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BCS Written EEE (892) study materials
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BCS Written EEE (892) practice questions

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Question 1Electrical Circuits and Network Analysis

In a purely resistive AC circuit, what is the phase relationship between the applied voltage and the current?

  1. Current lags voltage by 45 degrees
  2. Current is in phase with voltage Correct answer.
  3. Current lags voltage by 90 degrees
  4. Current leads voltage by 90 degrees

Explanation

In a resistor, voltage and current reach their peaks and zero crossings at the same instant, so the phase angle between them is zero. Reactance, not resistance, causes phase shift.

Question 2Electrical Circuits and Network Analysis

The unit of electrical conductance is the:

  1. Siemens Correct answer.
  2. Ohm
  3. Farad
  4. Henry

Explanation

Conductance is the reciprocal of resistance and is measured in siemens, formerly called the mho. A resistance of one ohm corresponds to one siemens of conductance.

Question 3Electrical Circuits and Network Analysis

Three 6-ohm resistors are connected in parallel. What is the equivalent resistance?

  1. 2 ohms Correct answer.
  2. 6 ohms
  3. 18 ohms
  4. 3 ohms

Explanation

For n equal resistors in parallel, the equivalent is R/n, giving 6/3 = 2 ohms. Equivalently, conductances add: 1/6 + 1/6 + 1/6 = 1/2 siemens.

Question 4Electrical Circuits and Network Analysis

Kirchhoff's current law is a direct statement of which conservation principle at a circuit node?

  1. Conservation of flux linkage
  2. Conservation of charge Correct answer.
  3. Conservation of energy
  4. Conservation of momentum

Explanation

KCL states that the algebraic sum of currents entering a node is zero, meaning charge cannot accumulate at an ideal node. It therefore expresses conservation of electric charge.

Question 5Electrical Circuits and Network Analysis

A 12 V source drives 3 A through a resistor. What power is dissipated?

  1. 15 W
  2. 0.25 W
  3. 36 W Correct answer.
  4. 4 W

Explanation

Power equals voltage times current, so 12 V x 3 A = 36 W. The resistance would be 4 ohms, and I squared R also gives 36 W.

Question 6Electrical Circuits and Network Analysis

An ideal voltage source has which internal resistance?

  1. Resistance equal to half the load
  2. Infinite resistance
  3. Zero resistance Correct answer.
  4. Resistance equal to the load

Explanation

An ideal voltage source maintains its terminal voltage regardless of load current, which requires zero internal series resistance. Any internal resistance would cause the terminal voltage to drop as current rises.

Question 7Electrical Circuits and Network Analysis

Which instrument connection is required to measure current through a branch accurately?

  1. Ammeter in series with the branch Correct answer.
  2. Voltmeter in series with the branch
  3. Ammeter in parallel with the branch
  4. Ohmmeter in parallel with the branch

Explanation

An ammeter must carry the branch current, so it is inserted in series. Its ideally zero resistance then causes negligible voltage drop and does not disturb the circuit.

Question 8Electrical Circuits and Network Analysis

A coil has 0.2 H inductance and carries a steady 5 A direct current. What is the stored magnetic energy?

  1. 0.5 J
  2. 2.5 J Correct answer.
  3. 5 J
  4. 25 J

Explanation

Inductor energy is one half L times I squared, so 0.5 x 0.2 x 25 = 2.5 J. The steady current means no induced voltage, but the field still stores energy.

Question 9Electrical Circuits and Network Analysis

A capacitor of 10 microfarad is charged to 100 V. What charge does it store?

  1. 1 mC Correct answer.
  2. 0.1 mC
  3. 10 mC
  4. 100 mC

Explanation

Charge equals capacitance times voltage, so 10 microfarad x 100 V = 1000 microcoulomb = 1 mC. The stored energy would be one half C V squared, or 50 mJ.

Question 10Electrical Circuits and Network Analysis

At resonance in a series RLC circuit, what is the impedance?

  1. Equal to the capacitive reactance
  2. At its minimum and purely resistive Correct answer.
  3. At its maximum and purely reactive
  4. Equal to the inductive reactance

Explanation

At series resonance the inductive and capacitive reactances are equal and cancel, leaving only resistance. The impedance magnitude is therefore minimum and the current is maximum.

Question 11Electrical Circuits and Network Analysis

A 50 Hz sinusoidal voltage has a period of:

  1. 20 ms Correct answer.
  2. 0.02 s per cycle squared
  3. 50 ms
  4. 2 ms

Explanation

Period is the reciprocal of frequency, so 1/50 s = 0.02 s = 20 ms. Each cycle therefore lasts twenty milliseconds.

Question 12Electrical Circuits and Network Analysis

The RMS value of a pure sine wave with peak value 10 A is approximately:

  1. 10 A
  2. 6.37 A
  3. 14.14 A
  4. 7.07 A Correct answer.

Explanation

For a sinusoid, RMS equals peak divided by the square root of two, so 10/1.414 = 7.07 A. RMS is the equivalent direct current that produces the same heating.

Question 13Electrical Circuits and Network Analysis

In a balanced three-phase star connection, the line voltage relates to the phase voltage as:

  1. Line voltage equals root-three times phase voltage Correct answer.
  2. Line voltage equals three times phase voltage
  3. Line voltage equals phase voltage divided by root three
  4. Line voltage equals phase voltage

Explanation

In a star connection the line voltage is the phasor difference of two phase voltages 120 degrees apart, giving a factor of square root of three. Line current equals phase current in star.

Question 14Electrical Circuits and Network Analysis

A circuit has a resistance of 3 ohms in series with an inductive reactance of 4 ohms. What is the impedance magnitude?

  1. 7 ohms
  2. 1 ohm
  3. 5 ohms Correct answer.
  4. 12 ohms

Explanation

Impedance magnitude is the square root of R squared plus X squared, giving the square root of 9 plus 16, which is 5 ohms. Resistance and reactance add vectorially, not arithmetically.

Question 15Electrical Circuits and Network Analysis

The power factor of a circuit is defined as:

  1. The product of real and reactive power
  2. The ratio of apparent power to real power
  3. The ratio of reactive power to apparent power
  4. The ratio of real power to apparent power Correct answer.

Explanation

Power factor equals real power divided by apparent power, which also equals the cosine of the impedance angle. It indicates how much of the supplied apparent power does useful work.

Question 16Electrical Circuits and Network Analysis

The reciprocity theorem applies to a network that contains:

  1. Only dependent sources
  2. Only bilateral linear elements Correct answer.
  3. Only time-varying elements
  4. Only non-linear elements

Explanation

Reciprocity holds for linear bilateral networks, where interchanging the source and response positions leaves the measured response unchanged. Dependent sources and non-linear elements generally destroy the property.

Question 17Electrical Circuits and Network Analysis

The Thevenin equivalent of a network consists of:

  1. Two equal voltage sources in series
  2. A current source in parallel with a resistance
  3. A voltage source in series with a resistance Correct answer.
  4. A voltage source in parallel with a resistance

Explanation

Thevenin's theorem replaces any linear two-terminal network with an open-circuit voltage source in series with the equivalent resistance seen looking back. Norton's equivalent, by contrast, uses a current source in parallel.

Question 18Electrical Circuits and Network Analysis

A source of 10 V with a 2-ohm internal resistance feeds a 2-ohm load. What power is delivered to the load?

  1. 25 W
  2. 12.5 W Correct answer.
  3. 6.25 W
  4. 50 W

Explanation

Total resistance is 4 ohms, so current is 2.5 A. Load power is I squared R, or 6.25 x 2 = 12.5 W, which is the maximum-power-transfer condition.

Question 19Electrical Circuits and Network Analysis

In superposition analysis of a linear network with two independent sources, each source is treated by:

  1. Deactivating it as an open circuit for both source types
  2. Replacing voltage sources with open circuits and current sources with short circuits
  3. Doubling its value while halving the other
  4. Replacing voltage sources with short circuits and current sources with open circuits Correct answer.

Explanation

Superposition sums the responses from each independent source acting alone. Setting a voltage source to zero volts creates a short, while zero current creates an open circuit.

Question 20Electrical Circuits and Network Analysis

An RC series circuit has R = 1 kilo-ohm and C = 1 microfarad. What is the time constant?

  1. 1 millisecond Correct answer.
  2. 1 second
  3. 1 microsecond
  4. 1000 seconds

Explanation

The RC time constant is R times C, so 1000 ohms x 1e-6 F = 1e-3 s, or 1 ms. After one time constant a charging capacitor reaches about 63 percent of final voltage.

Question 21Electrical Circuits and Network Analysis

In a series RLC circuit, what happens to the current as the source frequency moves far above resonance?

  1. Current rises toward a maximum
  2. Current falls because inductive reactance dominates Correct answer.
  3. Current becomes purely resistive and maximum
  4. Current stays constant at the resonant value

Explanation

Above resonance the inductive reactance grows with frequency while capacitive reactance shrinks, so total impedance rises and current falls. The circuit becomes increasingly inductive.

Question 22Electrical Circuits and Network Analysis

A two-port network has an ABCD matrix with A = 2, B = 5 ohms, C = 0.5 siemens and D = 2. Which condition indicates reciprocity?

  1. B equals C
  2. A equals D
  3. AD minus BC equals zero
  4. AD minus BC equals one Correct answer.

Explanation

A reciprocal two-port satisfies AD - BC = 1, the determinant condition of the transmission matrix. Passive networks of R, L and C elements always meet it.

Question 23Electrical Circuits and Network Analysis

Using Laplace analysis, the impedance of a capacitor with zero initial charge is:

  1. sL
  2. R + sC
  3. 1/(sC) Correct answer.
  4. sC

Explanation

Transforming the capacitor law i = C dv/dt gives I(s) = sC V(s), so V/I = 1/(sC). An initial voltage appears as a series source of value v(0)/s.

Question 24Electrical Circuits and Network Analysis

A network function has a pole exactly on the imaginary axis of the s-plane, away from the origin. The natural response is:

  1. A sustained sinusoid Correct answer.
  2. A constant value
  3. A growing exponential
  4. A decaying exponential

Explanation

A pole at plus or minus j omega corresponds to an undamped natural response oscillating at that angular frequency. The real part of the pole, which would set decay or growth, is zero.

Question 25Electrical Machines and Transformers

In an ideal transformer, the primary-to-secondary voltage ratio is determined by which quantity?

  1. The winding resistance
  2. The load power factor
  3. The turns ratio Correct answer.
  4. The core saturation flux

Explanation

Faraday's law applied to both windings sharing the same core flux gives V1/V2 = N1/N2, so the turns ratio sets the voltage transformation. Core flux magnitude and load conditions do not change this ideal relationship.

Question 26Electrical Machines and Transformers

A transformer core is built from thin laminated steel sheets primarily to reduce which loss?

  1. Hysteresis loss
  2. Copper loss
  3. Stray load loss
  4. Eddy current loss Correct answer.

Explanation

Laminations interrupt the paths of induced eddy currents, raising the effective resistance and sharply reducing eddy-current power loss. Hysteresis loss depends on the magnetic material and frequency, not on lamination thickness.

Question 27Electrical Machines and Transformers

The open-circuit test on a transformer is normally performed with rated voltage applied to one winding while the other is left open. What does this test primarily determine?

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

Explanation

With the secondary open, only a small magnetizing current flows, so the measured power is essentially core loss and the results give the shunt branch of the equivalent circuit. Copper loss is negligible because current is very small.

Question 28Electrical Machines and Transformers

A three-phase induction motor runs at a slip of 4 percent. If the synchronous speed is 1500 rpm, what is the rotor speed?

  1. 1440 rpm Correct answer.
  2. 1380 rpm
  3. 1560 rpm
  4. 1500 rpm

Explanation

Rotor speed equals synchronous speed multiplied by (1 minus slip), so 1500 times 0.96 gives 1440 rpm. The rotor always runs below synchronous speed in motoring mode.

Question 29Electrical Machines and Transformers

In a separately excited DC generator, the generated EMF is directly proportional to which combination?

  1. Load current and terminal voltage
  2. Field current and armature reaction
  3. Armature current and armature resistance
  4. Flux per pole, speed, and number of conductors Correct answer.

Explanation

The EMF equation E = (p times phi times N times Z)/(60 times a) shows the generated voltage rises with pole flux, rotational speed, and armature conductors. Armature current affects terminal voltage only through internal drops.

Question 30Electrical Machines and Transformers

Which starting method is commonly used to limit the high starting current of a large squirrel-cage induction motor?

  1. Brush lifting
  2. Commutator tapping
  3. Field weakening
  4. Star-delta starter Correct answer.

Explanation

A star-delta starter applies reduced phase voltage during run-up, cutting starting current to about one third of the direct-on-line value. The motor is then switched to delta for normal running.

Question 31Electrical Machines and Transformers

For a transformer, the voltage regulation at a given load is defined as which quantity?

  1. The ratio of primary current to secondary current
  2. The rise in secondary voltage from no load to full load, expressed as a fraction of rated voltage Correct answer.
  3. The ratio of core loss to copper loss
  4. The percentage drop in efficiency from no load to full load

Explanation

Regulation compares the no-load secondary voltage with the secondary voltage at the specified load, expressed as a per-unit or percentage of rated voltage. It reflects the series impedance drop.

Question 32Electrical Machines and Transformers

A 50 Hz transformer has 200 primary turns and 50 secondary turns. If the primary is connected to 400 V, what is the secondary voltage under no load?

  1. 50 V
  2. 100 V Correct answer.
  3. 200 V
  4. 1600 V

Explanation

The turns ratio is 200 to 50, or 4 to 1, so the secondary voltage is 400 divided by 4, giving 100 V. The ideal transformer relationship is V1/V2 = N1/N2.

Question 33Electrical Machines and Transformers

The short-circuit test on a transformer is usually conducted with reduced voltage applied to one winding while the other is shorted. Why is reduced voltage used?

  1. To keep rated current flowing while limiting core flux and core loss Correct answer.
  2. To eliminate leakage flux entirely
  3. To measure the magnetizing current directly
  4. To saturate the core for accurate readings

Explanation

With the secondary shorted, only a small voltage is needed to circulate rated current, so core flux and core loss stay negligible and the measured power is essentially copper loss. This yields the series impedance parameters.

Question 34Electrical Machines and Transformers

Two single-phase transformers with equal turns ratio but unequal leakage impedances are operated in parallel. How is the load shared between them?

  1. In inverse proportion to their leakage impedances Correct answer.
  2. In direct proportion to their leakage impedances
  3. Only by the transformer with lower impedance
  4. Equally regardless of impedance

Explanation

Parallel transformers share current inversely with their series impedances, so the unit with lower impedance carries more current. Loading must be checked so no unit exceeds its rating.

Question 35Electrical Machines and Transformers

An induction motor develops maximum torque when which condition holds?

  1. Slip equals unity
  2. Rotor reactance is much larger than rotor resistance
  3. Rotor resistance is zero
  4. Rotor resistance equals rotor leakage reactance at standstill Correct answer.

Explanation

The torque-slip curve peaks when the rotor resistance per phase equals the rotor leakage reactance referred to the rotor at standstill. This condition defines the breakdown slip.

Question 36Electrical Machines and Transformers

Adding external resistance to the rotor circuit of a wound-rotor induction motor mainly achieves which effect?

  1. Higher synchronous speed
  2. Higher starting torque with reduced starting current Correct answer.
  3. Lower core loss at full load
  4. Improved power factor at no load

Explanation

External rotor resistance increases rotor circuit resistance, which raises starting torque and limits starting current. It also shifts the breakdown slip to a higher value.

Question 37Electrical Machines and Transformers

In a DC shunt motor, what happens to speed if the field flux is reduced while the load torque remains constant?

  1. Speed becomes zero
  2. Speed remains unchanged
  3. Speed decreases
  4. Speed increases Correct answer.

Explanation

For a DC motor, torque is proportional to flux times armature current, so reduced flux demands higher armature current to hold load torque. Back EMF and hence speed rise accordingly.

Question 38Electrical Machines and Transformers

Which condition must be satisfied for a DC shunt generator to build up voltage?

  1. Armature must rotate in the reverse direction
  2. Load must be connected before starting
  3. Field resistance must exceed the critical value
  4. Field winding must be connected so its MMF aids the residual magnetism Correct answer.

Explanation

Voltage build-up requires residual flux and a field connection that reinforces it, so the induced EMF grows cumulatively. If the field opposes the residual flux, the machine fails to excite.

Question 39Electrical Machines and Transformers

The efficiency of a transformer is maximum when which condition is met?

  1. Core loss is zero
  2. Copper loss equals core loss Correct answer.
  3. Copper loss is twice the core loss
  4. Load current is at rated value

Explanation

Transformer efficiency peaks when variable copper loss equals constant core loss, which occurs at a specific load fraction. At rated load the copper loss may exceed core loss, so peak efficiency often occurs below full load.

Question 40Electrical Machines and Transformers

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

  1. 50 Hz
  2. 1.5 Hz Correct answer.
  3. 3 Hz
  4. 0.5 Hz

Explanation

Rotor frequency equals slip times supply frequency, so 0.03 times 50 gives 1.5 Hz. As slip falls with load, rotor frequency decreases toward zero at synchronous speed.

Question 41Electrical Machines and Transformers

Which statement best describes the role of a commutator in a DC machine?

  1. It reduces the magnetic flux in the air gap
  2. It provides mechanical support to the field poles
  3. It regulates the field current automatically
  4. It converts alternating EMF in the armature coils into unidirectional terminal voltage Correct answer.

Explanation

The commutator and brushes act as a mechanical rectifier, so the alternating EMF induced in rotating coils appears as a direct voltage at the terminals. In motoring mode the same action distributes current to produce unidirectional torque.

Question 42Electrical Machines and Transformers

In a transformer, the no-load current is small compared with rated current mainly because of which reason?

  1. The secondary is always loaded
  2. The core presents high permeability, so a small magnetizing current sets up the working flux Correct answer.
  3. Copper resistance is very small
  4. Leakage reactance is very large

Explanation

A high-permeability core requires only a modest magnetizing MMF to establish rated flux, so the magnetizing component of no-load current is a small fraction of rated current. The core-loss component adds a small in-phase term.

Question 43Electrical Machines and Transformers

A transformer has a per-unit series impedance of 0.05 on its own base. If the base voltage is 400 V and the base power is 100 kVA, what is the approximate base impedance?

  1. 0.16 ohm
  2. 1.6 ohm Correct answer.
  3. 0.016 ohm
  4. 16 ohm

Explanation

Base impedance equals base voltage squared divided by base power, so 400 squared divided by 100000 gives 1.6 ohm. Per-unit impedance is then multiplied by this base value to obtain ohms.

Question 44Electrical Machines and Transformers

A 4-pole, 50 Hz, 3-phase induction motor has a full-load slip of 5 percent. What is the approximate full-load rotor speed?

  1. 1500 rpm
  2. 1575 rpm
  3. 750 rpm
  4. 1425 rpm Correct answer.

Explanation

Synchronous speed is 120 times frequency divided by poles, giving 1500 rpm. Applying 5 percent slip gives 1500 times 0.95, or 1425 rpm.

Question 45Electrical Machines and Transformers

A transformer delivers rated kVA at 0.8 power factor lagging. If its per-unit series resistance is 0.02 and per-unit reactance is 0.06, what is the approximate per-unit voltage regulation?

  1. About 0.080
  2. About 0.020
  3. About 0.052 Correct answer.
  4. About 0.060

Explanation

Approximate regulation equals R cos(phi) plus X sin(phi), so 0.02 times 0.8 plus 0.06 times 0.6 gives 0.016 plus 0.036, about 0.052 per unit. The quadrature term is neglected in this approximation.

Question 46Electrical Machines and Transformers

In a separately excited DC motor, armature reaction mainly causes which effect at heavy load?

  1. A shift in the magnetic neutral axis and weakening of the main field Correct answer.
  2. An increase in the main field flux
  3. A rise in the generated back EMF
  4. Elimination of commutation sparking

Explanation

Armature MMF distorts and partially demagnetizes the main field, shifting the neutral axis in the direction of rotation for a generator and against it for a motor. Compensating windings or interpoles are used to counter this effect.

Question 47Electrical Machines and Transformers

A 3-phase induction motor is operated from a variable-frequency drive with constant volts-per-hertz control. What is the primary purpose of keeping the ratio constant?

  1. To hold slip at unity
  2. To increase the number of poles
  3. To keep rotor resistance constant
  4. To maintain approximately constant air-gap flux and avoid saturation Correct answer.

Explanation

Air-gap flux is proportional to applied voltage divided by frequency, so holding the ratio constant keeps flux near its rated value. This avoids core saturation at low frequency and preserves torque capability.

Question 48Electrical Machines and Transformers

Two identical 100 kVA transformers have per-unit impedances of 0.04 and 0.06 on their own bases and are paralleled on the same bus. If the total load is 150 kVA, what is the approximate loading of the lower-impedance unit?

  1. 90 kVA Correct answer.
  2. 60 kVA
  3. 75 kVA
  4. 100 kVA

Explanation

Load divides inversely with per-unit impedance, so the 0.04 unit takes 0.06 divided by 0.10, or 60 percent of the total, giving about 90 kVA. The 0.06 unit carries the remaining 60 kVA.

Question 49Power Systems (Generation, Transmission, Distribution, Protection)

In a three-phase transmission line, the surge impedance is 400 Ω and the surge impedance loading (SIL) is to be found for a line voltage of 220 kV. Which value is closest to the SIL?

  1. 55 MW
  2. 400 MW
  3. 242 MW
  4. 121 MW Correct answer.

Explanation

SIL equals V²/Zc using line-to-line voltage in volts and surge impedance in ohms. Substituting 220 kV and 400 Ω gives 121 MW, a useful reference for loading a line near its natural power.

Question 50Power Systems (Generation, Transmission, Distribution, Protection)

Which quantity remains constant along an ideal, lossless transmission line under steady-state operation?

  1. Current magnitude
  2. Phase angle
  3. Active power Correct answer.
  4. Voltage magnitude

Explanation

A lossless line has no series resistance or shunt conductance, so no real power is consumed along its length. Active power entering equals active power leaving, while voltage, current and phase angle all vary with distance.