SAIL Rourkela Operator-Cum-Technician Trainee(OCTT) Electrical 2019 Solved Question Paper PDF Part Two
Questions-26.Turns compensation is used in current transformers primarily for reduction of:
(A) Phase angle error
(B) Both ratio and phase angle errors
(C) Ratio error, reduction in phase angle error is incidental
(D) None of the above
Solution-Turns compensation is used in current transformer primarily for reduction of ratio error and reduction in phase angle error is incidental.
Questions-27.The energy in a lead acid battery is stored in a form of
(A) Di-electrostatic charge
(B) Non flowing current
(C) Chemical energy
(D) Nuclear energy
Solution-The energy in a lead acid battery is stored in a form of chemical energy in electrolyte at use time converted in electrical energy.
Questions-28.If supply frequency increases, the skin effect is
(B) Remains same
(D) None of these
Solution-Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor.
The skin effect is inversely proportional to skin depth
Skin effect ∝ 1/ Skin depth
Skin depth ∝ 1/√πμδf
So when frequency f will be increase skin effect will be increase
Questions-29.A transistor can be used as
(C) As a diode
(D) All of these
Solution-A transistor can be used as switch,amplifier and and a diode which act as a rectification work.
Questions-30.If the iron core of a transformer is replaced by an air core then the total iron loss in a transformer will
(C) Remain unchanged
(D) Become zero
Solution-In transformer air offer more reluctance in comparison to the iron so flux is decreases then iron loss decrees.
Questions-31.When the dc motor is at rest, the value of the back emf is
(A) Equal to the supply voltage
(B) Less than the supply voltage
(C) Greater than the supply voltage
Solution-At the starting of induction motor when speed is zero then net back should also be zero.
Because N ∝ Eb/φ
When N=0, Then Back EMF (Eb)=0.
Questions-32.A line which connects a distributor to substation is called:
(D) Service main
Solution-Feeder is a line which connects a distributor to substation.
Service main is a line which connects the distributor to consumer terminal.
Questions-33.An RLC series circuit is resonating at 150 Hz, its impedance at 50 Hz will be
(C) Only resistive
(D) Dependent on RLC elements values
Solution- In case of RLC series resonance circuit ,if frequency is less than the resonant frequency then it will be act as capacitive circuit and if frequency is more than the resonant frequency then it will be act as inductive circuit
Questions-34.LVDT is a
(A) Displacement transducer
(B) Velocity transducer
(C) Acceleration transducer
(D) Pressure transducer
Solution-LVDT stands for linear variable differential transformer.Which is used for linear displacement or position measurement.
Questions-35.In v/f control of three phase induction motor, which of these variable is fixed
(A) The maximum torque
(B) Synchronous speed
(C) The starting torque
(D) The slip for maximum torque
Solution-By changing v/f control of three phase induction motor, if frequency changes then synchronous speed changes,or speed changes then slip changes causes starting torque changes.but maximum torque is fixed.because it depends on reactance.
Questions-36.Transposition of conductors is done to
(A) Save conductor
(B) Increase stability
(C) To have equality in line parameters
(D) To interchange values of voltage of current at other end
Solution-In order to reduce inequality of inductance,and also inductive interference due to communication lines running adjacent to power lines,transposition is done.After transposition inductance of every phase becomes approximately equal and they have equality in line parameter.
Questions-37.In the circuit shown the current through 5 ohm resister is
Applying KVL in mesh ABC
Here current is taken in opposite so current through 5 ohm resister is 0.4
Questions-38.Transformer is used to change the values of
(D) Power factor
Solution-Transformer is used to change the values of voltage and current here frequency and power are fixed and power factor depends on load.
Questions-39.Open circuit test of transformer gives
(A) Hysteresis loss
(B) Eddy current loss
(C) Sum of hysteresis and eddy current loss
(D) Copper loss
Solution-In transformer open circuit test is performed to determine core loss ,here core loss consist of Hysteresis loss and eddy current loss.
Questions-40.In which type of fault maximum short-circuit current occurs
(A) Three-phase faults to ground
(B) Double line fault to ground
(C) Single line fault to ground
(D) None of these
Solution-In three phase faults to ground (LLLG) fault the maximum short-circuit current flow in a circuit.
Questions-41.In a 3-phase induction motor, maximum torque is obtained at starting if
(A) Rotor resistance = Rotor reactance
(B) Rotor resistance < Rotor reactance
(C) Rotor resistance >Rotor reactance
(D) Rotor resistance = Stator reactance
Solution-In a 3-phase induction motor ,Maximum torque is obtained when S=1,
We know that S=R/X ,When S=1 then R=X at stand still.
Questions-42.A single–phase ac regulator is used to convert
(A) Fixed ac voltage to variable magnitude ac voltage of same frequency
(B) Fixed ac voltage to variable frequency ac voltage of same magnitude
(C) Fixed ac voltage to variable frequency ac voltage through dc link
(D) Fixed ac voltage to variable magnitude variable frequency ac voltage
Solution-A single-phase ac regulator is used to convert fixed ac voltage to variable magnitude ac voltage of the same frequency , here always remember that regulator only control voltage. not frequency so here frequency is constant.
Questions-43.The equivalent resistance of the circuit is
(A) 200 Ω
(B) 400 Ω
(C) 600 Ω
(D) 1600 Ω
Solution-Here 400Ω,200Ω and 200Ω in series so total=400+200+200=800Ω
Now 800Ω and 800 Ω in parallel,It equivalent=800*800/1600=400 Ω
400Ω is in series with 200Ω,200Ω=so total=400+200+200=800Ω
Again 800Ω and 800 Ω in parallel,It equivalent=800*800/1600=400 Ω
Finally 400 Ω,100Ω and 100Ω in series so total=400 Ω+100Ω+100Ω=600Ω
Questions-44.The laplace transform of a constant value function is
Solution-The laplace transform of a constant value function is (k)=1/S.
Questions-45.Which of the following connections is best suited for 3-phase, 4 wire service
(A) Δ – Δ
(B) Δ – Y
(C) Y – Y
(D) Y – Δ
Solution-As we know that in secondary distribution system we use 3-phase ,4 wire service ,for taking neutral system in secondary side we use Δ – Y system here star system in secondary side gives neutral system .
Questions-46.Buchholz’s relay is used for protection of
(A) Over head lines
(B) Underground cables
Solution-Buchholz’s relay is used for all kind of internal fault protection in transformer,it is basically used in oil immersed transformer.
Questions-47.A delta/delta connected three-phase transformer can be connected in parallel with
Solution-For maintain same phase displacement in parallel connection of transformer, Delta-delta connected transformer are connected with star-star connected transformer.It is also used for protection purpose of each other.
Questions-48.Three resistances of 30 ohm, 15 ohm and 5 ohm are connected in parallel, their combine resistance will be
(A) Greater than 30 ohm
(B) Between 30 ohm to 15 ohm
(C) Between 15 ohm to 5 ohm
(D) Less than 5 ohm
Solution-Here parallel equivalent total resistance 1/R=1/30+1/15+1/5=1+2+6/30=9/30 Or R=30/9=3.33
In parallel connection of resistance the overall equivalent resistance is always less than the small resistance.So combined resistance is always less than the 5 ohm.
Questions-49.How is an electric element capacitor specified
(A) Ohmic value and Voltage
(B) Ohmic value and Wattage
(C) Voltage and capacitance
(D) Wattage and capacitance
Solution-Capacitance is the property of the capacitor to store energy in dielectric field,its unit is farad.and capacitor deals with current or voltage here average power is always equal to zero.
Questions-50.Maxwell Bridge is used for the measurement of
Solution-Maxwell bridge is used for the measurement of unknown inductance.