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THDC JE practice questions

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Question 1Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a hydroelectric plant, the function of the draft tube downstream of a reaction turbine is to:

  1. Increase the pressure at the runner inlet to raise the output
  2. Convert pressure head into additional jet velocity for the runner
  3. Recover the kinetic energy of the exit water and allow the runner to be set above tailwater level Correct answer.
  4. Prevent the turbine from overspeeding during load rejection

Explanation

A draft tube diffuses the flow leaving the runner, converting much of the residual velocity head back into pressure and permitting the runner to be placed above tailwater without losing effective head. This recovery improves overall efficiency and reduces cavitation risk. It does not add head at the inlet or govern speed control.

Question 2Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The specific speed of a hydraulic turbine is defined as the speed of a geometrically similar machine that would develop:

  1. Unit power at unit rotational speed
  2. Design power at rated head
  3. One horsepower at maximum efficiency
  4. One kilowatt at one metre of head Correct answer.

Explanation

Specific speed normalises turbine performance so machines of different sizes can be compared; it is the rotational speed of a similar unit producing 1 kW under 1 m of head. The value is used to classify the runner type, for example Pelton (low), Francis (medium) and Kaplan (high). It is a similarity parameter, not a site operating condition.

Question 3Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The primary purpose of a surge tank in a hydroelectric water-conductor system is to:

  1. Absorb pressure transients and limit water-hammer effects following sudden load changes Correct answer.
  2. Provide standby storage for irrigation releases
  3. Increase the available gross head at the powerhouse
  4. Filter sediment before it enters the turbine

Explanation

A surge tank placed near the powerhouse gives the water column a free surface that can rise or fall, absorbing the pressure rise on load rejection and supplying water on load acceptance. This limits water-hammer pressures and reduces governor response time. Sediment control and storage are handled by other structures.

Question 4Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a Pelton wheel, the bucket splitter angle is designed so that the jet is deflected through approximately:

  1. 165 degrees Correct answer.
  2. 180 degrees
  3. 90 degrees
  4. 45 degrees

Explanation

Pelton buckets split the jet and turn each half through about 165 degrees, close to but not fully reversing the flow. This near reversal extracts maximum momentum while avoiding the jet striking the back of the following bucket. A full 180-degree reversal would cause interference between the jet and the incoming buckets.

Question 5Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A three-phase induction motor draws a line current of 20 A at 0.85 power factor from a 415 V supply. The input power is approximately:

  1. 21.2 kW
  2. 7.1 kW
  3. 12.2 kW Correct answer.
  4. 3.5 kW

Explanation

For a balanced three-phase load, input power equals sqrt(3) times line voltage times line current times power factor. Substituting gives 1.732 x 415 x 20 x 0.85, which is about 12.2 kW. Forgetting the power factor or the square-root-three factor leads to the other listed values.

Question 6Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The main reason for using a star-delta starter with a squirrel-cage induction motor is to:

  1. Increase the starting torque above the direct-on-line value
  2. Reduce the starting current drawn from the supply by connecting the windings in star during run-up Correct answer.
  3. Provide stepless speed control during normal running
  4. Improve the power factor at full load

Explanation

During starting the windings are connected in star, so each phase sees only 1/sqrt(3) of the line voltage and the line current falls to about one third of the direct-on-line value. Once the motor accelerates, the windings are reconnected in delta for normal running. Starting torque is also reduced, so this method suits light-start loads.

Question 7Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a first-angle orthographic projection drawing, the view placed below the front view represents the:

  1. Right-side view
  2. Left-side view
  3. Bottom view
  4. Top view Correct answer.

Explanation

First-angle projection places the object between the observer and the plane, so the view seen from above is projected onto a plane below and appears under the front view. This is the opposite of third-angle projection, where the top view sits above the front view. Side views follow the same logic, with the left view appearing on the right.

Question 8Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A simply supported beam of span 6 m carries a central point load of 40 kN. The maximum bending moment is:

  1. 240 kN·m
  2. 30 kN·m
  3. 60 kN·m Correct answer.
  4. 120 kN·m

Explanation

For a simply supported beam with a central point load, each reaction equals half the load, that is 20 kN. The maximum moment occurs at mid-span and equals the reaction multiplied by half the span, giving 20 x 3 = 60 kN·m. Treating the full load over the full span overestimates the moment.

Question 9Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A rectangular channel 4 m wide carries a discharge of 8 m³/s at a depth of 1 m. The Froude number is approximately:

  1. 2.56
  2. 0.64 Correct answer.
  3. 1.56
  4. 0.40

Explanation

Velocity equals discharge divided by flow area, so 8 / (4 x 1) = 2 m/s. The hydraulic depth of a rectangular channel equals the flow depth, 1 m. Froude number is velocity divided by the square root of g times depth, giving 2 / sqrt(9.81) = 0.64, which indicates subcritical flow.

Question 10Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a Francis turbine operating at design conditions, water enters the runner radially and leaves:

  1. Radially inward at a smaller radius
  2. Radially outward at the same radius
  3. Tangentially with no axial component
  4. Axially at the runner exit Correct answer.

Explanation

A Francis runner is a mixed-flow machine: flow enters radially through the guide vanes and runner inlet and turns to discharge axially at the runner exit into the draft tube. This mixed-flow path distinguishes it from purely radial or purely axial machines. The axial exit keeps leaving losses low and suits the draft tube.

Question 11Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A 500 kVA, 11 kV/415 V distribution transformer has a per-unit impedance of 0.05. The short-circuit current on the low-voltage side for a bolted fault is approximately:

  1. 69.5 kA
  2. 13.9 kA Correct answer.
  3. 1.39 kA
  4. 0.72 kA

Explanation

The rated low-voltage current is 500,000 / (sqrt(3) x 415) = 695 A. For a bolted fault with a source of negligible impedance, the short-circuit current equals rated current divided by per-unit impedance, giving 695 / 0.05 = 13.9 kA. Using the high-voltage side or omitting the square-root-three factor gives the other values.

Question 12Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The core of a power transformer is laminated primarily to reduce:

  1. Eddy-current losses Correct answer.
  2. Dielectric losses in the oil
  3. Copper losses in the windings
  4. Hysteresis losses

Explanation

Laminations interrupt the paths of induced eddy currents in the core, greatly reducing eddy-current loss, which varies with the square of lamination thickness. Hysteresis loss depends on the core material's magnetic properties and is reduced by choosing suitable steel, not by lamination. Copper and dielectric losses arise elsewhere.

Question 13Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A 100 ohm resistor and a 150 ohm resistor are connected in parallel. The equivalent resistance is:

  1. 250 ohm
  2. 75 ohm
  3. 60 ohm Correct answer.
  4. 125 ohm

Explanation

For two resistors in parallel, the equivalent resistance is the product divided by the sum: (100 x 150) / (100 + 150) = 15,000 / 250 = 60 ohm. The result must be smaller than the smallest branch resistance, which immediately rules out values above 100 ohm. Adding the resistances applies to a series connection.

Question 14Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The main advantage of a moving-iron instrument over a moving-coil instrument is that it:

  1. Consumes no power
  2. Can measure both alternating and direct current Correct answer.
  3. Has a more uniform scale
  4. Is unaffected by external magnetic fields

Explanation

A moving-iron instrument responds to the magnitude of the magnetic field produced by the coil, so it deflects for both AC and DC and reads RMS values on AC. Moving-coil instruments respond to average torque direction and are suited mainly to DC unless a rectifier is added. The moving-iron scale is typically non-uniform and its field is weaker.

Question 15Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a differential pressure flow measurement setup using an orifice plate, the flow rate is proportional to:

  1. The cube root of the differential pressure
  2. The differential pressure directly
  3. The square of the differential pressure
  4. The square root of the differential pressure Correct answer.

Explanation

For an orifice or venturi, the Bernoulli relationship gives velocity proportional to the square root of the pressure drop across the restriction. Hence volumetric flow varies with the square root of the measured differential pressure. This non-linearity is why transmitters often include square-root extraction.

Question 16Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A thermocouple measures temperature based on the:

  1. Change in resistance of a platinum element with temperature
  2. Seebeck effect, where a temperature difference between junctions produces an emf Correct answer.
  3. Piezoelectric charge generated by thermal stress
  4. Thermionic emission from a heated filament

Explanation

A thermocouple consists of two dissimilar metals joined at a measuring junction and a reference junction; a temperature difference between them generates a small emf by the Seebeck effect. Cold-junction compensation is needed because the emf depends on the difference between junction temperatures. Resistance thermometers use the resistance-change principle instead.

Question 17Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a control loop, a controller with proportional-plus-integral action is used mainly to:

  1. Guarantee a critically damped response for all processes
  2. Eliminate steady-state offset while providing proportional corrective action Correct answer.
  3. Eliminate the need for a final control element
  4. Measure the process variable directly

Explanation

The proportional term gives a corrective output proportional to error, but alone leaves a steady offset. The integral term accumulates error over time and drives that offset to zero at steady state. Derivative action, not integral action, is used to anticipate rate of change and improve damping.

Question 18Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A gear train has a 20-tooth driving gear meshing with a 60-tooth driven gear. If the driving gear rotates at 900 rpm, the driven gear speed is:

  1. 2700 rpm
  2. 1800 rpm
  3. 150 rpm
  4. 300 rpm Correct answer.

Explanation

The speed ratio of a gear pair is inversely proportional to the tooth numbers, so the driven speed equals the driving speed multiplied by the ratio of driving teeth to driven teeth: 900 x (20/60) = 300 rpm. Torque increases in the same proportion that speed decreases. Multiplying instead of dividing gives an impossible speed-up for a reduction pair.

Question 19Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a four-stroke diesel engine, the camshaft rotates at:

  1. One quarter of the crankshaft speed
  2. Half the crankshaft speed Correct answer.
  3. Twice the crankshaft speed
  4. The same speed as the crankshaft

Explanation

A four-stroke cycle requires two crankshaft revolutions to complete one cycle of intake, compression, power and exhaust. Each valve opens once per cycle, so the camshaft makes one revolution per two crankshaft revolutions, that is half the crank speed. In a two-stroke engine the camshaft would run at crank speed if a cam is used.

Question 20Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A simply supported beam of rectangular section has width b and depth d, with d equal to twice b. If the depth is doubled while the width is halved, the section modulus about the horizontal axis changes by a factor of:

  1. 0.5
  2. 8
  3. 4 Correct answer.
  4. 2

Explanation

Section modulus for a rectangle is bd squared divided by six. Doubling depth multiplies the depth-squared term by four, while halving the width multiplies by one half, so the net factor is four times one half times two, giving a factor of four overall. The strong dependence on depth explains why deepening a beam is efficient.

Question 21Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A pump delivers 0.05 m³/s against a total head of 30 m with an overall efficiency of 75 percent. The shaft power required is approximately:

  1. 9.8 kW
  2. 26.2 kW
  3. 14.7 kW
  4. 19.6 kW Correct answer.

Explanation

Hydraulic power equals density times gravity times discharge times head: 1000 x 9.81 x 0.05 x 30, which is about 14.7 kW. Dividing by the overall efficiency of 0.75 gives a shaft power of about 19.6 kW. Multiplying by efficiency instead of dividing gives a value below the hydraulic power, which is impossible.

Question 22Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A 3-phase, 50 Hz, 4-pole induction motor runs at 1440 rpm. The slip is:

  1. 5 percent
  2. 2.5 percent
  3. 96 percent
  4. 4 percent Correct answer.

Explanation

Synchronous speed equals 120 times frequency divided by poles: 120 x 50 / 4 = 1500 rpm. Slip is the difference between synchronous and actual speed divided by synchronous speed: (1500 - 1440) / 1500 = 0.04, or 4 percent. Expressing the ratio of actual to synchronous speed gives the wrong quantity.

Question 23Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a hydroelectric station, two identical 11 kV generators are connected to a common bus. If the bus voltage is to be held constant while the reactive power sharing is adjusted, the excitation of one generator is increased. The result is that the generator:

  1. Runs at a higher frequency than the other generator
  2. Loses synchronism immediately
  3. Supplies more real power while reactive power stays constant
  4. Supplies more reactive power while its real power output remains set by the prime mover Correct answer.

Explanation

For a generator connected to an infinite bus, real power is governed by the mechanical input from the turbine and the load angle, while excitation controls the internal emf and hence reactive power. Raising excitation increases the reactive power export without changing the real power set by the prime mover. Frequency is fixed by the bus, so both machines stay in synchronism.

Question 24Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A closed-loop control system has an open-loop transfer function with a gain of 10 and a feedback factor of 0.2. The closed-loop gain, assuming a simple negative-feedback configuration, is approximately:

  1. 3.33 Correct answer.
  2. 0.5
  3. 10
  4. 2

Explanation

For negative feedback, closed-loop gain equals forward gain divided by one plus the product of forward gain and feedback factor. Here that is 10 / (1 + 10 x 0.2) = 10 / 3 = 3.33. Feedback reduces the gain but improves stability and reduces sensitivity to parameter variation. Ignoring the feedback term leaves the open-loop gain.

Question 25Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A steel rod of length 2 m and cross-sectional area 500 mm² carries an axial tensile load of 100 kN. Taking Young's modulus as 200 GPa, the elongation is:

  1. 4 mm
  2. 0.5 mm
  3. 1 mm
  4. 2 mm Correct answer.

Explanation

Stress equals load divided by area: 100,000 N / 500 mm² = 200 N/mm². Strain equals stress divided by modulus: 200 / 200,000 = 0.001. Elongation is strain times length: 0.001 x 2000 mm = 2 mm. Errors in unit conversion of area or modulus produce the other values.

Question 26Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a hydroelectric powerhouse, the surge tank is installed on the penstock primarily to:

  1. absorb pressure rise and provide water during load rejection or sudden demand Correct answer.
  2. remove suspended silt before water enters the runner
  3. increase the gross head available at the turbine inlet
  4. step up the generator terminal voltage

Explanation

A surge tank is a free-surface reservoir connected near the turbine end of the penstock. It limits water-hammer pressure rise when the turbine gates close suddenly and supplies water quickly when load demand increases, improving transient stability of the water column.

Question 27Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The function of a draft tube in a reaction turbine installation is to:

  1. regulate the flow rate by throttling
  2. prevent cavitation at the spiral casing inlet
  3. increase the head of water entering the runner
  4. convert the kinetic energy leaving the runner into useful pressure head Correct answer.

Explanation

Water leaving a reaction turbine runner still carries significant velocity. A gradually diverging draft tube decelerates this flow, recovering part of the kinetic energy as pressure and allowing the runner to be set above tailrace level, which raises overall efficiency.

Question 28Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a Pelton wheel, the bucket splitter angle is kept close to 180 degrees minus a small margin mainly so that:

  1. water leaves with an axial velocity component
  2. the nozzle can be eliminated
  3. the wheel can run fully submerged in the tailrace
  4. the jet is deflected through a large angle, maximising momentum transfer Correct answer.

Explanation

The force on a bucket depends on the change in momentum of the jet. Deflecting the jet through nearly 180 degrees gives the largest possible velocity reversal, but a small margin is retained so the outgoing water does not strike the back of the following bucket.

Question 29Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A centrifugal pump is required to deliver 0.04 m³/s against a total head of 25 m. Taking water density as 1000 kg/m³ and pump efficiency as 80 percent, the shaft power input is approximately:

  1. 15.7 kW
  2. 12.3 kW Correct answer.
  3. 8.0 kW
  4. 9.8 kW

Explanation

Hydraulic power equals density times gravity times discharge times head, giving about 9.81 kW. Dividing by the 0.80 efficiency gives a shaft input of roughly 12.3 kW.

Question 30Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

For a rectangular beam section of width b and overall depth D, the section modulus about the horizontal centroidal axis is:

  1. bD²/6 Correct answer.
  2. bD²/3
  3. bD/6
  4. bD³/12

Explanation

The second moment of area about the centroidal axis is bD³/12 and the extreme fibre distance is D/2. Dividing the two gives the section modulus bD²/6, which converts bending moment directly into extreme fibre stress.

Question 31Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A simply supported beam of span L carries a uniformly distributed load w per unit length over its entire span. The maximum shear force occurs:

  1. at the supports and equals wL/2 Correct answer.
  2. at quarter span and equals wL/4
  3. at the supports and equals wL
  4. at midspan and equals wL/8

Explanation

For a symmetric UDL on a simply supported beam, each reaction is wL/2. Shear force is maximum at the supports and reduces linearly to zero at midspan, while bending moment is maximum at midspan.

Question 32Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A column of effective length 3 m has both ends hinged and a least radius of gyration of 25 mm. Its slenderness ratio is:

  1. 60
  2. 300
  3. 120 Correct answer.
  4. 75

Explanation

Slenderness ratio is effective length divided by least radius of gyration. With both ends hinged the effective length equals the actual length, so 3000 mm divided by 25 mm gives 120.

Question 33Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The minimum number of equilibrium equations available for a rigid body in three-dimensional space is:

  1. four
  2. nine
  3. three
  4. six Correct answer.

Explanation

A general three-dimensional force system can produce three translational and three rotational effects, so six independent equilibrium equations are available. A planar system needs only three.

Question 34Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A soil sample has a void ratio of 0.60 and specific gravity of solids of 2.70. Its dry unit weight is approximately:

  1. 13.2 kN/m³
  2. 20.0 kN/m³
  3. 16.6 kN/m³ Correct answer.
  4. 18.5 kN/m³

Explanation

Dry unit weight equals specific gravity times unit weight of water divided by one plus void ratio. Thus 2.70 times 9.81 divided by 1.60 gives about 16.6 kN/m³.

Question 35Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a concrete mix, increasing the water-cement ratio while keeping all other proportions the same generally:

  1. increases workability and reduces compressive strength Correct answer.
  2. reduces workability and reduces compressive strength
  3. increases both workability and compressive strength
  4. has no effect on strength once the mix is fully compacted

Explanation

More water lubricates the mix and raises slump, but it also increases capillary porosity in the hardened paste. The excess voids reduce the strength, which is why the water-cement ratio is capped by durability and strength criteria.

Question 36Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The ruling minimum radius of a horizontal curve on a national highway is governed mainly by:

  1. the pavement thickness and subgrade strength
  2. the design speed and the maximum allowable superelevation and friction Correct answer.
  3. the sight distance available at the curve
  4. the width of the shoulder

Explanation

The minimum radius follows from the equilibrium of centrifugal force against superelevation and side friction, so it depends on design speed and the permitted values of those two factors. Pavement and shoulder details do not enter the radius formula.

Question 37Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A trapezoidal channel has a bottom width of 3 m, side slopes of 1 horizontal to 1 vertical, and a flow depth of 1 m. Its wetted perimeter is approximately:

  1. 5.00 m
  2. 4.41 m
  3. 6.41 m
  4. 5.83 m Correct answer.

Explanation

Each sloped side has a length equal to the depth times the square root of two, about 1.414 m. Adding the 3 m bed to two such sides gives a wetted perimeter near 5.83 m.

Question 38Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a rapid sand filter used for municipal water treatment, backwashing is performed mainly to:

  1. dissolve alum remaining in the water
  2. increase the filtration rate during peak demand
  3. expand the bed and wash out trapped solids Correct answer.
  4. disinfect the filter media with chlorine

Explanation

During filtration, particles accumulate in the filter bed and raise head loss. Upward backwash flow expands the bed so that the grains separate and the retained floc is carried away to waste, restoring the filtration capacity.

Question 39Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The sag in a transmission line conductor is 4 m at a span of 200 m with equal-level supports. The approximate length of the conductor in the span is:

  1. 204.00 m
  2. 200.08 m
  3. 201.60 m
  4. 200.53 m Correct answer.

Explanation

For a parabolic conductor with small sag, the extra length over the span is approximately 8s² divided by three times the span. With s equal to 4 m and span 200 m, the addition is about 0.53 m.

Question 40Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

The string efficiency of a suspension insulator string is defined as:

  1. the number of discs divided by the string voltage
  2. the voltage across the disc nearest the conductor divided by the string voltage
  3. the voltage across the whole string divided by the number of discs times the voltage across the disc nearest the conductor Correct answer.
  4. the leakage current divided by the string voltage

Explanation

Shunt capacitance to earth causes unequal voltage distribution, with the disc nearest the line experiencing the highest stress. String efficiency compares the ideal uniform distribution with the actual worst-case disc voltage, always being less than unity.

Question 41Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a short 11 kV feeder, the voltage drop is computed mainly from:

  1. the skin effect in the conductors
  2. the shunt capacitance of the line
  3. the corona loss of the line
  4. the current and the series resistance and reactance of the line Correct answer.

Explanation

For short lines the shunt capacitance is negligible, so the series impedance dominates. Voltage drop is approximately the current times the resistance and reactance components, evaluated with the load power factor.

Question 42Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A 3-phase, 400 V, 50 Hz induction motor has a synchronous speed of 1500 rpm and runs at 1470 rpm at full load. The frequency of the rotor currents is:

  1. 1 Hz Correct answer.
  2. 2 Hz
  3. 50 Hz
  4. 0.5 Hz

Explanation

Slip is the difference between synchronous and actual speed divided by synchronous speed, giving 30/1500 or 0.02. Rotor frequency equals slip times supply frequency, so 0.02 times 50 Hz is 1 Hz.

Question 43Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a DC shunt generator, the residual magnetism of the field poles is essential because it:

  1. reduces the eddy current loss in the armature core
  2. prevents armature reaction from distorting the field
  3. maintains constant terminal voltage under load
  4. provides the initial small emf needed to start voltage build-up Correct answer.

Explanation

A self-excited shunt generator cannot build voltage from zero. Residual flux induces a small emf, which drives a field current that reinforces the flux until the magnetisation curve intersects the field resistance line.

Question 44Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

An SCR in a controlled rectifier is turned on by:

  1. applying a negative gate voltage while it is reverse biased
  2. reducing the anode current below the holding current
  3. raising the junction temperature above the rated value
  4. applying a gate current pulse while it is forward biased Correct answer.

Explanation

An SCR blocks forward voltage until a gate pulse injects carriers that trigger regenerative conduction. The device must be forward biased for the gate pulse to latch it into the on state.

Question 45Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A 6-pole, 50 Hz, 3-phase induction motor is supplied at reduced voltage so that the torque at a given slip falls to one quarter. The voltage has been reduced to:

  1. 25 percent of rated value
  2. 75 percent of rated value
  3. 50 percent of rated value Correct answer.
  4. 12.5 percent of rated value

Explanation

Induction motor torque at a given slip is proportional to the square of the applied voltage. Reducing torque to one quarter therefore requires halving the voltage, since the square of one half is one quarter.

Question 46Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

In a balanced Wheatstone bridge used for resistance measurement, the bridge is balanced when:

  1. all four arms have equal resistance
  2. the ratio of the two adjacent arms equals the ratio of the other two arms Correct answer.
  3. the supply voltage is at its maximum
  4. the galvanometer shows maximum deflection

Explanation

At balance no current flows through the galvanometer, which occurs when the products of opposite arms are equal. Equivalently, the ratio of resistances in one branch equals the ratio in the other branch.

Question 47Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A strain gauge with a gauge factor of 2.0 is bonded to a steel member and experiences a strain of 500 microstrain. The fractional change in resistance is:

  1. 0.01
  2. 0.0005
  3. 0.001 Correct answer.
  4. 0.002

Explanation

The gauge factor is the fractional resistance change divided by the strain. Multiplying 2.0 by 500 microstrain, that is 500 times ten to the minus six, gives 0.001.

Question 48Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A piezoelectric transducer is preferred for measuring dynamic pressure because it:

  1. measures temperature simultaneously with pressure
  2. generates a charge proportional to the applied force and has high frequency response Correct answer.
  3. requires no charge amplifier for signal conditioning
  4. produces a steady output for static pressure indefinitely

Explanation

Piezoelectric crystals produce charge proportional to applied force and respond very quickly, making them suitable for fluctuating pressures. Their charge leaks away, however, so they cannot hold a static reading for long.

Question 49Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A first-order measurement system has a time constant of 2 s. The time required for its step response to reach about 63 percent of the final value is:

  1. 1 s
  2. 0.5 s
  3. 4 s
  4. 2 s Correct answer.

Explanation

For a first-order system the response is exponential with time constant tau, and it reaches 63.2 percent of the final value after exactly one time constant. Here that is 2 s.

Question 50Engineering Discipline (Civil / Electrical / Mechanical / Electronics & Instrumentation) - technical core

A bimetallic strip used in a temperature switch works because the two bonded metals have different:

  1. melting points
  2. thermal conductivities
  3. electrical resistivities
  4. coefficients of thermal expansion Correct answer.

Explanation

When heated, the two metals expand by different amounts and the bonded strip bends toward the metal with the lower expansion coefficient. This deflection can actuate a contact in a thermostat.