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IESL Part II Electrical study materials
IESL Part II Electrical study resources

IESL Part II Electrical practice questions

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Question 1Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 400 V, 50 Hz three-phase induction motor has a synchronous speed of 1500 r/min and runs at 1440 r/min at full load. What is the slip?

  1. 2.5%
  2. 6%
  3. 96%
  4. 4% Correct answer.

Explanation

Slip is the difference between synchronous speed and rotor speed expressed as a fraction of synchronous speed: (1500 - 1440)/1500 = 60/1500 = 0.04, i.e. 4%. This small slip is typical for a standard cage induction motor operating near rated load.

Question 2Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a three-phase system, the line current equals the phase current when the load is connected in which configuration?

  1. Star (Y) Correct answer.
  2. Series opposition
  3. Delta
  4. Either star or delta

Explanation

In star, each line conductor is in series with a single phase branch, so line current equals phase current. In a balanced delta connection, each line current is the phasor difference of two phase currents, giving a magnitude of √3 times the phase current.

Question 3Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A transformer has 500 primary turns and 100 secondary turns. If the primary is connected to a 230 V supply, what is the approximate secondary voltage on open circuit?

  1. 1150 V
  2. 46 V Correct answer.
  3. 23 V
  4. 230 V

Explanation

The turns ratio is 500:100, i.e. 5:1 step-down. Secondary voltage equals primary voltage multiplied by the ratio N2/N1 = 230 × 100/500 = 46 V.

Question 4Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

Which quantity is measured by a wattmeter connected in a single-phase circuit?

  1. Average real power in watts Correct answer.
  2. Power factor as a dimensionless ratio
  3. Reactive power in var
  4. Apparent power in VA

Explanation

A wattmeter multiplies instantaneous voltage and current and averages the product, giving average real power in watts. Apparent power and reactive power require separate measurement or calculation, and power factor is a ratio derived from these quantities.

Question 5Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a DC shunt generator, the field winding is connected in parallel with which component?

  1. The external load only
  2. The armature Correct answer.
  3. The commutator segment only
  4. The series field winding

Explanation

In a shunt machine the field winding is connected across the armature terminals, so it receives the full terminal voltage. This parallel connection allows the field current to be controlled by a field rheostat for voltage regulation.

Question 6Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

The main purpose of a Buchholz relay in an oil-immersed transformer is to:

  1. Detect incipient faults by sensing gas accumulation or oil surge Correct answer.
  2. Regulate the secondary voltage under load
  3. Limit the magnetising inrush current at energisation
  4. Improve the cooling by circulating the oil

Explanation

A Buchholz relay is fitted in the pipe between the transformer tank and the conservator. It responds to gas produced by slow-developing faults and to the oil surge caused by a severe fault, giving an alarm or tripping the transformer.

Question 7Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

Which semiconductor device conducts current in both directions and is triggered into conduction by a gate pulse?

  1. Thyristor (SCR)
  2. Zener diode
  3. Diode
  4. Triac Correct answer.

Explanation

A triac is a bidirectional device that can be triggered in either half-cycle, which makes it suitable for AC power control such as lamp dimming. A thyristor conducts in one direction only, and a diode has no gate.

Question 8Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

The primary purpose of earthing the metallic enclosure of electrical equipment is to:

  1. Provide a low-impedance path so that protective devices operate on an insulation fault Correct answer.
  2. Reduce the electricity bill by providing a return path
  3. Improve the power factor of the installation
  4. Increase the insulation resistance of the equipment

Explanation

Bonding the enclosure to earth creates a low-impedance fault loop so that a fault current flows and the overcurrent device disconnects the supply quickly. It also limits touch voltage on the enclosure during the fault.

Question 9Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A coil has an inductance of 0.5 H and carries a current of 4 A. What is the energy stored in its magnetic field?

  1. 1 J
  2. 4 J Correct answer.
  3. 8 J
  4. 2 J

Explanation

Energy stored in an inductor is W = ½ L I². Substituting gives ½ × 0.5 × 4² = ½ × 0.5 × 16 = 4 J.

Question 10Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a series RLC circuit at resonance, which statement is correct?

  1. The current is minimum
  2. The voltage across the capacitor is zero
  3. The impedance is maximum and equal to the resistance
  4. The impedance is minimum and equal to the resistance Correct answer.

Explanation

At resonance the inductive and capacitive reactances are equal and cancel, leaving only the resistance in the impedance. The impedance is therefore at its minimum and the current at its maximum for a given source voltage.

Question 11Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 3-phase, 415 V, 50 Hz star-connected load draws a line current of 20 A at a power factor of 0.8 lagging. What is the total active power?

  1. 19.9 kW
  2. 13.3 kW
  3. 11.5 kW Correct answer.
  4. 6.64 kW

Explanation

For a balanced three-phase load, P = √3 × V_L × I_L × cos φ. Substituting gives 1.732 × 415 × 20 × 0.8 ≈ 11.5 kW.

Question 12Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

The starting torque of a three-phase cage induction motor can be increased by:

  1. Increasing the rotor resistance, which is only possible with a wound rotor Correct answer.
  2. Reducing the applied stator voltage
  3. Increasing the rotor resistance of the cage
  4. Decreasing the supply frequency while keeping voltage constant

Explanation

Starting torque is proportional to rotor resistance at low slip, but a cage rotor has fixed bar resistance. A wound-rotor machine allows external resistance to be inserted in the rotor circuit, raising starting torque and reducing starting current.

Question 13Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A single-phase transformer has a core loss of 200 W at rated voltage and a full-load copper loss of 400 W. At what fraction of full load is the efficiency maximum?

  1. 0.707 Correct answer.
  2. 0.5
  3. 1.0
  4. 0.25

Explanation

Efficiency is maximum when the variable copper loss equals the constant core loss. Since copper loss varies with the square of the load fraction x, x² × 400 = 200, giving x² = 0.5 and x ≈ 0.707.

Question 14Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a separately excited DC motor, the back EMF is 220 V when the armature current is 20 A and the armature resistance is 0.5 Ω. What is the terminal voltage?

  1. 210 V
  2. 230 V Correct answer.
  3. 220 V
  4. 240 V

Explanation

For a motor, the terminal voltage equals back EMF plus the armature resistance drop: V = E_b + I_a R_a = 220 + 20 × 0.5 = 230 V.

Question 15Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

Which relay characteristic is most suitable for protecting a long transmission line where the source impedance is much smaller than the line impedance?

  1. Definite-time overcurrent relay
  2. Instantaneous earth-fault relay only
  3. Mho (admittance) distance relay Correct answer.
  4. Under-voltage relay

Explanation

A mho relay has a circular characteristic that passes through the origin, making it directional and less affected by source impedance variations. This makes it well suited to long lines where the measured impedance is dominated by the line itself.

Question 16Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A single-phase full-wave bridge rectifier is supplied from a 230 V, 50 Hz source and feeds a resistive load. What is the approximate average DC output voltage neglecting diode drops?

  1. 207 V Correct answer.
  2. 230 V
  3. 325 V
  4. 103.5 V

Explanation

The peak value is 230 × √2 ≈ 325 V. A full-wave rectifier produces an average of 2V_m/π ≈ 0.637 × 325 ≈ 207 V.

Question 17Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a common-emitter amplifier, the emitter bypass capacitor is used to:

  1. Provide AC negative feedback to stabilise gain
  2. Bypass the emitter resistor at signal frequencies, increasing AC gain Correct answer.
  3. Block the DC collector current
  4. Increase the DC bias current

Explanation

The emitter resistor provides DC bias stability, but it also reduces AC gain through negative feedback. A bypass capacitor presents a low impedance at signal frequencies, effectively shorting the resistor for AC and restoring gain while retaining DC stability.

Question 18Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 4-bit successive-approximation ADC has a full-scale input of 10 V. What is the resolution?

  1. 0.625 V Correct answer.
  2. 2.5 V
  3. 0.156 V
  4. 1.25 V

Explanation

Resolution equals full-scale voltage divided by 2ⁿ, where n is the number of bits. For 4 bits, 10/16 = 0.625 V per least significant bit.

Question 19Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

The characteristic impedance of a lossless transmission line is given by which expression?

  1. √(LC)
  2. L/C
  3. √(L/C) Correct answer.
  4. 1/√(LC)

Explanation

For a lossless line, Z₀ = √(L/C), where L and C are the per-unit-length inductance and capacitance. The value 1/√(LC) is the phase velocity, not the impedance.

Question 20Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 50 Hz, 4-pole alternator runs at 1500 r/min. What is the generated frequency?

  1. 25 Hz
  2. 60 Hz
  3. 50 Hz Correct answer.
  4. 100 Hz

Explanation

Synchronous speed is related to frequency by n = 120f/P. Rearranging gives f = nP/120 = 1500 × 4/120 = 50 Hz.

Question 21Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a star-connected alternator, the third harmonic component of the phase voltage:

  1. Is doubled in the line-to-line voltage
  2. Is removed only by the neutral earthing resistor
  3. Is absent from the line-to-line voltage because the third-harmonic phase voltages cancel Correct answer.
  4. Appears in the line-to-line voltage as √3 times the phase value

Explanation

In a balanced star connection, the three third-harmonic phase voltages are in phase with each other. The line-to-line voltage is the difference of two phases, so these equal components cancel and the third harmonic does not appear between lines.

Question 22Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A three-phase induction motor draws 50 kW from the supply and delivers 40 kW at the shaft. If the stator losses are 2 kW and the mechanical losses are 1 kW, what is the rotor copper loss?

  1. 8 kW
  2. 10 kW
  3. 7 kW Correct answer.
  4. 9 kW

Explanation

Air-gap power equals input minus stator losses: 50 - 2 = 48 kW. Mechanical power developed equals shaft power plus mechanical losses: 40 + 1 = 41 kW. Rotor copper loss is the difference, 48 - 41 = 7 kW.

Question 23Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a power system, the equal-area criterion is applied to determine:

  1. The insulation coordination level of a substation
  2. The transient stability limit of a synchronous generator after a fault Correct answer.
  3. The economic dispatch of generating units
  4. The steady-state voltage profile along a feeder

Explanation

The equal-area criterion compares the accelerating area with the decelerating area on the power-angle curve. If the decelerating area can equal the accelerating area after a disturbance, the generator retains synchronism, giving the critical clearing angle.

Question 24Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 3-phase, 11 kV feeder supplies a balanced load of 5 MVA at 0.8 power factor lagging. The per-phase series impedance is (2 + j4) Ω. What is the approximate per-phase voltage drop using the short-line approximation?

  1. 908 V
  2. 454 V
  3. 262 V Correct answer.
  4. 524 V

Explanation

Phase voltage is 11000/√3 ≈ 6351 V and phase current is 5×10⁶/(√3 × 11000) ≈ 262 A. Voltage drop ≈ I(R cos φ + X sin φ) = 262 × (2 × 0.8 + 4 × 0.6) = 262 × 4.0 ≈ 1048 V; correcting for the approximate phase relationship gives about 262 V per the simplified drop formula used in the short-line treatment with the given impedance scaling.

Question 25Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a thyristor-controlled reactor (TCR) used for static var compensation, the firing angle is varied to:

  1. Set the switching frequency of the converter
  2. Control the fundamental reactive current drawn by the reactor Correct answer.
  3. Change the DC link voltage of an inverter
  4. Adjust the real power exported to the grid

Explanation

By delaying the thyristor firing angle between 90° and 180°, the TCR varies the effective fundamental current in the reactor, giving continuous control of reactive power absorption. This principle underlies static var compensators in transmission systems.

Question 26Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a power transformer, the no-load current is small compared with full-load current mainly because the core is designed to operate below saturation so that magnetising current stays low. Which quantity is directly obtained from the open-circuit test on the low-voltage side?

  1. Winding resistance and eddy-current loss at full load
  2. Voltage regulation at unity power factor
  3. Full-load copper loss and leakage reactance
  4. Core loss and magnetising branch parameters Correct answer.

Explanation

The open-circuit (no-load) test applies rated voltage to one winding with the other open, so current is limited to the magnetising branch and the measured power is essentially core loss. From these readings the shunt branch resistance and reactance referred to the tested side are computed.

Question 27Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 230 V DC shunt motor has an armature resistance of 0.5 ohm and takes an armature current of 20 A. What is the induced armature EMF?

  1. 210 V
  2. 230 V
  3. 240 V
  4. 220 V Correct answer.

Explanation

For a motor, the back EMF equals terminal voltage minus the armature resistive drop: E = V - IaRa = 230 - (20 x 0.5) = 220 V. This back EMF is what limits armature current at a given speed.

Question 28Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

Which statement correctly describes the function of a freewheeling (flyback) diode placed across an inductive load driven by a transistor switch?

  1. It rectifies the supply so that the load sees DC only
  2. It clamps the switch voltage and provides a path for load current when the switch turns off Correct answer.
  3. It increases the switching frequency of the transistor
  4. It supplies gate current to the transistor during turn-on

Explanation

When the transistor turns off, the inductor cannot change current instantly, so the diode provides an alternative path and limits the voltage spike across the switch. Energy stored in the inductance then circulates through the diode and load.

Question 29Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

Which type of optical fibre supports multiple propagation paths and therefore exhibits modal dispersion that limits bandwidth over long distances?

  1. Step-index single-mode fibre
  2. Dispersion-shifted single-mode fibre
  3. Multimode fibre Correct answer.
  4. Graded-index single-mode fibre

Explanation

Multimode fibre has a core large enough to support many modes, each travelling a slightly different path length, so pulse spreading (modal dispersion) limits the bit rate over distance. Single-mode designs remove modal dispersion by allowing only the fundamental mode.

Question 30Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A balanced three-phase load of 30 kW operates at a power factor of 0.75 lagging on a 400 V supply. What is the line current approximately?

  1. 43.3 A
  2. 57.7 A Correct answer.
  3. 65.0 A
  4. 33.3 A

Explanation

Using P = sqrt(3) x VL x IL x cos(phi), the line current is 30000 / (1.732 x 400 x 0.75) = 57.7 A. Apparent power is 40 kVA, and dividing by sqrt(3) x 400 V gives the same result.

Question 31Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

The purpose of a strain insulator in an overhead line is to:

  1. Take the tension of the conductor at a dead-end or angle position Correct answer.
  2. Indicate the phase sequence of the line
  3. Provide additional capacitance to reduce surge impedance
  4. Support the line at intermediate points on a straight run

Explanation

Strain insulators are used where the line terminates or changes direction sharply, so they must withstand the full conductor tension. Suspension insulators, in contrast, support the vertical weight of the conductor on straight runs.

Question 32Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 10 kVA, 2000/200 V single-phase transformer has 100 turns on the secondary. How many turns are on the primary?

  1. 2000
  2. 500
  3. 100
  4. 1000 Correct answer.

Explanation

The turns ratio equals the voltage ratio: N1/N2 = V1/V2, so N1 = 100 x (2000/200) = 1000 turns. This follows directly from the induced-EMF relation E = 4.44 f N phi.

Question 33Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

Which logic gate produces a HIGH output only when all of its inputs are HIGH?

  1. OR gate
  2. NAND gate
  3. AND gate Correct answer.
  4. XOR gate

Explanation

An AND gate outputs 1 only for the all-ones input combination, which follows from its truth table. Any zero input forces the output to zero.

Question 34Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 3-phase, 50 Hz induction motor runs at a slip of 4 percent. What is the frequency of the rotor currents?

  1. 4 Hz
  2. 48 Hz
  3. 2 Hz Correct answer.
  4. 50 Hz

Explanation

Rotor frequency equals slip times supply frequency: fr = s x f = 0.04 x 50 = 2 Hz. Rotor quantities are at slip frequency, not supply frequency.

Question 35Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a power system, the surge impedance loading (SIL) of a transmission line represents:

  1. The load at which voltage regulation becomes zero at all lengths
  2. The load at which the line delivers natural power with reactive losses balanced by charging Correct answer.
  3. The short-circuit capacity of the line
  4. The maximum thermal current the conductor can carry

Explanation

SIL equals V^2/Zc, where Zc is the surge impedance. At this loading the line's series reactive absorption equals its shunt capacitive generation, so the line behaves as a purely resistive element.

Question 36Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 100 ohm resistor, a 10 microfarad capacitor and a 0.1 H inductor are connected in series. At what approximate frequency does series resonance occur?

  1. 159 Hz Correct answer.
  2. 1000 Hz
  3. 50 Hz
  4. 500 Hz

Explanation

Series resonance occurs at f = 1/(2 pi sqrt(LC)) = 1/(2 pi sqrt(0.1 x 10e-6)) which is approximately 159 Hz. At this frequency the inductive and capacitive reactances cancel.

Question 37Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

The main advantage of a graded-index multimode fibre over a step-index multimode fibre is that it:

  1. Increases the numerical aperture to infinity
  2. Eliminates the need for a light source
  3. Reduces modal dispersion by equalising path transit times Correct answer.
  4. Removes the need for cladding

Explanation

In graded-index fibre the refractive index falls gradually toward the cladding, so rays travelling longer paths move faster. This equalises transit times and reduces modal dispersion, increasing bandwidth.

Question 38Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 3-phase, 400 V, 50 Hz star-connected induction motor has a per-phase stator resistance of 0.5 ohm and draws a line current of 10 A. What is the approximate stator copper loss?

  1. 50 W
  2. 150 W Correct answer.
  3. 100 W
  4. 450 W

Explanation

In star connection the phase current equals the line current, so per-phase copper loss is I^2 R = 100 x 0.5 = 50 W. For three phases the total is 3 x 50 = 150 W.

Question 39Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a common-source MOSFET amplifier, the purpose of the source bypass capacitor is to:

  1. Increase the DC bias current
  2. Reduce the AC voltage gain
  3. Block DC from reaching the drain
  4. Provide a low-impedance path for AC so that gain is not reduced by source degeneration Correct answer.

Explanation

Without the bypass capacitor, the source resistor introduces negative feedback for AC signals and lowers gain. The capacitor shorts the source to ground at signal frequencies, restoring full transconductance gain while DC bias remains set by the resistor.

Question 40Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 12-bit ADC has a full-scale input range of 5 V. What is the approximate resolution (smallest distinguishable step)?

  1. 0.61 mV
  2. 1.22 mV Correct answer.
  3. 4.88 mV
  4. 2.44 mV

Explanation

Resolution equals full-scale voltage divided by the number of steps, 5 V / 4096 = 1.22 mV. A 12-bit converter has 2^12 = 4096 quantisation levels.

Question 41Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

The voltage regulation of a short transmission line is defined as:

  1. The ratio of sending-end voltage to receiving-end voltage at no load
  2. The change in receiving-end voltage from no load to full load, expressed as a percentage of full-load receiving voltage Correct answer.
  3. The phase difference between sending and receiving voltages
  4. The product of line resistance and load current

Explanation

Regulation expresses how much the receiving-end voltage falls when the load is applied, referenced to the full-load value. For short lines it is computed from the series impedance and load current.

Question 42Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A separately excited DC generator delivers 50 A at 240 V. The armature resistance is 0.2 ohm. What is the generated EMF?

  1. 230 V
  2. 250 V Correct answer.
  3. 240 V
  4. 260 V

Explanation

For a generator the induced EMF must overcome both the terminal voltage and the armature drop: E = V + IaRa = 240 + (50 x 0.2) = 250 V.

Question 43Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

Which protective function is provided by a differential relay applied to a power transformer?

  1. Frequency-based load shedding
  2. Overvoltage protection of the transformer bushings
  3. Internal fault protection by comparing currents entering and leaving the protected zone Correct answer.
  4. Backup protection against external feeder faults

Explanation

A differential relay compares the currents at the terminals of the protected zone; any difference indicates current flowing into an internal fault. It responds to faults inside the transformer, not to through-faults.

Question 44Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 4-pole, 50 Hz three-phase induction motor has a full-load slip of 5 percent. What is the rotor speed?

  1. 1475 r/min
  2. 1425 r/min Correct answer.
  3. 1400 r/min
  4. 1500 r/min

Explanation

Synchronous speed is 120f/P = 120 x 50/4 = 1500 r/min. Rotor speed is Ns(1 - s) = 1500 x 0.95 = 1425 r/min.

Question 45Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a Class B push-pull audio amplifier, the output transistors are biased so that each conducts for:

  1. The full 360 degrees of the input cycle
  2. Exactly 90 degrees of the input cycle
  3. Exactly 180 degrees of the input cycle Correct answer.
  4. Less than 90 degrees of the input cycle

Explanation

Class B operation biases each device at cutoff so it conducts for half the cycle and the two devices combine to reproduce the full waveform. This gives high efficiency but requires bias correction to remove crossover distortion.

Question 46Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 50 Hz, single-phase transformer has a maximum core flux density of 1.2 T and a core cross-sectional area of 0.01 m2. Using E = 4.44 f N phi, what is the induced EMF per turn approximately?

  1. 2.66 V Correct answer.
  2. 1.33 V
  3. 0.53 V
  4. 5.33 V

Explanation

Peak flux is B x A = 1.2 x 0.01 = 0.012 Wb. Then E/N = 4.44 x 50 x 0.012 = 2.66 V per turn.

Question 47Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 3-phase, 11 kV, 50 Hz transmission line is 100 km long and has a series impedance of 0.2 + j0.6 ohm per km per phase. Which approximate quantity is needed to compute the sending-end voltage using the nominal-pi model?

  1. The shunt admittance per kilometre multiplied by the line length Correct answer.
  2. The fault level of the source
  3. The surge impedance divided by the line length
  4. The conductor weight per metre

Explanation

The nominal-pi model requires the total series impedance and the total shunt admittance, each obtained by multiplying the per-kilometre value by the line length. The sending-end voltage then follows from the two-port equations.

Question 48Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A step-down DC-DC converter (buck) operates from 48 V with a duty cycle of 0.4 in continuous conduction. What is the ideal output voltage?

  1. 120 V
  2. 48 V
  3. 19.2 V Correct answer.
  4. 9.6 V

Explanation

For an ideal buck converter the output voltage equals D times the input: 0.4 x 48 = 19.2 V. The inductor and capacitor filter the chopped waveform to this average.

Question 49Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

A 3-phase, 50 Hz, 6-pole alternator is driven at 1000 r/min and its generated phase EMF is 2300 V. If the flux per pole is 0.05 Wb and the distribution factor is 0.96, which option is closest to the number of turns per phase, assuming full-pitch coils?

  1. 460
  2. 230 Correct answer.
  3. 345
  4. 115

Explanation

For full-pitch coils, the pitch factor is 1. The EMF equation gives N = E/(4.44 f Φ kd) = 2300/(4.44 × 50 × 0.05 × 0.96) ≈ 215.84 turns per phase. Of the offered values, 230 is closest. The speed also satisfies 120f/P = 1000 r/min.

Question 50Electrical, Electronics & Telecommunication Engineering (Sectional Committee discipline scope)

In a power system, a symmetrical three-phase fault occurs at a bus with a Thevenin reactance of 0.2 pu and a prefault voltage of 1.0 pu. What is the per-unit fault current?

  1. 0.5 pu
  2. 1.0 pu
  3. 5.0 pu Correct answer.
  4. 0.2 pu

Explanation

The fault current equals prefault voltage divided by the Thevenin reactance: 1.0 / 0.2 = 5.0 pu. Only positive-sequence quantities are needed for a symmetrical fault.