.  low voltage

C.  full armature voltage

D.  very high voltage

View Answer:

Answer: Option B

Solution:

135.  When the speed of alternator increases, the frequency

A.  decreases

B.  remains the same

C.  increases

D.  changes

View Answer:

Answer: Option C

Solution:

136.  Steam turbo-alternators are much smaller in size than water-turbine alternators for a given output. This is so because

A.  steam turbo alternators are built with smaller capacities

B.  steam turbo alternators run at high speed

C.  steam turbo alternators have long rotors

D.  all of these

View Answer:

Answer: Option B

Solution:

137.  For a coil having a span of 2/3 or pole pitch, the coil span factor is

A.  0.8

B.  0.866

C.  0.786

D.  0.966

View Answer:

Answer: Option B

Solution:

138.  Distributing the armature winding in alternators is more than one number of slots per pole per phase results in

A.  reduction of irregularities produced in the waveform

B.  economy of materials used in the winding

C.  less weight of the entire armature

D.  increased of generated emf per phase

View Answer:

Answer: Option A

Solution:

139.  When the power factor of load is unity, the armature flux of an alternator will be

A.  demagnetizing

B.  square wave form

C.  cross-magnetizing

D.  none of these

View Answer:

Answer: Option C

Solution:

140.  An alternator is capable of delivering power at a particular frequency. The frequency can be increased by

A.  increasing armature speed

B.  reversing the armature rotation

C.  increasing the current supplied in the field electromagnets

D.  reversing the field polarity

View Answer:

Answer: Option A

Solution:

141.  If the input of the prime mover of an alternator is kept constant but the excitation is increased then

A.  kVA will be leading

B.  kVA will be lagging

C.  kW will be changed

D.  the power factor of the load will remain constant

View Answer:

Answer: Option B

Solution:

142.  For two alternators operating in parallel, if the load shared by one of them is to be increased, its field excitation is

A.  to be strengthened keeping input torque same

B.  to be weakened keeping input torque same

C.  to be kept constant but input torque should increase

D.   none of these

View Answer:

Answer: Option C

Solution:

143.  A four-pole, three-phase, 50-Hz star connected AC generator has 24 stator slots. There are 6 conductors per slot and a sinusoidally distributed flux per pole of 65mWb. Find the terminal voltage on open circuit, if the coil span factor is unity.

A.  546.05 V

B.  579.44 V

C.  568.48 V

D.  558.62 V

View Answer:

Answer: Option B

Solution:

144.  A 2,500 kVA, three-phase, 60-Hz, 6.6 kV wye-connected alternator has a field resistance of 0.45 Ω and an armature resistance of 0.05 Ω per phase. The field current at full load 0.85 power factor is 200 A. The stray power losses amount to 82 kW. Calculate the efficiency of the alternator at full-load, 0.85 power factor lagging.

A.  93.7%

B.  94.8%

C.  95.2%

D.  96.3%

View Answer:

Answer: Option C

Solution:

145.  A 6.6 kV, Y-connected, 3-phase alternator has a synchronous reactance of 6 Ω per phase and a negligible resistance. At a certain field current the armature is known to induce 5.8 kV per phase. Calculate the maximum power that can be developed by the alternator at this excitation.

A.  12 MW

B.  11 MW

C.  9.8 MW

D.  10 MW

View Answer:

Answer: Option B

Solution:

146.  A three-phase alternator has 6 slots per pole per phase. Determine the distribution factor.

A.  0.977

B.  0.956

C.  0.963

D.  0.948

View Answer:

Answer: Option B

Solution:

147.  A three-phase star connected AC generator is rated at 2.5 MVA, 11 kV, 60 Hz. The generator has a negligible resistance and a synchronous reactance of 20 ohms per phase. To what value will the terminal voltage rise when full load at 80% pf lagging is switched-off?

A.  14.2 kV

B.  13.5 kV

C.  15.3 kV

D.  12.7 kV

View Answer:

Answer: Option A

Solution:

148.  A 5000 kVA, 3-, 10 kV, 50 Hz alternator runs at 1500 rpm connected to constant frequency, constant voltage busbars. If the moment of inertia of entire rotating system is 1.5 x 104 kg-m2 and the steady short circuit current is 5 times the normal full load current. Calculate the natural time period of oscillation.

A.  1.5 sec

B.  1.4 sec

C.  1.3 sec

D.  1.2 sec

View Answer:

Answer: Option B

Solution:

149.  A three phase wye connected ac generator is rated 1500 kVA, 11000 V. The effective resistance and synchronous reactance per phase are 1.5 ohms and 30 ohms respectively. To what value will the terminal voltage rise when full-load at 0.8 pf lagging is switch off.

A.  14 kV

B.  13 kV

C.  15 kV

D.  12 kV

View Answer:

Answer: Option A

Solution:

150.  A 100 MVA 10 kV turbo-alternator has a current limiting reactor of 0.162 ohms in its neutral. The per unit voltage of the reactor reactance based on the generator rating is

A.  0.05

B.  0.5

C.  0.025

D.  0.25

View Answer:

Answer: Option A

Solution:

151.  A three-phase, 6-pole, 72-slot armature of a synchronous generator is wound using a two layer lap coil having 20 conductors per coil with a pitch of 83.33%. The flux per pole is 4,800,000 Maxwells. If the rotor of the generator is driven at a speed of 1200 rpm, calculate the induced emf per phase. Note: There are 2 coils in 1 slot.

A.  5,680 V

B.  5,390 V

C.  5,564 V

D.  5,854 V

View Answer:

Answer: Option A

Solution:

152.  A 2-pole three phase 50 Hz Y-connected AC generator has 9 slots per pole and 6 conductors per slot. What is the flux per pole if the voltage on open circuit is 1.1 kV line-to-line? Assume a coil span of unity.

A.  0.1657 Wb

B.  0.1742 Wb

C.  0.1507 Wb

D.  0.1728 Wb

View Answer:

Answer: Option A

Solution:

153.  A 1 MVA, 11 kV, three-phase Y-connected synchronous generator has a synchronous reactance of 5 ohms and a negligible resistance per phase. At a certain field current the generator delivers rated load at 0.9 lagging power factor at 11 kV. For the same excitation, what is the terminal voltage at 0.9 leading power factor full-load?

A.  11.6 kV

B.  11.4 kV

C.  11.3 kV

D.  11.5 kV

View Answer:

Answer: Option B

Solution:

154.  A 2500 kVA, 6600 V synchronous turbo alternator is operating at full load and 0.8 pf lagging. This machine has a synchronous reactance per phase of 10.4 ohms and negligible resistance. Calculate the percentage voltage regulation.

A.  48.56%

B.  42.35%

C.  45.84%

D.  43.96%

View Answer:

Answer: Option D

Solution:

155.  Two alternators are connected in parallel. The total load is 4250 kW at 0.85 power factor lagging. Alternator operates with a load of 2125 kW at 0.707 power factor lagging. Determine the kVA load of alternator B.

A.  2245 kVA

B.  2078 kVA

C.  2185 kVA

D.  2410 kVA

View Answer:

Answer: Option C

Solution:

156.  The best and the most accurate method for determining the voltage regulation is

A.  synchronous impedance method

B.  mmf method

C.  emf method

D.  Pottier triangle method

View Answer:

Answer: Option D

Solution:

157.  A 3-phase, 8-pole generator is delta connected. The terminal voltage is 2400 V while the line current is 500 amperes. If the machine is converted to wye, what will be the terminal voltage in volts?

A.  4800 V

B.  4157 V

C.  4600 V

D.  3800 V

View Answer:

Answer: Option B

Solution:

158.  A 3-phase, 8-pole delta connected generator has a line current of 500 amperes. If converted in wye, what is the tolerable current?

A.  371.5 A

B.  288.7 A

C.  245.1 A

D.  315.2 A

View Answer:

Answer: Option B

Solution:

159.  A single-phase alternator gives 250 amperes at 1,200 volts. What is the output of the machine in KVA?

A.  300

B.  450

C.  400

D.  350

View Answer:

Answer: Option A

Solution:

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