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# 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

Choose the letter of the best answer in each questions.

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

A.ย  power factor of the load

B.ย  driving torque of the prime mover

C.ย  its field excitation

D.ย  none of these

Solution:

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

A.ย  pf becomes more lagging

C.ย  output current decreases

D.ย  output kW decreases

Solution:

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

A.ย  increasing field excitation

B.ย  decreasing field excitation

C.ย  increasing input to prime movers

D.ย  decreasing input to prime movers

Solution:

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

A.ย  increasing field excitation

B.ย  decreasing field excitation

C.ย  increasing the speed of prime movers

D.ย  decreasing the speed of prime movers

Solution:

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

A.ย  diesel engines

B.ย  water turbines

C.ย  steam turbines

D.ย  none of these

Solution:

106.ย  Damper windings are used in alternators to

A.ย  prevent hunting

B.ย  reduce windage losses

C.ย  achieve synchronism

D.ย  none of these

Solution:

107.ย  The rating of an alternator is expressed in

A.ย  kW

B.ย  HP

C.ย  kVA

D.ย  kVAR

Solution:

Unsolved problems

108.ย  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.ย  96.5 A

B.ย  99.8 A

C.ย  64.2 A

D.ย  80.2 A

Solution:

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

A.ย  50 Hz

B.ย  40 Hz

C.ย  60 Hz

D.ย  25 Hz

Solution:

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

A.ย  180 degrees mechanical

B.ย  90 degrees mechanical

C.ย  60 degrees mechanical

D.ย  45 degrees mechanical

Solution:

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

A.ย  0.5

B.ย  0.66

C.ย  0.866

D.ย  0.707

Solution:

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

A.ย  unity

B.ย  zero lagging

Solution:

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

A.ย  unity

B.ย  zero lagging

C.ย  0.8 lagging

Solution:

114.ย  A commercial alternator has

A.ย  rotating armature and stationary field

B.ย  stationary armature and rotating field

C.ย  both armature and field rotating

D.ย  both armature and field stationary

Solution:

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

A.ย  increase the magnitude of flux produced

B.ย  make the machine lighter in weight

C.ย  minimize the eddy current loss

D.ย  minimize the hysteresis loss

Solution:

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

B.ย  prevent hunting

C.ย  make the rotor dynamically balanced

D.ย  reduce armature reaction

Solution:

117.ย  A 4 โpole 50-Hz synchronous machine runs at

A.ย  750 rpm

B.ย  1500 rpm

C.ย  3000 rpm

D.ย  1440 rpm

Solution:

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

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

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

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

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

Solution:

119.ย  Voltage regulation of an alternator may be negative when

A.ย  the load power factor is lagging

D.ย  the machine is run at very low speeds

Solution:

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

A.ย  0.966

B.ย  0.833

C.ย  1.0

D.ย  3.454

Solution:

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

A.ย  0.96

B.ย  1.0

C.ย  0.5

D.ย  0.707

Solution:

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

A.ย  armature flux on the main flux

B.ย  heat produced on the armature windings

C.ย  armature current on the output

D.ย  armature flux on the output

Solution:

123.ย  An infinite bus-bar should maintain

A.ย  infinite frequency and infinite voltage

B.ย  constant frequency and constant voltage

C.ย ย  constant frequency but variable voltage

D.ย  variable frequency and variable voltage

Solution:

124.ย  ______ refers to an AC generator.

A.ย  Commutator

B.ย  Motor (AC)

C.ย  Rotor

D.ย  Alternator

Solution:

125.ย  In huge alternators, the moving part is

A.ย  brushes

B.ย  armature

C.ย  poles

D.ย  none of these

Solution:

126.ย  In case of alternator, the stator frame serves

A.ย  to protect the whole machine

B.ย  as a return path for the flux

C.ย  to hold the armature stampings

D.ย  to ventilate the armature

Solution:

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

C.ย  do not obey Lenzโs law

D.ย  obey Lenzโs law

Solution:

128.ย  High speed alternators have a rotor construction

A.ย  similar to d.c. machines

B.ย  of non-salient type

C.ย  of salient-type

D.ย  none of these

Solution:

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

A.ย  high d.c. voltage

B.ย  high a.c. voltage

C.ย  low d.c. voltage

D.ย  low a.c. voltage

Solution:

130.ย  Alternators are generally designed to generate

A.ย  variable frequency

B.ย  definite currents

C.ย  definite frequencies

D.ย  definite power factor

Solution:

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

A.ย  rotating field

B.ย  rotating armature

C.ย  either rotating field or rotating armature

D.ย  none of these

Solution:

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

A.ย  twenty four

B.ย  forty eight

C.ย  thirty two

D.ย  none of these

Solution:

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

A.ย  smaller because speed is high

B.ย  smaller because speed is less

C.ย  larger because speed is high

D.ย  none of these

Solution:

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

A.ย  extra high tension voltage

B.ย  low voltage

C.ย  full armature voltage

D.ย  very high voltage

Solution:

135.ย  When the speed of alternator increases, the frequency

A.ย  decreases

B.ย  remains the same

C.ย  increases

D.ย  changes

Solution:

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

A.ย  steam turbo alternators are built with smaller capacities

B.ย  steam turbo alternators run at high speed

C.ย  steam turbo alternators have long rotors

D.ย  all of these

Solution:

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

A.ย  0.8

B.ย  0.866

C.ย  0.786

D.ย  0.966

Solution:

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

A.ย  reduction of irregularities produced in the waveform

B.ย  economy of materials used in the winding

C.ย  less weight of the entire armature

D.ย  increased of generated emf per phase

Solution:

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

A.ย  demagnetizing

B.ย  square wave form

C.ย  cross-magnetizing

D.ย  none of these

Solution:

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

A.ย  increasing armature speed

B.ย  reversing the armature rotation

C.ย  increasing the current supplied in the field electromagnets

D.ย  reversing the field polarity

Solution:

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

B.ย  kVA will be lagging

C.ย  kW will be changed

D.ย  the power factor of the load will remain constant

Solution:

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

A.ย  to be strengthened keeping input torque same

B.ย  to be weakened keeping input torque same

C.ย  to be kept constant but input torque should increase

D.ย ย  none of these

Solution:

143.ย  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.ย  546.05 V

B.ย  579.44 V

C.ย  568.48 V

D.ย  558.62 V

Solution:

144.ย  A 2,500 kVA, three-phase, 60-Hz, 6.6 kV wye-connected alternator has a field resistance of 0.45 ฮฉ and an armature resistance of 0.05 ฮฉ 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.ย  93.7%

B.ย  94.8%

C.ย  95.2%

D.ย  96.3%

Solution:

145.ย  A 6.6 kV, Y-connected, 3-phase alternator has a synchronous reactance of 6 ฮฉ 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.ย  12 MW

B.ย  11 MW

C.ย  9.8 MW

D.ย  10 MW

Solution:

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

A.ย  0.977

B.ย  0.956

C.ย  0.963

D.ย  0.948

Solution:

147.ย  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.ย  14.2 kV

B.ย  13.5 kV

C.ย  15.3 kV

D.ย  12.7 kV

Solution:

148.ย  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.ย  1.5 sec

B.ย  1.4 sec

C.ย  1.3 sec

D.ย  1.2 sec

Solution:

149.ย  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.ย  14 kV

B.ย  13 kV

C.ย  15 kV

D.ย  12 kV

Solution:

150.ย  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.ย  0.05

B.ย  0.5

C.ย  0.025

D.ย  0.25

Solution:

151.ย  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.ย  5,680 V

B.ย  5,390 V

C.ย  5,564 V

D.ย  5,854 V

Solution:

152.ย  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.ย  0.1657 Wb

B.ย  0.1742 Wb

C.ย  0.1507 Wb

D.ย  0.1728 Wb

Solution:

153.ย  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.ย  11.6 kV

B.ย  11.4 kV

C.ย  11.3 kV

D.ย  11.5 kV

Solution:

154.ย  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.ย  48.56%

B.ย  42.35%

C.ย  45.84%

D.ย  43.96%

Solution:

155.ย  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.ย  2245 kVA

B.ย  2078 kVA

C.ย  2185 kVA

D.ย  2410 kVA

Solution:

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

A.ย  synchronous impedance method

B.ย  mmf method

C.ย  emf method

D.ย  Pottier triangle method

Solution:

157.ย  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.ย  4800 V

B.ย  4157 V

C.ย  4600 V

D.ย  3800 V

Solution:

158.ย  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.ย  371.5 A

B.ย  288.7 A

C.ย  245.1 A

D.ย  315.2 A

Solution:

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

A.ย  300

B.ย  450

C.ย  400

D.ย  350

Solution:

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