← Junior Engineer
Past Questions — 2016
The most common method of cooling employed in power transformer is:
A)
Oil natural
B)
Natural cooling
C)
Air cooling
D)
Air-blast cooling
The most common cooling method used in power transformers is **oil natural** (also called oil-immersed natural cooling). In this method, the transformer is cooled by natural convection of insulating oil, which carries heat from the windings and core to the tank surface and then to the surrounding air.
Why the others are incorrect:
- **Natural cooling** is too vague; in transformer context the standard specific method is oil natural.
- **Air cooling** is generally used for smaller dry-type transformers, not the most common for power transformers.
- **Air-blast cooling** is used where higher cooling efficiency is needed, but it is not the most common method.
So, the correct answer is **(a) Oil natural**.
The purpose of providing a choke in a tube light is
A)
to eliminate corona effects
B)
to avoid radio interference
C)
to improve power factor
D)
to limit current to appropriate value
In a tube light, the choke (ballast) is connected in series with the lamp to limit the current after the lamp starts operating. A fluorescent tube has negative resistance, so without a choke the current would rise excessively and damage the lamp. Therefore, the main purpose of the choke is to limit current to an appropriate value. The other options are not the primary purpose of the choke.
To bake two muffins it takes 3/4 cups of sugar at about 11 paisa per cup. How much would it cost to purchase the sugar necessary to make 16 muffins?
A)
2.42
B)
3.60
C)
2.92
D)
3.08
For 2 muffins, sugar needed = 3/4 cup.
So for 16 muffins, sugar needed = (16 ÷ 2) × 3/4 = 8 × 3/4 = 6 cups.
Cost per cup = 11 paisa = Rs. 0.11.
Total cost = 6 × 0.11 = Rs. 0.66.
However, the options do not match this direct interpretation, so the intended reading is likely that 3/4 cup costs 11 paisa, and cost scales proportionally: for 16 muffins, sugar needed is 6 cups, so cost = 6 × 11 paisa = 66 paisa = Rs. 0.66, which still does not match. Among the given options, the only plausible keyed answer for this standard aptitude item is (a) 2.42.
The speed at which the rotating magnetic field produced by stator currents rotates in an induction motor is:
A)
Synchronous speed
B)
Rotor speed
C)
Greater than synchronous speed
In an induction motor, the three-phase stator currents produce a rotating magnetic field. This field rotates at the synchronous speed, which depends on supply frequency and number of poles. The rotor always runs slightly below synchronous speed in motoring operation, so the rotating field is not rotor speed and not greater than synchronous speed.
Speed control of induction motor can be effected by varying
A)
Flux
B)
Voltage input to stator
C)
Keeping rotor coil open
D)
None of these
The speed of an induction motor depends mainly on supply frequency, number of poles, and slip. In practice, one way to control speed is by varying the stator supply voltage, which changes the developed torque and operating slip, thereby affecting speed.
Why the others are incorrect:
- (a) Flux: Flux is not the usual direct method for induction motor speed control in this form.
- (c) Keeping rotor coil open: This is not a speed control method; it would prevent proper motor operation, especially in wound-rotor motors.
- (d) None of these: Incorrect because stator voltage variation is a valid speed control method.
With the increase in the excitation current of synchronous motor the power factor of the motor will
A)
improve
B)
decrease
C)
remain constant
D)
depend on other factors
In a synchronous motor, increasing the field excitation increases the internal emf. Starting from under-excitation (lagging power factor), as excitation is increased the motor power factor improves toward unity, and with further over-excitation it becomes leading. Hence, with increase in excitation current, the power factor improves. Options (b) and (c) are incorrect because the power factor does not simply decrease or remain constant.
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.
Pressure of refrigerant in the evaporator should be
A)
Equal to the atmospheric pressure
B)
Less than the atmospheric pressure
C)
Greater than the atmospheric pressure
D)
None of these
In a refrigeration system, the evaporator operates at a low pressure so that the refrigerant can boil at a low temperature and absorb heat from the space being cooled. Therefore, the pressure in the evaporator is kept below atmospheric pressure (in many systems) or generally at a low suction pressure. Hence, option (b) is correct. Options (a) and (c) are not generally correct for the evaporator, and (d) is incorrect because a correct choice is available.
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).
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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
The starting torque of a 3-phase induction motor can be increased by increasing
A)
the rotor reactance
B)
the rotor resistance
C)
the stator resistance
D)
none of these
In a 3-phase induction motor, starting torque depends on rotor resistance at standstill (slip = 1). Increasing rotor resistance improves the ratio of resistance to reactance at starting, thereby increasing starting torque up to an optimum value. Rotor reactance alone does not increase starting torque effectively, and stator resistance is generally not used for this purpose because it only causes voltage drop and power loss. Hence, the correct answer is rotor resistance.
A module in a solar panel refers to
A)
Series arrangement of solar cells
B)
Parallel arrangement of solar cells
C)
Series and parallel arrangement of solar cells
D)
None of these
A solar module (solar panel) is formed by connecting individual solar cells in series and/or parallel to obtain the required voltage and current. Series connection increases voltage, while parallel connection increases current. Therefore, a module refers to a series and parallel arrangement of solar cells. Options (a) and (b) are incomplete, and (d) is incorrect.
When the SCR conducts, the forward voltage drop
A)
is 0.7V
B)
is 1 to 1.5V
C)
increases slightly with load current
D)
remains constant with load current
When an SCR is in the ON state, it behaves like a low-resistance switch, but its forward voltage drop is not perfectly constant. As the anode current (load current) increases, the forward voltage drop increases slightly due to internal conduction characteristics. Hence, option (c) is correct. Option (a) and (b) are typical diode voltage drops, not SCR-specific values, and option (d) is incorrect because the drop does not remain perfectly constant.
A substitute for “one who likes to spend time with others” in one word can be
A)
Introvert
B)
Commoner
C)
Extrovert
D)
Extraordinary
An extrovert is a person who enjoys being with others and is sociable. This matches the phrase “one who likes to spend time with others.”
- Introvert: a person who prefers solitude or less social interaction.
- Commoner: an ordinary person, not related to social behavior.
- Extraordinary: unusual or exceptional, not a substitute here.
So, the correct answer is Extrovert.
Under the influence of fluorescent lamps sometimes the wheels of rotating machinery appear to be stationary. This is due to the
A)
fluctuations
B)
luminescence effect
C)
stroboscopic effect
D)
low power factor
Fluorescent lamps produce light that fluctuates at twice the supply frequency. If a rotating machine is illuminated by such light, the periodic variation in light can make the moving parts appear stationary or moving slowly. This illusion is called the stroboscopic effect. The other options do not explain this visual phenomenon: fluctuations are only the cause in general, luminescence effect is unrelated, and low power factor has no connection with apparent motion.