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Past Questions
Theoretical maximum efficiency of wind power is about
A)
30%
B)
48%
C)
59%
D)
65%
The theoretical maximum efficiency of a wind turbine is given by the Betz limit, which states that no wind turbine can capture more than 59.3% of the kinetic energy of wind. Hence the closest option is 59%. The other options are either too low or unrealistically high for the theoretical limit.
A Distribution transformer is selected on the basis of:
A)
Voltage regulation
B)
Efficiency
C)
All day efficiency
D)
All of these
A distribution transformer is judged mainly by its performance in service. The important selection criteria include: (1) voltage regulation, because the output voltage should remain nearly constant under varying load; (2) efficiency, because losses must be low; and (3) all-day efficiency, which is especially important for distribution transformers since they remain energized all day but carry varying load. Therefore, all the given factors are relevant.
In an Auto Transformer, the Primary and Secondary are Coupled.
A)
Only Magnetically
B)
Only Electrically
C)
Magnetically as well as Electrically
D)
None of these
In an auto transformer, a single continuous winding acts as both primary and secondary. Therefore, the primary and secondary are connected electrically through the same winding, and they are also magnetically coupled by the common core flux. Hence the correct answer is magnetically as well as electrically. Option (a) is incorrect because coupling is not only magnetic, and option (b) is incorrect because it is not only electrical. Option (d) is also incorrect.
The efficiency of the solar cell is about
A)
25%
B)
15%
C)
40%
D)
60%
A typical commercial solar cell has an efficiency of about 15% under standard conditions. This means only about 15% of the incident solar energy is converted into electrical energy. Option (a) 25% is higher than the usual commercial value, while 40% and 60% are much too high for ordinary solar cells.
In an a.c. motor control, the ratio of voltage to frequency is maintained at constant value
A)
to make maximum use of magnetic circuit.
B)
to make minimum use of magnetic circuit.
C)
to maximize the current drawn from the supply to provide torque.
D)
to minimize the current drawn from the supply to provide torque.
In AC motor control, keeping the V/f ratio constant maintains approximately constant air-gap flux. This allows the magnetic circuit to be used efficiently without over-fluxing or under-fluxing. Hence, constant V/f is used to make maximum use of the magnetic circuit. The other options are incorrect because the purpose is not to maximize or minimize supply current directly, but to control flux and torque properly.
Major insulation in a transformer is the insulation between the:
A)
LV winding and core
B)
LV winding and HV winding
C)
Turns of the windings
D)
Both (a) and (b)
In a transformer, the major insulation is the insulation that withstands the highest electric stress. This includes: (1) insulation between the windings and the core, and (2) insulation between the LV and HV windings. Insulation between turns of the same winding is called minor insulation. Therefore, the correct answer is Both (a) and (b).
What does it mean to say “don’t judge a book by its cover”?
A)
Don’t believe everything you hear
B)
In judging a person or an object, do not look at outward appearance alone
C)
Well-bound books often have cheap covers
D)
Someone else’s situation always seems better than our own
The idiom “don’t judge a book by its cover” means that outward appearance may be misleading, so one should not form an opinion based only on looks. Option (b) states this meaning correctly. Option (a) is about hearsay, (c) is literal and not idiomatic, and (d) refers to comparison/envy, which is unrelated.
When a sodium vapor lamp is switched on, initially the color is
A)
Pink
B)
Yellow
C)
Green
D)
Blue
When a sodium vapour lamp is first switched on, the sodium is not yet fully vaporized and the discharge occurs through the starting gas. This gives a pinkish glow initially. As the lamp warms up, sodium vapour increases and the light changes to the familiar yellow color. Hence, the initial color is pink.
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 synchronous position. It occurs mainly when the load on the motor changes suddenly or varies, causing the rotor to swing back and forth before settling. A constant load does not normally cause hunting. Friction in bearings and air-gap size are not the direct causes of hunting.
Which of the following transformer connections will give highest secondary voltage?
A)
Delta primary, delta secondary
B)
Delta primary, star secondary
C)
Star primary, delta secondary
D)
Star primary, star secondary
In a transformer, the secondary line voltage depends on the winding connection. For the same turns ratio, a star-connected secondary gives line voltage equal to √3 times the phase voltage, while a delta-connected secondary gives line voltage equal to the phase voltage. Therefore, a delta-primary, star-secondary connection produces the highest secondary line voltage among the given options. The other options either give delta secondary (lower line voltage) or star-star, which does not boost the secondary line voltage as much as delta-star.
Thyristor switching circuits are used
A)
to reduce the stator voltage
B)
to increase the stator voltage
C)
to keep the stator voltage control
D)
none of these
Thyristor switching circuits are used in AC voltage controllers to regulate the stator voltage supplied to a motor. By controlling the conduction angle of thyristors, the effective RMS stator voltage can be adjusted. Hence, they are used to keep the stator voltage under control. Options (a) and (b) are too specific and not generally correct, and (d) is incorrect because option (c) matches the purpose.
Power MOSFET is a
A)
Voltage controlled device
B)
Current controlled device
C)
Frequency controlled device
D)
None of these
A power MOSFET is controlled by the gate-to-source voltage. The gate draws negligible steady-state current because the gate is insulated by an oxide layer, so it is classified as a voltage-controlled device. It is not current-controlled like a BJT, and frequency control is not its mode of control.
What would happen if we operate a 60 Hz Transformer on 50 Hz Source of Supply?(and how can we do that?)
A)
Current will decrease (so increase the current)
B)
Current will increase (so decrease the current)
C)
Current will be same in both cases.
D)
No Effect (We can do that without changing anything)
For a transformer, the induced emf per turn is proportional to frequency: E ∝ f. If a 60 Hz transformer is operated on a 50 Hz supply at the same applied voltage, the flux in the core increases because Φ ∝ V/f. Higher flux can push the core toward saturation, causing the magnetizing current to rise sharply. So, the current increases. To operate it safely on 50 Hz, we must reduce the applied voltage in proportion to frequency, i.e., keep V/f constant: V50 = V60 × (50/60).
The crawling in the induction motor is caused by
A)
improper design of the machine
B)
low supply voltage
C)
high loads
D)
harmonics developed in the motor
Crawling in an induction motor is the tendency to run at a speed much lower than the normal speed, often near one-seventh of synchronous speed. It occurs due to the presence of harmonic torques, especially the 7th harmonic produced in the motor. These harmonics create a stable running point at low speed, causing crawling. Hence, the correct answer is harmonics developed in the motor. The other options are not the primary cause: improper design may affect performance, low voltage mainly reduces torque, and high load causes slip increase but not crawling specifically.
Best protection is provided by HRC fuses in case of
A)
Open circuits
B)
Short circuits
C)
Overloads
D)
None of these
HRC (High Rupturing Capacity) fuses are designed to interrupt very large fault currents safely and quickly. Their best protection is therefore against short-circuit faults. They are not the best choice for open circuits, and overload protection is generally better handled by other protective devices such as relays or circuit breakers with suitable settings.
Direct current is preferred over alternating current for testing of ac transmission lines and cables because:
A)
Heavy charging currents will be drawn and so a large sized transformer is required if tested with ac
B)
Transmission lines and cables should not be tested with dc
C)
The transformers required for testing cannot be used for long distances
D)
All of these
For testing AC transmission lines and cables, DC is preferred because if AC is used, the line/cable capacitance causes heavy charging current. This demands a very large testing transformer and makes AC testing impractical. Option (b) is incorrect because transmission lines and cables can be tested with DC; in fact, DC is commonly used for high-voltage testing. Option (c) is irrelevant to the reason for choosing DC. Therefore, the correct answer is (a).
In air blast circuit breakers, the pressure of air is of the order of
A)
100mm Hg
B)
1 kg/cm2
C)
20 to 30 kg/cm2
D)
200 to 300 kg/cm2
In an air-blast circuit breaker, compressed air is stored at a high pressure, typically in the range of about 20 to 30 kg/cm², and this air is used to blow out the arc. Option (a) is too low, (b) is much lower than the usual operating pressure, and (d) is excessively high for standard air-blast circuit breakers.
The instrument which is cheapest for dc measurement is:
A)
Moving iron
B)
PMMC
C)
Hot-wire
D)
Electro-dynamo
For DC measurements, the moving iron instrument is generally the cheapest among the given options. It is simple in construction and less expensive than PMMC instruments. PMMC is more accurate but costlier and mainly used for DC. Hot-wire and electro-dynamo instruments are more complex and costly, so they are not the cheapest choice.
Electrostatic type instruments are mainly used for measurement of:
A)
Heavy currents
B)
Low currents
C)
Low voltages
D)
High voltages
Electrostatic instruments work on the principle of electrostatic attraction/repulsion. Since the deflecting torque is very small, they are mainly suitable for measuring very high voltages and not currents. They are not used for heavy or low current measurement, and low voltages are usually measured by other instruments.
A system having connected load of 100 kW, peak load of 80 kW, base load of 20 kW and average load of 40 kW, will have a load factor of
A)
40%
B)
50%
C)
60%
D)
80%
Load factor = (Average load / Peak load) × 100. Here, average load = 40 kW and peak load = 80 kW. So, load factor = (40/80) × 100 = 50%. Connected load and base load are not needed for calculating load factor. Hence the correct option is (b) 50%.
If the height of transmission towers is increased, which of the following parameters is likely to change?
A)
Resistance
B)
Inductance
C)
Capacitance
D)
None of these
When the height of transmission towers is increased, the conductors are placed farther from the ground. This increases the effective distance between the line conductors and the earth, which changes the electric field distribution and reduces the shunt capacitance to ground. Among the given options, the parameter affected is capacitance. Resistance is not affected by tower height, and inductance is mainly determined by conductor spacing and geometry rather than tower height alone.
Which of the following is not the distribution system normally used?
A)
3 phase-4 wire
B)
3 phase-3 wire
C)
Single phase - 3 wire
D)
Single phase -4 wire
In distribution systems, the commonly used arrangements are 3-phase 4-wire, 3-phase 3-wire, and single-phase 3-wire systems. A single-phase 4-wire system is not normally used in standard distribution practice. Therefore, option (d) is the correct answer.
On what factor does the operating speed of the relay depend upon?
A)
Rate of flux built up
B)
Armature core air gap
C)
Spring tension
D)
All of these
The operating speed of an electromechanical relay depends on multiple factors: the rate at which flux builds up in the magnetic circuit, the air gap between the armature and core, and the spring tension opposing motion. A faster flux build-up and smaller air gap generally make the relay operate faster, while higher spring tension slows it down. Therefore, all the given factors affect relay operating speed.
Demand factor is the
A)
Maximum Demand / Average Demand
B)
Maximum Demand / Connected Load
C)
Average Demand / Maximum Demand
D)
Connected Load / Maximum Demand
Demand factor is defined as the ratio of maximum demand to the total connected load. So, Demand Factor = Maximum Demand / Connected Load. Option (a) is the load factor relation if reversed incorrectly, option (c) is the inverse of load factor, and option (d) is the reverse of demand factor.
Protective relays can be designed to respond to
A)
Light intensity, impedance
B)
Temperature, resistance, reactance
C)
Voltage and current
D)
All of these
Protective relays may be designed to operate based on many electrical quantities depending on the protection scheme. They can respond to light intensity in special relays, impedance in distance relays, temperature in thermal relays, resistance/reactance in impedance-based relays, and also voltage and current in overvoltage, undervoltage, overcurrent, and related relays. Therefore, all the listed quantities are possible inputs for protective relays.