EE TRIVIA 3
If the supply frequency to the transformer is increase, the iron loss will A. decrease B. increase C. do not change D. vary
B. increase
The type of the motor that does not have a commutator is the A. repulsion motor B. induction motor C. dc shunt motor D. ac series motor
B. induction motor
Synchronizing power of a synchronous machine is A. equal to synchronous reactance B. inversely proportional to the synchronous reactance C. directly proportional to the synchronous reactance D. none of these
B. inversely proportional to the synchronous reactance
In a power transformer, the winding which is nearer to the iron core of the transformer A. is a H.V. winding B. is L.V. winding C. may be H.V. or L.V. winding D. is sandwiching of the two windings
B. is L.V. winding
If the input of the prime mover of an alternator is kept constant but excitation is increased then A. kVA will be leading B. kVA will be lagging C. kW will be change D. the power factor of the load remains constant
B. kVA will be lagging
An over-excited synchronous motor take A. lagging current B. leading current C. lagging and leading current D. none of these
B. leading current
Distribution transformers have good all-day efficiency due to A. low copper loss B. low iron loss C. low copper as well as iron loss D. low power loss
B. low iron loss
It is advisable to avoid line starting of induction motor and use starter because A. starting torque is very high B. motor takes five to seven times its full load current C. it will pick-up very high speed and may go out of step D. it will run in reverse direction
B. motor takes five to seven times its full load current
What kind of rotor is most suitable for turbo-alternators which are designed to run at high speed? A. salient-pole type B. non-salient pole type C. both A and B D. none of the above
B. non-salient pole type
Synchronous motors are A. self-starting B. not self-starting C. essentially self-starting D. none of these
B. not self-starting
In a synchronous motor, which loss does not vary with load? A. copper losses B. hysteresis losses C. windage losses D. none of the above
C. windage losses
The best and the most accurate method for determining the voltage regulation is A. synchronous impedance method B. mmf method C. emf method D. Potier Triangle Method
D. Potier Triangle Method
What is the efficiency of transformer compared with that of electrical motors of the same power? A. much smaller B. somewhat smaller C. much higher D. about the same
D. about the same
If Ө is the angle measured in electrical degrees between any point on the pole face and center of the pole then for setting a sinusoidal wave as output, the air gap at that point should be such that A. air gap α sin θ B. air gap α cos θ C. air gap α 1 / sinθ D. air gap α 1/cos θ
D. air gap α 1/cos θ
A good transformer oil should be absolutely free from A. sulphur B. alkalies C. moisture D. all of the above
D. all of the above
Short circuit test on the induction motor can be used to find out A. the transformation ratio of the induction motor B. power scale of circle diagram C. copper losses in the induction motor D. all of the above
D. all of the above
The induction motors are designed for A. three phase B. two phase C. single phase D. all of the above
D. all of the above
Which of the following is a correct relation for an induction motor? A.rotor efficiency = actual speed of rotor/synchronous speed B. rotor on losses = slip x rotor input C. rotor input = 2pi x gross torque x synchronous speed D. all of the above relations are correct
D. all of the above relations are correct
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
C. cross magnetizing
The induction motor differs from the synchronous motor in that it(s) A. requires a dc source for its rotor B. does not require a rotating magnetic field C. current is induced in its rotor D. current I conducted in rotor
C. current is induced in its rotor
The steel for construction of transformer core is made so as to have A. low permeability and high hysteresis B. low permeability and low hysteresis C. high permeability and low hysteresis D. high permeability and high hysteresis loss
C. high permeability and low hysteresis
The voltage applied to the field of a rotating cylindrical rotor type alternators A. high dc voltage B. high ac voltage C. low dc voltage D. low ac voltage
C. low dc voltage
The speed regulation of a 3-phase synchronous motor is A. 5% B. 1% C. 0.4% D. zero
D. zero
In a 3-phase synchronous converter, the ratio of ac current to dc current is equal to A. 0.943 B. 0.612 C. 1.60 D. 0.866
A. 0.943
If XS = te synchronous reactance of the machine, Xe = leakage reactance and Xa = reactance due to armature flux then A. Xa = XS - Xe B. Xa = XS + Xe C. Xa = Xe - XS D. Xe = XaXS
A. Xa = XS - Xe
Synchronous motor has A. constant speed B. slight drop in speed on full load C. high speed at low torque D. low speed at high torque
A. constant speed
Just like a dc shunt motor the speed of induction motor operating in stable region A. decreases with increase in load B. increases with increase in load C. decreases with decrease in load D. none of the above is true
A. decreases with increase in load
When a 3-phase synchronous generator is supplying a zero p.f. lagging load the armature field affects the main field in the following way: A. directly opposes it B. cross magnetizes it C. augments it directly D. indirectly augments it
A. directly opposes it
Dampers in large generators A. increase stability B. reduce voltage fluctuations C. reduce frequency fluctuations D. decrease stability
A. increase stability
In a 3-phase induction motor, the torque A. increases with the increase in supply voltage B. decreases with the increase in supply voltage C. has no relation with the supply voltage D. A and B depending upon the situation
A. increases with the increase in supply voltage
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
A. lagging
An alternator is said to be over-excited if it is operating at A. leading p.f. B. lagging p.f. C. unity p.f. D. zero voltage regulation
A. leading p.f.
42. For synchronizing three phase alternators, the additional requirement is that the phase rotation A. must be the same B. is clockwise C. is anti-clockwise D. none of the above
A. must be the same
No load primary input is practically equal to the iron losses in the transformer because A. primary current is small B. secondary current is small C. both the currents are small D. no load is not equal to iron losses
A. primary current is small
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 the above
A. rotating field
Squirrel cage induction motor differs from the slip ring type in that it has no A. slip rings B. rotor winding C. windings on the stator D. rotating part
A. slip rings
In a synchronous motor, the torque angle is A. the angle between the rotating stator flux and rotor poles B. the angle between the magnetizing current and back emf C. the angle between the supply voltage and the back emf D. none of the above
A. the angle between the rotating stator flux and rotor poles
When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected, then A. the motor stops B. it runs at the same speed C. it runs at lower speed D. it runs at very high speed
A. the motor stops
When the excitation of an unloaded salient-pole synchronous motor suddenly gets disconnected A. the motor stops B. it runs as a reluctance motor at the same speed C. it runs as a reluctance motor at a lower speed D. none of the above
A. the motor stops
A 3-phase synchronous motor is running clockwise. In case the direction of its field current is reversed A. the motor will continue to run in the same direction B. the motor will run in the reverse direction C. the motor will stop D. the winding of the motor will burn
A. the motor will continue to run in the same direction
A 3-phase, 400 V, 50 Hz salient pole synchronous motor is running on no load. If there is break in the excitation winding of the motor A. the motor will stop B. the winding will get overheated C. the motor will run as reluctance motor at the rated rpm D. the motor will run as reluctance motor at lower rpm
A. the motor will stop
If the input to the prime-mover of an alternator is kept constant but the excitation is changed then A. the reactive component of the output is changed B. the active component of the output is changed C. the p.f. of the load remains constant D. the p.f. of the load varies
A. the reactive component of the output is changed
For a 3-phase, wound-rotor induction motor, as increase in rotor resistance affects the motor performance in the following way A. the starting current decreases B. the motor efficiency decreases C. the maximum torque is reduced D. the starting current decreases
A. the starting current decreases
The armature reaction of an alternator will be cross-magnetizing of the power factor of the load is A. unity B. less than unity C. more than unity D. zero leading
A. unity
Torque under running condition is maximum at the slip S A. which makes rotor reactance/phase equal to rotor resistance/phase B. which makes rotor reactance/phase equal to rotor resistance/phase greater than rotor resistance/phase C. which makes rotor reactance/phase lesser than rotor resistance/phase D. which is independent of rotor resistance
A. which makes rotor reactance/phase equal to rotor resistance/phase
Ideal transformer assumptions do not include A. zero reactance of windings B. zero resistance of windings C. no leakage flux D. no saturation of core
A. zero reactance of windings
The full load copper loss in a transformer is 400 watts. At half load, the copper loss will be A. 400 watts B. 100 watts C. 200 watts D. 50 watts
B. 100 watts
Three 10 : 1 single phase, step-down transformers are connected in wye-delta. For a primary line-to- line voltage of 3980 V, find the corresponding line voltage on the secondary side. A. 398 V B. 230 V C. 3980 V D. 2300 V
B. 230 V
The speed of a synchronous motor connected to 50 cycle supply is 1000 rpm. The number of poles it must have A. 4 B. 6 C. 8 D. 10
B. 6
While conducting short-circuit test on a transformer, the following side is short-circuited: A. H.V. side B. L.V. side C. Primary side D. Secondary side
B. L.V. side
In any transformer the voltage per turn in primary and secondary remains A. always different B. always same C. always in ratio of k D. sometimes same
B. always same
Each of two alternators operating in parallel are adjusted for unity power factor. Increasing the field current of one A. causes its power factor to become leading B. causes its power factor to become lagging C. changes its speed D. increases its load
B. causes its power factor to become lagging
Eddy current losses in transformer core are reduced by A. increasing the thickness of laminations B. decreasing the thickness of laminations C. taking the wire of higher gauge for winding the transformer D. decreasing the air gap in the magnetic circuit
B. decreasing the thickness of laminations
The induced emf in the transformer secondary will depend upon A. maximum flux in core and frequencies only B. frequency, flux and number of turns in the secondary C. frequency of the supply only D. number of turns in secondary only
B. frequency, flux and number of turns in the secondary
The principle of operation of the induction motor is based on A. Faraday's law B. rotating magnetic field C. Flemming's right hand rule D. Ohm's law
B. rotating magnetic field
In an induction motor A. rotor runs at a speed equal to the speed of the stator field B. rotor runs at a speed which is always lesser than the speed of the stator field C. rotor runs at a speed which is always higher than the speed of the stator field D. B and C both are correct
B. rotor runs at a speed which is always lesser than the speed of the stator field
Pitch factor is defined as the ratio of the emf's of A. distributed winding to full pitch winding B. short pitch coil to full pitch coil C. full pitch winding to short pitch winding D. full pitch winding to concentrated winding
B. short pitch coil to full pitch coil
The laminations are made from A. low carbon steel B. silicon steel sheet C. nickel alloy steel stampings D. chrome steel sheets
B. silicon steel sheet
If the air gap of the induction motor is increased A. the magnetizing current will decrease B. the magnetizing current will increase C. the power factor will decrease D. none of the above is correct
B. the magnetizing current will increase
In a slip-ring induction motor resistance is connected in rotor phases A. to limit starting current B. to increase the starting current C. to limit starting current and increase starting torque D. none of the above statement is correct
B. to increase the starting current
Short-pitch coils in the alternator are used A. to reduce the size of the alternator B. to reduce the harmonics from the generated emf of the alternator C. to reduce the copper loss in the alternator D. to reduce the core loss in the alternator
B. to reduce the harmonics from the generated emf of the alternator
The minimum armature current of a synchronous motor corresponds to operation at A. 0.8 power factor lagging B. unity power factor C. zero power factor leading D. zero power factor lagging
B. unity power factor
Squirrel cage induction motor has A. zero starting torque B. very small starting torque C. very high starting current D. extra starting torque and it starts up from rest
B. very small starting torque
Transformer is used to change the values of A. frequency B. voltage C. power D. power factor
B. voltage
A change of 5% in the supply voltage of a 3-phase induction motor will produce to its torque a change of approximately A. 5% B. 25% C. 10% D. 20%
C. 10%
A 3-phase, 400 V, 50 Hz, 4-pole synchronous motor has a load angle of 10 degrees electrical. The equivalent mechanical degree will be A. 10 degrees B. 5 sqrt. 2 degrees C. 5 degrees D. 1 degree
C. 5 degrees
The synchronous speed of a 50 cycle 10 pole motor is A. 500 rpm B. 150 rpm C. 600 rpm D. 1000 rpm
C. 600 rpm
The maximum power developed in a synchronous motor occurs at a coupling angle of A. 0 degree B. 45 degrees C. 90 degrees D. 180 degrees
C. 90 degrees
At every instant, the direction of secondary current in a transformer must be such as to oppose any change in flux. This is in accordance with A. Faraday's law B. Joule's law C. Lenz's law D. Coulomb's law
C. Lenz's law
The armature current of the synchronous motor has large values of A. low excitation only B. high excitation only C. both low and high excitation D. moderate excitation
C. both low and high excitation
The frequency of the rotor current in an induction motor is given by (If f is the supply frequency, f' is the rotor frequency and S is the slip) A. f = Sf' B. f' = Sf' C. f' = Sf D. S = f f'
C. f' = Sf
A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen? It will A. start after some time B. continue to run at lower speed without damage C. get heated and consequently get damaged D. continue to run then stop after some time
C. get heated and consequently get damaged
When the speed of an alternator increases, the frequency A. decreases B. remains the same C. increases D. changes
C. increases
When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor A. decreases to half the original value B. remains the same as the original value C. increases to two times the original value D. tends to become zero
C. increases to two times the original value
The transformer oil used in transformers provide A. cooling and lubrication B. insulation and lubrication C. insulation and cooling D. insulation, cooling and lubrication
C. insulation and cooling
The resistance of low voltage side of a transformer A. is equal to resistance of its high voltage side B. is more than the resistance of its high voltage side C. is less than the resistance of its high voltage side D. B or C
C. is less than the resistance of its high voltage side
Synchronous motor is capable of being operated at A. lagging power factor only B. leading power factor only C. lagging and leading power factor D. none of these
C. lagging and leading power factor
The principle of working of a transformer is A. static induction B. dynamic induction C. mutual induction D. self induction
C. mutual induction
Two of the power supply terminals to a 3-phase induction motor got interchanged during reconnection after maintenance of the motor, when put back into service, the motor will A. fail to rotate B. rotate in the same direction as it was prior to maintenance C. rotate in the reverse direction to that prior to maintenance D. stop
C. rotate in the reverse direction to that prior to maintenance
The magnitude of mutual flux in a transformer is A. low at low loads and high at high loads B. high at low loads and low at high loads C. same at all levels D. varies at low loads and constant at high loads
C. same at all levels
The rotor speed of an induction motor is A. the same as the synchronous speed B. faster than the synchronous speed C. slower than the synchronous speed D. determined by frequency and the number of poles
C. slower than the synchronous speed
The synchronous motor is not inherently self-starting because A. the rotating magnetic field does not have enough poles B. the rotating magnetic field is produced by only 50 Hz frequency currents C. the force required to accelerate the rotor is to the synchronous speed in an instant is absent D. the starting devices to accelerate the rotor to near synchronous speed is absent
C. the force required to accelerate the rotor is to the synchronous speed in an instant is absent
In a synchronous motor, as load is applied, the motor takes more armature current because A. back emf decreases causing an increase in motor current B. the rotor strengthens the rotating field causing more motor current C. the rotor by shifting its phase backward causes motor to take more current D. the increased load has to take more current
C. the rotor by shifting its phase backward causes motor to take more current
In an ideal transformer on no-load, the primary applied voltage is balanced by A. the secondary voltage B. the drop across resistance and reactances C. the secondary induced emf D. the primary induced emf
C. the secondary induced emf
Synchronous motor always runs at A. a speed less than synchronous speed B. a speed more than synchronous speed C. the synchronous speed D. none of these
C. the synchronous speed
A 400 kW, 3-phase, 440 V, 50 Hz induction motor has a speed of 950 rpm on full load. The machine has 6 poles. The slip of the machine will be A. 0.06 B. 0.10 C. 0.04 D. 0.05
D. 0.05
A 3-phase, 4 pole, 24 slot alternator has its armature coils short-pitched by one slot. Its pitch factor will be A. 1.0 B. 0.966 C. 0.96 D. 0.94
D. 0.94
For a uniformly distributed winding the value of distribution factor is A. unity B. 0.96 C. 0.866 D. 0.955
D. 0.955
If the no-load speed of a squirrel cage induction motor connected to a three phase 50 cycle supply is 740 rpm. The motor has A. 2 poles B. 4 poles C. 6 poles D. 8 poles
D. 8 poles
The driving power from the prime mover driving an alternator is lost but the alternator remains connected to the supply network and field supply is on. The alternator will A. get burnt B. behaves as an induction motor C. behaves as synchronous motor but will rotate in reverse direction to that corresponding to generator action D. behaves as a synchronous motor and will rotate in the same direction
D. behaves as a synchronous motor and will rotate in the same direction
Adjustment of field excitation on one of the two alternators operating in parallel will A. increase its load B. change its frequency C. decrease its load D. change its power factor
D. change its power factor
Improvement in the power factor in an induction motor A. decreases torque B. increases the torque C. increases the current D. increases the torque and decreases the current due to increased impedance
D. increases the torque and decreases the current due to increased impedance
To reverse the direction of rotation of a three phase induction motor, it necessary to A. interchange all the three line connections B. reverse the polarity of the rotor circuit C. increase the resistance of the rotor circuit D. interchange any two of the three line connections
D. interchange any two of the three line connections
The rotor of an induction motor gets its power from A. the line voltage B. slip rings C. a centrifugal switch D. magnetic induction
D. magnetic induction
In order to transfer the load from one alternator to another alternator, when they are operating in parallel A. field current is increased B. field current is decreased C. speed is decreased D. prime mover power input is increased
D. prime mover power input is increased
In a synchronous motor, the synchronizing power comes into action when A. rotor speed is equal to synchronous speed B. rotor speed is less than synchronous speed C. rotor is speed is more than synchronous speed D. rotor speed is either less or more than synchronous speed
D. rotor speed is either less or more than synchronous speed
A certain motor is driven by a 1750 rpm dc shunt motor. If the power supply is to be changed to three phase, 50 cycles ac, the most suitable replacement motor would be A. synchronous motor B. capacitor motor C. ac commutator motor D. squirrel cage induction motor
D. squirrel cage induction motor
The motor in which stator and rotor magnetic field rotate at the same speed is A. induction motor B. reduction motor C. universal motor D. synchronous motor
D. synchronous motor
The primary and the secondary voltages are A. 180 degrees out of phase in a transformer B. always in phase in a transformer C. 90 degrees out of phase in a transformer D. 30 degrees or 60 degrees our of phase in a transformer
A. 180 degrees out of phase in a transformer
The concentric windings are used in core type transformer with A. LT winding placed next to core B. HT winding placed next to core C. LT winding on the outer side D. HT winding on the outer side
A. LT winding placed next to core
The path of the magnetic flux in a transformer has A.low reactance B.high reactance C.high conductivity D.low resistance
A. Low reactance
The no-load current of a transformer in terms of full load current is usually A. 1 to 3% B. 3 to 9% C. 9 to 12% D. 12 to 20%
A. 1 to 3%
Mark the correct statement. A. The percentages of resistance, reactance and impedance have the same value whether referred to primary or secondary of the transformer. B. A good transformer must have regulation as high as possible. C. If the power factor is leading the regulation of a good transformer will be higher than when it is lagging. D. None of the above is correct.
A. The percentages of resistance, reactance and impedance have the same value whether referred to primary or secondary of the transformer.
Eddy current losses in a transformer are minimized by laminating the core, the lamination being insulated from each other by A. a light coat of core plate varnish B. thin sheet of mica C. thick paper of insulation D. any one of the above
A. a light coat of core plate varnish
What type of core is used for a high frequency transformer? A. air core B. closed iron core C. aluminum core D. open iron core
A. air core
Conservator consists of A. an air-tight metal drum fixed at the top of the tank B. drum placed at the bottom of the tank C. overload protection D. none of these
A. an air-tight metal drum fixed at the top of the tank
Preferably, the resistance between the primary and the secondary of a transformer should be A. as low as possible B. as high as possible C. low or high depending upon whether it is step up or step down respectively D. high or low depending upon whether it is step up or step down respectively
A. as low as possible
The use of higher flux density in transformer design A. decreases the weight per kVA B. increases the weight per kVA C. increases the weight per kW D. decreases the weight per kW
A. decreases the weight per kVA
One of the advantages of a short pitch winding lies in A. elimination or suppression of harmonics B. reduction of voltage C. increasing the inductance D. reduction of the circulating current
A. elimination or suppression of harmonics
In transformer the purpose of breather is to A. extract moisture of the air B. to take insulating oil from conservator C. to provide cooling to the winding D. to provide insulation to the winding
A. extract moisture of the air
Cross-over windings are used for A. high voltage winding of small rating transformers B. low voltage winding of small rating transformers C. high voltage winding of large rating transformers D. low voltage winding of large rating transformers
A. high voltage winding of small rating transformers
The special silicon steel is used for laminations because A. hysteresis losses are reduced B. eddy current losses are reduced C. both the above losses are reduced D. none of these
A. hysteresis losses are reduced
As compared to an amplifier a transformer cannot A. increase output power B. increase the output current C. increase the output voltage D. decrease the output current
A. increase output power
An alternator is capable of delivering power at a particular frequency. The frequency can be increased by A. increasing the armature speed B. reversing the armature rotation C. increasing the current supplied to the field electromagnets D. reversing the field polarity
A. increasing the armature speed
The reactance of a transformer is determined by its A. leakage flux B. common core flux C. size of the core D. permeability of the material of the core
A. leakage flux
Distributing the armature winding of alternator 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 winding C. less weight of the entire armature D. increase of generated emf per phase
A. reduction of irregularities produced in the waveform
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. larger because speed is less
A. smaller because speed is high
The transformation of energy from primary of the transformer to secondary takes place due to A. the flux linkage between the two windings B. the difference in number of turns of primary and secondary windings C. the changing current in two windings D. none of these
A. the flux linkage between the two windings
If we increase the flux density in the case of a transformer A. the size of the transformer will reduce B. the distortion in the wave shape will reduce C. the hysteresis and eddy current losses will reduce D. none of the above is true
A. the size of the transformer will reduce
A winding is short pitched by 60 degrees electrical. Its pitch factor is A. 0.5 B. 0.866 C. 0.707 D. 0.966
B. 0.866
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
B. 0.866
Which of the following is a correct statement about eddy currents? A. Eddy currents improve the efficiency of a motor. B. Eddy currents heat up the metal parts. C. Eddy currents do not influence the movement. D. Eddy currents are used for arc welding.
B. Eddy currents heat up the metal parts.
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 verticate the armature
B. as a return path for the flux
One of the advantages of distributing the winding in alternators is to A. reduce harmonics B. improve voltage wave form C. reduce the amount of copper D. decrease the value of the voltage
B. improve voltage wave form
Synchronous reactance consists of two components, namely A. armature resistance and leakage reactance B. leakage reactance and fictitious reactance C. armature resistance and fictitious reactance D. leakage reactance and capacitive reactance
B. leakage reactance and fictitious reactance
The slip rings employed in a three-phase alternator in hydrostation are insulated for A. extra high tension voltage B. low voltage C. full armature voltage D. very high voltage
B. low voltage
What is common in the two windings of a transformer? A. electric circuit B. magnetic circuit C. winding wire gauge D. dielectric circuit
B. magnetic circuit
Electric power is transformed from one coil to the other coil in a transformer A. physically B. magnetically C. electrically D. electromagnetically
B. magnetically
If in a transformer the secondary turns are doubled and as the same time the primary voltage is reduced by half, then the secondary voltage will A. be halved B. not change C. be four times as high D. be reduced to a quarter
B. not change
High speed alternators have a rotor construction A. similar to dc machines B. of non-salient type C. of salient type D. of silent type
B. of non-salient type
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
B. steam-turbo alternators run at high speed
The magnetic coupling between the primary and secondary windings of a transformer may be increased by A. increasing the number of laminations of core material B. using the magnetic core of low reactance C. changing the turns ratio of the transformer D. no means, because it is constant
B. using the magnetic core of low reactance
Regulation of an alternator is defined as the rise in terminal when full load is thrown off expressed as percentage of the terminal for the same value of excitation and speed. A. current B. voltage C. rating D. power factor
B. voltage
Alternators mostly work 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. don't obey Ohm's law
B. work on Faraday's law
Which of the statement given below is true about autotransformer? A. It has two separate windings connected in series externally. B. It can only step down the voltage. C. It has only one winding. D. It is most suitable for power transformation.
C. It has only one winding.
What is the typical use of an autotransformer? A. Toy transformer B. Control transformer C. Variable Transformer D. Isolating Transformer
C. Variable Transformer
The imaginary or fictitious part of synchronous reactance takes care of A. copper losses B. voltage regulation C. armature reaction D. inductive reactance
C. armature reaction
Synchronization of alternators means A. connecting alternators in series B. transferring load from one alternator to another C. connecting alternators in parallel D. adjusting the excitation of the alternators
C. connecting alternators in parallel
The main function of the iron core in a transformer is to A. provide strength to the windings B. to decrease the hysteresis loss C. decrease the reluctance of the magnetic path D. reduce eddy current losses
C. decrease the reluctance of the magnetic path
Alternators are usually designed to generate A. variable frequency B. definite currents C. definite frequencies D. definite power factor
C. definite frequencies
High voltage alternators are usually of the rotating field type. This means the generated voltage is connected A. through slip rings to the load B. through slip rings of the rotating field C. directly to the load D. directly to the rotor
C. directly to the load
30. Distribution factor K4 is defined as the ration of emf's of A. distributed winding to full pitch winding B. concentrated winding to distributed winding C. distributed winding to concentrated winding D. full pitch winding to distributed winding
C. distributed winding to concentrated winding
Power transformers are designed to have maximum efficiency at A. no load B. half load C. near full load D. little more than full load
C. near full load
Which is the common method of cooling a power transformer? A. air cooling B. air-blast cooling C. oil cooling D. natural cooling
C. oil cooling
Current transformer secondary is connected to an ammeter of 5 A range to measure line current of 1000 A. The ammeter is to be serviced. It can be removed A. without interfering with supply B. with interfering with supply C. only after switching off the main current path D. only before switching off the main current path
C. only after switching off the main current path
Two transformers when operating in parallel will share the load depending upon their A. magnetizing current B. leakage reactance C. per unit impedance D. efficiency
C. per unit impedance
In huge alternators, the moving part is A. brushes B. armature C. poles D. rotor
C. poles
To synchronize alternator, three conditions must be fulfilled, namely A. same voltage ratio, same frequency and voltages should be in phase with respect to the external circuit B. same voltage, same frequency and voltages should be in phase opposition with respect to the external circuit C. same voltage, same frequency and voltages should be in phase opposition with respect to local series circuit D. same voltage ratio, different frequencies and proper phase relation of the voltages
C. same voltage, same frequency and voltages should be in phase opposition with respect to local series circuit
The maximum current that can be supplied by alternators depends upon the A. strength of the excite voltage B. number of poles C. strength of the magnetic field D. maximum heat dissipation
C. strength of the magnetic field
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 the same B. to be weakened keeping input torque the same C. to be kept constant but input torque should be increased D. to be kept constant but input torque should be decreased
C. to be kept constant but input torque should be increased
All day efficiency of a transformer η A A. η A = electrical efficiency of transformer B. η A > electrical efficiency of transformer C. η A < electrical efficiency of transformer D. none these may be true
C. η A < electrical efficiency of transformer
A transformer is working at its maximum efficiency. Its iron loss is 500 watts. Its copper loss will be A. 300 watts B. 250 watts C. 350 watts D. 500 watts
D. 500 watts
The efficiency of a transformer is normally in the range of A. 50% to 70% B. 60% to 75% C. 80% to 90% D. 90% to 98%
D. 90% to 98%
The size of a transformer core will depend on A. frequency B. flux density of the core material C. area of the core D. A and B both
D. A and B both
A transformer operates A. always at unity power factor B. has its own power factor C. at power factor below a particular value D. at power factor depending on the power factor of the load
D. at power factor depending on the power factor of the load
43. The method most commonly used for synchronizing the alternators in a power house is: A. dark lamp method and a synchroscope B. dark lamp method only C. bright lamp method D. bright lamp method and a synchroscope
D. bright lamp method and a synchroscope
From what source does a rotating field type alternator receive its exciter voltage? A. from an externally controlled ac voltage B. from an externally controlled dc voltage C. from a self excited source like shunt generator D. from an auxiliary generator driven by main generator shaft
D. from an auxiliary generator driven by main generator shaft
Transformers are rated in A. kW B. kV C. kWH D. kVA
D. kVA
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
D. none of these
Synchronous impedance of an alternator is defined as the ratio of A. short circuit voltage to short circuit current B. open circuit voltage to short circuit current C. short circuit current to no load current D. open circuit voltage to short circuit current for same value of excitation
D. open circuit voltage to short circuit current for same value of excitation
Two transformers operating in parallel will share the load depending upon their A. rating B. leakage reactance C. efficiency D. per unit impedance
D. per unit impedance
In an ideal transformer on no load, the voltage applied across the primary winding of the transformer is balanced by A. the emf induced in the primary winding B. the drop across the impedance of primary side C. the drop across the impedance of secondary side D. the secondary induced emf
D. the secondary induced emf
The basic property of the transformer is that it changes the voltage level of an a.c. signal A. without changing the power B. without changing its shape C. without changing its frequency D. without changing power, frequency or shape
D. without changing power, frequency or shape