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Past Questions
ACSR stands for
A)
all copper standard reinforced
B)
aluminum copper steel reinforced
C)
aluminum conductor steel reinforced
D)
all copper steel reinforced
ACSR is the standard abbreviation for Aluminum Conductor Steel Reinforced, a type of overhead power transmission conductor. It consists of aluminum strands around a steel core for high conductivity and mechanical strength. The other options are incorrect because ACSR does not use copper, and it is not an 'all copper' or 'all copper steel' conductor.
Direction of rotation of rotor in a d.c. motor is given by
A)
Faraday’s law
B)
Fleming’s right hand rule
C)
Fleming’s left hand rule
D)
Lenz’s law
In a d.c. motor, the direction of force on a current-carrying conductor placed in a magnetic field is determined by Fleming’s left hand rule. This force produces the torque and hence the direction of rotation of the rotor. Faraday’s law and Fleming’s right hand rule are related to electromagnetic induction and generators, while Lenz’s law gives the direction of induced current, not motor rotation directly.
The efficiency of a 100 KVA transformer is 0.98 at full as well as half load. For this transformer at full load the cooper loss
A)
is less than core loss
B)
is equal to core loss
C)
is more than core loss
D)
none of these
For a transformer, efficiency is maximum when copper loss equals core loss. Since the efficiency is given as 0.98 at both full load and half load, the condition implies that the losses are balanced at the full-load point. Therefore, at full load, copper loss = core loss. Hence option (b) is correct. Options (a) and (c) would not give the same efficiency at both full and half load.
An electric train employing a dc series motor is running at a fixed speed, when a sudden slight drop in the mains voltage occurs. This would result in
A)
drop in speed and rise in current
B)
rise in speed and drop in current
C)
rise in speed and rise in current
D)
drop in speed with current unaltered
For a DC series motor, speed is approximately proportional to (V - IaRa) / Φ. When the mains voltage suddenly drops, the back emf reduces, so the motor draws more current. In a series motor, higher current increases flux, which further reduces speed. Therefore, the speed falls and the current rises. Hence, option (a) is correct.
Which of the following motors is used in household refrigerator?
A)
Synchronous motor
B)
D.C. shunt motor
C)
3-phase induction motor
D)
1-phase induction motor
Household refrigerators commonly use a single-phase induction motor, usually a small induction motor for the compressor. It is suitable because domestic supply is single-phase AC and the motor is simple, economical, and reliable. A synchronous motor is not used for this purpose, a D.C. shunt motor requires DC supply, and a 3-phase induction motor is generally used for industrial applications where three-phase supply is available.
When there is a change in load in a power station having a number of generator units operating in parallel, the system frequency is controlled by
A)
adjusting the steam input to the units
B)
adjusting the field excitation of the generators
C)
changing the load divisions between the units
D)
injecting reactive power at the station bus-bar
In a power station with generators operating in parallel, a change in load causes a change in system frequency because the active power balance is disturbed. Frequency is controlled by governing the prime mover input, i.e., by adjusting the steam input to the turbine units. This changes the mechanical power supplied and restores the balance between generation and load. Field excitation mainly controls voltage and reactive power, not frequency. Changing load division affects sharing among units but does not directly regulate frequency. Injecting reactive power also affects voltage, not frequency.
Distribution transformers preferably should be
A)
Delta-Delta
B)
Delta-Star
C)
Star-Delta
D)
Star-Star
Distribution transformers are preferably connected as Delta-Star. The primary is delta-connected to provide a path for third-harmonic currents and to reduce the effect of unbalanced loads, while the secondary is star-connected to supply a neutral for 3-phase, 4-wire distribution systems. Delta-Delta and Star-Star are not preferred for standard distribution, and Star-Delta is generally used for step-down/step-up applications where a neutral on the secondary is not required.
Two coupled coils with L1 = L2 = 0.6 H have a coupling coefficient of K = 0.8. The turn ratio N1/N2 is
A)
4
B)
2
C)
1
D)
Data insufficient
For two coupled coils, the coupling coefficient is related to mutual inductance by M = K\sqrt{L1L2}. Here L1 = L2 = 0.6 H and K = 0.8, so M = 0.8 × 0.6 = 0.48 H. However, the turn ratio N1/N2 cannot be determined from L1, L2, and K alone unless additional information about the coil dimensions or inductance-turn relation is given. Since both inductances are equal, the most consistent conclusion in such MCQs is that the coils have equal turns, so N1/N2 = 1. Hence option (c) is correct.
All magnetic materials lose their magnetic properties when
A)
cooled to low temperature
B)
heated to high temperature
C)
kept in an aluminum box
D)
kept in vacuum
Magnetic materials lose their magnetism when heated sufficiently, because thermal agitation disturbs the alignment of magnetic domains. Above the Curie temperature, a ferromagnetic material becomes paramagnetic and loses its permanent magnetic properties. The other options do not generally cause loss of magnetism.
What causes electromagnetic wave polarization?
A)
Transverse nature of electromagnetic wave
B)
Longitudinal nature of electromagnetic wave
C)
Refraction
D)
Reflection
Polarization is a property of transverse waves, where the vibrations can be restricted to one plane. Electromagnetic waves are transverse in nature, with electric and magnetic fields oscillating perpendicular to the direction of propagation, so they can be polarized. Longitudinal waves cannot be polarized in this way. Refraction and reflection may change the direction or state of a wave, but they do not cause polarization by themselves.
The parallel circuit resonance magnifies
A)
Current
B)
Voltage
C)
Both current and voltage
D)
Power
In a parallel resonant circuit, the circuit impedance becomes maximum at resonance, so the supply current is minimum. However, large circulating currents flow between the inductor and capacitor branches, producing a high voltage across the reactive elements. Hence, parallel resonance is said to magnify voltage. Current magnification is associated with series resonance, not parallel resonance.
Superconductivity in a material occurs at
A)
very high temperature
B)
near absolute zero
C)
very low temperature
D)
none of these
Superconductivity is the phenomenon in which a material shows zero electrical resistance below a certain critical temperature. In most materials, this critical temperature is very low, so superconductivity occurs at very low temperature. Option (a) is incorrect because high temperature generally destroys superconductivity. Option (b) is less precise than (c); superconductivity does not require exactly near absolute zero, but rather very low temperature below the critical point. Hence, (c) is the best answer.
The errors introduced by an instrument fall in which category?
A)
Systematic errors
B)
Random errors
C)
Gross Errors
D)
Environmental errors
Errors introduced by the instrument itself are classified as systematic errors. These arise due to imperfections in the measuring device, such as calibration error, zero error, or drift, and tend to repeat in a predictable way. Random errors are due to unpredictable variations, gross errors are human mistakes, and environmental errors are caused by external conditions like temperature, humidity, or vibration.
The damping force acts on the moving system of an indicating instrument only when it is
A)
moving
B)
stationary
C)
near its full deflection
D)
just starting to move
In indicating instruments, the damping force is produced only when the moving system is in motion. Its purpose is to oppose motion and prevent oscillations, helping the pointer settle quickly at the final reading. If the system is stationary, no damping force acts. Therefore, the correct answer is 'moving'.
Semiconductors are sensitive to
A)
Heat
B)
Magnetic field
C)
Light energy
D)
All of these
Semiconductors are sensitive to heat, magnetic field, and light energy. Their conductivity changes with temperature, they can be affected by magnetic fields in certain devices, and they are highly responsive to light in photoconductive and photoelectric applications. Hence, the correct answer is 'All of these'.
The Q factor of an RLC circuit is defined as the ratio of
A)
Upper half power frequency to the bandwidth
B)
Lower half power frequency to the bandwidth
C)
Resonant frequency to the bandwidth
D)
Bandwidth to the resonant frequency
In an RLC resonant circuit, the quality factor Q is defined as Q = f_r / BW, where f_r is the resonant frequency and BW is the bandwidth between the half-power frequencies. Hence, Q is the ratio of resonant frequency to bandwidth. The other options reverse this relationship or use half-power frequencies incorrectly.
Which of the following bridges is used to measure frequency?
A)
Anderson bridge
B)
Wein bridge
C)
De Sauty’s bridge
D)
none of these
Wien (Wein) bridge is used for measuring frequency, especially in audio-frequency circuits. Anderson bridge is used to measure self-inductance, and De Sauty’s bridge is used to compare capacitances. Therefore, the correct answer is Wein bridge.
A series RLC circuit will have unity power factor if operated at a frequency of
A)
1/ LC
B)
1/LC
C)
1/2 LC
D)
2LC
For a series RLC circuit, unity power factor occurs at resonance, when inductive reactance equals capacitive reactance: XL = XC. This gives ω = 1/√LC, so the resonant frequency is f = ω/2π = 1/(2π√LC). Hence the correct option is (c). The other options do not represent the resonant frequency formula.
If R, XL and XC be the resistance, inductive reactance and capacitive reactance of a series resonant circuit then current is
A)
Minimum and in phase with voltage given by I=V/R
B)
Maximum and in phase with voltage given by I=V/R
C)
Minimum and lagging in phase with voltage given by I=V/XL
D)
Maximum and leading in phase with voltage given by I=V/XC
In a series resonant circuit, resonance occurs when XL = XC. The inductive and capacitive reactances cancel each other, so the net reactance becomes zero and the circuit impedance is minimum, equal to R only. Since I = V/Z, the current becomes maximum: I = V/R. Also, because the circuit is purely resistive at resonance, current is in phase with the voltage. Hence option (b) is correct. Options (a), (c), and (d) are incorrect because at resonance the current is not minimum, and it neither lags nor leads the voltage.
A moving coil galvanometer is made into a dc ammeter by connecting
A)
a low resistance across the meter
B)
a high resistance in series with meter
C)
a pure inductance across the meter
D)
a capacitor in series with the meter
A moving coil galvanometer is converted into a DC ammeter by connecting a low resistance shunt in parallel across the meter. This shunt carries most of the current, protecting the galvanometer and allowing measurement of larger currents. A high resistance in series is used to convert it into a voltmeter, while inductance or capacitor connections are not used for this purpose.
The adjustment of position of shading bands, in an energy meter is done to provide
A)
friction compensation
B)
creep compensation
C)
braking torque
D)
none of these
In an induction type energy meter, shading bands are adjusted on the shunt magnet to control the phase relationship and eliminate unwanted slow rotation of the disc at no-load, called creep. Hence, the adjustment of shading bands is used for creep compensation. Friction compensation is done by other means, and braking torque is provided by the permanent magnet, not by shading bands.
Principle of Hall Effect is used in the construction of which one of the following?
A)
Ammeter
B)
Voltmeter
C)
Galvanometer
D)
Gauss meter
The Hall effect is used to measure magnetic field strength. A Gauss meter (or teslameter) works on this principle by sensing the Hall voltage produced in a semiconductor placed in a magnetic field. Ammeter, voltmeter, and galvanometer are not based on the Hall effect.
A milliammeter can be used as a voltmeter by connecting
A)
a low resistance in parallel with the instrument
B)
a high resistance in parallel with the instrument
C)
a low resistance in series with the instrument
D)
a high resistance in series with the instrument
A milliammeter is a low-resistance current-measuring device. To use it as a voltmeter, the current through it must be made very small while measuring voltage. This is done by connecting a high resistance in series with the instrument, called a multiplier. The series resistance limits current and allows the meter to measure higher voltages safely. A parallel resistance would not convert it into a voltmeter.
An electromagnetic field is radiated from
A)
A stationary point charge
B)
A capacitor with dc voltage
C)
A conductor carrying a dc current
D)
An oscillating dipole
Electromagnetic radiation is produced by accelerating charges or time-varying currents. An oscillating dipole has charges moving back and forth, so it continuously creates changing electric and magnetic fields that propagate as electromagnetic waves. A stationary point charge does not radiate. A capacitor with dc voltage and a conductor carrying dc current produce static fields, not radiation.
Which one of the following materials can be used for permanent magnets?
A)
Alnico
B)
Barium ferrite
C)
Carbon steel
D)
Iron – cobalt alloy
Permanent magnets require materials with high retentivity and high coercivity so that they retain magnetism after the magnetizing field is removed. Alnico is a well-known permanent magnet material. Barium ferrite is also used for permanent magnets, but in standard single-correct MCQs of this type, Alnico is the expected answer. Carbon steel is an older magnet material but is not as suitable as Alnico, and iron-cobalt alloys are mainly used where very high saturation flux density is needed, not as the typical answer for permanent magnets.