100+ Electrical MCQ Questions in Induction Motor

 

101.In a 3-phase squirrel cage induction motor, skewing of rotor slots reduces

  1. a) parasitic torque and noise but increases pull-out torque
  2. b) parasitic torque and noise but increases starting torque

c)noise but increases pull-out torque and parasitic torque

d)parasitic torque, noise pull-out torque and starting torque

Answer: parasitic torque, noise pull-out torque and starting torque

 

102.A 6-pole,50Hz wound rotor induction motor when supplied at the rated voltage and frequency with slip-ring open-circuited, developed a voltage of 100 V between any two rings. If rotor is driven by an external means at 1000 rpm opposite to then direction of starter field, the frequency of voltage across a slip-rings will be

  1. a) zero b)50 Hz

c)100 Hz      d)200 Hz

Answer: zero

 

103.Emf induced in the rotor of a 3-phase induction motor is proportional to the

  1. a) relative velocity between starter field and rotor conductor
  2. b) voltage applied to the starter

c)slip

d)all of these

Answer: relative velocity between starter field and rotor conductor

 

104.The shunt resistance circuit component obtained by no-load test of an induction motor is representation of

  1. a) windage and friction loss only
  2. b) core loss only

c)core loss and windage and friction loss

d)copper loss only

Answer: core loss and windage and friction loss

 

105.During no-load test, an induction motor draws power

  1. a) for core loss and windage and friction loss
  2. b) for core loss and copper loss

c)for copper loss and windage and friction loss

d)only for the very small copper loss.

Answer: for core loss and windage and friction loss

 

106.If rotor power output of 3-phase induction motor is 15 kW, then rotor copper losses at a slip of 4% will be

a)600 W                          b)625 W

c)650 W                          d)700 W

Answer: 600 W

 

107.A 6-pole, 3-phase,60 Hz induction motor runs at 1000 rpm developing maximum torque. Rotor resistance per phase is 1.2 ohms. Neglecting starter impedance, for developing maximum torque, the external resistance to be connected in series with each rotor phase will be

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a)7.2 ohms                     b)6 ohms

c)1.44 ohms                   d)1.2 ohms

Answer: 6 ohms

 

108.A 3-phase induction motor has a full-load slip of 3 percent at normal voltage. Which of the following will be the value or slip of the motor if it develops same torque theoretically while operating at 110 percent of its normal voltage?

a)2.48%                       b)0.248%

c)0.483%                     d)4.83%

Answer:2.48%

 

109.The power input to a 415 V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 40 kW. If starter losses are 1 Kw and friction and windage losses total 2 kW, then efficiency of the motor is

a)92.5%                     b)91%

c)90%                        d)88%

Answer:92.5%

 

110.The rotor of a three phase, 5 kW, 400 V, 50 Hz, slip ring induction motor is wound for 6 poles while its starter is wound for 4 poles. The approximate average no load steady state speed when this motor is connected to 400 V, 50 Hz supply is

a)1500 rpm                b)500 rpm

c)0 rpm                      d)1000 rpm

Answer:1000 rpm

 

111.I f a three phase slip ring induction motor is fed from the rotor side with starter winding short circuited, then frequency of the current flowing in the short circuited starter is

  1. a) slip frequency
  2. b) supply frequency

c)frequency corresponding to rotor speed

d)zero

Answer: slip frequency

 

112.In an induction motor, if air gap is increased, then

  1. a) speed will reduce
  2. b) efficiency will improve

c)power factor will be lowered

d)breakdown torque will reduce

Answer: power factor will be lowered

 

113.An induction motor having full load torque of 60 Nm when delta-connected develops a starting torque of 120 Nm. For the same supply voltage, if motor is changed to star-connection to star-connection, then starting torque developed will be

a)40 Nm                        b)60 Nm

c)90 Nm                        d)120 Nm

Answer:40 Nm

 

114.Unbalanced supply voltage given to a 3-phase, delta-connected induction motor will cause

  1. a) zero sequence currents
  2. b) less heating of the motor

c)negative sequence components current

d)all of these

Answer: negative sequence components current

 

115.If rotor of an induction motor is assumed to be purely resistive, then angle between resultant flux density wave and rotor mm wave is

  1. a) dependent upon the load
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b)180°

c)90°

d)45°

Answer:90°

 

116.If an induction motor when started on load does not accelerate up to full speed but runs at 1/7th of the rated speed, then it is called

  1. a) locking b) plugging

c)crawling                        d) clogging

Answer: crawling

 

117.If a squirrel-cage induction motor having a rated slip of 2% on full-load has a starting torque of 0.5    full-load torque, then starting current is

  1. a) equal to full-load current
  2. b) twice full-load current

c)four times full-load current

  1. d) five times full-load current

Answer: five times full-load current

 

118. Assume that starter impedance of an induction motor is negligible. The frequency of voltage applied to the motor is increased keeping V/f constant. The maximum motor torque will

  1. a) increase and will occur at smaller values of slip
  2. b) increase and will occur at larger values of slip
  3. c) remains constant but will occur at smaller values of slip
  4. d) remains constant but will occur at larger values of slip

Answer: remains constant but will occur at smaller values of slip

 

119.For controlling the speed of an induction motor, the frequency of the supply is increased by 10%. For maximum torque to remain constant, the supply voltage must

  1. a) be increased by 10%
  2. b) remain constant

c)be decreased by 10%

d)be reduced or increased by 20%

Answer: be increased by 10%

 

120.For a constant torque load, addition of a rotor resistance in a slip ring induction motor

  1. a) reduces starter current as well as slip
  2. b) reduces starter current but increases slip

c)causes no change in starter current but increases its slip

  1. d) causes no change in starter current but reduces its slip

Answer: reduces starter current but increases slip

 

121.By adding resistance in the rotor circuit of a slip ring induction motor, starting current

  1. a) and torque both reduce (compared to direct on-line starting)
  2. b) and torque both increase
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c)reduces but starting torque increases

d)increase but starting torque decreases

Answer: reduces but starting torque increases

 

122.If an induction machine is run at above synchronous speed, it acts as a/an

  1. a) synchronous motor
  2. b) induction motor

c)induction motor

d)none of these

Answer: induction motor

 

123.Starting current of an induction motor is five times the full-load current while full-load slip is 4%. The ratio of starting torque to full-load torque is

a)0.6                     b)0.8

c)1.0                     d)1.2

Answer:1.0

 

124.If rotor power output of 3-phase induction motor is 15 kW, then rotor copper losses at a slip of 4% will be

a)600 W                      b)625 W

c)650 W                      d)700 W

Answer:625 W

 

125.The power input to an induction motor is 40 kW when it is running at 5% slip. The starter resistance and core loss are assumed negligible. Torque developed in synchronous watt is

a)42 kW                       b)40 kW

c)38 kW                       d)2 kW

Answer:40 kW

 

126.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

  1. a) under-excited
  2. b) over-excited

c)normally excited

d)disconnected from the common terminals

Answer: over-excited

 

127.The blocked rotor test of squirrel-cage induction motor determines, its equivalent

  1. a) series resistance and reactance as seen from the starter
  2. b) shunt resistance and reactance as seen from the starter
  3. c) series resistance and reactance as seen from the rotor
  4. d) shunt resistance and reactance as seen from the rotor

Answer: series resistance and reactance as seen from the starter

SYNCHRONOUS MACHINE

128.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°

Answer:18°

 

129.Synchronous generators used in thermal power stations have

  1. a) cylindrical rotors
  2. b) slip-rings and brushes
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c)starter slots in multiples of 6

d)all of these

Answer: slip-rings and brushes

 

130.A synchronous motor is operating on no-load at unity power factor. If field current is increased, the power factor will become

  1. a) leading and the current will decrease
  2. b) leading and the current will increase

c)lagging and the current will decrease

d)leading and the current will increase

Answer: leading and the current will increase

 

131.In a non-salient pole synchronous machine, distribution of field mm around the air gap is a

  1. a) sinusoidal wave
  2. b) rectangular wave

c)stepped triangular wave

d)flat topped stepped wave

Answer: flat topped stepped wave

 

  1. Mmf of a sinusoidally distributed current sheet is
  2. a) triangular shifted from the current by 90°
  3. b) triangular in phase with the current wave

c)sinusoidal shifted from the current wave by 90°

d)sinusoidal in phase with the current wave

Answer: sinusoidal shifted from the current wave by 90°

 

133.In a synchronous machine, the voltage induced by armature reaction flux acts like the voltage drop in

  1. a) inductive reactance
  2. b) resistance

c)capacitive reactance

d)inductive impedance

Answer: inductive reactance

 

134.Synchronous reactance is

  1. a) difference of armature leakage reactance and reactance equivalent of armature reaction.
  2. b) same as armature leakage reactance

c)reactance equivalent of armature reaction

d)sum of armature leakage reactance and reactance equivalent of armature reaction.

Answer:  sum of armature leakage reactance and reactance equivalent of armature reaction.

 

135.A synchronous motor is floating on infinite mains at on load. If its excitation is now increased, it will draw

  1. a) unity power factor current
  2. b) zero power factor lagging current

c)zero power factor loading current

d)no current

Answer: zero power factor loading current

 

136.Armature reaction at of a synchronous generator at rated voltage zero power factor lagging is

  1. a) magnetizing
  2. b) demagnetizing

c)cross-magnetizing

d)both magnetizing and cross-magnetizing

Answer: demagnetizing

 

137.If a 400 V, 50 Hz, star connected, 3 phase squirrel cage induction motor is operated from a 400 V, 75 Hz supply, the torque that the motor can now provide while drawing rated current from the supply

  1. a) reduces
  2. b) increases
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c)remains the same

d)increase or reduces depending upon the rotor resistance

Answer: reduces

 

138.A synchronous generator is feeding power to infinite busbar at unity power factor. Its excitation is now increased. It will feed

  1. a) the same power but at a leading power factor.
  2. b) the same power but at a lagging power factor.

c)more power at unity power factor.

d)less power at unity power factor.

Answer: the same power but at a leading power factor

 

139.If the excitation of a turbo generator feeding lagging current to infinite busbar is increased, then

  1. a) the generator would feed more lagging kVAr
  2. b) the generator would feed more leading kVAr
  3. c) the generator would feed more leading kVAr

d)there would be no change

Answer: the generator would feed more lagging kVAr

 

140.If the steam input to a turbo generator connected to infinite busbar in increased, the generator will feed more

  1. a) real power to busbar and its power angle will increase
  2. b) leading kVAr to busbar but its power angle will not change

c)real power to busbar and its power angle will decrease

  1. d) lagging kVAr to busbar but its power angle will not change

Answer: real power to busbar and its power angle will increase

141.If    a   synchronous generator is rated 10 MVA,11 Kv,0.85pf,50Hz, then the significance of 0.85 pf is that

  1. a) load of pf lower than 0.85 cannot be supplied
  2. b) the prime mover power rating is 0.85 MW

c)load of pf higher than 0.85 cannot be supplied.

d)it is not of any significance

Answer: the prime mover power rating is 0.85 MW

 

 

142.In a generating synchronous machine carrying load (usual symbols are used)

  1. a) E leads V by angle 𝒮
  2. b) E lags V by angle 𝒮

c)E and V are in phase

d)E and V are in phase opposition

Answer: E leads V by angle 𝒮

 

143.synchronous generator is operating at constant load while its excitation is adjusted to give unity pf current. If the excitation is now increased, the power factor will

  1. a) lead
  2. b) remain at unity
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c)lag

d)become zero

Answer: lag

 

144.In a cylindrical rotor synchronous machine, the phase addition of starter and rotor mmf is possible because

  1. a) two mmf are rotating in opposite direction
  2. 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

Answer: two mmfs are stationary with respect to each other

 

145.The starter of a salient pole synchronous generator is excited from a low voltage 50 Hz supply while the rotor is made to run at a slip of 0.05.the reluctance presented to the rotating starter mmf is

  1. a) constant
  2. b) varies sinusoidally at 525 Hz

c)varies sinusoidally at 0.25 Hz

d)varies sinusoidally at 2.5 Hz

Answer: varies sinusoidally at 2.5 Hz

 

146.A synchronous generator is loaded and the armature current lags behind the excitation emf by angle 𝜓. The electrical angle between the field axis and the axis of the armature reaction field will be

a)90°+𝜓                          b)90°-𝜓

c)90°                                 d) 𝜓

Answer: 90°-𝜓

 

147.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. The excitation emf of the motor will be

a)2.0                                     b)1.5

c)1.0                                     d)0.5

Answer:1.5

 

148.The pole shoes of a salient pole synchronous machine are so shaped at that air-gap is least in the middle and increase somewhat towards the edges. This type of construction

  1. a) increase flux/pole for a given excitation
  2. b) facilitates machining of pole shoes

c)eliminates a particular harmonic

d)reduces the harmonic content of the induced voltage wave.

Answer: reduces the harmonic content of the induced voltage wave.

 

149.Two synchronous generators G1 and G2 are operating in parallel and sharing the load equally. To shift part of the load from G2 to G1 while keeping the frequency constant,

  1. a) lower the frequency-power characteristics of G1 or raise that of G2
  2. b) raise the frequency-power characteristic G1 or lower that of G2
  3. c) raise the frequency-power characteristic G1 and lower that of G2
  4. d) lower the frequency-power characteristics of G1 and raise that of G1G2
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Answer: raise the frequency-power characteristic G1 and lower that of G2

 

150.Two synchronous generators G1 and G2 are operating in parallel and are equally sharing the kVAr (lagging) component of load. To shift part of the KVAR load from G1 to G2 while keeping the terminal voltage fixed and

  1. a) lower the field current of G1 or raise that of G2
  2. b) raise the field current of G1 or lower that of G2

c)raise the field current of G1 and lower that of G2

d)lower the field current of G2 and raise that of G2

Answer: raise the field current of G1 and lower that of G2

 

151.The armature current upon symmetrical 3-phase short-circuit of a synchronous machine (armature resistance is negligible)

  1. a) constitutes q-axis current only
  2. b) constitutes d-axis current only

c)has both d-axis and q-axis components

d)short-circuit current cannot be divided into d-and q-axis components

Answer: constitutes d-axis current only

 

152.The phase sequence of a three-phase alternator will reverse if

  1. a) the field current is reversed keeping the direction of rotation same
  2. b) the field current is remains the same but the direction of rotation is reversed
  3. 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: the field current is reversed keeping the direction of rotation same

 

153.A synchronous generator connected to an infinite bus is over excited. Considering only the reactive power, from the point of view of the system, the machine acts as

  1. a) a capacitor b) an inductor

c)a resistor                     d) none of these

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Answer: none of these

 

154.A cylindrical rotor synchronous motor is switched on the supply with its field windings shortest on themselves. It will

  1. a) not start
  2. b) start but not run at synchronous speed

c)start as an induction motor and then run as synchronous motor

d)start and run at synchronous speed

Answer: start but not run at synchronous speed

 

155.In a 400 kV network,350 kV is recorded at a 400kV bus. The reactive power absorbed by a shunt rated for 50MVAr,400kV connected at the bus is

a)61.73MVAr       b)55.56MVAr

c)45MVAr            d)40.5MVAr

Answer:40.5MVAr

 

156.Efficiency of electrical, machines should be calculated by measuring

  1. a) output and input
  2. b) losses and output

c)losses and input

d)losses

Answer: losses

 

157.Damper winding is provided in a polyphaser synchronous motor in order to

  1. a) dampen out noise of the machine
  2. b) prevent hunting

c)provide starting torque

d)provide a cylindrical structure to reduce wind friction

Answer: prevent hunting

 

158: A single-phase,2000 V alternator has an armature resistance and reactance of 0.8 ohms and 4.94 ohms respectively. The voltage regulation of the alternate at 100 a load at 0.8 leading power-factor is

a)7%        b)-8.9%

c)14%      d)0%

Answer: -8.9%

 

159.A three phase alternator is wound with a 60-degree phase-spread armature windings and develops 300 kVA. if armature is reconnected utilizing all the coils for single phase operation with a phase spread of 180 degrees, then new rating of the machine is

a)100kVA        b)200kVA

c)250kVA        d)300kVA

Answer:200kVA

 

160.If a 50Hz,4-pole turbo-generator rated 100MVA,11kV has given a mechanical input of 80MW suddenly for an electrical load of 50MW then speed of the rotor after 10 cycles will be, if machine inertia constant is 8MJ/MVA.

a)1512rpm            b)1628rpm

c)337.5rpm           d) none of these

Answer:1512rpm

 

161.A salient pole synchronous motor is running with normal excitation. If excitation is reduced to zero

  1. a) it becomes an induction motor
  2. b) it becomes a reluctance motor

c)it remains a synchronous motor

d)the motor would stop

Answer: it becomes a reluctance motor

 

162.An unexcited single phase synchronous motor is

  1. a) reluctance motor
  2. b) universal motor
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c)repulsion motor

d)AC series motor

Answer: universal motor

 

163.If power to the deck field at a 15kVA (input)synchronous motor is 6 per cent of the rates arc input, then direct current that excites the field at 120 voltaic

a)7.5 amps

b)125 amps

c)15 amps

d)none of these

Answer:7.5 amps

 

164.If a 10 pole 25Hz alternator is directly coupled to and is driven by a 60Hz synchronous motor, then number of poles in a synchronous motor are

a)48        b)12

c)24        d) none of these

Answer:24

 

165.The torque angle at a synchronous machine operating from a constant voltage bus is usually defined as the space angle between

  1. a) rotor mmf wave and starter mmf wave
  2. b) rotor mmf wave and resultant flux density

c)starter mmf wave and resultant flux density

d)none of these

Answer: rotor mmf wave and starter mmf wave

 

166.A 3-phase synchronous machine is synchronized with an invite bus. If steam input to synchronous machine is increased, then synchronous machine starts working as

  1. a) alternator at a leading pf
  2. b) alternator at a lagging pf

c)synchronous motor at a leading pf

d)induction generator at a lagging pf

Answer: alternator at a leading pf

 

167.A 3-phase alternator is supplying power to infinite bus at lagging.pf. if with constant steam input, alternator excitation is increased, then

  1. a) load angle 𝒮 decreases and pf increases
  2. b) 𝒮 increases and pf decreases

c)both 𝒮 and pf decreases

d)both 𝒮 and pf increases

Answer: both 𝒮 and pf decreases

 

168.Which one of the following methods would give a higher than actual value of regulation of an alternator?

  1. a) ZPF method b) MMF method

c)EMF method      d) ASA method

Answer: EMF method

 

169.Distributed armature winding in a 3-phase alternator

  1. a) reduces the phase belt harmonics
  2. b) increases utilization of armature iron and copper

c)increases rigidity and mechanical strength of the winding

d)reduces copper in the overhang of the winding

Answer: increases rigidity and mechanical strength of the winding

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170.During blocked-rotor test on an induction motor, the power is drawn mainly for

  1. a) copper loss and core loss
  2. b) copper loss

c)core loss

d)copper loss, core loss, and windage and friction loss

Answer: copper loss

 

171.During short-circuit test on a slip ring induction motor, the rotor is

  1. a) short-circuited but is free to rotate
  2. b) open-circuited but is blocked from rotation

c)short-circuited but is blocked from rotation

d)open-circuited but is free to rotate

Answer: short-circuited but is blocked from rotation

 

172.In a synchronous machine, field flux is ahead of the armature field axis, in the direction of rotation, then machine is working as

  1. a) asynchronous motor
  2. b) asynchronous generator

c)synchronous motor

d)synchronous generator

Answer: synchronous generator

 

173.Damping winding in a synchronous motor is generally used to

  1. a) prevent hunting and provide the starting torque
  2. b) reduce eddy currents

c)minimise vibrations

d)reduce noise level

Answer: prevent hunting and provide the starting torque

 

174.If a synchronous condenser is installed to current the 2400kVA ,0.65 power factor load in a plant to unity, then kilovolt ampere taring of condenser is

a)27kVA

b)1560kVA

c)1825kVA

d)3690kVA

Answer:1825kVA

 

175.The per unit impedance of a synchronous machine is 0. 242.if base voltage is increased by 1.1 times, then per unit value will be

a)0.266       b)0.242

c)0.220       d)0.200

Answer:0.200

 

176.If dimensions of all the parts of a synchronous generator, and the number of field and armature turns are doubled, then generated voltage will change by factor of

a)1       b)2

c)4       d)8

Answer:8

 

177.During hunting of synchronous motor

  1. a) negative phase sequence current is generated
  2. b) harmonics are developed in the armature circuit

c)damper bar develops torque

d)field excitation increases

Answer: damper bar develops torque

 

178.During disturbance of a synchronous machine, the rotor swings from A to B before finally settling down to a steady state at point C on the power angle curve. The speed of the machine during oscillation is synchronous at point(s)

  1. a) A and B b) A and C
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c)B and C        d) only at C

Answer: only at C

 

179.Voltage across the open-circuited field terminals of a synchronous machine under slip test is

  1. a) d.c.
  2. b) d.c. of slip frequency

c)a modulated supply frequency ac voltage with slip frequency envelope

d)arc. of supply frequency

Answer: d.c. of slip frequency

 

180.If two mechanically coupled alternators deliver power at 50Hz and 60Hz respectively, then highest speed of the alternator is

a)3600 rpm          b)3000 rpm

c)600 rpm            d)500 rpm

Answer:600 rpm

 

181.If 1.8° step,4-phase stepper motor has a total of 40 teeth on 8 poles of starter, then number of rotor teeth for this motor will be

a)40        b)50

c)100      d)80

Answer:100

 

182.If input of the prime mover of an alternator is kept constant but excitation is charged, then

  1. a) active component of the output is changed
  2. b) reactive component of the output is changed

c)power factor of the load remains constant

d)power factor of the load changes from lagging to leading

Answer: reactive component of the output is changed

 

183.When two alternators are running in parallel, their KVA load share is changed by changing their……. while their kW load share is changed by changing their…….

  1. a) excitation, driving torque
  2. b) driving torque, excitation

c)excitation, excitation

d)driving torque, driving torque

Answer:excitation,driving torque

 

184.A synchronous machine working in the motor made, is fed from an infinite bus and is delivering half full-load. If an increase in field current causes an increase in the armature current, then the motor will

  1. a) deliver reactive power and active power to the bus
  2. b) absorb reactive power and active power from the bus

c)absorb reactive power from the bus and deliver active power to the bus

d)deliver reactive power to the bus and absorb active power from the bus

Answer: deliver reactive power to the bus and absorb active power from the bus

 

185.Advantage of providing damper winding in alternators is/are

  1. a) elimination of harmonic effects
  2. b) provide a low resistance path for the currents due to unbalancing of voltage
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c)oscillations are provided when two alternators operate in parallel

d)all of these

Answer: all of these

 

186.Two alternator A and B are sharing a resistive load (pp.=1) equally. Now if excitation of alternator A is increased, then

  1. a) alternator A will become lagging and alternator B will become leading
  2. b) alternator A will become leading and alternator B will become lagging

c)both alternators will continue to operate a unity power factor

d)both alternators will operate on lagging power factor

Answer: alternator A will become lagging and alternator B will become leading

 

187.In a phase of synchronous converter the position of minimum copper loss

  1. a) is the tapping point whatever may be the power factor
  2. b) varies with the power factor and this position is displaced from the taping point by the power factor angle

c)is middle of the phase whatever may be the power factor

d)varies with the power factor and this position is displaced from the middle of the phase by the power factor angle

Answer: varies with the power factor and this position is displaced from the middle of the phase by the power factor angle

 

188.Driving power from the prime mover driving the alternator is lost but the alternator remains connected to the supply also remains on. The alternator will

  1. a) get burnt
  2. b) behave as an induction motor and will rotate in the opposite direct

c)behave as a synchronous motor and will rotate in the same direction

d)behave as a synchronous motor but will rotate in a reverse direction to that corresponding to generator action

Answer: behave as a synchronous motor and will rotate in the same direction

 

 

189.A 3-phase synchronous motor connected to infinite bus is operating at half full-load with normal excitation. When load and the synchronous motor is suddenly increased, its speed will

  1. a) first decrease and then become synchronous
  2. b) first increases and then become synchronous

c)fluctuate around synchronous speed and then become synchronous

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d)remain unchanged

Answer: fluctuate around synchronous speed and then become synchronous

 

SERVO/STEPPER MOTOR

190.A two-phase servomotor develops maximum torque at

  1. a) forward speed of one-half of the synchronous speed
  2. b) backward speed of one-half of the synchronous speed

c)synchronous speed

d)a speed of twice the synchronous speed

Answer: forward speed of one-half of the synchronous speed

 

191.If an arc. servomotor has one of its winding excited by arc, then voltage measured at the other winding with rotor running will be

  1. a) zero
  2. b) proportional to rotor speed

c)proportional to the square of the rotor speed

d)constant independent of speed

Answer: proportional to rotor speed

 

192.In an ac servomotor, the control winding is supplied with input voltage of

  1. a) same frequency as the reference winding voltage but with 90° phase difference.
  2. b) same frequency as the reference windings voltage and also having same phase.

c)any frequency compared to the reference winding but with 90°

d)any frequency compared to the reference winding but of the same phase.

Answer: same frequency as the reference winding voltage but with 90° phase difference.

 

193.An ac servomotor exhibits

  1. a) two times constants which depend upon rotor inertia and mechanical friction
  2. b) two time constants which depend upon rotor inertia, the slope of its torque speed characteristic and mechanical friction.

c)a dominant time constant which depends upon rotor inertia and mechanical friction

d)a dominant time constant which depends upon rotor inertia, the slope of its torque-speed characteristic and mechanical friction.

Answer: a dominant time constant which depends upon rotor inertia, the slope of its torque-speed characteristic and mechanical friction.

 

194.A 3-phase 4-pole alternator has 48 starter slots carrying 3-phase distributed winding. If each coil of the winding is short chorded by one slot pitch, then winding factor is given by

a)1/16 cot(7.5°)                          b)1/8 cot(7.5°)

c)1/16 sin(7.5°)                          d)cos(7.5°)

Answer: 1/8 cot(7.5°)

 

195.Two 3-phase ac generator is such that one has twice the linear dimensions of the other. The field windings of each are excited to give identical sinusoidal air-gap flux density waveform. Both have same number of starter slots and identical winding patterns. The conductor/slot in the big generator is Ktimes that of the smaller one. The value of K to get equal no load voltage at the same frequency is

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a)8                                b)4

c)1/2                            d)1/4

Answer:1/2

 

196.A large a.c. generator, supplying to an infinite bus, has a sudden short-circuit occurring at its terminals. Assuming prime mover input and voltage behind the transient reactance to remain constant immediately after the fault, acceleration of the generator rotor is

  1. a) inversely proportional to moment of inertia of the machine
  2. b) inversely proportional to square of the voltage

c)directly proportional to square of the short-circuit current

d)directly proportional to short-circuit power

Answer: inversely proportional to moment of inertia of the machine

 

197.In a transformer the phase angle between primary and secondary terminal voltage is

a)90°; primary voltage leading the secondary voltage

  1. b) 90°; primary voltage lagging the secondary voltage

c)a few degrees, primary voltage leading the secondary voltage

  1. d) a few degrees, primary voltage lagging the secondary voltage

Answer: a few degrees, primary voltage leading the secondary voltage

 

198.A 3-phase alternator is connected to the transformer. The hysteresis and eddy current losses of the transformer are respectively 300 W and 400 W. If speed of the alternator is reduced by10%, then hysteresis and eddy current losses of the transformer will be respectively

a)228 W and 262.44 W

b)243 W and 324 W

c)243 W and 360 W

d)270 W and 400 W

Answer: 243 W and 360 W

 

199.A transformer has a voltage regulation of 3% at full load, 0.8 power factor lagging. At full load unit power factor, the regulation will be

a)3%                       b) negative

c)less than 3%        d) more than 3%

Answer: negative

 

200.In a 4:1 turn ratio transformer, the leakage reactance as referred to the secondary side is 0.05 pu. The leakage reactance as referred to the primary is

a)0.05 ohm                     b)0.05 pu

c)0.8 pu                          d)0.2 pu

Answer:0.05 pu

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