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Past Questions
Transformer core are laminated in order to
A)
Simplify its construction
B)
Minimise eddy current loss
C)
Reduce cost
D)
Reduce hysteresis loss
Transformer cores are laminated to break up the path of circulating currents induced by the alternating magnetic flux. This greatly increases the resistance to these currents and therefore minimizes eddy current loss. Option d is incorrect because lamination does not reduce hysteresis loss; hysteresis loss depends on the magnetic properties of the core material. Option a is incorrect because lamination is not done to simplify construction. Option c is incorrect because lamination is not primarily for reducing cost, though it may indirectly affect material usage and efficiency.
In 3ph induction motor, the rotor field rotates at synchronous speed with respect to
A)
Stator
B)
Rotor
C)
Stator flux
D)
None of these.
In a 3-phase induction motor, the rotor currents produce a rotor magnetic field that rotates at synchronous speed with respect to the rotor itself. This is because the rotor current frequency is the slip frequency, and the rotor field speed relative to the rotor is synchronous. Hence, option (b) Rotor is correct.
Why others are incorrect:
- (a) Stator: The rotor field does not rotate at synchronous speed with respect to the stator; with respect to the stator it rotates at nearly synchronous speed, but not exactly in the same sense of this question.
- (c) Stator flux: The stator flux itself rotates at synchronous speed, but the question asks about the rotor field, not stator flux.
- (d) None of these: Incorrect because the correct reference frame is the rotor.
Choose the correct alternative that will continue the same pattern and replace the question mark in the given series: 120, 99, 80, 63, 48 ?
A)
35
B)
38
C)
39
D)
40
The pattern is the differences between consecutive terms: 120→99 = -21, 99→80 = -19, 80→63 = -17, 63→48 = -15. These are consecutive odd numbers decreasing by 2 each time, so the next difference should be -13. Therefore, 48 - 13 = 35. Hence the correct option is a (35). The other options are incorrect because 38, 39, and 40 do not match the required next term from the difference pattern.
If the flux per pole of a shunt-wound DC generator is halved, the generated e.m.f. at constant speed
A)
is doubled
B)
is halved
C)
remains the same
D)
none of these
For a DC generator, the generated e.m.f. is given by E = (P Φ Z N) / (60 A), where Φ is the flux per pole and N is the speed. At constant speed, E is directly proportional to Φ. Therefore, if the flux per pole is halved, the generated e.m.f. is also halved.
Why the other options are wrong:
- a (is doubled): e.m.f. would double only if flux doubled, not halved.
- c (remains the same): e.m.f. depends on flux, so it cannot remain unchanged when flux changes.
- d (none of these): incorrect because the correct result is explicitly that e.m.f. is halved.
Arrange the words given below in a meaningful sequence: 1. Police 2. Punishment 3. Crime 4. Judge 5. Judgement
A)
3, 1, 2, 4, 5
B)
1, 2, 4, 3, 5
C)
5, 4, 3, 2, 1
D)
3, 1, 4, 5, 2
The logical sequence is: Crime → Police → Judge → Judgement → Punishment. A crime is committed first, then the police investigate/arrest, the judge gives a judgement, and punishment follows. So the correct order is 3, 1, 4, 5, 2, which matches option d. Option a is wrong because it places punishment before judge and judgement. Option b is wrong because it starts with police and punishment before the crime and judge. Option c is wrong because it begins with judgement, which cannot come before crime, police action, and the judge’s decision.
The snubber circuit is used in thyristor circuits for
A)
Triggering
B)
dv/dt protection
C)
di/dt protection
D)
phase shifting
A snubber circuit in thyristor circuits is primarily used for dv/dt protection. It limits the rate of rise of voltage across the thyristor so that the device does not turn on unintentionally due to its internal capacitances.
Why others are incorrect:
- di/dt protection is mainly addressed by series inductance or current-limiting elements, not the snubber’s primary function.
- phase shifting is not the purpose of a snubber circuit; phase control is done by the triggering circuit.
- triggering is performed by gate pulses, not by the snubber circuit.
A solar cell converts
A)
heat energy into electrical energy
B)
solar energy into electrical energy
C)
heat energy into light energy
D)
solar energy into light energy
A solar cell (photovoltaic cell) converts solar energy, specifically sunlight, directly into electrical energy by the photovoltaic effect. Option (b) is correct. Option (a) is incorrect because a solar cell does not convert heat energy into electrical energy; that describes a thermoelectric or heat engine process. Option (c) is incorrect because it does not convert heat energy into light energy. Option (d) is incorrect because a solar cell does not convert solar energy into light energy; it converts light from the sun into electricity.
Traic cannot be used in
A)
Ac voltage regulator
B)
Cycloconverter
C)
Solid state type of switch
D)
Inverter
TRIAC is a bidirectional thyristor mainly used for AC power control, so it can be used in AC voltage regulators and as a solid-state AC switch. It is also used in some cycloconverter-related AC control circuits. However, a TRIAC cannot be used in an inverter because an inverter requires controlled conversion from DC to AC, typically using devices arranged for forced commutation and unidirectional current control; TRIACs are not suitable for inverter operation. Hence, option d is correct. Option a is incorrect because TRIACs are commonly used in AC voltage regulators. Option b is incorrect because TRIACs may be used in AC power conversion/control applications such as cycloconverter circuits. Option c is incorrect because a TRIAC is itself a solid-state switch.
Thermal storage of energy is possible in the form of
A)
sensible heat
B)
latent heat
C)
chemical reaction
D)
any of the above.
Thermal energy can be stored in multiple forms: as sensible heat (by raising the temperature of a material), as latent heat (during phase change), and even through thermochemical/chemical reactions in some thermal storage systems. Therefore, the correct choice is "any of the above." Option a is correct as one valid form, option b is also correct as another valid form, and option c is correct because chemical reaction-based storage is a recognized thermal energy storage method.
The hunting in a synchronous motor takes place when
A)
friction in bearings is more
B)
air gap is less
C)
load is variable
D)
load is constant
Hunting in a synchronous motor is the oscillation of the rotor about its steady synchronous position. It occurs when the load on the motor changes suddenly or repeatedly, i.e., under variable load conditions, because the rotor cannot instantly adjust to the new torque demand and starts swinging.
Why others are incorrect:
- a) More friction in bearings causes mechanical losses and may reduce speed margin, but it is not the usual cause of hunting.
- b) A smaller air gap affects magnetic characteristics and may improve synchronizing torque; it does not directly cause hunting.
- d) Constant load gives steady operation and minimizes hunting, so it is the opposite of the condition that causes it.
The maximum theoretical efficiencies of solar sales could be around
A)
99%
B)
60%
C)
48%
D)
1%.
The maximum theoretical efficiency of solar cells is around 48% for an ideal single-junction photovoltaic cell under standard solar spectrum assumptions (Shockley–Queisser limit). Hence, option (c) 48% is correct. Option (a) 99% is impossible because real solar cells cannot convert nearly all incident solar energy into electricity due to fundamental thermodynamic and spectral losses. Option (b) 60% is higher than the theoretical limit for a single-junction cell, so it is incorrect. Option (d) 1% is far too low and does not represent the maximum theoretical efficiency.
When a reverse bias is applied to a diode, it will
A)
Raise the potential barrier
B)
Lower the potential barrier
C)
Increases the majority-carrier a current greatly
D)
None of these above
In reverse bias, the applied voltage opposes the junction’s built-in electric field, widening the depletion region and increasing the potential barrier. This prevents majority carriers from crossing the junction, so current remains very small (only leakage current). Therefore, option (a) is correct. Option (b) is incorrect because reverse bias does not lower the barrier; it raises it. Option (c) is incorrect because majority-carrier current does not increase greatly in reverse bias. Option (d) is incorrect because one of the given statements is correct.
An unexcited single phase synchronous motor is
A)
Reluctance motor
B)
Repulsion motor
C)
Universal motor
D)
AC series motor
An unexcited single-phase synchronous motor is a reluctance motor. It operates without DC excitation on the rotor; instead, the rotor aligns itself with the rotating magnetic field due to the tendency of the magnetic circuit to take the path of minimum reluctance, producing synchronous speed operation.
Why the others are incorrect:
- Repulsion motor: This is a commutator motor that works on repulsion torque, not a synchronous motor.
- AC series motor: This is essentially a universal motor type, not a synchronous motor.
- Universal motor: A series-wound commutator motor that can run on AC or DC; it is not synchronous.
Therefore, the correct answer is a) Reluctance motor.
The capacity of cell is measured in
A)
Watt-hour
B)
Watts
C)
Amperes
D)
Ampere-hour
The capacity of a cell is measured in ampere-hour (Ah), which indicates the amount of electric charge the cell can deliver over time. For example, a 10 Ah cell can supply 10 amperes for 1 hour, or 1 ampere for 10 hours. Watt and watt-hour are units of power and energy respectively, not cell capacity in the usual battery-rating sense. Ampere is only a unit of current, not capacity.
The effect of increasing the length of air-gap in the induction motor will be increase the
A)
Power factor
B)
Speed
C)
Magnetising current
D)
Air gap flux
Increasing the air-gap length increases the magnetic reluctance of the magnetic circuit. To produce the same air-gap flux, the motor must draw more magnetising current. Hence, the magnetising current increases. The other options are incorrect: air-gap flux does not increase for a given supply; it tends to decrease or require more current to maintain. Power factor becomes worse (lower), not higher, because the magnetising current increases. Speed is not directly increased by a larger air-gap; in fact, a larger air-gap generally reduces performance and efficiency rather than increasing speed.
The principle of operation of a 3ph induction motor is most similar to that of a
A)
Synchronous motor
B)
Repulsion-start induction motor
C)
Transformer with a shorted secondary
D)
Capacitor start, induction motor
A 3-phase induction motor works on transformer action: the stator produces a rotating magnetic field, which induces EMF and current in the rotor. The rotor circuit is effectively short-circuited, so current flows due to induction and produces torque. Hence, its principle of operation is most similar to a transformer with a shorted secondary.
Why others are incorrect:
- a) Synchronous motor: runs at synchronous speed and does not operate by rotor induction in the same way.
- b) Repulsion-start induction motor: this is a special single-phase starting arrangement, not the basic operating principle of a 3-phase induction motor.
- d) Capacitor start, induction motor: again a single-phase motor starting method; it is not the fundamental principle of 3-phase induction motor operation.
In a three phase voltage source inverter operating in square wave mode, the output line voltage is free from
A)
3rd harmonic
B)
7th harmonic
C)
11th harmonic
D)
13th harmonic
In a three-phase voltage source inverter operating in square-wave mode, the phase voltages contain triplen harmonics (3rd, 9th, 15th, etc.). However, in the line voltage, these triplen harmonics cancel out because they are in phase in all three phases and subtract out when line-to-line voltages are formed. Therefore, the output line voltage is free from the 3rd harmonic.
Why the others are incorrect:
- 7th harmonic: not a triplen harmonic, so it is generally present in the line voltage.
- 11th harmonic: also present in the line voltage.
- 13th harmonic: also present in the line voltage.
Hence, the correct answer is a) 3rd harmonic.
A 4-pole three-phase induction motor has a synchronous speed of 25 revls. The frequency of the supply to the stator is:
A)
50 Hz
B)
100 Hz
C)
25 Hz
D)
12.5 Hz
For an induction motor, synchronous speed is given by Ns = 120f / P. Here Ns = 25 rev/s = 25 × 60 = 1500 rpm and P = 4. So, f = Ns × P / 120 = 1500 × 4 / 120 = 50 Hz. Hence option (a) is correct.
Why others are incorrect: (c) 25 Hz would give Ns = 750 rpm for a 4-pole motor, not 1500 rpm. (d) 12.5 Hz would give only 375 rpm. (b) 100 Hz would give 3000 rpm. None of these match the given synchronous speed.
The direction of rotation of universal motor can be reversed by
A)
reversing the supply terminals
B)
switching over from ac to dc
C)
interchanging the brush leads
D)
any of these
A universal motor is a series motor designed to operate on either AC or DC. Its direction of rotation depends on the relative directions of armature current and field current. Therefore, the rotation can be reversed by any method that reverses the current in either the armature or the field while keeping the other unchanged. Reversing the supply terminals reverses both armature and field currents together, so the torque direction remains the same; switching from AC to DC does not reverse rotation at all; and interchanging the brush leads reverses the armature current relative to the field, thereby reversing rotation. Hence, all listed methods are valid in the intended sense, so the correct answer is 'any of these'.
The starting torque of a capacitor start motor is
A)
zero
B)
low
C)
same as rated torque
D)
more than rated torque.
A capacitor-start motor develops a high starting torque because the capacitor creates a large phase difference between the main and auxiliary winding currents, producing a strong rotating magnetic field at start. Hence its starting torque is more than the rated torque. Option (a) zero is incorrect because capacitor-start motors are specifically designed to have non-zero, high starting torque. Option (b) low is incorrect because the starting torque is not low; it is comparatively high. Option (c) same as rated torque is incorrect because starting torque is typically different from and usually higher than the rated (full-load) torque.
Y is in the East of X which is in the North of Z. If P is in the South of Z, then in which direction of Y, is P?
A)
North
B)
South
C)
South-East
D)
None of these
Let Z be at the origin. Since X is north of Z, X is directly above Z. Y is east of X, so Y is directly to the right of X. P is south of Z, so P is directly below Z. Relative to Y, P lies down and to the left, i.e. in the South-West direction. Since South-West is not among the options, the correct choice is d) None of these. Option a (North), b (South), and c (South-East) are all incorrect because P is neither directly above, directly below, nor down-right of Y.
Which of these is not a renewable source of energy
A)
The sun
B)
Natural gas
C)
Wind
D)
Ocean tidal energy
Natural gas is not a renewable source of energy because it is a fossil fuel formed over millions of years and cannot be replenished on a human timescale. The sun, wind, and ocean tidal energy are renewable because they are naturally replenished continuously and are not depleted by normal use.
Which of the following area is preferred for solar thermal electric plants ?
A)
mountain tops
B)
hot arid zones
C)
coastal areas
D)
high rainfall zones.
Solar thermal electric plants are best suited to hot arid zones because they require strong, direct solar radiation (high DNI), clear skies, and low cloud cover for efficient heat collection and power generation. Arid regions also usually have large open land areas, which are ideal for installing solar collectors and mirrors.
Why the others are incorrect:
- Coastal areas: often have higher humidity, cloudiness, and corrosion from salty air, which reduce efficiency and increase maintenance.
- Mountain tops: may receive good sunlight, but terrain is difficult, land is limited, and construction/maintenance are costly.
- High rainfall zones: frequent clouds and rain greatly reduce direct solar radiation, making them unsuitable for solar thermal plants.
Arrange the words given below in a meaningful sequence: 1. Word 2. Paragraph 3. Sentence 4. Letters 5. Phrase
A)
4, 1, 5, 2, 3
B)
4, 1, 3, 5, 2
C)
4, 2, 5, 1, 3
D)
4, 1, 5, 3, 2
The meaningful sequence is from the smallest unit of language to the largest: Letters (4) form a Word (1), words combine to make a Phrase (5), phrases help form a Sentence (3), and sentences together make a Paragraph (2). So the correct order is 4, 1, 5, 3, 2, which matches option a. Option b is incorrect because it places Sentence before Phrase and Paragraph before Sentence. Option c is incorrect because it places Paragraph before Phrase and Sentence. Option d is incorrect because it places Paragraph before Sentence but after Phrase, which breaks the logical progression.
The main purpose of performing open-circuit test on a transformer is to measure its
A)
Copper loss
B)
Core loss
C)
Efficiency of transformer
D)
Insulation resistance
The open-circuit (no-load) test on a transformer is mainly used to determine the core (iron) loss. In this test, the secondary is left open and rated voltage is applied to the primary, so the no-load current is small and the copper loss is negligible. Hence the power measured is essentially the core loss. Option a is incorrect because copper loss is obtained mainly from the short-circuit test, where current is made rated and core loss is negligible. Option c is incorrect because efficiency is not directly measured by the open-circuit test; it is calculated using both open-circuit and short-circuit test results. Option d is incorrect because insulation resistance is measured by a megger, not by the open-circuit test.