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SAIL Rourkela Operator-Cum-Technician Trainee(OCTT) Electrical 2019 Solved Question Paper PDF Part Three
Questions-51.The voltage ratio transfer function of an active filter is given by
The above transfer function is for a
(A) Low pass filter
(B) High pass filter
(C) Band pass filter
(D) Band reject filter
Solution-In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels
Standard transfer function of band reject filter is given by
Questions-52.Distributed winding and fractional pitching employed in A.C. machines result in
(A) Increase of e.m.f. and reduction of harmonics
(B) Reduction of both e.m.f. and harmonics
(C) Increase in both e.m.f. and harmonics
(D) No effects on both e.m.f. and harmonics
Solution-In ac machine if distributed winding is used then it improve the waveform and keep waveform purely sinusoidal.or short pitched winding decreases emf due to shorter length of winding is used it also reduce harmonics and gives harmonics free sinusoidal wave form .
Questions-53.Two perfectly matched silicon transistors are connected as shown in figure.
The value of the current I is
(A) 0 mA
(B) 2.3 mA
(C) 4.3 mA
(D) 7.3 mA
Solution-Given β=1000 for each transistor,Vbe=0.7 V
From figure I current flowing through 1 KΩ resistance,Applying KVL to 1 KΩ resistance loop we get
I=4.3 V/1 KΩ=4.3 mA
Questions-54.The presence of ac source resistance in rectifiers effectively leads to
(A) Increase in output dc voltage
(B) Reduction in the load current ripples
(C) Increase in the load current ripple
(D) Reduction in output dc voltage
Solution-The presence of ac source resistance in rectifiers increases ripple in output dc source so it reduces the output dc voltage.Here due to presence of ripple in output dc voltage the output dc voltage is decreases.
Questions-55.For the measurement of the earth resistance of a given earth electrode
(A) Collecting electrode should be very near to the electrode under test
(B) Collecting electrode should touch the electrode under test
(C) Collecting electrode should be far away the electrode under test
(D) None of these
Solution-For failure of one earth electrode other collecting electrode should be work satisfactorily and does’t effect the other earth electrode,It should be placed far away the electrode under test
Questions-56.The forward dynamic resistance of a junction diode
(A) Decreases with increasing forward current
(B) Increase with increasing forward current
(C) Is unaffected by the forward current
(D) Doubles for every 10°C rise in temperature at constant current
Solution-As we know that resistance is inversely proportional to current.If forward current increases then forward dynamic resistance decreases.
Questions-57.For charging of the transmission line,choose the correct sequence of the following activities; (I) closing of the circuit breaker, (II) opening of the earthing switch and (III) closing of the isolator
Solution-For charging of the transmission line following sequence should be follow
- Opening of the earthing switch
- Closing of the isolator
- Closing of the circuit breaker
For maintenance time following sequence should be follow
- Closing of the earthing switch
- Opening of the isolator
- Opening of the circuit breaker.
Questions-58.The most preferred motor used in the food mixer is
(A) dc series motor
(B) Squirrel cage induction motor
(C) Reluctance motor
(D) Universal motor
Solution-Universal motor should be used in ac or dc operation.Which speed is very high.So it is used in food mixer ,electric saver,grinders etc.
Questions-59.In the closed loop control system, proportional integral (PI) controller is preferred compared to proportional controller (P)
(A) Fast response
(B) High gains
(C) Zero steady state error
(D) Low overshoot
Solution-Proportional integral (PI) controller-In PI output power equals to the sum of proportion and integration coefficients.PI control provides zero steady state error and is insensitive to interference of the measurement channel.
Questions-60.A salient pole synchronous motor is running at no load. Its excitation is reduced to zero
(A) It will stop
(B) It will remain running at synchronous speed
(C) It will loose synchronism
(D) It is uncertain
Solution-If excitation is reduced to zero. Than because of damper winding is also used in synchronous motor it always running at synchronous speed.
Questions-61.The function of commutator in the dc motor:
(A) To collect current from the conductors
(B) To change ac into dc
(C) To conduct the current to the brushes
(D) To change dc into ac
Solution-A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit.
In case of dc motor the function of commutator is to convert dc into ac.
Questions-62.To calculate the power factor which of the following is odd one
(A) True power/apparent power
Solution-Power factor is defined as the following way-
- True power / Apparent power
- cos φ
Questions-63.The size of a feeder is determined primarily by
(A) The current it is to carry
(B) The percentage variation of voltage in the feeder
(C) The voltage across the feeder
(D) The distance over which the transmission is made
Solution-Feeder is always determined primarily by its current carrying capacity because there is no tapping taken on this so current is always same in feeders.
Questions-64.The ratio of voltage and current in a closed circuit
(B) Remains constant
Solution-For a closed circuit the resistance value of its is always constant so if resistance value is constant then the ratio of voltage and current is also constant.
Questions-65.Energy consumed by a heater of rating 1000 watts by operating it for a period of 2 hours will be
(A) 1 unit
(B) 2 unit
(C) 2.5 unit
(D) 4 unit
Solution-Energy consumed by 1000 watt rating of heater at using 2 hours is-1000×2=2000 watt-hour
As we know that 1000 watt-hour =1 unit.
So 2000 watt-hour/1000 watt-hour=2 unit.
Questions-66.A single-phase preventer trips frequently in the motor. What is the reason for it
(A) Improper phase sequence
(B) Low supply voltage
(C) Abnormal fluctuation in the voltage
(D) Relay of signal phase prevention is not energized
Solution-In three phase motor if supply voltage is same in all three phases then motor running satisfactorily. Then if one of the three phases in motor ,voltage fluctuation happen abnormally then a single phase preventer trips the motor frequently so save the motor from low or abnormal voltage or heating problem.
Questions-67.Equivalent resistance between point A and B in following figure is
(A) 10 Ohm
(B) 12.4 Ohm
(C) 6.8 Ohm
(D) 16 Ohm
Solution-Here 4 ohm is parallel with 6 ohm so net resistance in parallel connection is
Reqv=2.4 after that adding series resistance then total resistance should be=2.4+10=12.4 Ω
Questions-68.Induction generator is more suitable for supplying
(A) Resistive load
(B) Inductive load
(C) Capacitive load
(D) A combination of these
Solution-We know that a induction generator works in negative slip region,Then its speed is more than the synchronous speed.Here it consume leading reactive power and supply lagging reactive power.In -ve slip region it is best suitable for supplying capacitive load.
Questions-69.In the measurement of power by two watt meter method, when reading of two watt–meters are equal and opposite, it can be concluded that power factor is
Questions-70.Ferranti effect in overhead lines is experienced when
(A) Sending end voltage is more than receiving end voltage
(B) Sending end voltage is less than receiving end voltage
(C) Sending end voltage is equal to receiving end voltage
(D) Voltage at all points in line is equal
Solution-At Ferranti effect which occurs under no load condition or light load condition under this condition due to capacitance effect the receiving end voltage is greater than the sending end voltage
Questions-71.In which of the following amplifier configurations, the power gain is the largest
(D) None of the above
Solution-Comparison amplifier configurations
Questions-72.The Hopkinsons test on d.c. Machine is conducted in order to determine:
(A) Rated losses of d.c. machine
(B) No-load losses of d.c. machine
(C) core losses of d.c. machines
(D) Copper losses of d. c. machines
Solution-Hopkinson test on dc machine is conducted in order to find full load cu loss or rated loss of dc machine
Questions-73.In a single-phase, fully controlled rectifier with continuous dc load current, the number of thyristors conduct during commutation
Solution-The single phase fully controlled rectifier allows conversion of single phase AC into DC. Total four thyristors conduct during commutation of single phase fully controlled rectifier
Questions-74.In an inductor, its time constant is 10msec. and power loss is 10W at 1 A current flowing through it, its inductance is
(A) 1 H
Solution-Power loss =10W
L=Time Constant*R=10 m sec*10=100 mH
Questions-75.A 4-pole three-phase induction motor has a synchronous speed of 25 rev/s. The frequency of the supply to the stator is
(A) 50 Hz
(B) 100 Hz
(C) 25 Hz
(D) None of these
Solution- Ns=f/p So,f=Ns×P=25×=50Hz
Questions-76.A Lissajous patterns are used to measure
(A) Voltage and frequency
(B) Frequency and Phase shift
(C) Frequency and amplitude distortion
(D) Amplitude and flux
Solution-In CRO Lissajous patterns are used to find frequency, phase shift and waveform.
Questions-77.No load current of a three-phase squirrel cage induction motor at rated voltage is
(A) 30% to 50% of the rated current
(B) The rated current
(C) 5% to 10% of the rated current
(D) 5 to 7 times of the rated current
Solution-In induction motor the air gap between stator and rotor is more so about 30 to 40% rated current is required to setup flux in this gap.
Questions-78.A three-phase induction motor has 4 poles and operates with a slip of 0.04 with 50 Hz supply for a certain load. The speed of the rotor magnetic field with respect to stator is
(A) 1440 rpm
(B) 60 rpm
(C) 1500 rpm
(D) 0 rpm
Solution-Here given P=4 ,S=0.04 and f=50 Hz
Rotor magnetic field (Ns)=120*f/p=120*50/4=1500
Both stator or rotor magnetic field rotates with the synchronous speed.
And stator is always fix not rotate So speed of the rotor magnetic with respect to stator is=Ns-0=Ns=1500 rpm.
Questions-79.DC generator used for voltage drop compensation is known as
(C) Cumulative compound
(D) Differential compound
Solution-DC series generator is used to compensate voltage drop as a booster in transmission line.
Questions-80.A dc shunt motor is running at rated speed with rated supply voltage. If the armature voltage is reduced to half, then the speed of the motor becomes
(A) Half of the rated speed
(B) Double of the rated speed
(C) Slightly less than the rated speed
(D) Slightly more than the rated speed
Solution-As we know that speed is directly proportional to back emf And Eb=V-IaRa
Using this formula =N1/N2=V1/V2
If armature voltage is reduced to half then back emf also reduced to half,and speed is directly proportional so it should be also reduced to half.